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Simplified Theory of Breath Testing & Breath Testing Instrumentation

It is appropriate at this point to discuss the basic theory of breath testing1. The underlying theory behind all breath test devices is that there is a uniform and standard ratio of breath alcohol to blood alcohol in the ratio of 2100 to 1. In theory, 2100 cubic centimeters of exhaled breath will contain the same per cent by weight of alcohol as one cubic centimeter of whole blood2. This relationship is commonly expressed as the 2100:1 ratio and is also variously referred to by the terms “partition ratio,” “conversion ratio,” and “breath to blood ratio.”

All breath testing devices currently employed in the United States use the 2100:1 ratio to estimate blood alcohol. The underlying basis for this ratio is predicated on a phenomenon known as Henry’s Law which states in a closed system, at a given temperature and pressure, for each chemical compound that is dissolved in another liquid compound (e.g., ethanol in water) the concentration of the volatile substance dissolved in the liquid is directly proportional to the vapor pressure of the volatile substance above the liquid. Henry’s Law is accepted as basic science, much as the laws of gravity and magnetism are accepted as facts of science. Among the contested issues in breath testing is not the validity of Henry’s Law, but deciding the proper breath to blood ratio for use as an evidentiary test, determining the temperature of the exchange process, and ascertaining whether the human respiratory system is in fact a “closed” system3.

Breath testing theory is based on the measurement of alcohol contained in deep-lung or “alveolar” air. As blood flows into the lungs to exchange carbon dioxide for oxygen, a part of the alcohol flowing in the blood stream is also exchanged and exhaled4. Henry’s Law provides the basis for estimating the amount of alcohol in the blood stream by measuring the amount of alcohol in exhaled breath. The 2100:1 ratio is used to estimate the amount of ethanol in the whole blood by measurement of the amount of ethanol in an expired breath sample.

The 2100:1 ratio is based on a 1972 study by the National Safety Council that determined 2100 cubic centimeters of lung air at 34 degrees centigrade5 will closely equal the amount of alcohol present in one cubic centimeter of blood. However, there are variances between each person, although the statistical variance is usually slight. Although the 2100:1 ratio has been systematically challenged nationwide by members of the criminal defense bar, no U.S. state court has yet struck down the statutorily mandated presumptive ratio6. In Alabama, the 2100 to 1 ratio is the statutory standard: “Percent by weight of alcohol in the blood shall be based upon grams of alcohol per 100 cubic centimeters of blood or grams of alcohol per 210 liters of breath7.”

Breath Alcohol Testing Instruments: Breath alcohol instruments used in the United States currently employ three types of analytical technology to measure alcohol content in the breath. These detection techniques used and the current instrumentation are listed below:

Primary Detection Principle Instrument
Infrared SpectrometryBAC DataMaster DMT8 (including
BAC DataMaster and cdm series)
Intoxilyzer 5000/80009 series
Electromechanical Oxidation/Fuel CellIntoximeter EC/IR II10 (including EC/IR)
Dual Detector: Infrared/Fuel CellAlcotest 7110 Mk III
Alcotest 951011

Infrared Spectroscopy: Breath test instruments using infrared spectroscopy either as the sole method of analysis or in combination with a secondary method are the most common type of evidentiary breath testing instruments currently in use12. All infrared breath testing instruments use infrared light (IR) absorption to detect the presence and concentration of ethyl alcohol (ethanol).

Ethanol is one of a family of alcohols which includes methanol (methyl alcohol or “wood alcohol”), 1-Propanol (propyl alcohol), 1- butanol (butyl alcohol), 2-Propanol (isopropyl alcohol or “rubbing alcohol”), and ethanediol (ethylene glycol or “antifreeze”). All alcohol compounds consist of three elements: carbon, hydrogen, and oxygen. It is the molecular structure of each alcohol compound that differentiates one type of alcohol from other alcohol compounds. All alcohol compounds have an oxygen-hydrogen ending in the molecular structure, and are commonly referred to as “hydroxyl compounds.”

Common Alcohol Compounds

Common Name IUPAC Formula
Methyl alcoholMethanolCH3OH
Ethyl alcoholEthanolCH3CH2OH
n-Propyl alcohol1-PropanolCH3CH2CH2OH
Isopropyl alcohol2-Propanol(CH3)2CHOH
n-Butyl alcohol1-ButanolCH3(CH2)2CH2OH

Infrared analysis relies on the absorption of various wavelengths of infrared light. The amount of infrared light that is absorbed during analysis can then be measured and the alcohol concentration determined. The advantages of using infrared analyzers for mass breath testing, and the rapid replacement of the wet chemical method during the past two and half decades, are based on infrared instruments’ speed of analysis, simplicity of operation, the elimination of hazardous chemicals, reduced potential of operator error or deliberate manipulation, the ability of the instrument to print test results, and computer processing of the data generated.

