The starting point for selection is not the HPLC Safety Cap itself, but the specific application. The first step is to determine which solvent bottles are used, how many supply tubing lines are required and what their outer diameters are, which eluents are used, and what requirements the analytical method places on the solvent supply.

Together with its integrated components, an HPLC Safety Cap forms a controlled closed system for supplying solvents to an HPLC system. The bottle opening and capillary feedthroughs are sealed against uncontrolled air exchange. When eluent is drawn from the bottle during HPLC operation, negative pressure develops. The integrated air valve for pressure equalization opens as required and allows filtered ambient air to flow in until the pressure has equalized. The valve then closes again so that the system remains largely closed off from the laboratory environment.

Internationally, these closure systems are also referred to as “Mobile Phase Caps”, “HPLC Solvent Bottle Caps” or “Solvent Reservoir Caps”. Throughout this guide, the term HPLC Safety Cap, which is established in HPLC practice, is used consistently.

The solvent bottle used is a key selection criterion. Many HPLC laboratories use 1-litre glass laboratory bottles, often with a GL 45 thread. Depending on the application, however, bottles with other volumes, materials or thread types may also be used. The bottle type and, in particular, the bottle thread determine which HPLC Safety Cap can be mounted securely and leak-tight.

Analytical or Preparative Application

First, determine whether the application is analytical or preparative HPLC.

In analytical HPLC, supply tubing with outer diameters of 1.6 mm, 2.3 mm or 3.2 mm is commonly used. These dimensions correspond to the standard fittings and feedthroughs used with HPLC Safety Caps.

Preparative laboratory applications, by contrast, operate at higher flow rates. They therefore require larger supply tubing and appropriately sized connections, for example for tubing outer diameters of 4.0 mm, 4.76 mm or 6.35 mm.

The type of application therefore directly determines which tubing diameters, fittings and feedthroughs must be considered when configuring the HPLC Safety Cap. The detailed selection of tubing and fittings is covered in a later section.

Eluents Used and HPLC Operating Mode

An eluent is the solvent or solvent mixture that transports the sample through the chromatographic system. Depending on the method, a single eluent, a constant eluent mixture, or multiple eluents with a changing composition may be used.

Another important starting point is therefore whether the HPLC is operated isocratically or with a gradient.

Isocratic HPLC

In isocratic HPLC, a single eluent or a constant eluent mixture is used throughout the entire analysis.

Gradient HPLC

In gradient HPLC, two or more eluents from different solvent bottles are usually used. The composition of the mobile phase changes during the analytical run.

A simple isocratic HPLC system therefore places different requirements on the solvent supply than a gradient system using several mobile phases. For each solvent bottle, it must be determined how many supply tubing lines are required and what their outer diameters are. This directly determines the required number and type of capillary feedthroughs or connections in the HPLC Safety Cap.

The eluents and additives used are particularly relevant to the selection of the HPLC Safety Cap when they impose special requirements on the chemical resistance or freedom from contamination of the media-contacting components.

For conventional HPLC applications, PTFE is the proven standard because of its very high chemical resistance and inertness. This applies particularly to the media-contacting core of the HPLC Safety Cap and to the supply tubing.

For PFAS-sensitive applications, replacing a single component is not sufficient. A consistently PFAS-free material combination is required, for example a non-fluorinated HDPE bottle, a PEEK core and PEEK tubing, PE fittings and PE blind plugs, a GreenLine air valve, and UHMW-PE suction filters.

The specific material selection must be assessed for each component and application. Chapter 4 discusses the chemical and analytical suitability of the materials in detail.

When taking stock of the existing setup, the technical data and current configuration of the HPLC solvent supply should also be reviewed. Particularly relevant are the number of solvent channels, the permissible flow range of the pump, and the design and outer diameter of the existing supply tubing or the tubing specified by the instrument manufacturer.

For selecting the HPLC Safety Cap, the actual outer diameter of the supply tubing is especially important because it determines the required fittings and feedthroughs.

Planned extensions or a later change of method should also be taken into account during selection.

Questions to Answer Before Selecting an HPLC Safety Cap

• Is the application analytical or preparative HPLC?
• Is the HPLC operated isocratically or with a gradient?
• How many solvent bottles are required?
• Which bottle types, bottle volumes and bottle threads are used?
• How many supply tubing lines are required?
• What are the outer diameters of the supply tubing?
• What flow rates are intended for the application?
• Which eluents and additives are used?
• Do they impose special requirements on the chemical resistance or freedom from contamination of the media-contacting components?
• Are PTFE components required for conventional HPLC applications, or is a consistently PFAS-free material combination needed?
• Are future extensions or changes to the application foreseeable?

An HPLC Safety Cap is more than simply a closure for the solvent bottle. It forms the central interface between the solvent bottle and the HPLC system and is part of a controlled closed solvent supply system.

For this system to function reliably, the HPLC Safety Cap, solvent bottle, supply tubing, fittings, air valve, suction filters and blind plugs must be technically matched to one another.

What Functions Does an HPLC Safety Cap Perform?

An HPLC Safety Cap does far more than act as a simple bottle closure. It provides the basis for a controlled closed solvent supply and performs two key functions: it reduces the release of solvent vapours into the laboratory environment and helps keep the composition of the mobile phase as constant as possible during HPLC operation.

Open or only loosely closed solvent bottles can release volatile solvent components into the laboratory air. The controlled closed design of the HPLC Safety Cap reduces these emissions and thereby supports occupational safety in the laboratory.

At the same time, evaporation from the solvent bottle is reduced. This is particularly relevant for premixed mobile phases because individual components of a solvent mixture can evaporate at different rates. This can change the mixing ratio. Such a change affects the chromatographic conditions and can, for example, lead to shifts in retention times and reduced reproducibility of the analysis.

Because the HPLC continuously draws eluent from the bottle during operation, the withdrawn volume must be replaced by incoming air. This function is performed by the integrated air valve. It opens when negative pressure develops, allows filtered ambient air to flow in, and then closes again. As a result, the solvent supply remains largely closed against uncontrolled exchange of air and vapours.

Which Components Are Part of the System?

Depending on the application, the specific design of individual components may vary. However, the basic functional areas are the same in all HPLC Safety Cap systems.

The HPLC Safety Cap first establishes the mechanical connection to the solvent bottle. This requires its thread to match the bottle thread exactly and the bottle opening to be sealed reliably. At the same time, it guides the supply tubing into the bottle in a controlled manner and secures it using suitable fittings and capillary feedthroughs.

The individual components perform different functions:

• HPLC Safety Cap with media-contacting core: establishes the connection to the solvent bottle, seals the bottle opening, and accommodates the other system components.
• Fittings and capillary feedthroughs: guide and secure the supply tubing and seal the annular gap between the tubing and the feedthrough against uncontrolled air exchange.
• Air valve: contains a filter membrane and a duckbill valve. When eluent withdrawal creates slight negative pressure, the duckbill valve opens only toward the inside of the bottle and allows filtered ambient air to flow in. In the opposite direction, an intact valve remains closed and thus reduces the escape of solvent vapours by this route.
• Supply tubing: transports the eluent from the bottle to the HPLC system.
• Suction filters: are located at the lower end of the supply tubing and retain particles already present in the eluent or solvent bottle, or introduced into the system during handling, before they can enter the HPLC system.
• Blind plugs: seal unused connections and prevent uncontrolled air exchange.

Only the technically coordinated interaction of these components creates a controlled closed solvent supply system. An HPLC Safety Cap with the correct bottle thread is therefore not yet a complete solution. It is equally important that the number and outer diameters of the supply tubing, the fittings, feedthroughs, air valve, suction filters and blind plugs match the specific application.

Protection Against Contamination and Changes in the Mobile Phase

In addition to reducing solvent vapours, the HPLC Safety Cap helps keep the prepared quality and composition of the mobile phase as constant as possible during HPLC operation.

The required pressure equalization does not occur uncontrolled through an open bottle neck, but in a controlled manner through the air valve. The integrated filter membrane reduces the ingress of airborne particles with the incoming ambient air.

Reduced evaporation additionally helps preserve the prepared mixing ratio of the mobile phase as far as possible.

The suction filter performs a complementary function. It retains particles already present in the eluent or solvent bottle, or introduced into the system during handling, before they can enter the HPLC system through the supply tubing.

The HPLC Safety Cap does not purify the mobile phase. However, it helps reduce contamination from the laboratory environment, particulate ingress and evaporation-related changes, thereby helping to preserve the prepared quality of the mobile phase during use.

Clear and Reproducible System Configuration

A correctly configured HPLC Safety Cap also provides structure to the entire solvent supply. Each supply tubing line is routed through a defined connection and securely fixed. Unused connections are sealed with blind plugs, while the air valve is installed in the designated connection.

This is particularly relevant for HPLC systems with multiple solvent bottles and supply tubing lines. Clear tubing routing simplifies installation, inspection and maintenance and reduces the risk of mix-ups, incorrectly assigned lines or unintended changes to the configuration.

The original configuration should also remain clearly traceable after a bottle change or reassembly. Clear labelling of the solvent bottles and supply tubing can provide additional security.

Do Not Consider Any Component in Isolation

System performance does not depend solely on the quality of the HPLC Safety Cap. Even a technically high-quality cap cannot perform its function reliably if, for example:

• the bottle thread does not match
• an unsuitable fitting is used
• the capillary feedthrough does not match the outer diameter of the supply tubing
• a connection is left open
• the air valve is missing, damaged or not functioning
• the suction filter is unsuitable or incorrectly positioned.

