Introduction

Autosampler vial septa are manufactured from a variety of elastomeric materials designed to provide reliable sealing while allowing repeated needle penetrations during HPLC, UHPLC, GC, and LC-MS analyses.

One specification commonly associated with septa is the Shore A hardness rating. Understanding this measurement can help analysts select the most appropriate septum for their analytical application and instrument requirements.

What Is Shore Hardness?

Shore hardness is a standardized measurement used to describe the hardness or firmness of elastomeric materials such as rubber and flexible plastics.

The scale was developed by:  Albert F. Shore  and remains one of the most widely used methods for characterizing elastomer hardness.

What Does Shore A Mean?

The letter designation identifies the hardness scale used.

For chromatography vial septa:

Shore A is the most common scale because it is intended for:

• Flexible elastomers
• Silicone rubber
• Soft plastics
• Molded rubber materials

This scale is well suited for the materials typically used in autosampler vial closures.

Understanding the Numerical Scale

The Shore A scale ranges from:  0 to 100

Where:

• 0 represents extremely soft material
• 100 represents extremely hard material

As the Shore A value increases, the material becomes firmer and more resistant to deformation.

Difference Between 45 Shore A and 55 Shore A

A septum rated at:  55 Shore A  is harder than a septum rated at:  45 Shore A

Because of the increased hardness, the 55 Shore A material generally exhibits:

• Greater resistance to deformation
• Increased firmness
• Enhanced puncture resistance

In contrast, a 45 Shore A septum is:

• Softer
• More compliant
• Easier to compress

How Septum Hardness Can Affect Performance

Septum hardness may influence several performance characteristics.

Softer Septa (Lower Shore A)

Potential characteristics include:

• Easier needle penetration
• Greater flexibility
• Increased compressibility
• Improved sealing in some applications

Harder Septa (Higher Shore A)

Potential characteristics include:

• Improved resistance to repeated punctures
• Greater dimensional stability
• Reduced deformation
• Enhanced durability in certain autosampler applications

The optimal hardness often depends on the instrument design and analytical workflow.

Why Septum Hardness Matters in Chromatography

Proper septum selection can contribute to:

• Reliable vial sealing
• Consistent autosampler performance
• Reduced evaporation
• Improved sample integrity
• Reduced risk of leaks

Because different instruments and sampling conditions place different demands on the septum, hardness is one of several factors considered when selecting vial closures.

Additional Factors Beyond Hardness

Although Shore A hardness is important, overall septum performance is also influenced by:

• Silicone formulation
• PTFE facing material
• Cure chemistry
• Extractables profile
• Chemical compatibility
• Temperature resistance

For this reason, hardness should be considered alongside other septum properties when selecting vial closure systems.

Conclusion

The terms 45 Shore A and 55 Shore A describe the hardness of the rubber material used in autosampler vial septa. Shore A is the standard hardness scale for flexible elastomers, with higher numbers indicating a firmer material. A 55 Shore A septum is harder than a 45 Shore A septum and may offer different puncture and sealing characteristics depending on the application. Understanding Shore hardness can help chromatographers choose the most appropriate vial closure for their analytical needs.