Basics of DUI Blood Testing

Basics of DUI Blood Testing

Parts of a Gas Chromatograph

Carrier Gas – Inert gas to carry the sample through the instrument and wont react with the sample.

Syringe – Draws up and introduces the appropriate amount of sample into the GC.

GC Oven – Provides heated environment to maintain volatility of the analytes isothermic temperature setting of 35 degrees Celsius

Column – Provide interactive forces to separate analytes in the sample.

Detector – Measure the response of a given analyte after it has been isolated by the column.

Amplifier – Increase the measured responses for analysis.

Necessary Maintenance of Instruments within Chromatograph

Change rubber septum – After the syringe punctures the septum from too many injections, septum particulates are dislodged leaving a small hole.

Bake out oven – Helps to clear out the column of higher molecular weight molecules (less volatile) that may have accumulated once reaching the lower isothermic temperature of the oven.

Check gas pressure regulators – Ensure there is sufficient gas flow for the entirety of the next run

Why is gas chromatography used in DUI blood tests?

Gas chromatography (GC) is mainly used for the qualitative analysis of samples: it answers the question “What chemicals are present in this sample, and in what relative proportions?” The basic idea with GC is that you inject a liquid into a long coil (or column) which is contained in an oven. Because oven heats the coil the liquid will vaporize and separate. Because the coil is long it will take the vapors some time to reach the end of the coil. Different vapors will exit the coil at different times. We can compare these times to standard values to determine the identity of the vapors, and therefore, the chemicals that were in the sample. In DUI related blood tests, the chemical sought after is ethanol.

Factors that can Influence the Separation of Chemicals

Boiling Point – The lower boiling point the shorter retention time

Column Temperature – The lower the column temperature the greater column interaction

Carrier Gas Flow Rate – The lower the flow rate the greater the column interaction

What is Ethanol?

Volatile, colorless liquid that has a slight odor. It burns with a smokeless blue flame.

Ethanol is the principal type of alcohol found in alcoholic beverages.

Alcoholic drink equivalents:  Beer -12 oz., Malt Liquor – 8 oz., Wine – 5 oz., Hard Liquor – 1.5 oz.


On average, one drink will produce a BAC of about 0.02.

Most light and moderate drinkers will feel some effects at this level. 

Other Volatile Compounds Searched for in DUI Investigation


Methanol toxicity remains a common problem in many parts of the developing world. It can be found in paints, resins, solvents, refrigerants, windshield washer fluid and many other everyday products


Familiar uses of acetone are as the active ingredient in nail polish remover and as paint thinner.


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It occurs widely in nature and is produced on a large scale in industry. Acetaldehyde occurs naturally in coffee, bread, and ripe fruit, and is produced by plants. It is also produced by the partial oxidation of ethanol by the liver enzyme alcohol dehydrogenase and may be a contributing factor to hangovers from alcohol consumption.


Otherwise known as “rubbing alcohol,” it is used in keyboard, LCD and laptop cleaning, is sold commercially as a whiteboard cleaner. A 75% diluted solution in water may be used as a hand sanitizer.

What is N-Propanol and Why is it Important?

Internal Standard for a DUI chromatograph.

It is a compound similar to the analyte of interest (i.e. ethanol) and is added to the sample and run.

Having the analyte and the internal standard elute in the same run helps eliminate run to run variability giving more precise results

Properties –

1)  It is similar to ethanol in its effects on human body, but 2-4 times more potent.

2)  It is a volatile, colorless liquid.


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