Vial caps play a crucial role in guarding the sample from spills, contamination, and evaporation. Ideally, caps should form an airtight seal and be inert. Caps may come with a silicone or polytetrafluoroethylene (PTFE) septum, which creates a tight seal. Needles can pierce the septum because the elasticity of the material allows it to reclose.

Screw Cap:

Most people have used screw caps to close their water or soda bottle. Screw caps for vials are not much different from the bottle caps we use every day. Screw caps are excellent at forming a tight seal. When you turn a screw cap,you apply pressure that holds the septum between the vial rim and the cap, and it won't move when it's pierced. Screw caps may have an opening to use with an autosampler or a solid top for storage purposes.

Crimp Cap:

Crimp caps often consist of an aluminum cap and PTFE/silicone septum. This type of cap tightly squeezes the septum between the vial and the cap, forming a superior seal and reducing evaporation. Crimp caps require the use of manual or automatic crimping tools.

Snap Cap:

A snap cap provides a moderate seal and doesn't require special tools because it can be snapped onto a vial and removed by hand. With an opening in the top, snap caps provide easy access for autosamplers. Snap caps are recommended for short-term storage and can be used with septa and metal O-rings.

Chromatography Vial Septa:

Septa are commonly made of PTFE and silicone. There are several benefits of silicone septa. For example, silicone can withstand extremely high and low temperatures and maintain its flexibility. It's also chemically inert and won't impact the sample's integrity. Lastly, silicone is resistant to UV radiation and is suitable for various sterilization methods, including steam autoclaving.
You have many options when it's time to select the septum. Common septum materials include:

PTFE/silicone:

A PTFE/silicone septum consists of pure silicone laminated with PTFE. This creates a high inertness and exceptional resealing abilities, even after multiple punctures. This is often a preferred choice for chromatography applications.

PTFE:

PTFE septa provide excellent resistance to solvents and are easy to penetrate. However, PTFE septa are not resealable and are recommended only for short-term storage and single-injection use.

Pre-slit PTFE/silicone:

Pre-slit septa feature a slit in the center, allowing easier penetration and sample removal. This type of septum is similar to a silicone septum without a slit because it also has excellent resealing capabilities. One difference is a pre-slit septum is slightly less tolerant of aggressive solvents. A pre-slit PTFE/silicone septum may be the ideal choice for improving autosampler reproducibility.

PTFE/red rubber:

PTFE/red rubber septa are a popular and affordable option for standard gas chromatography uses. These septa provide moderate resealing capabilities and high chemical inertness. It's not recommended to use PTFE/red rubber septa for multiple injections.Different Types of vials caps and septa has their own advantage and disadvantage. Choose the right caps and septa according to your sample's characteristics.

For knowledge of vials, please check this Aritcle :

What Vial Is Right for You?

Septa Selection for Autosampler Vials

5 Points Need to Consider When Choosing An Autosampler Vial