Selection should therefore not begin with a single product number. The complete system configuration should first be defined. Only then can it be determined which HPLC Safety Cap and which supplementary components are required for the specific application.

Selecting the correct HPLC Safety Cap starts with the solvent bottle. Even if the supply tubing, fittings and other components have been selected correctly, the system can only function reliably if the Safety Cap matches the bottle thread exactly.

The bottle type and, in particular, the bottle thread directly determine which version of the HPLC Safety Cap can be used.

Which Solvent Bottle Is Used?

Many HPLC laboratories use 1-litre glass laboratory bottles with a GL 45 thread. However, smaller or larger glass bottles, plastic bottles and special storage containers are also used.

Before selecting the HPLC Safety Cap, the following should therefore first be clarified:

• What material is the solvent bottle made of?
• What is its volume?
• Which bottle thread does it have?
• Is it a standard laboratory bottle or a special design?
• Should the existing bottle continue to be used, or should the solvent supply be standardised?

The bottle thread is particularly important for selection. Only if the internal thread of the HPLC Safety Cap and the external thread of the solvent bottle are technically compatible can the cap be mounted correctly and the bottle opening sealed reliably.

Why Thread Identification Is More Complex Than It First Appears

There is no single thread system used worldwide for bottle and container threads. In laboratory and packaging applications, GL, S, DIN, GPI and SPI threads are used, among others, as well as various manufacturer-specific or not clearly standardised designs.

As a result, threads with similar outer diameters may have different pitches, minor diameters, thread forms or thread lengths. Conversely, different thread designations may be so similar in their dimensions that a closure appears to fit the thread even though there is no unambiguous technical match.

The thread designation must therefore not be inferred solely from the bottle volume, material or external shape of the container. The description “1-litre laboratory bottle” is likewise insufficient for selecting an HPLC Safety Cap.

Which Dimensions Are Important for Thread Identification?

For the most reliable possible thread identification, several characteristics should be assessed together:

• Outer diameter of the container thread: largest diameter across the outer thread crests
• Minor diameter of the container thread: diameter at the root of the external thread, or in the lower area of the external thread
• Thread pitch: distance between two consecutive thread turns
• Thread length and thread design: especially for threads available in short and long versions
• Thread form: for example GL, S or GPI design

A caliper should be used wherever possible to measure the outer and minor diameters. The outer diameter alone is often insufficient for a reliable assignment. The nominal thread size alone is also insufficient to identify a bottle thread.

For example, GL 32 and S 32 have very similar outer diameters but differ, among other things, in thread pitch. GL 45 and S 45 are likewise close in their dimensions. Due to manufacturing tolerances, a closure may in some cases even screw onto a thread that is not an exact match.

Other easily confused designs include S 50 and S 51. For GPI 38, a short 38-400 version and a longer 38-430 version are also distinguished. The outer diameter and pitch may be identical while the thread length or design differs.

These examples show that the ability to screw a cap onto a bottle is not sufficient evidence of a technically correct threaded connection.

Systematically Identify the Bottle Thread

1. Check Existing Information

First, check markings on the bottle, information on the original closure, product numbers and product documentation from the container manufacturer. Embossed markings in the existing cap can also provide initial clues.

However, such information should only be used as guidance. A number embossed in the original closure does not necessarily correspond to the actual thread designation.

2. Measure Outer and Minor Diameters

The outer diameter and minor diameter of the container thread should preferably be determined using a caliper. The measured values can then be compared with the SCAT thread identification table.

3. Compare Pitch and Thread Design

If several threads are possible based on the measured diameter, the thread pitch, thread length and visible thread form must also be considered. This comparison is especially important for very similar threads.

4. Allow for Manufacturing Tolerances

The stated dimensions are reference values. Depending on the container manufacturer, actual dimensions may deviate from the table values due to manufacturing tolerances. A small difference between the measured value and the reference dimension therefore does not automatically mean that the thread is a different type.

If in Doubt, Have the Container or Closure Checked

Because of the large number of different thread systems, similar dimensions and manufacturer-dependent production tolerances, a bottle or container thread cannot always be identified unambiguously from measurements or photographs.

If doubts remain after measurement, selection of the HPLC Safety Cap should not be based on an estimate. Customers can therefore send the existing container or its original closure to SCAT Europe for inspection.

The sample can be used to determine which thread design is actually present and which HPLC Safety Cap or thread adapter is suitable for it.

Particularly for special threads, older containers, closures without clear markings or very similar thread designs, this inspection provides a reliable basis for selection.

Typical Threads on HPLC Solvent Bottles

Which threads are most common in practice depends strongly on the bottle manufacturer and the solvent bottle used.

GL threads are widely used on conventional laboratory glass bottles. GL 45 is the most common thread for laboratory glass bottles. Depending on bottle size, manufacturer and application, other designs may also be used, for example:

• GL 28
• GL 32
• GL 38 or GPI 38-400
• GPI 38-430
• GL 40
• GL 45
• S 40/41
• S 45
• other special and manufacturer-specific threads

Knowing the bottle manufacturer or brand can help with identification, but it does not replace a technical check. A manufacturer may use different container series and thread types.

Why an Incorrect Thread Is Problematic

An HPLC Safety Cap must not merely appear to screw onto the bottle. The thread, seal and bottle opening must fit together in such a way that the cap can be mounted straight, completely and without excessive force.

An unsuitable combination can result in problems such as:

• The Safety Cap can only be screwed on partially.
• The cap sits at an angle or is mechanically stressed.
• The seal does not fully contact the bottle opening.
• The system is not reliably sealed against uncontrolled exchange of air and vapours.
• The bottle thread or the Safety Cap may be damaged.
• A connection that only appears to fit may loosen during a bottle change or during operation.

A stiff or difficult assembly should therefore not be compensated for by applying greater force. If the HPLC Safety Cap cannot be mounted evenly and without cross-threading, the thread size and thread design must be checked again.

When Is a Thread Adapter Useful?

Thread adapters are available for bottles or containers with different thread types. They make it possible to use an HPLC Safety Cap on a solvent bottle for which no directly matching cap version is available.

A thread adapter may be useful, for example, when:

• an existing special-purpose bottle is to continue to be used
• the bottle type is specified for methodological or operational reasons
• different bottle threads within a laboratory are to be standardised
• no directly matching HPLC Safety Cap is available.

In principle, however, an HPLC Safety Cap that directly matches the bottle thread should be preferred. Every additional connection increases system complexity and creates another point at which the thread, fit and seal must all be correct.

A thread adapter should therefore be used selectively and only when technically necessary or operationally appropriate.

Also Check Bottle Geometry and Installation Conditions

In addition to the thread, the practical installation situation should also be considered. Together with the air valve, fittings and supply tubing, an HPLC Safety Cap requires sufficient space above and beside the solvent bottle.

The following should be checked in particular:

• available height in the solvent cabinet or on the HPLC system
• distance from other solvent bottles
• stress-free routing of the supply tubing
• a safe and stable bottle position
• good accessibility for bottle changes, inspection and maintenance.

A technically compatible threaded connection alone is not sufficient if the fully assembled HPLC Safety Cap does not have enough space in the intended setup or if the supply tubing has to be sharply bent or routed under tension.

Before selecting an HPLC Safety Cap, the following questions in particular should therefore be answered:

• Which bottle type is used?
• What volume and material does the solvent bottle have?
• Which thread designation is stated on the bottle or original closure?
• What values were measured for the outer and minor diameters?
• What are the thread pitch and thread design?
• Has the thread been identified unambiguously, or are several assignments possible?
• Is a directly matching HPLC Safety Cap available?
• Is a thread adapter required?
• Can the fully assembled Safety Cap be installed without mechanical stress and with good accessibility?
• Should the container or original closure be sent to SCAT Europe for inspection?
• Should the existing range of bottles be standardised over the long term?

After determining the bottle type and bottle thread, the next question is: Which materials should be used for the media-contacting components of the HPLC solvent supply?

The media path begins in the solvent bottle itself. Media-contacting components are all parts and surfaces that come into direct contact with the eluent or with the solvent-containing gas phase generated from it. Depending on the design, these include in particular:

• the solvent bottle
• the media-contacting core of the HPLC Safety Cap
• supply tubing
• fittings and capillary feedthroughs
• suction filters
• seals and other connection elements
• where applicable, components of the air-venting system.

The outer cap body may be made of a different material, such as polypropylene. For chemical and analytical assessment, the components that come into direct contact with the eluent or solvent-containing gas phase are the most relevant.

Material selection must therefore not be limited to a single component. The decisive factor is that all media-contacting components are chemically and analytically suitable for the specific application.

Why Material Selection Matters

During HPLC operation, media-contacting materials are continuously or regularly exposed to solvents, buffers and additives. Two requirements in particular must be distinguished:

Chemical resistance: The material must not swell excessively, become brittle, deform or otherwise be damaged by the eluent.

Analytical suitability: As far as possible, the material should not release constituents into the mobile phase that could affect sensitive analyses. Unwanted adsorption and desorption effects should likewise be kept to a minimum.

Extractables are constituents that can be released from a material under defined test conditions. Leachables are substances that migrate into the medium under actual conditions of use. Conventional routine analysis may therefore have different requirements from highly sensitive trace, metal-ion or PFAS analysis.

