Measurement of acetaldehyde in exhaled breath using a laser absorption spectrometer Pratyuma C. Kamat,1 Chad B. Roller,1,* Khosrow Namjou,1 James D. Jeffers,1 Ali Faramarzalian,2 Rodolfo Salas,1 and Patrick J. McCann3 1Ekips Technologies, Incorporated, Norman, Oklahoma 73069 School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14853 3School of Electrical and Computer Engineering, University of Oklahoma, Norman, Oklahoma 73019 2
*Corresponding author:
[email protected] Received 16 October 2006; accepted 13 December 2006; posted 5 January 2007 (Doc. ID 76095); published 12 June 2007
A high-resolution liquid-nitrogen-free mid-infrared tunable diode laser absorption spectroscopy (TDLAS) system was used to perform real-time measurement of acetaldehyde concentrations in human exhaled breath following ingestion of an alcoholic beverage. Acetaldehyde absorption features were measured near 5.79 m 共1727 cm⫺1兲 using a IV–VI semiconductor laser, a 100 m long path optical gas cell, and secondharmonic detection coupled with wavelength modulation. Acetaldehyde levels were measured with a minimum detection limit of 80 ppb for 5 s integration time. The variations in exhaled acetaldehyde levels over time were analyzed prior to and following ingestion of two different amounts of white wine. A method to calibrate acetaldehyde measurements internally using water vapor absorption lines was investigated to eliminate the need for system calibration with gas standards. The potential of a TDLAS system to be used as a noninvasive clinical tool for measurements of large volatile compounds with possible applications in cancer detection is demonstrated. © 2007 Optical Society of America OCIS codes: 000.1430, 140.3070, 300.1030, 300.6340.
1. Introduction
It has been known for decades that increased levels of certain marker molecules in exhaled breath correlate with certain diseases [1]. Examples include elevated levels of acetone in exhaled breath of uncontrolled diabetic patients and elevated ammonia levels in exhaled breath in renal failure patients [2,3]. Recent technological advances in the detection of specific gas phase breath metabolites offer the promise of improved diagnosis of various diseases. Tunable diode laser absorption spectroscopy (TDLAS) is a potential candidate technology for realtime analysis of a number of volatile molecules in exhaled breath with clinical importance. Examples include the measurement of exhaled nitric oxide [4 – 6], carbon monoxide [7], and ethane [8], to name a few. The advantages of TDLAS include suitably high sensitivities for many molecules, high molecular 0003-6935/07/193969-07$15.00/0 © 2007 Optical Society of America
selectivity, and fast response times. The expansion of TDLAS to the measurement of large molecular species containing greater than six atoms marks an important expansion of demonstrated capabilities. The present work focuses on applying the TDLAS technique to the quantification of acetaldehyde 共C2H4O兲 in exhaled breath upon consumption of ethanol 共C2H6O兲 in alcoholic beverages. Acetaldehyde is known to be an intermediate in the metabolism of ethanol in the liver [9]. The enzyme alcohol dehydrogenase (ADH) converts ethanol to acetaldehyde, which is then converted to acetic acid by the enzyme acetaldehyde dehydrogenase. It is also produced in the human body by all tissues with high ADH activity, e.g., liver, intestine, kidney, and bone marrow [10,11]. It has also been shown that acetaldehyde can be formed from ethanol via colonic microbial ADH in the gastrointestinal tract by fermenting sugars to ethanol and oxidizing exogenous ethanol to acetaldehyde [12– 14]. Furthermore, since the capacity of intestinal mucosa and flora to metabolize acetaldehyde fur1 July 2007 兾 Vol. 46, No. 19 兾 APPLIED OPTICS
3969
ther is limited, acetaldehyde accumulates locally in the areas of the digestive tract covered by microbes [15,16]. Similarly, oral microflora have been shown to produce high concentrations of acetaldehyde from ethanol [17]. Ethanol oxidation and consequent acetaldehyde production by gut microbes after moderate alcohol drinking are responsible for the acetaldehyde accumulation in the mouth [17,18]. This accumulation of acetaldehyde in the mouth and in saliva after the consumption of ethanol results in high concentrations of acetaldehyde in exhaled breath [19]. The detection of acetaldehyde in human breath after the consumption of ethanol has a long history. The first paper covering the presence of acetaldehyde in human breath was published in 1926 [20]. Endogenous acetaldehyde is also