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Breath Alcohol Analysis

Dateline: 04/27/98

By Alan Bruzel

Blood alcohol content (BAC) is the amount of alcohol present in a 100 milliliter (mL) volume of blood. (This article will use the word alcohol rather than the proper chemical name, ethanol.) A brief explanation of the units involved in blood alcohol measurement is in order. Percent, from the Latin per centum, means by the hundred, so one may express a concentration of 80 milligrams (mg) of alcohol in 100 mLs of blood as 80mg%. Since 80 mg is 0.08 grams, 0.08 grams of alcohol in 100 mLs may be written as 0.08%. In other words, 80 mg% is equal to 0.08% which is equal to 80 mg/dL (deciliter; 100 mLs). This value can also be described as 0.08 BAC. All of these methods of expressing blood alcohol concentration are in use; BAC will be used here.

Sixteen states in the US define intoxicated drivers as those having at least a 0.08 BAC. The remaining 34 states and the District of Columbia use a 0.10 BAC legal limit. The United Kingdom mandates a 0.08 BAC limit, and the US Department of Transportation imposes a 0.02 BAC limit on its transportation and safety sensitive employees. Compliance with these standards, in part to reduce the more than 17,000 alcohol-related traffic fatalities per year in the US, has engendered a variety of alcohol detection devices.

In non-alcoholics, alcohol begins to uncoil its physiological effects at 0.02 BAC, but does not reduce psychomotor performance at levels less than 0.05 BAC. It produces stereotypical drunken behavior at 0.10 BAC, and kills by respiratory depression at 0.50 BAC. Although the validity of results from breath alcohol analyzers, and the multiplication factor needed to convert breath alcohol readings into BAC are routinely challenged by defense attorneys, this article will address only the apparatus used and the chemistries involved.

Gas chromatography, used for blood alcohol analysis in controlled laboratory situations, has yielded to the simpler and quicker analysis of breath samples required by routine police work. The first breath alcohol analyzer was developed by Glenn Forrester in 1937. It consists of an empty balloon and a glass tube filled with potassium dichromate crystals wetted with sulfuric acid. The suspect blows up the balloon, which the police officer then attaches to the tube, allowing the expired breath to pass through and react with the dichromate crystals. Alcohol in the breath sample reduces the yellow dichromate (VI) to green chromium (III). Silver nitrate is present as a catalyst to bring the reaction to completion. The amount of green crystals produced is proportional to the amount of alcohol in the breath sample. Current technology has replaced this apparatus with a device housing either a fuel cell, an infrared spectrometer, or a semiconductor. An onboard computer calculates the BAC, and displays the result on a digital readout.

The alcohol molecule absorbs light in only certain regions of the infrared spectrum. The more alcohol that is present in a sample, the more absorbance in these particular regions. Infrared spectrometry can quantitate alcohol concentrations in neat samples of gin, scotch, and white wine using alcohol's absorbance at 7.5 microns (1330 cm^-1). For evidential breath analysis, a sample is tested at more than one wavelength; 3.4 microns, for example, will detect acetone as well as alcohol. Measurement at 3.4, 3.5, and 3.8 microns, ensures identification and quantitation of alcohol even in the presence of acetone, isopropanol, and methanol. Infrared detectors are available that measure a 0.100 BAC plus or minus 0.002. This method of breath alcohol analysis is accepted by all states in the US, and is the most popularly used breath alcohol measurement technology. Unlike the fuel cell, an infrared alcohol detector monitors the entire volume of expired air, allowing examination of air from deep in the lungs.

Another portable device uses a fuel cell for alcohol detection. Exhaled breath contacts the upper platinum surface of the cell, and any alcohol present is oxidized to acetic acid, two protons, and two electrons. The upper surface is thus rendered electron rich. The protons combine with oxygen and electrons on the lower surface of the cell forming water and making this region electron poor. A meter placed in a circuit connecting the upper and lower surfaces measures the electrical current produced, and displays readings corresponding to the amount of alcohol converted into acetic acid. The fuel cell reacts primarily with alcohol, less strongly with isopropanol and methanol, and not at all with acetone and gasoline. It will also detect carbon monoxide, but only at a level that is not commensurate with human life – interference from carbon monoxide is therefore not a credible excuse for a positive reading. Fuel cell detectors used for evidential breath analysis can measure a 0.100 BAC plus or minus 0.005. They show a short-lived fatigue effect, however, and cannot be used continuously without allowing time for recovery. A product that prevents a drunk driver from starting his vehicle uses a fuel cell alcohol sensor. A "rolling retest" periodically checks the driver's sobriety, preventing another person from starting the car and then turning over the controls to the inebriated driver.

Other breath alcohol devices employ semiconductors, whose electrical conductance changes in the presence of alcohol. They give an accuracy of plus or minus 0.005 at 0.100 BAC. Saliva alcohol testers are also popular because the alcohol content of saliva parallels that of blood for two hours after drinking. Both of these technologies are primarily used for personal monitoring rather than for law enforcement.

These devices quantitate the amount of alcohol in a subject's breath, but cannot measure the duration of alcohol abuse. Elevated blood methanol, however, is a marker for chronic alcoholism. Methanol accumulates in alcoholics because most alcoholic beverages contain some methanol as an impurity, and because methanol is not metabolized in the presence of alcohol. Blood methanol monitoring would complement breath alcohol testing in assessing an individual's recovery from this debilitating addiction.

Recommended Web resource for additional information:

Chemical Analysis of Wine
An article from this Web site examining new developments in wine chemistry.

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