Selecting the Material of the Solvent Bottle

Borosilicate Glass as the Standard

For conventional HPLC applications, borosilicate glass laboratory bottles have become the established standard. Borosilicate glass is dimensionally stable, transparent and chemically resistant to many typical HPLC eluents. The fill level can be checked from the outside, and the bottles are available in common volumes and thread sizes.

In particular, 1-litre borosilicate glass laboratory bottles with a GL 45 thread are widely used in many laboratories. For most conventional HPLC applications, borosilicate glass is therefore the proven standard solution.

HDPE Bottles for PFAS-Sensitive Applications

For PFAS-sensitive applications, a suitable non-fluorinated HDPE solvent bottle may be appropriate. HDPE is fluorine-free and does not belong to the PFAS substance group. This requires that the container has not been fluorinated and that no fluorine-containing coatings or other fluorinated components are used.

Glass surfaces can adsorb PFAS compounds that are already present in a solution or on the container surface. At trace levels, such interactions can affect recovery and reproducibility. Under changed conditions, previously adsorbed PFAS compounds may also be released again.

Adsorption alone, however, does not create PFAS contamination. For PFAS to migrate from a container surface into an originally PFAS-free eluent, corresponding residues must already be present. Possible sources of ingress include manufacturing, cleaning, packaging, closure components, storage, previous use and handling in the laboratory.

Plastic containers can also be contaminated with traces of PFAS through manufacturing, storage or handling. An HDPE bottle alone therefore does not guarantee a PFAS-free solvent supply. For PFAS-sensitive applications, the entire relevant media path must consist of suitable PFAS-free components and should be controlled using blanks or system blanks.

For conventional HPLC methods, borosilicate glass remains the proven standard solution. For PFAS-sensitive applications, a qualified, non-fluorinated HDPE bottle may be the more suitable alternative because it supports a fluorine-free media path and reduces adsorption effects associated with glass.

Other Container Materials

In addition to borosilicate glass and HDPE, other materials can be used for special applications:

• Stainless steel: suitable for larger preparative, industrial or process-oriented systems. Special connection techniques, cleaning and validation effort, possible corrosion, metal-ion ingress and interactions with metal-affine analytes must be considered.
• Polypropylene: fluorine-free, break-resistant and resistant to many aqueous media. However, its suitability depends on the proportion of organic solvent, temperature, contact time and possible extractables/leachables.
• PFA or FEP: highly chemically resistant, but as fluoropolymers they are not suitable for consistently PFAS-free solvent supply systems.
• Quartz and special glass: technically possible, but of secondary importance for conventional HPLC solvent supply because of higher costs, lower standardisation and limited availability.

PTFE as the Standard for Conventional HPLC Applications

PTFE, polytetrafluoroethylene, is one of the established materials used in HPLC laboratory technology. It has very broad chemical resistance to numerous organic solvents, acids and bases. At the same time, PTFE is largely inert and has low surface energy.

These properties make PTFE the proven standard for conventional HPLC applications, particularly for:

• the media-contacting core of the HPLC Safety Cap
• supply tubing
• fittings and capillary feedthroughs
• other media-contacting components of the supply system.

PTFE provides a broad chemical safety margin and can be used reliably in many cases even with changing methods and different typical HPLC eluents. SCAT Europe therefore uses PTFE as the standard material in the media-contacting area of conventional HPLC Safety Caps.

Why PE as a Safety Cap Core Is Not Equivalent to PTFE

Simple or heavily cost-optimised cap systems may use media-contacting cores made of PE. PE is generally resistant to many media. However, its suitability depends more strongly on the specific material grade, the eluent used, the temperature and the duration of contact.

Depending on the application, the following effects may be relevant:

• swelling or changes in material properties
• embrittlement or deformation
• increased permeability to certain solvents
• possible extractables/leachables.

PE is not fundamentally unsuitable for all HPLC applications. However, as the material for a Safety Cap core that is permanently in contact with the medium, it does not provide the same broad chemical safety margin as PTFE. SCAT Europe therefore does not use PE as a substitute for PTFE in the media-contacting core of a conventional HPLC Safety Cap.

This assessment must not be generalised to every PE component. In the PFAS-free GreenLine configuration, PE is used specifically for fittings and blind plugs. In these components, the material performs a different mechanical and analytical function from the core of the HPLC Safety Cap. HDPE as the material of a solvent bottle must likewise be assessed separately.

PEEK for PFAS-Sensitive and Deliberately Fluorine-Free Applications

PEEK is a high-performance, PFAS-free thermoplastic. For HPLC solvent supply, PEEK offers two distinct advantages in particular.

Analytical reason: PFAS-sensitive applications

In PFAS analysis, a consistently PFAS-free media path helps reduce potential ingress of fluorine-containing substances and unwanted background signals. PEEK can replace PTFE particularly in the Safety Cap core and in supply tubing.

A typical PFAS-free SCAT material combination includes:

• qualified, non-fluorinated HDPE solvent bottle
• HPLC Safety Cap with PEEK core
• PEEK supply tubing
• PE fittings and PE blind plugs
• PFAS-free GreenLine air valve
• UHMW-PE suction filters.

A single PEEK component or an HDPE bottle does not in itself create a completely PFAS-free supply system. Material purity, manufacturing, packaging, cleaning, previous use and handling can also influence potential background signals. The complete system should therefore be controlled using blanks or system blanks.

Material-strategy reason: reducing fluorinated materials

PEEK can also be used independently of a specific PFAS analysis when a laboratory deliberately wants to reduce fluorinated materials in the media path. Provided that chemical resistance to the eluents used is assured, PEEK offers a demonstrably PFAS-free alternative to PTFE.

This does not mean that PEEK is inherently more environmentally friendly than PTFE. A complete environmental assessment would have to compare manufacturing, energy use, service life, reuse and disposal over the entire life cycle. What can be stated reliably is that PEEK reduces the use of fluorinated materials in the relevant media path.

Correctly Positioning PTFE and PEEK

PTFE and PEEK are established materials for HPLC solvent supply, but they meet different requirements.

PEEK is suitable for many typical eluents and additives, including water, methanol, acetonitrile, common buffer systems, dilute organic acids and low-concentration chromatographic additives.

Chemical limitations may arise in particular with unusually aggressive, strongly oxidising or highly concentrated acidic media. Resistance should also be assessed for the specific application when unusually high TFA concentrations, elevated temperatures or long contact times are involved.

PEEK is therefore not categorically better than PTFE. The decisive factor is which material meets the chemical, analytical and material-strategy requirements of the specific application.

Placing PTFE in the Context of the PFAS Discussion

PTFE belongs to the fluoropolymers and, under broad definitions, is classified within the PFAS substance group. However, it is not equivalent to low-molecular-weight PFAS such as PFOA or PFOS. As a high-molecular-weight, largely inert fluoropolymer, PTFE has different properties in terms of solubility, mobility and bioavailability.

The PFAS discussion surrounding PTFE therefore concerns not only the use of the finished component, but especially manufacturing, possible fluorinated processing aids, emissions, impurities, disposal and the long-term persistence of fluorine-containing materials.

For conventional HPLC applications, PTFE remains a proven standard because of its chemical resistance. For PFAS-sensitive or deliberately fluorine-free applications, PEEK provides a suitable alternative material strategy.

Assess Materials as a Complete System

Before selection, all components along the relevant media path should be evaluated together:

• solvent bottle
• media-contacting core of the HPLC Safety Cap
• fittings and capillary feedthroughs
• supply tubing
• suction filters
• seals and other connections
• where applicable, components of the air-venting system.

The following should be assessed in particular:

• chemical resistance
• adsorption and desorption effects
• possible extractables/leachables
• possible analytical background signals
• material purity and manufacturing quality
• previous use and cleaning
• suitability for the specific method.

Comparison of Materials for Solvent Bottles

Solvent Bottles and Solvent Containers

Comparison of Materials for Cores, Tubing and Connection Elements

Selection Questions for the Material Configuration

Before selection, the following questions in particular should be answered:

• Which eluents, buffers and additives are used?
• Is the application routine, trace, PFAS, metal-ion or another particularly sensitive form of analysis?
• Do all relevant media-contacting components need to be PFAS-free?
• Are the bottle, core, tubing, fittings, blind plugs, valve and suction filters coordinated as a system?
• Has chemical resistance been checked for the specific application?
• Are blanks or system blanks required to monitor possible background signals?

Selecting an HPLC Safety Cap may initially appear straightforward. In practice, however, numerous details must be coordinated: the number, material and dimensions of the supply tubing, fittings, connection threads, suction filters, tubing routing, and clear assignment of the solvent bottles.

Many problems only become apparent during installation or a bottle change. An unsuitable fitting, an open connection or an inappropriate threaded port can prevent the HPLC Safety Cap from performing its function as a controlled closed supply system.

SCAT HPLC Safety Caps were therefore developed around typical situations encountered in laboratory practice. Colour-coded fittings, complete equipment for the analytical connections, supplied blind plugs, replaceable labelling fields and the freely rotating core form a coordinated system. Details that are barely noticeable in a superficial product comparison can be decisive for safe installation and immediate readiness for use.