present in the exhaled breath of healthy persons who have not ingested alcohol and have been reported to range from 0 to 104 ppb [21]. Recent work involving in vitro analysis has shown that incubated lung cancer cells in a tissue culture flask emit volatile acetaldehyde in concentrations proportional to the number of cancer cells [22]. It is speculated that molecular markers produced by cancer cells could in turn be detected in the exhaled breath to aid in the early diagnosis and prognosis of various cancers such as lung cancer and prostate cancer. This assumption is strengthened by recent studies showing that dogs can distinguish lung and breast cancer patients from healthy individuals,
just by sniffing the exhaled breath samples, with 88% diagnostic sensitivity and 98% diagnostic specificity [23]. Dogs have also been able to detect bladder cancer patients by smelling the odor of urine [24]. A sensor array analysis technique, known as “electronic nose” also has been employed for the detection of lung cancer based on exhaled breath analysis with 71.4% diagnostic sensitivity and 91.9% diagnostic specificity [25]. The aim of the present study was to investigate the feasibility and performance of TDLAS for real-time measurement of exhaled acetaldehyde and to quantify the acetaldehyde levels in exhaled breath after ethanol consumption. Also given are the development plans and related issues of TDLAS systems to validate their use for future measurements of acetaldehyde in clinical studies. 2. Experimental Methods
In this study, a TDLAS system equipped with a double heterostructure IV–VI mid-infrared semiconductor diode laser (Ekips Technologies, Inc., Norman, Oklahoma) with output power of ⬃1 mW, a tuning rate of 0.01 cm⫺1兾mA, and a linewidth of 0.001 cm⫺1 operating near 5.8 m was used for the real-time detection of acetaldehyde in exhaled breath. The schematic of the system is shown in Fig. 1. The details of the system are briefly described below. The IV–VI la-
Fig. 1. System schematic of the TDLAS spectrometer designed to measure acetaldehyde. Major components include a cryostat, Herriott multipass cell, electronics, and breath collection device. 3970
APPLIED OPTICS 兾 Vol. 46, No. 19 兾 1 July 2007
ser source, a HgCdTe photovoltaic detector, a foil heater, and a temperature sensor were mounted in the cryostat housing. The temperature of the cryostat was maintained at 101 K by using a closed cycle refrigeration system (Brooks Automation, Petaluma, California). An integrated heater located just beneath the laser maintained constant laser operating temperatures with an accuracy of 0.01 K. Various optical mirrors and a lens were used to direct the laser beam into a pressure controlled astigmatic 100 m optical path length Herriott cell (3.2 l volume, Aerodyne, Billerica, Massachusetts) and onto the HgCdTe photodetector. Two function generators provided the control waveforms for tuning and modulating the laser across the acetaldehyde absorption of interest near 1727 cm⫺1. An ac modulation current 共42 kHz triangle waveform) was superimposed onto the 100 Hz sweep signal to modulate the laser emission wavelength for the purpose of harmonic detection. The detector signal after preamplification was sampled at twice the modulation frequency by a lock-in amplifier to yield second-harmonic (2f) spectra. An analog-to-digital converter acquired 500 data points per scan from the output of the lock-in amplifier. A computer stored the individual laser scans and performed a running coverage of 200 sequential scans to reduce highfrequency electronic noise and improve the signal-tonoise ratio. Slight thermal variations of the laser can cause wavelength drift of the absorption spectra with respect to the injection current. A strong H2O absorption line with an unambiguous peak at 1727.32 cm⫺1 was used to align the acquired spectra in memory prior to averaging. A diaphragm pump (Vacuubrand, Essex, Connecticut) was used to reduce the gas-cell pressure to 26 Torr and induce a 1 L兾min flow that was held constant with mass flow controllers (Alicat Scientific, Tucson, Arizona). Measurements were performed at a pressure of 26 Torr to reduce line broadening and interference between the H2O and the acetaldehyde absorption line features. A pressure transducer (MKS Instrument, Andover, Massachusetts) was used to measure the pressure inside the gas cell. Teflon tubing was used for all the connections in the gas sampling system. The breath collection device mouthpiece was designed to collect single exhalations and consisted of a T-shaped piece connected to a disposable mouthpiece, a negative pressure release valve, and a 0.25 in. 共0.64 cm兲 inner diameter Teflon tubing to direct breath through the flow controller and into the gas cell [6]. To perform breath measurements, the volunteers were instructed to exhale with some pressure for a period of 30 s. The TDLAS system was used for the detection of acetaldehyde levels in exhaled breath pre- and post-ethanol consumption. The volunteers who participated in this study donated three breath samples just prior to consuming 375 ml of wine (Pinot Grigio, Sparks Winery, Oklahoma, 12.5% alcohol). Informed consent was provided by each volunteer according to a protocol approved by the
