How does gas chromatology detect alcohol in the blood?

Gas chromatology, also known as gas chromatography, is a
way to separate volatile gasses or liquids from one another so they can be identified.
In the case of a blood sample being examined for ethanol, the sample is first prepared
by dilution with a known concentration of another (non-consumable) alcohol, usually
propanol or butanol, for comparison purposes. It is then injected into a vaporization
chamber. A constant flow of an inert gas such as nitrogen carries the vaporized blood
sample into a separation column. The different volatile chemicals in the sample interact
with the column by adhering and moving thorough the column by capillary action. Lighter
molecules move through the column relatively quickly, while heavier molecules take more
time. This difference in motion results in the different sized molecules becoming
separated into groups as they move through the chamber. As each group exits the chamber,
it passes through a detector.

The detector allows the
instrument to note the retention time within the column for each substance. The
retention times are compared to a known standard and the presence of absence of various
molecules is confirmed. The size of the peak corresponds with the sample volume;
comparing the size of the ethanol peak with that of the alcohol added during sample
preparation allows a calculation of blood alcohol as a percent
volume.

In most cases, an additional detection step is
added in the form of mass spectroscopy. In this case, the detection system also shines
light of a known wavelength through each substance as it exits the separation chamber.
The amount that the substance refracts the light beam is recorded and again is compared
to a known database. This provides a second confirmation of the identity of the
substances present in the sample.