IMPROVING THE SPECIFICITY OF INFRARED BREATH ANALYZERS. * D. G. Hutson SYNOPSIS

Transcription

1 IMPROVING THE SPECIFICITY OF INFRARED BREATH ANALYZERS * D. G. Hutson SYNOPSIS In the advent of infrared forensic breath-alcohol testing instruments, courts are still raising the question of specificity. Past, present, and developing techniques for achieving a measured degree of specificity are reviewed. Also covered are the practical and theoretical levels of interfering substances that could be present in a drunk driver. INTRODUCTION Most of the popular breath testing instruments now being used operate on the principal of infrared absorption. A beam of light (infrared energy) is passed through a chamber containing the subject breath sample. An interference filter placed in the beam's path allows energy only of the desired wavelength to strike the detector, this wavelength usually being in the region of 3.3 to 3.5 microns. Ethanol absorbs energy from the beam and quantitation is possible by measuring this decrease in energy. Absorption of energy in this region is from C-H stretching. This choice of wavelength accounts for the almost total lack of specificity since literally thousands of hydrocarbons have this structure. In the Scandinavian journal, Work, Environment, & Health, Dr. Irma Astrand reported on the "uptake of solvents in the blood and tissue of man." The report showed 30-minute exposures to toluene, styrene, white spirits, gasoline, diethyl ether, and a number of other substances could give breath concentration that would interfere greater than.01% BAC (blood alcohol concentration) on IR's operating in the 3.4 micron region. Other investigators have reported significant levels of acetone can be produced in the breath of diabetics and persons on stringent diets. To date, IR's have successfully discriminated only against acetone using either a 2-filter system on the IR or a Taguchi cell operating in the normal fixed voltage configuration. Neither method has been shown to discriminate effectively against other substances that could be in the breath. * Caldetect, Inc., Richmond, California 94801, U.S.A. 653

2 METHODS Discriminating against acetone by the 2-filter system can be understood by referring to Figure 1. This shows the IR spectrum of acetone for wavelengths from 2.5 to 14 microns. Also in the 3.4 micron region is superimposed the absorption curve for ethyl alcohol. Figure 2 shows an expanded view of the absorption spectrum in this region. Acetone may be discriminated against by measuring the absorption using 2 different narrow band pass filters as shown by Filter A and B. The absorption ratio A/B will decrease if acetone is present compared to ratio for ethyl alcohol. This technique depends on the absorption spectrum for the interference substance being shifted toward shorter wavelengths, thus limiting its ability to discriminate against a large number of substances such as the other alcohols, methane, and butane. The Taguchi cell has been widely in breath screening instruments for the detection of alcohol. This active element is an "N" type semiconductor of tin dioxide cintered to the outside surface of a ceramic cylinder about 1.5 mm in diameter by 4 mm long. The oxide coating also incorporates a small amount of palladium or platinum which acts as a catalyst to enhance the reaction to certain gases. A small coiled heater mounted inside the cylinder heats the element to about 270 C when the manufacturer's recommended operations voltage is applied (Figure 3A). In the circuit shown in Figure 3, a voltage is applied across the semiconductor element and the current which flows produces a voltages output across the resistor. Oxygen absorbed in the surface of the semiconductor reduces the number of free electrons present and reduces the current flow across the surface of the semiconductor. When the element is exposed to hydrocarbon gases, such as alcohol, the gas is oxidized on the surface of the semiconductor which frees up some of the bound electrons and the current across the cell increases. Most chemical reactions that incorporate a catalyst to enhance or accelerate the reaction usually have an optimum temperature range for a particular reaction to take place. It was strongly suspected this might be the case with the Taguchi cell. Alcohol and acetone standards were prepared using water solutions at 34 C. For compounds that were not soluble in water, a test subject would breathe concentrated vapors from an open container and blow into the instrument as a test subject. No attempt was made to establish equilibrium between breath concentrations and blood concentrations since this would take several hours to as much as several days in the case of acetone. 654

