In the Laboratory
W
Measuring Breath Alcohol Concentrations with an FTIR Spectrometer Adam Kniesel and Michael K. Bellamy* Department of Chemistry, Northwest Missouri State University, Maryville, MO 64468; *
[email protected] Experiments that deal with breath testing for ethanol by undergraduate chemistry students have been published in this Journal (1, 2). However, none of these articles discusses the use of infrared spectrometers. This article describes how to measure breath alcohol concentrations with an FTIR spectrometer in an instrumental analysis course. Undergraduate students are enthusiastic about learning about modern realworld applications. As part of this exercise, the absorbance of breath samples and Henry’s law are used to calculate blood alcohol concentrations at increasing times after use of a mouthwash. Also, a defense attorney’s claim that eating bread can cause a positive reading for ethanol is investigated.
Equipment and Materials
Theory IR breath analyzers employ interference filters to measure the absorbance of ethanol in breath. The absorbance of radiation of 3.80 µm corresponds to a point in the baseline, while the absorbance of 3.40 µm is used to quantify ethanol. Up to three other wavelengths can be used to detect, and in some cases, quantify interfering compounds. A good overview of the technology of IR breath analyzers is available on the Web (3). In the body, ethanol partitions between blood at the surface of capillaries and air in alveoli (3, 4). Henry’s law is assumed to apply to this system; that is, the concentration of ethanol in the blood is assumed to be directly proportional to the partial pressure of ethanol in the air above the blood, [ethanol]blood = k[ethanol]air where k is the Henry’s law constant. Thus, if a person’s breath alcohol concentration is measured, and the Henry’s law constant is known at breath temperature (34 ⬚C), then the concentration of ethanol in a person’s blood can be calculated. In this experiment, aqueous reference ethanol solutions are used to prepare a calibration curve. The curve is used to relate absorbance measurements of breath samples with blood alcohol concentrations. An explanation of how Beer’s law and Henry’s law relate to the calibration curve is given in the Supplemental Material.W The concentrations of the reference solutions must be corrected before they are used to make a calibration curve owing to the different partition coefficients that ethanol has when partitioned between water兾air versus blood兾air. Sample calculations are given in the Supplemental Material.W Experimental Overview A calibration curve, with corrected concentration values, is used to calculate a person’s blood alcohol concentration after gargling with a commercial mouthwash. A study reported that a person’s blood alcohol concentration decreases 1448
exponentially after gargling with mouthwash. For the most concentrated mouthwash it took about six minutes for the subject to go below 0.08% blood alcohol (5). The procedure reported in the literature study is repeated in this laboratory exercise (5). The last part of this laboratory exercise involves testing data that an attorney posted on the Web. His data indicate that eating bread can give a positive reading with a commercial IR breath analyzer (6). The reported results are tested by using the calibration curve to calculate a person’s blood alcohol concentration after eating bread.
FTIR spectrometer Heated long-path IR cell (7) Ethanol Mouthwash Bread
Hazards Loose clothing should not be worn near vacuum pump. Do not swallow the mouthwash. Procedure Reference solutions are prepared and transferred to modified, 500-mL squeeze bottles (Table 1). The concentration of each ethanol solution must be corrected for the assumed 2100:1 ratio of breath to blood alcohol. An explanation of how and why this calculation is performed is given in the Supplemental Material.W The reference solutions are heated to breath temperature (34 ⬚C), and the IR cell is heated to about 50 ⬚C to
Table 1. Reference Solutions Solution
Concentrations of Reference Ethanol Solutions (g/100 mL)
Corrected Concentrations of Reference Ethanol Solutions (g/100 mL)
1
0
0
2
0.0404
0.0330
3
0.0632
0.0516
4
0.0812
0.0663
5
0.0976
0.0797
6
0.1196
0.0976
7
0.1813
0.1480
Journal of Chemical Education • Vol. 80 No. 12 December 2003 • JChemEd.chem.wisc.edu
In the Laboratory 0.35
0.18
0.30 0.25
Absorbance
prevent condensation of the sample. Headspace samples are collected of the reference solutions by flushing the gas through the IR cell (squeezing the air out of the bottle). A volunteer collects a spectrum of his or her breath before gargling to ensure that no ethanol is detected. Then the same volunteer gargles with 20 mL of a commercial mouthwash for 15 seconds. IR spectra of the volunteer’s breath are collected 0, 2, 4, 6, 8, 10, 12, 14, and 16 min after gargling. Another volunteer collects a spectrum of his or her breath to ensure that no ethanol is detected. The same volunteer then chews a piece of bread for 20 seconds. With the bread still in the mouth, an IR spectrum of the volunteer’s breath is collected.
