The variables of the Gas Chromatography process

I have posted before on Gas Chromatography and its principles. 

  1. Gas Chromatography and why is it is so important to Pennsylvania DUI arrests
  2. Crimping a Headspace Vial in Gas Chromatography: How not putting the "lid on the jar" can lead to disaster in a DUI case
  3. A large problem in Gas Chromatography: No uniform standard for GC run position or composition

As I have written before:

As headspace gas chromatography (as opposed to direct injection) is based upon Henry’s law. Henry’s Law states that at a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. Or stated differently, an equivalent way of stating the law is that the solubility of a gas in a liquid at a particular temperature is proportional to the pressure of that gas above the liquid. Headspace gas chromatography requires a closed system (i.e. no pressure leaking from the gaseous phase). As a result if the crimping of the headspace vial (which is typically a manual process) is not complete, then it is not a closed system. Hence, without assurances that the crimping is properly done, there is a large possible source of error as Henry’s Law is violated. It is indeed the entire principle upon which Headspace Gas Chromatography is based.

Henry’s Law depends upon several key variables In the entire apparatus reamin constant throughout the run to work.  These are:

  • Flow/pressure rate
  • Temperature of the headspace vial incubation oven
  • Temperature of the oven
  • Injection port temperature

If any of the variables change and there is a "cold spot" or a pressure dip, then Henry’s Law is violated.  The ultimate alleged and reported result is not accurate.  It is an isothermal process when Headspace Gas Chromatography with Flame Ionization Detector is used in ETOH determination.  But guess what is not recorded and reported at all times during the run of the analytical device? 

  • Flow/pressure rate
  • Temperature of the headspace vial incubation oven
  • Temperature of the oven
  • Injection port temperature

Interesting, huh?  In the Courtroom, we must not guess; we must not accept assertions based upon faith. it’s not Amen. But rather, data, facts and proof.



-Justin J. McShane, Esquire, Pennsylvania DUI Attorney

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Justin McShane

PA DUI attorney Justin J. McShane is the President/CEO of The McShane Firm, LLC - Pennsylvania's top criminal law and DUI law firm. He is the highest rated DUI attorney in PA as rated by Justin McShane is a double Board certified attorney. He is the first and so far the only Pennsylvania attorney to achieve American Bar Association recognized board certification in DUI defense from the National College for DUI Defense, Inc. He is also a Board Certified Criminal Trial Advocate by the National Board of Trial Advocacy, a Pennsylvania Supreme Court Approved Agency.

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