3 Figure 4 shows the Taguchi cell suspended in a chamber of about 25 cc volume. Breath enters Port 4 from the breath inlet line and exists Port 5 then passes through the infrared chamber. RESULTS AND DISCUSSION Figure 5 shows the relationship of the Taguchi cell detector output along the Y-axis and time along the X-axis. The output increases to a maximum while the subject is blowing and during this time the IR channel makes a measurement on the breath sample. After the subject has stopped blowing (Point A), the output changes slightly and levels out at Point B. This change is a result of the Taguchi cell's temperature restablizing after being exposed to the moving gas stream of the subject's breath. At Time B, the heater voltage on the Taguchi cell is programmed to decrease from 5 volts to 1.3 volts in 256 evenly spaced steps. This occurs over a time period of approximately 10 seconds. When the output reaches a peak, the heater voltage at that instant (0-256) is stored in memory. The family of curves shown are for different chemical elements; these are acetone, shown to peak at Step 130, isopropyl alcohol at Step 145, butane at Step 220, toluene at Step 100, and gasoline and paint thinners at Step 115. It is fortuitous that the step (temperature) at which ethyl alcohol peaks is separated by a wide margin from the common chemicals that might be inhaled or ingested by a person. In a practical circuit, a pure ethyl alcohol standard would be analyzed and the step number recorded in memory. This step number would then be compared to each breath sample run and when the difference exceeded a predetermined amount, the instrument would reject that reading for not being pure ethyl alcohol. In practice, it has been found that rejecting any sample that differs by more than 10% is adequate to insure an accurate reading from the infrared detector. Referring back to Figure 5, we note that the output signal from the electronic detector represented by Point B can be scaled to provide an output that is related to the concentration of the alcohol in the subject's breath sample. When this reading is used in conjunction with the peak step to assure that the content is ethyl alcohol, the detector can be used as a very specific low-cost screening instrument. Figure 6 shows the effect of changing the rate at which the sensor heater voltage is programmed. For ramp times of 5 seconds or less, the thermal time constant of the heater and the mass of the ceramic element prevent the sensor temperature from falling fast enough to follow faithfully 655

4 the ramp voltage applied to the heater. In practice, a ramp time of approximately 10 seconds is used; therefore, slight variations in the ramp time will have very little effect on the peak response time. Figure 7 is a typical plot of peak response measured over a 26-day period. The maximum variation did not exceed 5% for either ethyl alcohol or styrene. Additional testing over 3 months has shown this stability can be expected without any elaborate preselection of the Taguchi cell. CONCLUSIONS Using the Taguchi cell in this program mode provides a means of detecting and discriminating against those interfering substances that had been reported in high enough levels in the human breath to interfere with present infrared analyzers. REFERENCES Browning, E. (1965). Toxicity and Metabolism of Industrial Solvents. Amsterdam, London, New York: Elsevier. P.' 413. Haggard, H. W., Greenburg, L. A., and Turner, J. M. The physiological principles governing the action of acetone, together with determination of toxicity. Journal of Industrial Hygiene and Toxicology, 26: 133. Lindner, P., and Blackburn, G. L. (1976). Multi-disciplinary approach to obesity utilizing fasting modified by protein sparing therapy. Obesity and Bariatric Medicine, 5^: Del. 76. Mason, M., and Hutson, D. (1975). Review of acetone levels in diabetics and those levels present in industrial exposure. In Israelstam, S., and Lambert, S. (eds.), Alcohol, Drugs, and Traffic Safety. Toronto: Addiction Research Foundation. Stewart, R. D., and Roettner, E. A. (1964). Expired-air acetone in diabetes mellitus. New England Journal of Medicine, 270: Sulway, M. J., Trotter, M. D., Trotter, E., and Malins, J. M. (1971). Acetone in uncontrolled diabetes. Post Graduate Medical Journal (June Suppl.): Astrand, I. (1975). Uptake of solvents in the blood and tissue of man: A review. Scandinavian Journal of Work, Environment, & Health:

5 Figure 1. IR spectrum of acetone for wavelengths from 2.5 to 14 microns. I 1oo fcj eo iu o 2 ^ 46 X in2 r ks X F Z ^1 / T j Cc 2*5 W J J > WAVCLpM^tH CMCpaJS) E-THYL A LC O H O L A &TOMI= PHT&f* A * 5.45 fhltlsp - 3 ^ MlCKON^ Figure 2. IR specturm: expanded view for wavelengths 3.0 to 3.6 microns. Ethyl alcohol: solid line; acetone: dashed line. Filter A: 3.43 microns; Filter B: 3.35 microns. 657

6 100-mesh SUS 316 stainless steel gauze (double) Noble metal wire Sensor Heater coil Resin moulding Ni pin Figure 3a. The Taguchi cell. Figure 3b. Circuit involving the semiconductor element of the cell. 658

7 ^emi'compuct^p, pettcrok 7 Figure 4. The Taguchi cell in position. 5TYKEW& rolueweb ^ P R o r Y L a l c o h o l eth YL ALCOHOL F ^opasie ^UfAWg. SIGNAL e>tr! U$TH AcerotJerAlNT fhikimek qa^jlikie- C TlM^- (ST^-F^) Figure 5. Taguchi cell detector output versus time. 659

8 RAMP TIME- IN S C0K]pS> Figure 6. Effect of change in rate at which the sensor heater is programmed. P 6 \ K P-AY5 Figure 7. Plot of peak response over a 26-day period. 660