Selection in Five Steps

Five questions must be answered when configuring the solvent supply:

1. Which eluents must be delivered, and at what flow rate?
2. How many suction lines run from each solvent bottle to the HPLC system?
3. Which material should be used for the supply tubing?
4. Which outer and inner diameters and which tubing lengths are required?
5. Which HPLC Safety Cap, fittings and suction filters match this configuration?

An isocratic method using a premixed mobile phase has different requirements from a binary or quaternary gradient system with several separate eluents. The specifications of the HPLC manufacturer and the dimensions of existing suction lines must also be taken into account.

The outer diameter of the tubing determines the mechanical connection to the fitting. The inner diameter influences flow resistance, line volume and possible flow rate. The length must allow a stress-free connection between the solvent bottle and the HPLC system.

Supply Tubing Material: PTFE or PEEK?

Within an HPLC system, tubing made from different materials may be used. On the high-pressure side or in special applications, for example, stainless steel or fused silica may be used.

For the solvent supply considered here, between the solvent bottle and the HPLC pump, PTFE and PEEK are the main relevant materials.

PTFE is the proven standard solution for conventional HPLC applications. The material has broad chemical resistance and, because of its flexibility, is easy to route.

PEEK is PFAS-free. It is used in particular when fluorinated materials are to be avoided in the media path. PEEK is more dimensionally stable and mechanically stronger, but it is also stiffer than PTFE.

PEEK is generally suitable for typical HPLC eluents. For strongly oxidising, unusually aggressive or highly concentrated media, resistance should be checked specifically. The detailed material assessment is provided in Chapter 4.

Determine the Number of Connections and the Safety Cap Size

The decisive factor is the number of suction lines that run from the respective solvent bottle to the HPLC system.

A solvent bottle can:

• supply one pump channel
• supply several pump channels with the same eluent
• supply several HPLC instruments
• accommodate additional suction lines for special applications.

In a binary gradient system, for example, eluent A and eluent B are provided separately. A quaternary system may correspondingly operate with eluents A through D.

The type designation of the SCAT HPLC Safety Cap indicates the number of capillary feedthroughs: Safety Cap I, II, III, IV or VI have one, two, three, four or six connections respectively.

All Three Standard Sizes for Every Connection

The required fitting size depends on the actual outer diameter of the supply tubing. Tubing with outer diameters of 2.3 and 3.2 mm is widely used; depending on the HPLC system, 1.6 mm tubing is also used.

This means that a Safety Cap VI is supplied with a total of 18 fittings. At the same time, if only one supply line is used, the remaining five connections can be sealed securely.

When ordering an analytical SCAT HPLC Safety Cap, the tubing outer diameter does not have to be fixed to a single fitting size because all three standard sizes are supplied for every connection. By contrast, the maximum number of required tubing connections must be known before selecting the Safety Cap. The appropriate fitting is selected during installation; unused connections are sealed with blind plugs.

With some products on the market, by contrast, only one fitting for 3.2 mm tubing is included. If 1.6 or 2.3 mm is required, suitable fittings must be purchased separately. Since replacement fittings are often only available in pack quantities, this can lead to additional effort, delays and costs.

A meaningful product comparison must therefore consider not only the cap thread and number of connections, but also the following points:

• supplied fitting sizes
• number of fittings per connection
• number of blind plugs
• immediate readiness for use with the existing suction lines.

Clearly Label Solvent Bottles

In gradient systems, it must be clear which solvent bottle supplies each pump channel.

For this purpose, SCAT HPLC Safety Caps can be equipped with a replaceable labelling field. This can contain information such as:

• A, B, C or D
• name of the eluent
• composition of the mobile phase
• date of preparation or bottle change.

The labelling makes it easier to assign the solvent bottles correctly and reduces the risk of an incorrect connection during a bottle change or maintenance work.

The labelling field is an additional aid for assignment. It does not replace method documentation or the required hazardous-substance labelling.

Determine the Outer and Inner Diameter of the Tubing

Outer and inner diameter perform different functions:

• The outer diameter of the tubing determines the matching fitting and reliable sealing.
• The inner diameter of the tubing influences flow resistance, possible flow rate and line volume.

The following dimensions are used for SCAT PTFE supply tubing:

The wall thickness is calculated as follows:

Despite its larger outer diameter, the 3.2 mm tubing has a slightly smaller inner diameter of 1.6 mm than the 2.3 mm tubing, which has an inner diameter of 1.7 mm.

The two tubing sizes therefore have comparable hydraulic cross-sections. The larger outer diameter of the 3.2 mm tubing is mainly due to its thicker wall. This increases resistance to kinking, crushing and deformation. It is mechanically more robust, but also stiffer.

For PEEK tubing, different inner diameters may be available for the same outer diameter. Both dimensions must therefore always be checked for PEEK. The outer diameter alone is not sufficient for hydraulic sizing.

Correctly Match Tubing and Fitting

The fitting secures the supply tubing in the HPLC Safety Cap and seals the annular gap between the tubing and the feedthrough. Standard analytical SCAT fittings have a 1/4"-28 UNF external thread. The size designations 1.6, 2.3 and 3.2 mm, by contrast, refer to the outer diameter of the tubing. The colour coding shown in the table above enables quick assignment; colour codes used by other manufacturers may differ.

Do Not Estimate the Tubing Outer Diameter

If the outer diameter of existing tubing is not known, it should be measured.

SCAT Europe offers a free measuring template for capillaries and tubing. It can be used to match the tubing directly to the correct fitting.

A caliper can also be used. With soft PTFE tubing, however, care must be taken not to compress the material, as this can distort the measurement.

The Most Common Practical Errors

An HPLC Safety Cap is often installed without closer inspection because, at first glance, it appears to be a simple screw cap. This mainly leads to two safety-relevant errors.

Error 1: The Fitting Does Not Match the Tubing Outer Diameter

Tubing with an outer diameter of 1.6 mm can be passed through a fitting designed for 3.2 mm tubing. However, it will neither be securely clamped nor sealed in that fitting.

Possible consequences include:

• escape of solvent vapours
• ingress of ambient air and moisture
• changes in the mobile phase
• ingress of contaminants
• movement of the tubing
• impaired reproducibility.

A fitting is not suitable simply because the tubing can pass through it. It is suitable only if the tubing is securely clamped and completely sealed.

Error 2: An Unused Connection Is Left Open

An open connection creates a direct path between the solvent bottle and the laboratory environment.

This can result in:

• solvent vapours escaping into the laboratory air
• personnel being exposed to solvent vapours
• ambient air and moisture entering the bottle
• volatile components evaporating
• changes in the concentration and composition of the eluent
• ingress of contaminants.

Even a small threaded port can continuously release solvent vapours over the entire operating and standby period. An open connection is therefore not a harmless spare port, but a relevant occupational-safety risk.

The supplied blind plugs are therefore not optional accessories. They are part of the safety function.

Preparative Tubing Requires Dedicated Connections

Preparative liquid chromatography operates at higher flow rates. This requires supply tubing with larger outer diameters and dedicated fitting threads.

These fitting families are not interchangeable:

• A standard analytical fitting does not fit into a preparative connection.
• A preparative fitting does not fit into a standard analytical threaded port.
• 5/16"-24 UNF and 1/8" NPT fittings are likewise not interchangeable.

For preparative applications, the tubing outer diameter, fitting and threaded port in the HPLC Safety Cap must therefore already be specified at the time of ordering.

SCAT offers preparative and combined HPLC Safety Caps for this purpose. A combined version can, for example, have one standard analytical connection and an additional preparative connection.

Take Tubing Length and Suction Filter into Account

The supply tubing must be long enough to connect the solvent bottle and HPLC system without mechanical stress.

It should:

• be routed without tensile load
• allow sufficient freedom of movement for bottle changes
• not be kinked or crushed
• be routed as short and clearly as possible.

Tubing that is too short places stress on fittings and instrument connections. Unnecessarily long tubing increases line volume and lengthens flushing and changeover processes.

The tubing should be as short as technically practical and as long as required for safe handling.

A suction filter is normally fitted to the lower end of the tubing. The following should be checked:

• connection size
• material
• pore size
• chemical resistance
• PFAS-free design for PFAS-sensitive applications
• sufficient flow capacity.

The suction filter and supply tubing must be mechanically, chemically and analytically compatible.

Freely Rotating Core Prevents “Tubing Tangles”

When an HPLC Safety Cap is screwed on or off, the outer cap body rotates relative to the solvent bottle.

If the core and supply tubing rotated fully with it, the lines could wrap around one another, become twisted or kinked. In everyday laboratory work, this can quickly result in a confusing “tangle of tubing”.

Possible consequences include:

• kinked or twisted tubing
• mechanical stress on the fittings
• tensile forces on instrument connections
• displacement of the suction filters
• more difficult identification of the lines.

For this reason, the media-contacting PTFE or PEEK core in SCAT HPLC Safety Caps is mounted so that it can rotate freely. The outer cap body can turn as the cap is screwed on or off, while the core and tubing remain largely in position.

The freely rotating core is therefore more than a convenience feature. It supports orderly, low-stress and safe tubing routing.

Safety and Selection Check

The following points should be checked before selection and after installation:

• number of required supply tubing lines
• material and outer and inner diameter of each tubing line
• specifications of the HPLC manufacturer
• analytical or preparative application
• matching threaded port in the HPLC Safety Cap
• matching fitting size or fitting colour
• sufficient number of fittings and blind plugs supplied
• clear labelling of the solvent bottles
• suitable tubing length and matching suction filters
• orderly and stress-free tubing routing
• free rotation of the core
• sealing of every unused connection.