Western Institutional Review Board, Olympia, Washington. Measurements of exhaled acetaldehyde were performed every 30 s for the next 140 min. The same procedure was then repeated after the same subject consumed 187 ml of wine. 3. Acetaldehyde Measurements Using TDLAS
Acetaldehyde has a strong absorption band (the v4 band [26]) ranging from 1680 to 1820 cm⫺1 with a peak magnitude in the R branch at 1764 cm⫺1. The IV–VI laser in the TDLAS system was tuned to operate in the above spectral region by adjusting the current and temperature to 400 mA and 101 K, respectively. The absorption spectra for acetaldehyde and the operating region for the laser are shown in Fig. 2. The most promising acetaldehyde absorption features in terms of absorption magnitude near 1764 cm⫺1 were not accessible with the laser used in this study. Figure 3 illustrates 2f absorption waveforms obtained by the TDLAS system for room air showing H2O absorption lines and 3.8 ppm acetaldehyde in room air generated using the calibration system described below. The strongest accessible acetaldehyde absorption line obtained by the calibration system was found near 1727.1 cm⫺1, shown in Fig. 4, and was separated by 0.22 cm⫺1 from the nearest water line.
Fig. 2. Upper graph: Infrared absorption spectrum for acetaldehyde acquired using FTIR. Lower graphs: the 2f spectra acquired using the TDLAS system with H2O absorption lines matched with H2O lines in the HITRAN 2000 database to verify laser emission spectral regions. 1 July 2007 兾 Vol. 46, No. 19 兾 APPLIED OPTICS
3971
Fig. 3. Typical 2f laser absorption spectra for (a) ambient air containing H2O absorption lines and (b) ambient air ⫹ 3.8 ppm acetaldehyde.
Acetaldehyde concentrations were quantified by comparing acquired spectra of unknown concentration with a reference spectrum at a known concentration of 3.8 ppm using a least-squares-fitting routine [4]. The minimum detection limit for a 5 s integration time (200 coadds), which was the integration time used for the exhaled breath measurements, was determined to be 80 ppb based on the Allan variance. The Allan variance plot was constructed by repetitively sampling the constant inputs of zero air at time intervals of 0.5 s for a period of 15 min. As
Fig. 4. Acetaldehyde absorption feature near 1727.1 cm⫺1 was separated by 0.22 cm⫺1 from the nearest H2O line. The highlighted region in the subset was used to measure the breath acetaldehyde concentration upon ingestion of ethanol. The H2O absorption at 1727.32 cm⫺1 is used to align spectra and maintain wavelength stability. 3972
APPLIED OPTICS 兾 Vol. 46, No. 19 兾 1 July 2007
Fig. 5. Allan deviation plot acquired by measuring zero air devoid of C2H4O at every 0.5 s interval. The X axis refers to the integration time in seconds and the Y axis refers to the Allan deviation of the deduced acetaldehyde measured concentration. As can be seen the minimum detection limit is ⬃30 ppb at 45 s integration time and 80 ppb at 5 s integration time.
shown in Fig. 5, the measurement deviation decreases with the integration time (white-noise regime) and shows an optimum integration time of 45 s where a minimum detection limit of 30 ppb was obtained. The further improvement of this figure of merit is possible with faster electronics in order to collect more spectra in a given time. 4. Analysis of Exhaled Acetaldehyde
The acetaldehyde measurements were quantified using a simple calibration system (shown in Fig. 6) consisting of an acetaldehyde permeation tube, a U-tube, and a flow controller. The permeation tube, designed for a permeation rate of 8200 ng兾min at 30 °C, was placed in the U-tube holder, and the calibration system was maintained at 25 °C. A uniform carrier flow of 1 l兾min was maintained through the U-tube. A strong acetaldehyde absorption feature was observed at 1727.1 cm⫺1 with no apparent interference from water, ammonia, or methane absorption features.