The visible light that humans can see is only a small part of the light spectrum, which is part of the “electromagnetic spectrum.” The electromagnetic spectrum includes the increasingly shorter wavelength radiations of ultraviolet light, X rays, and gamma rays, and the increasingly longer wavelength radiations of infrared, microwave, and radio wave. The infrared region is that part of the electromagnetic spectrum that is just longer than visible light. Visible light covers the wavelength of 390-770 nanometers [0.39 to 0.77 microns]. The wavelength of light that we can readily see is 0.4 microns (blue light) to 0.7 microns (red light). Infrared region covers the segment of electromagnetic spectrum longer than 0.77 microns but shorter than microwave.

Electromagnetic Spectrum

Portion of the SpectrumWavelength in Microns
Radio wavesGreater than 10+3
Microwaves25 to 10+3
Far Infrared15 to 50
Mid Infrared2.5 to 15
Near Infrared0.7 to 2.5
Visible light0.4 (blue) to 0.7 (red)
Ultraviolet10-3 to 4 x 10-1
X-rays10-6 to 10-3
Gamma raysLess than 10-6

Light of a specific energy means light of specific wavelength can be measured. Light, like electromagnetic radiation in general, has properties of both waves and particles. Light has the properties of particulate matter and is emitted and absorbed in discrete amounts called “photons.” Each photon of light has a specific energy. The shorter the wavelength, the higher the energy of a photon of light. For example, ultraviolet light radiation has higher energy than infrared light radiation. This is well-known with the dangers associated with ultraviolet rays that can cause skin burning and cancer formation. Ultraviolet radiation is capable of producing actual chemical changes to the molecule by breaking down the bonds that hold the molecule together. The much less energetic infrared rays cannot break bonds, but can cause more subtle effects, such as bending or stretching the bonds connecting atoms. The absorption of light rays of specific energy to cause specific effects in molecules and atoms is the basis of spectroscopy13.

Light travels through space much as waves on the ocean, and are categorized according to speed, frequency, and amplitude. Waves of light travel at the speed of light, thus the only variables are frequency and amplitude. Infrared spectroscopy capitalizes on a foundational law of physics that infrared light can be absorbed at a particular wavelength by molecular bonds present in all molecules.

Infrared breath testing devices were first placed on the market in 1972 with the introduction of the “Intoxilyzer” produced by Omicron Systems Corporation. The rights to the original Intoxilyzer were acquired by CMI, Inc. located in Owensboro, Kentucky. The original Omicron Intoxilyzer was then sold as the CMI Intoxilyzer 4011. CMI subsequently replaced the 4011 with the Intoxilyzer 5000 series. The Intoxilyzer 5000 series employs the infrared spectrum of 3.4 micron for ethanol detection and added a series of filters to detect interferents14. The latest model, the Intoxilyzer 5000 EN, incorporates five infrared filters and a cooled detector for greater detector stability.

Infrared light emitted at 3.4 micron, 7.2 micron, 8.2 micron, 9.5 micron, and 11.4 micron will be absorbed by ethanol. Most currently manufactured infrared breath test devices, such as the Intoxilyzer 5000/8000 series, only use the 3.4 micron wavelength for ethanol detection. One problem with the use of the use of the lower wave length instrumentation is common alcohol compounds such as methanol or acetone are also detected at the 3.4 micron range, thus inhibiting specificity for ethanol.

In contrast, the current breath test instrument used by the state of Alabama, the Draeger 7110 Mk III, uses the 9.5 micron range for infrared light analysis and a fuel cell alcohol analyzer. The first feature that sets the Draeger 7110 apart from other infrared breath detection instruments is the design to utilize the infrared light at 9.5 micron wavelength.

Absorption of infrared light in the 9.5 micron region of the electromagnetic spectrum is characteristic of the carbon-oxygen single bond. Ethanol has two bonds — a C-H bond and C-O bond. Most organic compounds have C-H bonds, while relatively few have a C-O bond. Therefore, the potential for interference is much less when measurements are made in the 9.5 micron range of the infrared spectrum. However, a number of solvents and other alcohols do have the C-O bond, so absorbance of infrared light in the 9.5 range does not, in itself, assure specificity.

