Choosing the right septa and closures for your analysis is critical to ensure chemical compatibility with your sample and solvents, protect sample integrity and prevent contamination.
 
Table 1 can be used as a reference point when selecting the right closure system for your analysis. It summarizes the chemical compatibility of the most common septa materials with chemicals and solvents used in many standard chromatography applications. Please note that Table 1 indicates septa compatibility post-injection, this means after piercing with an autosampler syringe needle.

Table 1. Septa compatibility post-Injection

1. Will your application require repeat injections from the same vial?

• Repeated injections and sample storage: septa made of PTFE / Silicone / PTFE are the product of choice for because of the excellent resealing characteristics
• Single injection applications that do not require long-term sample storage: septa made of PTFE / Rubber are more economical solution

 

2. Do you want to use your chromatography vial for long-term sample storage? Are you afraid of gas venting?

• For long-term sample storage, crimp caps, which are attached / removed using crimping / decapping tools, provide best quality seals.
• If you are looking for a convenient closure system that does not require additional tools, screw caps can offer a good compromise of secure fit and ease of use.
• Snap cap vials are chosen for convenience purpose as well. Snap caps may be adequate for applications that do not require heating of the sample above room temperature and where some cap cracking and moderate sealing performance can be tolerated.
• When for septa selection, silicone rubber is usually thicker than natural rubber and therefore provides a tighter seal even after punctuation with a needle.

 

3. Facing trouble with vacuum formation during sample draw?

Try using pre-slit septa to provide proper venting of the vial during sample draw.

 

4. What type of autosampler needle are you using?

• Thin, fragile needle: soft and thin septa, such as Silicone / PTFE required.
• Blunt, thick needle: pre-cut septa in order to facilitate needle penetration and to prevent coring.

In general, silicone septa are more easily pierced than red rubber or butyl.
 

5. In which direction does the PTFE membrane in PTFE / Silicone and PTFE / Rubber caps face?

• The main function of the PTFE part of the liner is to create a nearly inert barrier between sample and septa material prior to injection. Therefore, it always faces in the direction of the sample.

 

6. What does the information “Shore A 45°” mean?

• The hardness of a material is called durometer and measured in ° Shore. Different scales are used to specify what type of substance is described. E.g., the “A” in “Shore A 45°” refers to the scale for flexible mold rubbers / plastic. Numbers go from 0 to 100. A high shore grade describes a hard liner whereas a low shore grade refers to a softer liner. The typical hardness for septa materials used in chromatography is Shore A 45°±5, but can also be Shore A 35°±5 or Shore A 50°±5. Both the thickness of a liner as well as the shore grade are important numbers to take into account when choosing the right type of needle for your application.