Fig. 6. Schematic of the calibration system consisting of an acetaldehyde permeation tube, U-tube holder, and a flow controller. The calibration system was used as an acetaldehyde source of known concentration to quantify exhaled acetaldehyde measurements.
Fig. 7. Variations in measured peak breath acetaldehyde concentration over time (in minutes) from a subject following the ingestion of 375 ml of ethanol. The initial peak is attributable to the mouth emissions, and the subsequent rise in acetaldehyde levels is attributable to the passage of acetaldehyde from stomach to blood.
The changes in measured maximum breath acetaldehyde concentrations post-ethanol consumption of 375 ml of wine are shown in Fig. 7. The first peak in acetaldehyde concentrations is relatively high with eventual rapid decrease. A second rise in the acetaldehyde concentration can be seen after ⬃30 min. These results are in good agreement with the results obtained by Smith and co-workers [19], who used selected ion flow tube mass spectroscopy (SIFT-MS) to detect acetaldehyde. They tested breath acetaldehyde and ethanol levels in three subjects upon ingestion of ethanol and found a similar initial spike in breath acetaldehyde concentrations in all three subjects followed by a second subsequent rise. Smith and co-workers [19] correlated the first rise in breath acetaldehyde levels to ethanol retention found in the mouth (saliva) after drinking, which later quickly decreases as saliva is recycled. The second rise in
Fig. 8. Data trends of breath acetaldehyde levels measured following the consumption of two different concentrations of ethanol (375 and 187 ml) by the same subject.
acetaldehyde levels was attributed to acetaldehyde entering the bloodstream after ethanol is metabolized in the liver. The onset of this second rise depends on the rate of gastric emptying of the ethanol. The comparative breath analysis trend data for the same subject obtained from breath acetaldehyde concentrations after the ingestion of the highest amount of wine 共375 ml兲 and the lowest 共187 ml兲 are shown in Fig. 8. The initial peak acetaldehyde concentration for the highest dose of wine 共375 ml兲 was significantly higher than the initial peak acetaldehyde concentration after the ingestion of the smaller dose of wine 共187 ml兲, 4.2 and 3 ppm, respectively. Moreover, the second rise in the acetaldehyde concentration and the retention of the acetaldehyde concentration in the blood stream were prolonged after the ingestion of the higher amount of wine 共375 ml兲. This is attributable to the increased time required for gastric emptying for larger amounts of ethanol than for the smaller dose. As mentioned earlier, an unambiguous absorption line for water near 1727.32 cm⫺1 was used to align the acquired spectra in the memory before averaging to minimize spectral smear. The exhaled breath is saturated with water vapor, hence the magnitude
Fig. 9. Measured breath acetaldehyde and H2O absorption trends following the consumption of two different amounts of wine (375 and 187 ml) by the same subject. (a) Trends of H2O and acetaldehyde absorption magnitudes when the laser power was relatively constant. The constant magnitude of the exhaled H2O absorption trends during exhalation indicates that the exhaled breath is consistently highly saturated with water vapor. (b) The effect of decreasing laser power on the H2O and acetaldehyde absorption trends. 1 July 2007 兾 Vol. 46, No. 19 兾 APPLIED OPTICS
3973
Fig. 10. Modified acetaldehyde absorption magnitudes using a correction factor derived from the changing water absorption trends from Fig. 9(b).