The air valve is a key functional component of the HPLC Safety Cap. It enables controlled, filtered pressure equalization in the solvent bottle while at the same time limiting the uncontrolled escape of solvent vapours.

Only the interaction between a tightly sealing HPLC Safety Cap and a functional air valve creates a solvent supply that is both controlled closed and pressure-equalised.

Why Is Pressure Equalization Necessary?

The HPLC pump continuously draws eluent from the supply bottle. The volume of liquid removed must be replaced by incoming air.

If the bottle is tightly closed and no air can flow in, increasing negative pressure develops. This worsens the suction conditions for the HPLC pump and may impair consistent delivery of the eluent.

However, a connection must not simply be left open for this reason. An open connection would allow pressure equalization, but would also:

• allow solvent vapours to escape into the laboratory air
• introduce dust and other contaminants into the eluent
• allow ambient moisture to enter
• promote evaporation losses
• affect the composition of the mobile phase.

The air valve resolves this conflict of objectives:

Air is allowed to enter the solvent bottle in a controlled manner, while solvent vapours should not escape uncontrolled.

How Does the Air Valve Work?

The air valve has two key functional elements:

• a filter membrane
• a check valve or duckbill valve.

The filter membrane cleans the incoming ambient air and retains dust and other particles.

Withdrawal of eluent creates negative pressure in the bottle relative to the surroundings. As soon as the resulting pressure differential reaches the opening pressure of the duckbill valve, it opens toward the inside of the bottle and allows filtered air to flow in.

Once the pressure has equalized, the duckbill valve closes again. This limits the escape of solvent vapours through the intended valve path. At the same time, evaporation losses are reduced and stable conditions for the mobile phase are supported.

Why Housing Leak-Tightness Is Critical

An air valve essentially consists of four components:

• lower housing section
• housing cover
• filter membrane
• duckbill valve.

For the valve to function as intended, the incoming air must follow a clearly defined path:

• through the filter membrane
• through the duckbill valve
• into the solvent bottle.

In SCAT air valves, the lower housing section, housing cover and membrane are permanently welded together. This reliably seals the joint between the two housing parts and the membrane.

The welded construction minimises a potential path for bypass air between the lower housing section and the cover. At the same time, it reduces the risk of solvent vapours escaping through this housing joint.

With a housing that is merely clicked or snapped together, leak-tightness additionally depends on the design of the joint. If this joint is not adequately sealed, bypass air can enter and circumvent the intended flow path.

Possible consequences include:

• unfiltered ingress of ambient air
• ingress of dust and particles
• uncontrolled ingress of ambient moisture
• escape of solvent vapours through the housing joint
• bypassing the defined opening pressure of the duckbill valve.

An air valve can function as intended only if, in addition to the filter membrane and duckbill valve, the valve housing is also sufficiently leak-tight.

The type of housing connection is therefore a technically relevant selection criterion, even though it is often not apparent in a superficial product comparison.

Assess Flammability Classification and Analytical Purity Separately

A UL 94 V-0 classification describes the flammability behaviour of a plastic under defined test conditions, not its chemical or analytical purity.

Special plastic formulations containing additional functional constituents may be used to achieve certain flammability classifications. In analytical applications, such additives are in principle a potential source of extractables/leachables or background signals.

This does not prove that a specific air valve contaminates the eluent. However, the flammability classification should be assessed separately from chemical and analytical suitability.

The valve housing is located in the air and vapour space of the solvent bottle and is therefore not completely outside the system. Incoming air passes through the valve, and solvent vapours or condensate may also come into contact with the internal valve surfaces.

For this reason, SCAT deliberately does not use additional flame-retardant additives. The focus is on high chemical resistance and reducing avoidable potential sources of contamination.

Do Not Confuse the Air Valve with an Exhaust Filter

Air valves and exhaust filters are used on different sides of the HPLC system.

The two components perform different functions and cannot be substituted for one another.

Which Air Valve Is Required?

The following questions are particularly important for selection:

• Is the application conventional or PFAS-sensitive?
• Is a standard or consistently PFAS-free supply path required?
• Does the eluent need to be sparged, or does the incoming air need to be dried?
• Does the connection thread match the HPLC Safety Cap?

Standard Air Valve

The standard air valve is intended for conventional analytical and preparative HPLC applications.

It features:

• a 1/4"-28 UNF connection thread
• a PTFE filter membrane
• a check-valve function
• a Luer Lock connection
• a labelling field for the scheduled replacement date.

The valve is screwed into the designated thread in the HPLC Safety Cap.

This valve version covers a broad range of flow rates from analytical to preparative applications. A separate valve size is therefore not normally required for these applications.

GreenLine Air Valve

A GreenLine version is available for PFAS-sensitive applications. In this version, the conventional PTFE filter membrane is replaced by a PFAS-free filter membrane.

As described in Chapter 4, a consistently PFAS-free media path requires the entire supply system to be configured accordingly. The GreenLine air valve should therefore be regarded as part of a fully coordinated PFAS-free solution.

Luer Lock Connection for Sparging or Air Drying

The Luer Lock connection can be used to extend the air valve for special applications.

Possible applications include:

• sparging the eluent with a suitable gas
• protecting oxidation-sensitive eluents
• connecting a drying tube
• reducing moisture ingress from the ambient air.

Whether such an additional function is required depends on the eluent and the analytical method.

What Happens If Valve Function Is Restricted?

As operating time increases, particles may accumulate on the filter membrane. At the same time, the opening and sealing characteristics of the duckbill valve may change.

The functional condition of the duckbill valve cannot be assessed reliably during normal laboratory operation. Neither its external appearance nor trouble-free operation of the HPLC system proves that the valve still opens at the intended negative pressure and seals reliably in the reverse direction.

If insufficient air can flow in because the filter membrane is clogged or the duckbill valve no longer opens reliably, increasing negative pressure develops in the solvent bottle.

The HPLC pump does not actively increase its suction power in response. Instead, suction conditions on the low-pressure side deteriorate. If the eluent can no longer flow in uniformly, the following effects are possible:

• gas bubbles or cavitation on the suction side
• fluctuating or reduced actual flow rate
• pressure fluctuations
• shifts in retention times
• problems during priming or flushing
• in extreme cases, interruption of eluent delivery.

If, by contrast, the duckbill valve no longer closes reliably, the HPLC system may continue to appear to operate without problems. At the same time, however:

• solvent vapours may escape
• evaporation losses may increase
• ambient moisture may enter the solvent bottle
• the composition of the mobile phase may change.

Trouble-free instrument operation is therefore not proof that the air valve is still functioning correctly.

Service Life and Replacement Interval

The air valve is a consumable item.

During operation, the filter membrane collects particles from the ambient air. At the same time, the filter membrane and duckbill valve are exposed to the atmosphere inside the solvent bottle and the solvent vapours it contains.

SCAT therefore specifies a service life of six months for both the standard and GreenLine air valves. After this period, the complete valve must be replaced preventively – even if no external change is visible and the HPLC system continues to operate without problems.

After six months, the duckbill valve may still be fully functional. However, reliable function can no longer be assumed.

Because opening pressure, air flow and reverse leak-tightness cannot be checked reliably during normal laboratory operation, replacement after six months should be understood as a preventive functional and safety replacement.

Earlier replacement is required if:

• the valve is visibly damaged
• heavy contamination is visible
• the valve has been wetted or flooded with eluent
• pressure equalization no longer functions reliably
• there are doubts about chemical or mechanical functionality.

Clearly Document the Replacement Date

A Timestrip® is solely a time indicator. It does not assess either the technical condition or the actual usage time of the air valve. The indicator merely shows that a defined period has elapsed since activation.

For clear operational control, the specific replacement date should therefore also be entered on the air valve during installation:

This approach provides a clear replacement date that can be read immediately. Especially where several HPLC systems are in use, it simplifies scheduled checks and timely replacement.

Avoid Typical Errors

The following errors occur particularly often in laboratory practice:

• The HPLC Safety Cap is operated without an air valve.
• A connection is left open instead of fitting the valve.
• The valve is not screwed in fully.
• A valve with a damaged or leaking housing continues to be used.
• The valve is used beyond its specified service life.
• The replacement date is not entered.
• A valve that has been wetted or flooded with eluent continues to be used.
• A standard valve with a PTFE membrane is used for a PFAS-sensitive application.
• The GreenLine valve is used even though the rest of the media path still contains fluorinated materials.
• A flammability classification is equated with chemical or analytical purity.
• The air valve and exhaust filter are confused with one another.

Selection and Inspection Check

Before commissioning, the following should be checked:

• standard or GreenLine version appropriate for the application
• complete assessment of the media path for PFAS-sensitive applications
• suitable material selection without unnecessary potential sources of contamination
• matching 1/4"-28 UNF connection thread
• sufficiently leak-tight and undamaged valve housing
• valve screwed in fully
• free Luer Lock connection or correctly connected additional function
• replacement date “Change by” entered
• documented replacement after six months
• earlier replacement in the event of damage, contamination or malfunction.

The suction filter is located at the lower end of the supply tubing inside the solvent bottle. It retains particles, precipitates and other solid contaminants before they can enter the suction line and the HPLC system.

Suction filters therefore perform a different function from the air valve:

• The air valve filters the incoming ambient air.
• The suction filter filters the liquid eluent before it enters the supply tubing.

