Introduction

The Intoxilyzer 5000 was released in 1982 as a more accurate test for alcohol concentration that uses the infrared transmittance of ethyl alcohol, as opposed to the Breathalyzer, which uses chemical reaction to determine presence of alcohol, which as we know from class, offers much error in equilibrium. Through a series of filters and infrared spectroscopy analysis, one’s alcohol content level can be easily determined. The sample (breath) chamber is small (81.4mL) and heated (48^oC) to keep one’s breath fluid and gaseous (zero condensation) as the breath moves through the open-system chamber, and infrared light is passed through the breath, then through absorption filters and its transmittance read. In a nutshell, the amount of light transmitted is analyzed according to the Beer-Lambert Law:

log(T) = log(I/I_o) = -σlN

Equation 1. Transmittance is the inverse of absorbance. It is a fractional measure of the amount of light transmitted by a body, in this case the breath sample. It is directly proportional to the intensity of light at the end of the chamber by T=I/I_o.

where T is transmittance, I is intensity at the end of the chamber, I_ois the initial intensity, σ is the absorption cross section (measure of particle-particle interaction), lis the length of the chamber and Nis the density of ethyl alcohol per unit volume. The reading of intensity at the end of the chamber is proportional to the amount of ethyl alcohol in the sample, which allows a direct correlation to the subject’s body, since the Beer-Lambert Law is independent of sample volume.

While accuracy of the Intoxilyzer 5000 is controversial, it is still used by many states to determine one’s blood alcohol content (BAC). To minimize controversy, the Intoxilyzer 5000 is used with a 20-minute observation period. That is, anyone under suspicion of being under the influence will be held for 20 minutes before and between testing to ensure that any residual alcohol (from mouthwash, most recent drink, disease, etc.) is minimized, giving a more accurate analysis of the subject in question.

How does the Intoxilyzer 5000 work?

The basic parts of an Intoxylizer are shown in the diagram below. A lamp generates a broadband IR beam (a broadband beam contains multiple-wavelengths). This beam then passes through the sample chamber, and is focused by a lens onto a filter wheel, which is spinning. The filter wheel contains narrow band filters that are specific for the wavelengths of the bonds in ethanol. The light passing through each filter is then detected by a photocell and converted to an electrical pulse, which is passed on to the microprocessor. The microprocessor interprets the pulses and calculates the BAC based on the absorption of infrared light.

Figure 1. Diagram of the Intoxilyzer 5000

IR Spectroscopy

Infrared spectroscopy involves the infrared region of the electromagnetic spectrum, which is the region of light that’s lower in energy than visible light. The region from approximately 4000 cm^-1to 400 cm^-1is most useful to chemists because it can be used to learn about a molecule’s structure, particularly its functional groups. The photon energies in this region are not large enough to excite electrons, but they can induce vibrational excitation. As seen in the diagram below, various bending and stretching vibrations lead to different peaks on an IR spectrum, and these peaks are specific to certain functional groups. The hydroxyl group of ethanol is the particular peak of interest in this discussion of the Intoxylizer.

Figure 2. The above is a sample diagram of an IR spectrum that shows where the peaks of various functional groups are expected to occur.

As shown in the figure below, ethyl alcohol absorbs between ~2.8-4 microns, as you may remember from organic chemistry, as a strong broad peak. The result from the Intoxylizer 5000 would be similar to the figure below if a subject was under the influence of alcohol. The IR spectroscopy reading of a subject not under the influence of alcohol would not show readings in this range. A concentrated sample would show a stronger peak than a dilute sample, which would lead to the determination of a subject’s BAC level, 0.08 being the legal limit.

Figure 3. Range of ethyl alcohol transmittance

Conclusion

The intoxylizer is one of many ways of measuring blood alcohol level. Unlike conventional breathalyzers, the intoxylizer utilizes IR spectroscopy to determine alcohol level.  IR spectroscopy uses differences in bond vibrational frequencies to identify compounds.  Absorbance and the Beer-Lambert Law can be used to determine concentration of the compound in question, in this case, ethanol.  Because IR spectroscopy can differentiate between different alcohols, the intoxylizer is one of the most accurate means of measuring alcohol level.

Bibliography

Freudenrich, Craig.  "How Breathalyzers Work"  20 October 2000.  HowStuffWorks.com. <http://electronics.howstuffworks.com/gadgets/automotive/breathalyzer.htm>  23 November 2010.

Head, William C. "The Intoxilyzer 5000." Drunk Driving Defense. Legal Brand Marketing, 2010. Web. 20 November 2010.

Reusch, William. "Infrared Spectroscopy." Introduction to Spectroscopy. Michigan State University, 1999. Web. 24 November 2010. <http://www2.chemistry.msu.edu:80/faculty/reusch/VirtTxtJml/Spectrpy/InfraRed/infrared.htm#ir1>.