of the water lines should be constant for every measured exhaled breath [Fig. 9(a)]. As such, any changes in laser power or gas cell pressure should be reflected directly on the magnitude of the absorption lines for water as well as the absorption of acetaldehyde. Therefore the absorption lines for water can be used to monitor the quality of the acquired acetaldehyde spectra and monitor fluctuations in transmitted laser power. Figure 9(b) shows the decrease in the magnitude of the water absorption lines after a gradual decrease in the laser power attributable to a sudden drop in the cooling efficiency of the commercial cryogenic system followed by automatic compensation in the laser current to keep the water line in the same location. A change in the laser power will affect both the magnitude of the acetaldehyde and the water vapor absorption features based on their close spectral proximity of 0.22 cm⫺1. However, noting the changes in acetaldehyde absorption magnitudes attributable to changes in laser power lines is difficult because the concentration of acetaldehyde is not known. An advantage of our TDLAS system, however, is the ability to use the water line to recover the true acetaldehyde concentration when laser power fluctuates. A correction factor determined from the water vapor absorption line assuming a constant concentration was applied to correct for the absorption magnitude of acetaldehyde. The corrected acetaldehyde concentrations are shown in Fig. 10 along with the distorted concentrations for comparison. Future work will examine the use of the water line to serve as an internal calibration standard to eliminate the need for calibration with a standard gas of known acetaldehyde concentration in much the same way that CO2 can be used to perform real-time internally calibrated exhaled NO concentration measurements [4 – 6]. 5. Conclusion
This study presented what is believed to be the first online measurements of acetaldehyde levels in hu3974
APPLIED OPTICS 兾 Vol. 46, No. 19 兾 1 July 2007
man breath using TDLAS. The results in this paper confirm the capability of a mid-infrared TDLAS using a IV–VI diode laser to measure acetaldehyde concentrations in breath from single exhalations. The experimentally determined minimum detection limits for acetaldehyde were estimated to be 80 ppb for a 5 s integration time and ⬃30 ppb for a 45 s integration time. As previously indicated, these studies lay a foundation for the development of breath analysis as a clinical tool for disease diagnosis and thereby reinforce the importance of real-time data measurements. Following the observations that acetaldehyde is emitted by cancer cells in vitro [22] there is a possibility that measurement in breath might lead to a new way to detect lung cancer in vivo. The present measurements are a prelude to such studies, and future work will involve measuring and comparing the levels of acetaldehyde in the breath of healthy individuals and of lung cancer patients. However, future improvements in the measurement capability of the TDLAS system described in this paper are required. Specifically, based on work from Smith and co-workers [21], to determine acetaldehyde levels in healthy individuals a minimum detection limit of 10 ppb is required. In addition, a measurement integration of no more than 3 s is desired to reduce exhalations times to 15 s or less. To accomplish both of these objectives, more advanced electronics are required to acquire more spectral coaverages for the desired integration time of 3 s. Finally, if one were to measure acetaldehyde features near 1764 cm⫺1, an improvement of a factor of ⬃2 would be expected in measurement sensitivity given that an interference-free line can be found. References 1. A. Manolis, “The diagnostic potential of breath analysis,” Clin. Chem. 29, 5–15 (1983). 2. C. N. Tassopoulos, D. Barnett, and T. R. Fraser, “Breathacetone and blood-sugar measurements in diabetes,” Lancet II, 1282–1286 (1969). 3. L. R. Narasimhan, W. Goodman, C. Kumar, and N. Patel, “Correlation of breath ammonia with blood urea nitrogen and creatinine during hemodialysis,” Proc. Natl. Acad. Sci. USA 98, 4617– 4621 (2001). 4. C. Roller, K. Namjou, J. D. Jeffers, M. Camp, A. Mock, P. J. McCann, and J. Grego, “Nitric oxide breath testing by tunablediode laser absorption spectroscopy: application in monitoring respiratory inflammation,” Appl. Opt. 41, 6018 – 6029 (2002). 5. C. Roller, K. Namjou, J. Jeffers, W. Potter, P. J. McCann, and J. Grego, “Simultaneous NO and CO2 measurement in human breath with a single IV–VI mid-infrared laser,” Opt. Lett. 27, 107–109 (2002). 6. K. Namjou, C. B. Roller, T. E. Reich, J. D. Jeffers, G. L. Mcmillen, P. J. McCann, and M. A. Camp, “Determination of exhaled nitric oxide distributions in a diverse sample population using tunable diode laser absorption spectroscopy,” Appl. Phys. B 85, 427– 435 (2006). 7. K. Namjou, P. J. McCann, and W. T. Potter, “Breath testing with a Mid-IR laser spectrometer,” in Application of Tunable Diode and Other Infrared Sources for Atmospheric Studies and Industrial Processing Monitoring II, A. Fried, ed., Proc. SPIE 3758, 74 – 80 (1999).
