Septa Seletion Guide for Chromatography

Choosing the right septa for your testing is important to prevent contamination, avoid damage and to allow high levels of repeatability. Ensuring compatibility with your instruments, compounds and solvents is an integral part of this.

There are many different materials used to produce the septa found in Ion (IC), Liquid (LC) and Gas (GC) chromatography vial caps. This document aims to guide you to make the correct material selection by providing a thorough technical overview.

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Images: Green 9-425 Open top Smooth Screw Cap with 9mm Pre-Slit Red PTFE/White Silicone Septa 1mm thick

Septa Selection Chart

As a quick guide, please see our Septa Selection Chart below. Further, more detailed guidance is provided in the remainder of the document.

Septa MaterialAdvantagesChemical ResistanceApplicationsOperating Temperatures
PTFEEconomical.Excellent.Single injection only. Can be penetrated by most normal gauge metal HPLC needles. Rarely used for GC.-200°C to 250°C
PTFE/SiliconeExcellent resealing capabilities and excellent at retaining purity.Excellent until punctured. Not suitable for chlorosilanes.Multiple injections then moderate resistance. Ideal for most HPLC and GC applications.-60°C to 200°C
Pre-slit PTFE/SiliconeReduces coring & prevents vacuum from inside vial.Excellent until punctured, then moderate resistance.Multiple injections. Highly recommend for autosamplers with thin gauge needles.-60°C to 200°C
PTFE/Silicone/PTFEResistant to coring & autoclavable.Excellent.Above average resealing. Multiple injections or applications with long periods between injections. Recommended for autosamplers with large diameter, blunt-tip needles.-60°C to 200°C
PTFE/Natural RubberGood reseal-ability. Moderately priced.Good chemical resistance.Multiple injections. Not as easy to penetrate as Red Rubber.-40°C to 120°C
PTFE/Red RubberEconomical.Excellent until punctured.Moderate resealability. Not recommended for holding samples for further analysis. General HPLC & GC.-40°C to 110°C
Grey Butyl StopperVery economical.Not suitable for chlorinated solvents, alkanes, benzenes or cyclohexanes.Suitable for low pressure applications, analysis of fixed gases. Provide absolute resistance to moisture penetration.-59°C to 125°C
Polyethylene (PE)Economical.Good.Single injection use with aqueous based sample mixtures.-20°C to 80°C
Polypropylene (PP)Economical.Excellent. Better solvent compatibility than PE, but slightly higher piecing force.Single injection use with aqueous based sample mixtures.-10°C to 130°C
Table 1 – Septa selection chart

Main factors to consider

  • Instrument – Different injector needles are used by different instruments. Pre-slit septa and having softer septa materials can assist with penetration, and reduce the chance of broken or bent needles.
  • Compound – A tight seal is critical to prevent compound loss if volatile compounds are being used.
  • Solvent – Septa integrity can be affected by some commonly used solvents. The chemical compatibility should therefore be checked beforehand.
  • Multiple injection – In this instance the reseal-ability of the septa should be a major consideration, and the septa should be able to withstand multiple injects to avoid debris.

Septa Materials – An Overview

PTFE

  • High chemical resistance against the majority of aggressive solvents.
  • Thin membrane allows for penetration by most normal gauge metal High Performance Liquid Chromatography (HPLC) needles.
  • Not resealable.
  • Should not be used with highly volatile solvents, short cycle times or multiple injections methods.
  • Rarely used for Gas Chromatography (GC).

PTFE/Silicone

  • The most versatile septum material, suitable for all types of chromatographic applications.
  • Typically, extractables are at lower levels in comparison to other resealable materials.
  • Provides highly consistent performance.
  • Effective resealing.
  • Low penetration force required.
  • PTFE limits exposure of the elastomer to the solvent.
  • Ideal for use in most HPLC and GC applications where reseal-ability and purity are required.

Pre-slit PTFE/Silicone

  • Share most of the same physical and chemical characteristics of PTFE/Silicone.
  • The slit allows for easier penetration of the needle and allows the release of any vacuum created in the vial.
  • Ability to withstand exposure to highly aggressive solvents slightly reduced.
  • Highly recommended for autosamplers with thin gauge needles or high volumes of injections where vacuums can be an issue.

PTFE/Natural Red Rubber

  • An economical choice.
  • Used primarily for routine analysis in GC with Flame Ionisation Detectors (FID), Thermal Conductivity Detector (TCD) & Flame Photometric Detector (FPD) & High Performance Liquid Chromatography (HPLC) with UV detectors or Refractive Index Detectors (RID).
  • Offer good reseal-ability and excellent chemical inertness before penetration.
  • Red rubber has a low Shore hardness allowing for easy needle penetration even with thin bore GC needles.
  • Not recommended for multiple injections with long run times or retention of samples for further analysis after initial penetration.

Grey Butyl Stopper

  • An economical septum.
  • Does not provide a PTFE film barrier.
  • Not suitable for use with alkanes, benzene, chlorinated solvents or cyclohexane.
  • Preferred for analysis of fixed gases.
  • Provides absolute resistance to moisture ingress.