0.20
0.12
0.15
0.098 0.08
0.10
0.06 0.04
0.05
Blank 0.00 950
Data Processing
990
1030
1070
1110
1150
ⴚ1
Wavenumber / cm
The spectra of the ethanol 1053 cm᎑1 band of the reference solutions, collected at 34 ⬚C, are shown in Figure 1. The calibration curve constructed using the maximum heights of the peaks at 1053 cm᎑1 is shown in Figure 2. The results obtained after a volunteer gargled with Equate antiseptic mouth rinse (26.9% ethanol) on two separate occasions are shown in Figure 3. Both curves predict that it takes 6.1 minutes for a person’s blood alcohol concentration to drop to the legal limit. A published study reported that it took 5.7 minutes to drop to the legal limit using a commercial instrument (5). A volunteer’s blood alcohol concentration was found to be 0.05% after chewing on Old Home Enriched White Bread. Using a commercial instrument, the same value was reported for “sliced white bread” (6). Discussion This experiment is a favorite with undergraduate students of instrumental analysis. Calibration curves produced using the procedure in this lab can be precise if great care is taken to control the temperature of the reference solutions. The results of determining the time for a person’s calculated blood alcohol concentration to fall below 0.08% after use of a commercial mouthwash agree well with published data. Also, the calculated blood alcohol concentration after eating bread agrees well with reported values. In this exercise students get hands-on experience installing and aligning accessories for FTIR spectrometers. They learn that gas-phase studies use a long-path cell to lower the detection limit of an analysis. The use of Henry’s law in calculations helps reinforce the concept of chemical equilibrium. At the instructor’s discretion, mixtures of different compounds could be sampled so that the effect of interfering compounds could be studied. Different methods of data pro-
0.30
Absorbance (1053 cmⴚ1)
Results
Figure 1. FTIR spectra of aqueous ethanol reference solutions. Spectra were recorded at breath temperature (34 ⬚C). Curve labels designate uncorrected ethanol concentration (%).
0.25 0.20 0.15 0.10
y = 1.7743x R 2 = .9689
0.05 0.00 0.00
0.05
0.10
0.15
0.20
Blood Ethanol Concentration (%) Figure 2. Calibration curve made from the spectra shown in Figure 1. The curve plots the maximum height of the 1053 cm᎑1 peak versus corrected concentrations. Concentrations were corrected for the difference in the partitioning of ethanol between air/water and air/blood. 1.4
Blood Alcohol Concentration (%)
A calibration curve is prepared by plotting either peak height or peak area of an ethanol peak versus corrected concentrations of the reference solutions, as is discussed in the Supplemental Material.W The calibration curve is used to convert the absorbance reading obtained following use of a mouthwash into blood alcohol concentrations. The curve is also used to calculate blood alcohol content after eating a slice of bread.
1.2
Trial 2
y = 0.8114 eⴚ0.3821x R 2 = .9784
1.0
0.8
Trial 1
Trial 1 ⴚ0.3293x
0.6
Trial 2
y = 0.5854 e R 2 = .9605
Legal Limit
0.4
0.2
Legal Limit 0.0 0
2
4
6
8
10
12
14
16
18
Time / min Figure 3. Results of gargling with a commercial mouthwash. The maximum height of the 1053 cm-1 peak is used along with the calibration curve in Figure 2 to calculate % blood alcohol.
JChemEd.chem.wisc.edu • Vol. 80 No. 12 December 2003 • Journal of Chemical Education
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In the Laboratory
cessing, including use of the five wavelengths commercial IR breath-testing equipment uses, could be compared (3). Finally, breath alcohol testing is somewhat controversial, and students will know that they are not intoxicated after having gargled with mouthwash. Consequently, after this experiment is performed, students enjoy discussing potential sources of error in the experiment, which helps them learn about sources of error in chemical measurements in general. Supplemental Material Detailed instructions for the students, including lab questions, and notes for the instructor are available in this issue of JCE Online. W
Acknowledgment The long-path IR cell was purchased with funds from the Applied Research Committee of Northwest Missouri State University.
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Literature Cited 1. Timmer, W. C. J. Chem. Educ. 1986, 63, 897–898. 2. Labianca, D. A. J. Chem. Educ. 1990, 67, 259–261. 3. Virginia Division of Forensic Science. Breath Test Operators Training Manual Intoxilyzer Model 5000. Download this document at http://www.dcjs.org/forensic/documents/ bamanual.pdf (accessed Sep 2003). 4. Saferstein, R. Criminalistics: An Introduction to Forensic Science, 7th ed.; Prentice Hall: Upper Saddle River, NJ, 2001; pp 263–269. 5. Modell, J. G.; Taylor, J. P.; Lee, Y. L. J. Am. Med. Assoc. 1993, 270, 2955–2956. 6. Intoxilyzer, A Breath Testing Device? http://www.aldrunkdrivinglawyer.com/pubsarticles/bread.htm (accessed Sep 2003). 7. Cell may be fabricated. See Douglass, G. J. Chem. Educ. 1980, 57, 389.
Journal of Chemical Education • Vol. 80 No. 12 December 2003 • JChemEd.chem.wisc.edu