8. K. D. Skeldon, G. M. Gibson, C. A. Wyse, L. C. McMillan, S. D. Monk, C. Longbottom, and M. J. Padgett, “Development of high-resolution real-time sub-ppb ethane spectroscopy and some pilot studies in life science,” Appl. Opt. 44, 4712– 4721 (2005). 9. G. Freund and P. J. O’Hollaren, “Acetaldehyde concentrations in alveolar air following a standard dose of ethanol in man,” Lipid Res. 6, 471– 477 (1965). 10. S. N. Wickramasinghe, A. N. Bond, H. A. Sloviter, and J. E. Saunders, “Metabolism of ethanol by human bone marrow cells,” Acta Haematol. 66, 238 –243 (1981). 11. L. F. Leloir and J. M. Munoz, “Ethyl alcohol metabolism in animal tissues,” Biochem. J. 32, 299 –307 (1938). 12. K. Jokelainen, A. Siitonen, H. Jousimies-Somere, T. Nosova, R. Heine, and M. Salaspuro, “In vitro alcohol dehydrogenasemediated acetaldehyde production by aerobic bacteria representing the normal colonic flora in man,” Alcohol Clin. Exp. Res. 20, 967–972 (1996). 13. M. Salaspuro, “Bacteriocolonic pathway for ethanol oxidation: characteristics and implications,” Ann. Med. 28, 195–200 (1996). 14. M. Salaspuro, “Microbial metabolism of ethanol and acetaldehyde and clinical consequences,” Addict. Biol. 2, 35– 46 (1997). 15. T. Koivisto and M. Salaspuro, “Aldehyde dehydrogenases of the rat colon: comparison with other tissues of the alimentary tract and the liver,” Alcohol Clin. Exp. Res. 20, 551–555 (1996). 16. T. Nosova, K. Jokelainen, P. Kaihovaara, R. Heine, H. Jousimies-Somer, and M. Salaspuro, “Characteristics of aldehyde dehydrogenases of certain aerobic bacteria representing human colonic flora,” Alcohol Alcohol 33, 273–280 (1998). 17. N. Homann, H. Jousimies-Somer, K. Jokelainen, R. Heine, and M. Salaspuro, “High acetaldehyde levels in saliva after ethanol consumption: methodological aspects and pathogenetic implications,” Carcinogenesis 18, 1739 –1743 (1997). 18. K. Jokelainen, R. P. Roine, H. Väänänen, M. Färkkilä, and M.
19.
20.
21.
22.
23.
24.
25.
26.
Salaspuro, “In vitro acetaldehyde formation by human colonic bacteria,” Gut. 35, 1271–1274 (1994). D. Smith, T. Wang, and P. Spanel, “On-line, simultaneous quantification of ethanol, some metabolites and water vapour in breath following the ingestion of alcohol,” Physiol. Meas. 23, 477– 489 (2002). A. W. Jones, “Measuring and reporting the concentration of acetaldehyde in human breath,” Alcohol Alcohol. 30, 271–285 (1995). C. Turner, P. Spanel, and D. Smith, “A longitudinal study of ethanol and acetaldehyde in the exhaled breath of healthy volunteers using selected-ion flow-tube mass spectrometry,” Rapid Commun. Mass Spectrom. 20, 61– 68 (2006). D. Smith, T. Wang, J. Sule-Suso, P. Spanel, and A. E. Haj, “Quantification of acetaldehyde released by lung cancer cells in vitro using selected-ion flow-tube mass spectrometry,” Rapid Commun. Mass Spectrom. 17, 845– 850 (2003). M. McCulloch, T. Jezierski, M. Broffman, A. Hubbard, K. Turner, and T. Janecki, “Diagnostic accuracy of canine scent detection in early- and late-stage lung and breast cancers,” Integr. Cancer Ther. 5, 30 –39 (2006). C. M. Willis, S. M. Church, C. M. Guest, W. A. Cook, N. McCarthy, A. J. Bransbury, M. R. T. Church, and J. C. T. Church, “Olfactory detection of human bladder cancer by dogs: proof of principle study,” Br. Med. J. 329(7468), 712–714 (2004). R. F. Machado, D. Laskowski, O. Deffenderfer, T. Burch, S. Zheng, P. J. Mazzone, T. Mekhail, C. Jennings, J. K. Stoller, J. Pyle, J. Duncan, R. A. Dweik, and S. C. Erzurum, “Detection of lung cancer by sensor array analyses of exhaled breath,” Am. J. Respir. Crit. Care Med. 171, 1286 –1291 (2005). H. Hollenstein and Hs. H. Günthard, “Solid state and gas infrared spectra and normal coordinate analysis of 5 isotopic species of acetaldehyde,” Spectrochim. Acta Part A 27, 2027– 2060 (1971).
1 July 2007 兾 Vol. 46, No. 19 兾 APPLIED OPTICS
3975
