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Basic Principles of Headspace Analysis

A headspace sample is usually prepared in a vial containing the sample, the dilution solvent, a matrix modifier and the headspace. Volatile components from complex sample mixtures can be extracted from non-volatile sample components and isolated in the headspace or gas portion of a sample vial. A sample of the gas in the headspace is injected into a GC system for separation of all of the volatile components.

Phases of the Headspace Vial

 

G = the gas phase (headspace)

The gas phase is commonly referred to as the headspace and lies above the condensed sample phase.

S = the sample phase

The sample phase contains the compound(s) of interest. It is usually in the form of a liquid or solid in combination with a dilution solvent or a matrix modifier.

Once the sample phase is introduced into the vial and the vial is sealed, volatile components diffuse into the gas phase until the headspace has reached a state of equilibrium as depicted by the arrows. The sample is then taken from the headspace.

Headspace Vial and Septa Issues

In order to achieve the best performance with headspace GC, careful attention should be paid to sample preparation and instrument setup. The vial and cap are a very important, and often overlooked, element in the headspace analysis preparation.

It is important to ensure the correct vial and cap are used!

  • Vial Size: Headspace vials are available in 6, 10 and 20ml sizes. Use a vial large enough to ensure adequate headspace or phase ratio without excessive dilution of the components of interest. Phase ratio is discussed later and should normally be at least 50% of the sample volume.
  • Bottom Profile: Headspace vials may be round-bottomed or flat-bottomed. Either type is suitable but round-bottomed vials tend to be stronger and may work more reliably when the autosampler moves the vials into and out of the incubator. Round-bottomed vials also tend to withstand higher pressures and are more suitable for elevated temperatures and applications such as derivatization.
  • Correct Crimping: Headspace vials must be correctly crimped. Ensure the crimper is not worn and is adjusted correctly. The cap should not turn after application to the vial. Find out more about correct vial crimping.
  • Septa Choice: Use septa suitable for the temperature of the system. Poor septa can result in bleed and so contaminate the headspace.
  • Cleanliness of Vial: Vials should be clean. Not all vials are the same and dirty or re-washed vials can cause ghost peaks or erroneous results.