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Gas Chromatography Blood Testing

Gas Chromatography

The most common method used by law enforcement for the testing of blood for ethanol is gas chromatography.  The specific method used is Headspace Gas Chromatography. 

For detection of drugs, a different related technique is used.  The method often used is Gas Chromatography with Mass Spectrometry (GC-MS).  

Headspace Gas Chromatography

Headspace Gas Chromatography (HS-GC) is a widely accepted method of testing blood for alcohol content and is considered reliable if done correctly. 

Chromatography is a separation science.  Headspace gas chromatography tests gases that have evaporated from a liquid into the "headspace" of a testing vial.  The liquid in DUI cases is blood.  A small amount of blood is placed into a vial and capped.  The vial is heated, and volatiles in the blood evaporate into the air immediately above the liquid. Ethanol is a volatile. 

The HS-GC device captures a small amount of the headspace gas and moves the gas into a "column."  A "column" in HS-GC is a long super thin glass tube.  In HS-GC, the headspace gas moves through the "column" and its various components separate from each other.  The time it takes a particular substance to pass through the column identifies the substance.  The amount of the substance is determined by measuring the amount of energy it gives off as it passes through a detector. 

The process involves the following steps:

  1. Collection (blood draw)
  2. Storage
  3. Sample Preparation
  4. Sample Testing

Any mistakes in the four steps can lead to unreliable and/or inaccurate results. 

Gas Chromatography with Mass Spectrometry

Gas Chromatography with Mass Spectrometry (GC-MS) is a related chromatography technique.  Forensic investigators employ it in investigations of fires and explosions, as well as for detecting and identifying drugs. The GC-MS process is capable of detecting very small amounts of chemicals, which makes it useful in these types of investigations.  GC-MS is the most sophisticated drug test available.

In GC-MS the sample preparation process is more complex than HS-GC.  In HS-GC, the substances of interest can be converted to gas simply by raising the temperature of the sample.  In GC-MS, the substances typically do not easily evaporate, so the sample preparation process must vaporize the sample. Also different is the type of detector used.  In HS-GC a flame ionization detector is used.  This is a fancy way of saying the tested substance is burned.  The amount of energy created in the burning process is measured and used to quantify the substance.  In GC-MS, instead of the simple flame ionization detector, a mass spec detector is used.  The mass spec detector blasts the sample with an electron beam and the sample is then moved into a magnetic tube where the mass spectrometer analyzes the fragments by their mass-to-charge ratios. Analysts are then able to measure the molecular fingerprints against a predetermined standard to determine each distinct compound. Both their retention time relative to the gas chromatography and their mass spectrum identifies the substances.

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