The suction filter complements proper preparation of the mobile phase, but does not replace it. Visibly contaminated eluents or mobile phases with visible precipitates should not be treated solely by means of the suction filter.

Likewise, the suction filter on the low-pressure side does not replace an in-line filter or column-protection filter on the high-pressure side of the HPLC system.

What Functions Does the Suction Filter Perform?

A suitable suction filter:

• retains particles and precipitates
• reduces the ingress of contaminants into the suction line
• protects the HPLC pump and downstream system components
• defines the suction point at the lower end of the supply tubing
• when suitably positioned, supports withdrawal of eluent from the lower part of the bottle.

At the same time, the filter must not restrict the required flow rate to an unacceptable degree. The suction filter, tubing and HPLC pump must therefore be considered as an interconnected system.

Which Criteria Determine Selection?

The following points are particularly relevant for selection:

• pore size
• effective filter area and filter geometry
• material
• required flow rate
• viscosity of the eluent
• connection to the supply tubing
• requirements of the analytical method

Pore size alone is therefore not sufficient for evaluating a suction filter.

Pore Size, Filter Area and Flow Rate

A smaller pore size retains finer particles. At the same time, a finer filter can cause higher flow resistance and may clog more quickly with particle-containing eluents or eluents prone to precipitation.

The following are also decisive for flow capacity:

• effective filter area
• filter geometry
• viscosity of the eluent
• analytical or preparative flow rate
• degree of filter contamination.

The principle “the smaller the pore size, the better” is therefore not universally correct.

A filter with a small pore size and a large effective filter area may have lower flow resistance than a small filter body with a larger pore size. Especially in preparative applications, it must therefore be checked whether the filter area and flow capacity are sufficient for the required flow rate.

The composition of the mobile phase is also relevant. Aqueous eluents, buffers and salt solutions are more prone to deposits, precipitation or microbial growth than pure organic solvents.

Selecting the Suction Filter Material

The filter material must be chemically compatible with the eluent and analytically suitable for the application.

SCAT offers two basic versions for use with supply tubing having an outer diameter of 3.2 mm:

PFA/PTFE version

The PFA/PTFE version with a 5 µm pore size is intended for conventional HPLC applications in which fluorinated materials in the media path are acceptable.

It provides fine particle retention and broad chemical resistance.

UHMW-PE version

For PFAS-sensitive applications, a fluorine-free version made of ultra-high-molecular-weight polyethylene, or UHMW-PE, is available.

As already described in Chapter 4, PFAS-sensitive applications require the entire media path to be configured accordingly. The PFAS-free suction filter should therefore be regarded as part of a coordinated supply system.

Metal- and Ion-Sensitive Applications

Depending on the manufacturer and application, other HPLC systems may also use suction filters made of stainless steel, titanium, glass or PEEK.

For applications sensitive to metals, ions or biocompatibility, it must additionally be checked whether a metallic suction filter is permissible or whether a polymeric version is required.

Possible reasons for selecting polymeric filters include:

• avoiding metallic surfaces
• reducing possible release of metal ions
• biocompatible applications
• sensitive LC-MS methods
• special ion-chromatography applications.

Material selection should therefore not be based solely on chemical resistance, but also on the analytical requirements of the method.

Check the Connection to the Supply Tubing

The suction filter must connect securely and mechanically to the supply tubing.

The SCAT versions described are intended for connection to supply tubing with an outer diameter of 3.2 mm.

Before selection, check:

• What is the outer diameter of the tubing?
• Does the filter fit directly onto the tubing?
• Is the connection sufficiently secure?
• Does the connection remain secure during bottle changes?
• Are the tubing and suction filter chemically compatible?
• Does the combination comply with the HPLC manufacturer’s specifications?

A loose connection may cause the filter to detach inside the solvent bottle or allow air to be drawn in at the connection point.

Position the Suction Filter Correctly

The suction filter must be fully immersed in the eluent and positioned so that the eluent can flow freely to the effective filter area.

The correct position depends on the filter geometry.

Flat bottom-of-the-bottle filters are specifically designed to rest on the bottom of the bottle. They allow solvent withdrawal down to a comparatively low residual volume.

With other filter shapes, however, it must be ensured that the effective filter area is not blocked by the bottle bottom or bottle wall.

The following general points must be observed:

• The filter must be fully immersed in the eluent.
• There must be sufficient free flow around the filter area.
• The filter must not hang above the liquid level.
• The tubing must not be kinked or under tensile stress.
• The filter must not shift uncontrollably during a bottle change.
• Its position must be suitable for the specific filter and bottle geometry.

The suction filter defines the suction point, but not every version automatically acts as a weight or so-called filter sinker. Whether it reliably holds the tubing in the lower part of the bottle depends on its material, weight and design.

Condition a New Suction Filter

Before first use or after replacing a filter, check whether the suction filter must be flushed or conditioned in accordance with the manufacturer’s instructions.

The following steps are particularly relevant:

• Touch the filter only with clean, chemical-resistant, powder-free gloves.
• If required, rinse the filter with a suitable high-purity solvent.
• Fully immerse the suction filter in the eluent.
• Completely fill the supply tubing with eluent.
• Remove air from the suction line.
• Flush or prime the HPLC pump in accordance with the manufacturer’s instructions.

A dry or incompletely wetted suction filter can make priming more difficult. Air remaining in the filter and tubing can lead to unstable delivery conditions.

When Must the Suction Filter Be Replaced?

There is no uniform, manufacturer-independent replacement interval for suction filters.

Replacement depends on:

• filter type
• manufacturer’s instructions
• eluent used
• operating time
• degree of contamination
• analytical sensitivity
• functional problems that occur.

More frequent inspection is advisable for aqueous mobile phases, buffers and long standby periods. Deposits, precipitates or microbial growth can impair filter permeability.

A clogged or contaminated suction filter can cause effects including:

• difficult or interrupted suction
• loss of pump prime
• fluctuating or reduced flow rate
• incorrect gradient profiles
• pressure fluctuations
• shifts in retention times
• broadened peaks
• contamination or background signals.

The suction filter should be replaced if:

• visible deposits or discolouration are present
• the eluent is no longer drawn in uniformly
• the HPLC pump repeatedly loses prime
• increased flow resistance is suspected
• the filter is damaged
• the connection to the tubing is no longer secure
• the filter has come into contact with an unsuitable or contaminated medium
• carryover must be avoided when changing methods.

After replacement, the filter and suction line must again be completely wetted, vented and flushed in accordance with the manufacturer’s instructions.

Cleaning or Replacement?

Whether a suction filter can be cleaned and reused depends on the material, filter design, contamination and manufacturer’s instructions.

Cleaning is appropriate only if:

• the filter is expressly suitable for cleaning
• the cleaning medium is chemically compatible
• no carryover into the next application is expected
• the filter structure is not damaged
• the original permeability can be restored reliably.

In the case of unknown contamination, analytically critical applications or damaged filter structures, replacement is usually the safer option.

A filter that appears clean is not automatically free of contaminants or deposits within the filter structure.

Avoid Typical Errors

The following errors occur particularly often in laboratory practice:

• The filter connection does not match the outer diameter of the tubing.
• The material and eluent are not sufficiently compatible.
• A fluorinated filter is used for a PFAS-sensitive application.
• An unsuitable metal filter is used for a metal- or ion-sensitive method.
• The filter hangs above the liquid level.
• The filter area is blocked by the bottle bottom or bottle wall.
• The tubing is too short, kinked or under tensile stress.
• The filter area and flow capacity are insufficient for the flow rate.
• A visibly contaminated or clogged filter continues to be used.
• A new filter is not sufficiently wetted.
• After a filter change, the suction line is not vented and the pump is not primed.
• The suction filter is expected to clean an eluent that is already visibly contaminated.

Selection and Inspection Check

Before commissioning, the following should be checked:

• suitable pore size
• sufficient filter area and flow capacity
• chemically and analytically suitable material
• PFAS-free version for PFAS-sensitive applications
• suitability for metal- or ion-sensitive methods
• matching connection for the tubing outer diameter
• secure connection between suction filter and tubing
• suitable position according to the filter geometry
• filter fully immersed
• free flow around the filter area
• kink-free, low-stress tubing routing
• complete wetting and venting
• replacement in the event of contamination, damage or functional abnormalities.

Once the individual components have been considered, they must be combined into a supply system that is technically coordinated and compatible in terms of materials.

The decisive factors are:

• analytical method
• eluents used
• number and dimensions of the suction lines
• required flow rate
• special requirements such as PFAS, metal or ion sensitivity.

An HPLC Safety Cap should therefore never be selected in isolation. Both functional paths must always be considered:

Eluent path:
Suction filter → supply tubing → fitting → HPLC pump

Air path:
Ambient air → filter membrane → valve → solvent bottle

The solvent bottle, HPLC Safety Cap, fittings, tubing, air valve and suction filter must be technically coordinated with one another and materially compatible.

Configuration in Seven Steps

For each solvent bottle, the following decisions should be made in sequence:

1. determine the solvent bottle and bottle thread
2. determine the number of required supply tubing lines
3. select the material and outer and inner diameters of the tubing
4. determine the matching fittings and threaded ports
5. select a standard or GreenLine air valve
6. select a suitable suction filter
7. seal unused connections and clearly label the bottle

The following application cases show how these decisions can be combined into typical configurations.