Polyethylene (PE)

  • Suitable for use with most HPLC autosamplers.
  • Not resealable and intended for single injection use.
  • Intended for single injection use with aqueous based sample mixtures.

Polypropylene (PP)

  • Suitable for use with most HPLC autosamplers.
  • Not resealable and intended for single injection use with aqueous based sample mixtures.
  • Offer better chemical resistance than PE, however, a slightly higher piercing force is required.

Chemical Resistance

The following table provides a guide for the chemical resistance of the septa materials mentioned above.

Please note: There are a lot of factors, such as temperature, concentration and the duration of exposure can affect the chemical resistance. So where necessary ensure to test your product under the actual conditions it will be subject to during experimentation.

A = Recommended, B = Suitable for most purposes, C = Use with are, D = Not advisable, – = Not tested.

The character in the ( ) denotes the chemical resistance of the seal after injection.

 Septa material
SolventPTFE /Red RubberPTFE /Natural RubberGrey ButylLDPEHDPEPPPTFEPTFE /Silicone/PTFE
Acetic acid aqueousA (A)A (B)A (B)A (A)A (A)A (A)A (A)A (A)
AcetoneA (A)A (C)A (A)D (D)B (B)B (B)A (A)A (A)
AcetonitrileA (A)A (A)A (A)A (A)
Alcohols (aromatic)A (B)A (D)D (D)D (D)B (B)A (A)A (A)
Alcohols (aliphatic)A (A)A (B)A (B)D (D)B (B)B (B)A (A)A (A)
Amyl AcetateA (A)A (D)A (C)D (D)D (D)A (A)A (C)
Aqueous solutions diluteA (A)A (A)A (A)A (A)A (A)A (A)A (A)
BenzeneA (D)A (D)D (D)D (D)D (D)D (D)A (A)A (C)
Butyl alcoholA (B)A (A)A (B)B (B)B (B)B (B)A (A)A (B)
Carbon disulphideA (D)A (D)D (D)D (D)D (D)D (D)A (A)A (A)
Carbon tetrachlorideA (D)A (D)D (D)D (D)D (D)D (D)A (A)A (C)
ChloroformA (D)A (D)D (D)D (D)D (D)D (D)A (A)A (C)
CyclohexaneA (D)A (D)D (D)A (A)A (C)
CyclohexanolA (D)A (D)D (D)D (D)D (D)B (B)A (A)A (B)
Diethyl etherA (D)A (D)D (D)D (D)D (D)D (D)A (A)A (B)
Dimethyl sulphoxideA (C)A (D)D (D)A (A)A (A)
DioxaneA (B)A (D)A (B)A (A)A (C)
EstersA (B)A (D)A (C)D (D)D (D)B (B)A (A)A (B)
Ethyl acetateA (B)A (D)A(B)D (D)D (D)B (B)A (A)A (B)
Ethyl alcoholA (A)A (A)A (A)D (D)B (B)B (B)A (A)A (A)
Ethylene chlorideA (D)A (D)A (C)D (D)D (D)D (D)A (A)A (C)
Ethylene glycolA (A)A (A)A (A)A (A)A (A)A (A)A (A)A (A)
FormaldehydeA (B)A (B)A (A)A (A)A (A)A (A)A (A)A (A)
GlycolA (A)A (A)A (A)A (A)A (A)A (A)A (A)A (A)
Halogenated hydrocarbonsA (D)A (C)A (B)D (D)D (D)D (D)A (A)A (A)
HexaneA (D)A (D)D (D)A (A)A (C)
Hydrochloric acid diluteA (A)A (C)A (A)A (A)A (A)A (A)A (A)A (A)
Iso-octaneA (D)A (D)D (D)A (A)A (C)
KetonesA (A)A (C)A (B)D (D)B (B)B (B)A (A)A (B)
MeOH/H2O/AcetonitrileA (A)A (-)A (A)A (B)
MethanolA (A)A (A)A (A)A (A)A (A)
Methyl chlorideA (C)A (D)A (C)D (D)D (D)D (D)A (A)A (A)
Methyl acetateA (B)A (C)A (A)D (D)D (D)B (B)A (A)A (B)
Methyl ethyl ketoneA (A)A (D)A (B)D (D)B (B)B (B)A (A)A (A)
Methylene chlorideA (D)A (D)D (D)D (D)D (D)D (D)A (A)A (B)
Nitric acid diluteA (A)A (D)A (B)A (A)A (A)A (A)A (A)A (B)
PentaneA (D)A (-)A (A)A (C)
Petroleum etherA (D)A (-)D (D)D (D)D (D)A (A)A (C)
Sodium hydroxideA (A)A (A)A (A)A (A)A (A)A (A)A (A)A (A)
Sulphuric acid diluteA (D)A (C)A (B)A (A)A (A)A (A)A (A)A (B)
SurfactantsA (A)A (-)A (A)A (A)
TolueneA (D)A (D)D (D)D (D)D (D)B (B)A (A)A (C)
TrichloroethyleneA (D)A (D)D (D)D (D)D (D)D (D)A (A)A (C)
WaterA (A)A (A)A (A)A (A)A (A)A (A)A (A)A (A)
Table 2 – Chemical resistance chart for common septa materials.

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