Using the Draeger 7110 instrument for breath analysis, from a single breath sample, two separate analyses can be obtained by two different methods. Each test serves as a check on the other. If different technology is used for the two tests, agreement between the two tests significantly decreases the likelihood that other volatile organic compounds were wrongly identified as ethanol15. A second distinct feature of the Draeger 7110 is that the instrument incorporates an electromechanical (fuel cell) detector in addition to infrared absorption detector. The infrared absorption process is nondestructive. When the breath sample passes through the chamber, the infrared light that is directed through the chamber excites the molecules of alcohol, but does not destroy them. After passing through the chamber, the breath passes through the electromechanical detector and is analyzed by the fuel cell where it is oxidized. The Draeger instrument use of two independent methods to analyze a single breath sample enhances credibility of result, assuming close agreement in the test results from the two different detection devices.

  1. This material was reviewed and revised by Thomas E. Workman, BS EE, MS EE, JD. Mr. Workman has acquired over forty years experience in computer science and electrical engineering and is a nationally recognized expert in the area of infrared breath testing instrumentation. Mr. Workman, a licensed attorney and adjunct professor of law, has testified as an expert witness in litigation involving computer technology and breath test instruments. ↩︎
  2. For purposes of relative comparison in size, 2100 cubic centimeters is approximately the capacity of a 2 liter soda bottle. A cubic centimeter is about the size of a regular sugar cube. ↩︎
  3. “The trick is how to formulate the proper ratio of alcohol found in the breath to the alcohol found in the blood.” State v. Johnson, 717 SW 2d 298 (Tenn. 1986) ↩︎
  4. The issue of whether or not exhaled breath alcohol concentration is closely related to alveolar (deep lung) air is vigorously contested in academic circles. Recent experimental research studies have demonstrated that alcohol exchanges dynamically with airway tissue both during inspiration and expiration. This research indicates that the presumed 2100 to 1 ratio may not be as reliable a measure of whole blood as previously indicated. M. Hlastala, Ph.D, Paradigm Shift for the Alcohol Breath Test, J. Forensic Science, March 2010, Vol. 55, No. 2 pp. 451-455 ↩︎
  5. Most forensic scientists recognize that thirty-four degrees centigrade (34 C) cannot be the true temperature of a living human’s blood. To convert from centigrade to farenheit, multiply by 9, divide by 5, and add 32 degrees. Thirty-four degrees centigrade is the same as 93.4 degrees farenheit, which is clearly not the temperature of the blood in the body core. Body core temperature is slightly higher than the “standard” oral temperature of 98.6. The average body core temperature of 99.3 farenheit is equivalent to 37.3 degrees centigrade. ↩︎
  6. See, generally, Annotation, Challenges to Use of Breath Tests for Drunk Drivers Based on Claim that Partition or Conversion Ratio Between Measured Breath Alcohol and Actual Blood Alcohol Is Inaccurate, 90 A.L.R. 4th 155 (1991). ↩︎
  7. See, Code of Alabama, 1975, section 32-5A-194 (a)(5) ↩︎
  8. DataMaster DMT is manufactured by National Patent Analytical Systems, 2090 Harrington Memorial Rd., P.O. Box 1435, Mansfield, Ohio 44901. ↩︎
  9. Intoxilyzer 5000EN and 8000 are manufactured by CMI, Inc., 316 East Ninth St., Owensboro, Kentucky 42303. ↩︎
  10. Intoximeter EC/IR II is manufactured by Intoximeters, Inc., 8110 Lackland Road, St. Louis, Missouri, 63114. ↩︎
  11. Alcotest 9510 and 7110 are analytically identical instruments that employ both an infrared and fuel cell detector. The manufacturer is Draeger Safety Diagnostics, Inc., 4040 W. Royal Lane, Ste. 136, Irving, Texas 75063. ↩︎
  12. To the editor’s best knowledge, no jurisdiction in the United States still utilizes the “wet chemical” oxidation/photometry method that was used in the Breathalyzer 900/900A models or the similarly designed Photo-Electric Intoximeter. ↩︎
  13. For a comprehensive understanding of the principles of infrared breath testing, see Erwin, Defense of Drunk Driving Cases, 3 rd Ed., Chapter 18A. ↩︎
  14. An interferent is a chemical which has a molecular structure that is similarly enough to ethanol, so that the molecules of the interferent will absorb infrared light at one or more of the frequencies used by the breath test instrument. ↩︎
  15. For an extended review and detailed analysis of the Draeger Alco-Test 7110 Mk III in actual use in one jurisdiction, see Workman, Massachusetts Breath Testing For Alcohol: A Computer Science Perspective, 8 J. High Tech. L. (2008) ↩︎

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