Case 1: Conventional Analytical HPLC

In a conventional analytical HPLC application, a solvent bottle is connected to one pump channel via one supply line.

A typical configuration may look as follows:

Safety Cap I has one tubing connection and is therefore suitable for supplying a single pump channel. The fitting is selected to match the actual outer diameter of the supply tubing.

If a second supply line is required, or if an additional connection is deliberately to be kept available, a Safety Cap II can be used instead. An unused connection must be sealed with the supplied blind plug.

This configuration is suitable for many conventional routine HPLC applications using organic or aqueous-organic mobile phases. Nevertheless, the chemical resistance of all media-contacting components must be checked for the specific eluent.

Case 2: Gradient Systems – Binary to Quaternary

Gradient systems operate with several separate eluents. Each eluent is usually provided in its own supply bottle with its own HPLC Safety Cap.

Each eluent path consists of:

• solvent bottle
• HPLC Safety Cap
• supply tubing
• air valve
• suction filter.

Clear labelling prevents eluents or pump channels from being confused during bottle changes. Replaceable labelling fields on the HPLC Safety Caps make assignment easier.

The individual eluent paths may be configured differently. An aqueous buffer, for example, may place different requirements on the material and suction filter than an organic eluent.

Case 3: Multiple Suction Lines from One Solvent Bottle

One solvent bottle can supply several pump channels or HPLC instruments with the same eluent. The required Safety Cap size depends on the number of suction lines.

Each tubing line must be sealed with a fitting that matches its outer diameter. Unused connections are sealed with the supplied blind plugs.

Bottle volume, total flow rate and sufficient flow capacity of the air valve and suction filters must also be taken into account.

Case 4: PFAS-Sensitive Analysis

For PFAS-sensitive applications, fluorinated materials must be consistently avoided in the relevant media path.

A typical PFAS-free configuration can be set up as follows:

The decisive factor is consistent consideration of the entire relevant media path. As described in Chapter 4, other media-contacting components of the analytical system must also be checked for PFAS suitability.

Case 5: Preparative HPLC

Preparative HPLC applications operate at higher flow rates and often require supply tubing with larger diameters.

One possible configuration includes:

Preparative tubing requires a threaded port that matches its size. Analytical and preparative fittings are not interchangeable; the exact assignment is shown in Chapter 5.

The following should be checked in particular when configuring the system:

• total flow rate
• inner diameter and suction resistance of the tubing
• viscosity of the eluents
• filter area and flow capacity of the suction filter
• required bottle volume
• safe handling of larger supply bottles.

Combined HPLC Safety Caps can have both analytical and preparative connections. The respective fitting families and threaded ports remain separate.

Case 6: Moisture- or Oxidation-Sensitive Eluents

For moisture- or oxidation-sensitive eluents, the Luer Lock connection of the air valve can be used for additional protective measures.

Possible applications include:

• connecting a drying tube
• drying the incoming ambient air
• sparging with a suitable protective gas
• reducing oxygen ingress.

One possible configuration includes:

The specific setup must match the sensitivity of the eluent and the analytical method.

When sparging, it must be ensured that:

• a suitable gas of sufficient purity is used
• no analytically relevant contaminants are introduced
• the gas supply is controlled
• no impermissible overpressure develops in the solvent bottle.

Final Configuration Check

After assembly, check:

• Is every connection correctly occupied or sealed with a blind plug?
• Is the supply tubing routed in an orderly and stress-free manner?
• Are all solvent bottles and pump channels clearly labelled?
• Are the flow capacities of the air valve and suction filter sufficient for the required flow rate?
• Has the replacement date of the air valve been entered?

Decision Matrix

Correct component selection is only the first step. Only proper installation ensures that the HPLC solvent supply can be operated as a controlled closed, reproducible and safe system.

Typical problems often arise not from product selection but during installation – for example due to unsuitable fittings, open connections, incorrectly adjusted tubing lengths, inappropriate line routing or insufficient venting.

This chapter describes the recommended procedure from preparation through initial commissioning. Detailed diagnosis of typical errors and malfunctions follows in Chapter 10.

Proper installation is therefore essential for reliable operation of the complete solvent supply system.

Check Before Installation

Before assembly, all components should be fully available and clearly assigned to the intended application.

Check that:

• bottle thread and HPLC Safety Cap match
• the number of tubing connections is sufficient
• the correct fittings are available
• blind plugs for unused connections are ready
• the air valve is present and undamaged
• the suction filters match the tubing and application
• the supply tubing is sufficiently long
• eluents and pump channels are clearly assigned
• all media-contacting components are chemically and analytically suitable.

The solvent bottle should be prepared and clearly labelled. In gradient systems, assignment to A, B, C or D is recommended before assembly begins.

Installation in Eight Steps

Step 1: Select the Correct Fittings and Insert the Tubing

Initially, the HPLC Safety Cap remains removed from the solvent bottle.

For each supply line, select the fitting that matches its actual outer diameter.

The tubing is passed through the fitting and the designated connection in the HPLC Safety Cap. At first, the fitting is inserted only far enough to allow the tubing length to be adjusted.

Make sure that:

• tubing and fitting match exactly
• the tubing is not crushed or damaged
• the fitting sits straight in the threaded port
• analytical and preparative fittings are not confused.

A fitting is not suitable simply because the tubing can be passed through it. It is suitable only if the tubing is reliably secured and completely sealed.

Step 2: Install the Suction Filter

Attach the suction filter intended for the application to the lower end of each supply line.

Check that:

• matching connection size
• secure mechanical connection
• chemically and analytically suitable material
• suitable pore size and flow capacity
• undamaged filter structure.

The suction filter must neither come loose nor slide along the tubing.

Step 3: Adjust Tubing Length and Filter Position

The tubing length is now adjusted to the solvent bottle being used.

To do this, first hold the HPLC Safety Cap loosely over the bottle opening. Adjust the tubing so that, after installation, the suction filter:

• is fully immersed in the eluent
• is positioned in the lower part of the bottle
• has sufficient free flow around it according to its design
• is not pressed inappropriately against the bottle bottom or wall.

Inside and outside the bottle, the tubing must:

• not be kinked
• not be twisted
• not be under tensile stress
• not form unnecessarily tight loops.

Once the correct insertion depth has been reached, secure and seal the tubing using the matching fitting.

Step 4: Seal Unused Connections

Seal every unused tubing connection with a blind plug.

The rule is:

An open connection creates an uncontrolled path between the solvent bottle and the laboratory environment. This can allow solvent vapours to escape and ambient air, moisture and contaminants to enter the bottle.

Step 5: Check the Air Valve

Before placing the HPLC Safety Cap on the bottle, check the air valve intended for the application.

Check that:

• valve version matches the application
• connection thread is correct
• valve is screwed in fully
• valve housing is undamaged
• Luer Lock connection is free or correctly connected.

For PFAS-sensitive applications, the GreenLine version must be used as part of the appropriately configured supply system.

Now enter the specific replacement date directly on the valve:

For applications using a drying tube or protective-gas supply, correctly install the intended additional component on the Luer Lock connection.

Step 6: Install the HPLC Safety Cap on the Solvent Bottle

Carefully insert the fitted suction filters into the solvent bottle. The filters and tubing must not snag, kink or twist around one another.

Then place the HPLC Safety Cap straight onto the bottle thread and screw it on fully.

Make sure that:

• bottle and cap threads match
• the cap is not cross-threaded
• threads and sealing surfaces are undamaged
• tubing and suction filters remain in their intended positions
• the cap sits securely without being subjected to excessive force.

The freely rotating cap body allows the cap to be screwed on and off without the media-contacting core and installed tubing rotating fully with it. This reduces twisted lines and mechanical stress.

Step 7: Label the Eluent Paths and Route the Tubing

After installation, clearly label the solvent bottle and supply path.

Possible information includes:

• A, B, C or D
• name of the eluent
• mixing ratio
• preparation date
• date of bottle change.

Then route the supply tubing to the intended pump channel.

Make sure that:

• clear assignment to the HPLC system
• kink-free tubing routing
• no tensile stress on the cap, fitting or instrument connection
• sufficient freedom of movement for a later bottle change
• no unnecessarily long or confusing tubing loops.

Step 8: Fill and Vent the Suction Lines

Before the first analytical run, the suction filters and supply tubing must be completely wetted and filled with eluent.

The exact procedure depends on the HPLC manufacturer’s instructions. The following steps are generally required:

• Fully immerse the suction filter in the eluent.
• Remove air from the suction line.
• Prime or flush the HPLC pump.
• Pump eluent until no visible air bubbles remain.
• Check for uniform delivery and stable operating conditions.

A dry or only partially wetted suction filter can make priming difficult. Air remaining in the suction filter or tubing can cause unstable delivery, pressure fluctuations or gradient disturbances.

Functional Check Before the First Run

Before the system is released for use, the following should be checked one final time:

• HPLC Safety Cap is seated straight and securely on the bottle
• air valve is fully installed and undamaged
• replacement date has been entered
• the correct fitting has been used for each tubing line
• every unused connection is sealed with a blind plug
• suction filters are fully immersed and have sufficient free flow around them
• tubing is routed without kinks and with minimal mechanical stress
• solvent bottles and pump channels are clearly labelled
• suction lines are completely filled and vented
• there are no visible air bubbles in the lines
• the HPLC pump delivers uniformly
• there are no visible leaks or other abnormalities.

