Headspace sampling is a method of introducing volatile compounds from virtually any sample matrix directly into a gas chromatography (GC) or GC/MS instrument. This technique is relatively simple when compared to other injection techniques, such as purge-and-trap, and keeps instruments cleaner than standard liquid injection.

The most common type of headspace sampling instrument is the autosampler, which consists of a vial that contains sample and a needle attached to a pump. Depending on the model, the needle is driven into the septum of the vial or the septum is held in place by a captive syringe as the sample is extracted.

When the sample is extracted, the needle is moved out of the vial and a valve is opened that transfers some of the pressure in the vial into a sample loop and vents it to surrounding atmosphere through a vent restrictor (not shown). This process requires a brief equilibration or settling time. Once a representative sample has been acquired, the needle is moved back into the vial.

Equilibration is a critical step in headspace sampling, and careful attention toward choosing appropriate temperatures and times during method development is crucial to ensuring robust and long-lived procedures. The equilibration temperature depends on the slowest-moving components of the sample, and the minimum equilibration time is determined by each component’s thermal migration rate.

The equilibration process is Headspace sampler also dependent on the vapor pressure of the sample, which is influenced by the surrounding ambient temperature and a compound’s viscosity. Consequently, a higher equilibration temperature is desirable for both greater headspace sensitivity and shorter equilibration times.

As the equilibration process progresses, the concentrations of each component partition between sample and headspace, with equilibrium achieved when the components attain constant values. The sampler is then set to repeat the equilibration procedure at a specified interval until the desired equilibrium is achieved.

Another important step in the equilibration process is thermostating, which controls the vial’s temperature and ensures that it remains at a temperature sufficient to hold the samples. Too-high thermostating temperatures can cause problems, such as leaks and septum bleeds.

For the sampler to operate at its optimal equilibration temperature and pressure, it must be calibrated, which is done with a known mixture that accurately matches the compounds being analyzed. This can be done with a variety of methods, including matrix matching and standard addition or multiple headspace extraction.

Then, the method must be calibrated again for each new sample. This step can be automated, but it’s still very important to experimentally determine the proper temperature and pressure to get an accurate analysis.

During the calibration process, analysts often have to account for the activity coefficient of the system, which is matrix-dependent and must be calculated. The activity coefficient is the denominator of a mathematical equation called the Clausius-Clapeyron equation that accounts for the concentration differences between a sample and a mixture. The activity coefficient is typically calculated by calculating the ratio of the sample concentrations to the corresponding concentrations in the matrix.