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:
- Collection (blood draw)
- Sample Preparation
- 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.