Blood alcohol testing (BAC) technology continues to improve in the fight to curb drunk driving and to better identify individuals who are driving while intoxicated. Among one of the most interesting DWI law-related technological topics involves the use of infrared technology.
Infrared technology is a mainstay in modern breath testing machines to identify individuals who drive while intoxicated. Infrared detection relies on an electronic analysis of the scope or spectra of a beam of infrared radiation passed through a breath sample.
Based on ancient astronomers’ analysis of stars’ chemical composition via a spectrometer to measure each star’s light spectrum, the science holds that no two chemical substances produce the same spectrum, similar to obtaining an individual’s fingerprints for identification. Infrared radiation ranges from between .75 and 100 microns (m). A micron equals one thousandth of a meter.
Qualitative versus Quantitative Analysis
In order to understand how an infrared breath testing device operates, one must examine both qualitative—the nature of the substance being analyzed—and quantitative—the concentration of the substance—components.
With respect to qualitative analysis, ethyl alcohol contains unique molecules that, when exposed to infrared radiation, absorb radiation that is unique to the substance, thus creating signature wavelengths. Whereas more than a single compound may absorb radiation at one or more similar wavelengths, it is critical to identify a compound after identifying all of its wavelengths. For example, alcohol absorbs infrared radiation at major peaks of 3.39, 3.48, 7.25, 9.18, 9.50, and 11.5 microns, as well as absorption of one micron on either side of the peak. No other compound absorbs infrared radiation at only and exactly those peaks. Alcohol also absorbs a lesser amount of radiation at secondary bands such as at 3.42 microns; again, unique to other substances.
As for quantitative analysis, since driving with some alcohol presence in the bloodstream is, in fact, legal, it becomes critical to develop technology to determine the precise amount of alcohol present in a given sample. Spectrometry uses Beer’s Law—or the Beer-Lambert Law—that provides that radiation absorbed by a substance that is subsequently dissolved in a non-absorbing solvent—such as deep lung air—is directly proportional to the substance’s concentration and path of radiation. Thus, if one knows the amount of absorbed radiation in a substance, then one can determine the substance’s concentration.
Infrared Technology in Breath Testing
There is evidence both confirming and questioning how applicable the Beer-Lambert formula is to breath-alcohol tests. Among the biggest questions are differences in how commercially available machines trap samples differently. Questions arise as to whether a sample contains deep lung air or simple mouth air because the latter can provide a false positive. However, more sophisticated infrared-based breath testers also measure the rate of cooling across a thermistor—or slope detector—that provides information as to whether the air sample was mouth or lung air.
Development of a commercially viable breath tester to measure every frequency would be highly expensive. Thus, breathalyzer manufacturers decide which frequency to analyze and then electronically filter out and subtract those other substances at or near the chosen frequency.
There are three infrared-based breath testers currently used today, each of which uses a different testing algorithm. First, the BAC DataMasterTM uses tri-fold sample chambers and mirrors to measure breath flow at certain micron ranges. The National Dreager AlcoTest 7110 MKIIITM is the newest commercially available breath testing device; however, its size makes it difficult to use as a mobile device. This device conducts two infrared tests—one with an electro-chemical fuel cell—and requires a smaller breath sample than is necessary for other breath testers. Finally, the IntoxilyzerTM 4011—currently used in New York—uses infrared technology at predetermined micron levels to determine the presence and concentration of alcohol in a subject’s breath sample. Each of these devices provide a computer printout report detailing alcohol presence and concentration for evidence in criminal proceedings, if necessary.