• Making Roads a Safer Place with Headspace GC Analysis

GC, MDGC

Making Roads a Safer Place with Headspace GC Analysis

Dec 22 2014

Many people have been affected by the effects of drink-driving. For some people, it can mean prison or the loss of a job. For others, it can be even more severe — around 3500 people killed or seriously injured due to drink driving in the UK each year.

Gas chromatography (GC) can be used to accurately determine the quantity of ethanol in blood, urine or saliva — for legal purposes to prosecute drivers and also for post-mortem analysis. Here we look at a little closer at headpace GC and one of the techniques used to prepare blood samples for analysis...

What can Headspace GC be used for?

Headspace GC is used for the analysis of volatile organic compounds — for example alcohol in blood,  solvent residues in pharmaceuticals — or even in the natural gas industry as discussed in this article: Methane, Ethylene, and Ethane in Water by Headspace-Gas Chromatography (HS-GC) with Flame Ionisation Detection (FID).

When samples contain large quantities of non-volatile materials, they can be difficult to remove from the GC system leading to column degradation and poor analytical performance. The answer is to remove the volatile organic compounds from the sample matrix.

The removal of the volatiles can involve several preparation steps to extract the required materials from the non-volatiles. This costs time and money. Headspace analysis can be the simplest and cheapest method to sample the volatile materials.

How it Works

The principle of headspace analysis is quite straightforward. The sample, a solid or liquid, is placed in a vial with a dilution solvent, sealed and mixed thoroughly. Room must be left in the vial — the headspace — for the volatile components of the sample matrix to escape from the sample. As the volatile components diffuse into the headspace, an equilibrium is set-up. A sample of the headspace is taken and analysed using a standard GC set-up.

The concentration of the volatile components in the headspace should be maximised — as this is the part that is sampled and analysed. To maximise the concentration of the volatiles, the partition coefficient for the volatile component in the relevant solvent/air system is used.

The partition coefficient (K) is simply the ratio of the analyte concentration in the sample (cs) and the analyte concentration in the gas phase (cg), so K = cs / cg. This means that:

  • Compounds with low K values have a higher concentration in the gas phase.
  • Compounds with high K values partition less readily into the gas phase.
  • The partition coefficient can be altered by changing the temperature or adding inorganic salts to the sample mix.
  • The volume of sample used and the volume of the headspace also affect the partition value. This ratio is known as the phase volume.

For alcohol in blood — no significant preparation is required. A sample of 100µL of blood is added to 1 mL of internal standard, usually propanol, in a 20 mL headspace vial.

A simple process sometimes used in tragic circumstances. Don’t add your name to the statistics.

Image Source: Tiremarks 

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