Even with a correctly selected and installed HPLC Safety Cap, abnormalities can occur during operation. The cause does not necessarily lie in the HPLC instrument itself. Installation errors, contaminated or damaged components, and unsuitable operating conditions within the solvent supply are often responsible.

This chapter is intended as a compact diagnostic tool. It links typical symptoms to possible causes on the solvent-supply side and describes suitable inspection steps.

Begin Troubleshooting Systematically

Before working on the solvent supply, stop the HPLC pump and use the required personal protective equipment.

Troubleshooting should then follow a fixed procedure:

1. Describe the symptom as precisely as possible.
2. Identify the affected eluent path or pump channel.
3. Visually inspect the accessible components.
4. Rule out simple and likely causes first.
5. Make changes one at a time and document their effect.

If several components are changed at the same time, it is difficult later to determine which measure actually resolved the problem.

Basic Check of the Solvent Supply

Before carrying out more extensive troubleshooting, check the following points:

• Is there sufficient eluent in the bottle?
• Is every connection sealed either with a suitable fitting or with a blind plug?
• Does every fitting match the outer diameter of the supply tubing?
• Are the tubing and suction filters fully wetted with eluent?
• Are the suction filters fully immersed and freely surrounded by eluent?
• Is the tubing routed without kinks and with minimal mechanical stress?
• Is the air valve fully screwed in and externally undamaged?
• Has the specified replacement interval for the air valve been observed?
• Are the solvent bottles and pump channels correctly assigned?

Many problems can already be identified through this basic check.

The internal function of the duckbill valve cannot be checked reliably during normal laboratory operation. If restricted pressure equalization is suspected and no other cause can be identified, the air valve should be replaced with a new valve known to be functional.

Quick Diagnosis

Priority Levels

Note: With flexible plastic bottles, restricted pressure equalization may cause visible deformation. With glass bottles, the problem is more likely to appear as suction problems, air bubbles or a decreasing flow rate. Their dimensional stability does not, however, mean that they are suitable for vacuum loads.

Notes on Interpretation

The table shows possible causes within the HPLC solvent supply. It does not replace a complete technical diagnosis of the HPLC system.

Shifts in retention time, pressure fluctuations or incorrect gradients can also be caused, for example, by the following components:

• HPLC pump
• mixing system
• degasser
• injector
• column
• temperature control
• instrument control system.

If the abnormality persists after the solvent supply has been checked, troubleshooting must be extended to the other components of the HPLC system.

For PFAS background signals, the following applies:

A PFAS-free HPLC Safety Cap alone does not create a completely PFAS-free analytical system.

The entire relevant media path and other potential sources of contamination in the laboratory must be included in the assessment.

When Should Operation Be Interrupted?

The affected solvent supply should not continue to be operated if:

• liquid is visibly leaking
• a connection is open or not securely sealed
• the cap, fitting, tubing, suction filter or air valve is visibly damaged
• a strong or newly occurring solvent odour cannot be localised
• the pump repeatedly draws in air or the suction line repeatedly fills with air bubbles despite repeated venting
• a plastic solvent bottle visibly collapses under negative pressure
• the suction filter is not securely connected to the tubing
• unknown contamination of the eluent is suspected
• safe assignment of the eluent paths cannot be ensured.

Operation may only be resumed once the cause has been identified, the affected component repaired or replaced, and safe function of the supply system restored.

If the cause remains unclear despite systematic inspection, SCAT Europe technical support should be consulted.

Final Check After Troubleshooting

After each measure, check:

• Are all connections securely sealed again?
• Have the correct fittings and blind plugs been used?
• Are the suction filters securely attached and fully immersed?
• Is the tubing routed without kinks and with minimal mechanical stress?
• Is the air valve fully screwed in and externally undamaged?
• Has the specified valve replacement interval been observed?
• Are the lines completely filled and vented?
• Are the eluent paths correctly labelled and assigned?
• Is the pump delivering uniformly?
• Are there no visible leaks, unusual odours or air bubbles?
• Has it been verified that the original symptom has actually been resolved?

The action taken and its result should be documented. If the problem occurs again, previous observations and measures can then be referenced more specifically.

A correctly installed HPLC Safety Cap requires little maintenance. Nevertheless, the complete solvent supply must be inspected regularly. This includes in particular the air valve, suction filter, supply tubing, fittings and blind plugs, solvent bottle and labelling.

The objectives of preventive maintenance are to:

• detect functional problems at an early stage
• avoid unplanned downtime
• maintain analytical quality
• reduce solvent losses
• maintain safe operation over the long term.

Many problems do not occur suddenly, but develop gradually through contamination, ageing, deposits or mechanical stress on individual components.

Maintain on a Defined Schedule

The solvent supply should be inspected according to a defined and documented schedule.

The following intervals provide practical guidance:

The specific inspection intervals must be adapted to operating intensity, the eluents used, analytical sensitivity and internal quality requirements. For quality-critical or regulated applications, the intervals should be defined in an internal work instruction.

Visual Inspection at Every Bottle Change

A bottle change is a suitable opportunity for a brief visual inspection because the suction filters, tubing and connections are directly accessible at that time.

Check the following:

• air valve externally undamaged
• valve replacement date not yet reached
• suction filter present in full and securely attached
• no visible deposits or discolouration
• tubing free from kinks, crushing or other damage
• fittings and blind plugs complete and undamaged
• bottle thread and sealing surfaces clean and intact
• labelling current and legible
• no visible leaks.

This inspection usually takes very little time, but can reveal developing functional problems at an early stage.

Replace the Air Valve Preventively

The air valve is a consumable or wear part. During operation, the filter membrane and duckbill valve in particular are exposed to ambient particles, solvent vapours and repeated pressure-equalization cycles.

Because opening pressure, air flow and retention function cannot be checked reliably during normal laboratory operation, the valve should not be assessed solely by its external appearance.

A preventive replacement interval of six months applies to both the standard air valve and the GreenLine air valve.

Replacement is carried out regardless of whether any external abnormalities are already visible.

The air valve should be replaced earlier in the event of:

• visible damage
• heavy external contamination
• contact with eluent
• suspected restricted pressure equalization
• recurring suction problems with no other identifiable cause
• unknown usage or storage history.

The replacement date should be entered on the labelling field immediately upon installation, or the replacement indicator should be activated. The indicator documents the passage of time but does not assess the actual technical condition of the valve.

Inspect Components Regularly

For reusable components, there is generally no uniform time-based replacement interval. Their condition must be assessed based on the application and any abnormalities observed.

For supply tubing, particular attention should be paid to the areas directly adjacent to:

• fittings
• instrument connections
• bottle holders
• tight bends
• fixing and guide points.

Even minor mechanical damage can affect sealing, suction conditions or flow rate.

Monitor Suction Filters According to the Application

There is no universally applicable, purely time-based replacement interval for suction filters. The required inspection and replacement cycle depends, among other things, on:

• eluent composition
• buffer and salt content
• operating time
• eluent standing time
• contamination risk
• analytical sensitivity
• filter material and filter geometry.

More frequent inspections are advisable in particular for:

• aqueous eluents
• buffer and salt solutions
• visible crystal or precipitate formation
• long standing times
• increased risk of microbial contamination
• recurring suction or flow problems.

A filter that appears clean externally is not necessarily freely permeable or free of internal deposits. In cases of unknown contamination, damage or critical analytical applications, replacement is generally safer than unvalidated cleaning.

Keep Solvent Bottles Clean and Clearly Labelled

The solvent bottle is part of the solvent supply and therefore part of the analytically relevant media path. Residues, precipitates or mix-ups can directly affect the composition of the eluent and thus the analysis.

After a method or eluent change and after long standing periods, check whether:

• the existing bottle can continue to be used
• suitable cleaning is required
• the suction filter must be cleaned or replaced
• the complete eluent path has been flushed sufficiently
• the materials used are suitable for the new eluent.

Cleaning must be carried out using a procedure suitable for the bottle material, residues and subsequent application. Unknown residues or contamination that cannot be removed reliably are grounds for replacing the bottle.

Each solvent bottle should carry at least the following information:

• eluent designation
• pump channel A, B, C or D
• preparation or make-up date
• where applicable, change date
• special notes such as PFAS-sensitive application, drying tube or protective gas.

Illegible, contradictory or outdated labels must be replaced immediately.

Document Maintenance Activities

For quality-critical applications, traceable documentation of inspections and measures performed is recommended.

The following should be documented in particular:

• date of inspection or maintenance
• affected HPLC system or eluent path
• replacement of the air valve
• cleaning or replacement of the suction filter
• replacement of tubing, fittings or blind plugs
• abnormalities identified
• corrective actions taken
• result of the subsequent functional check.

Documentation improves traceability and supports root-cause analysis if a problem occurs again.

Release After Maintenance Work

After cleaning, component replacement or another maintenance measure, the solvent supply should only be released for operation again once:

• all connections are correctly occupied or sealed with blind plugs
• suction filters are securely attached and fully immersed
• tubing runs without kinks and with minimal mechanical stress
• the air valve is fully screwed in and still within its service life
• solvent bottles and pump channels are clearly labelled
• suction lines have been completely filled and vented
• the pump delivers uniformly
• there are no visible leaks or other abnormalities.

If problems had previously been identified, it must also be verified that the original symptom has actually been eliminated.