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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 38. NO. IO. OCTOBER 1991

Tunable Diode Laser Spectroscopy for Isotope Analy sis-Detection of Isotopic Carbon Monoxide in Exhaled Breath Peter S. Lee, Richard F. Majkowski, and Thomas A. Perry

Abstract-A high resolution tunable infrared diode laser spectroscopy system was developed for isotope analysis with sensitivity at ppb levels. Such a system is ideally suited for detection and measurement of minute amounts of infrared active compounds present in a huge noninfrared active background such as air. The operation and capabilities of the system were demonstrated b measurin physiological levels of isotopic carbon monoxide, k I 6 Oand '3C160, naturally present in exhaled human breath with essentially no sample preparation. The simplicity in obtaining such data suggests that fundamental physiological information may be derived from noninvasive measurements. This makes the system potentially useful for many biomedical applications.

INTRODUCTION ADIOISOTOPES have been extensively used as tracers. Many investigations, however, preclude their use either because there are no suitable radioisotopes for some elements (e.g., the longest lived radioisotope of oxygen, oxygen-15, has a half-life of 2 min), or because radiation exposure raises health, environmental, or waste disposal concerns. The application of stable isotopes as tracers predates that of radioisotopes [l], [2] but, because of the lack of a simple and versatile detection method, routine application of stable isotopes has thus far been limited. Mass spectrometry is the traditional method for detection of stable isotopes, but it requires extensive efforts to distinguish and measure chemically different molecules with the same nominal mass; an example is detecting a minute amount of carbon monoxide I2CI60 in the high background of nitrogen I4Nl4N present in exhaled human breath. In previous papers, we presented a tunable diode laser spectroscopy system for isotope analysis [3], [4].This system combines the specificity of the infrared absorption spectra of isotopic molecules and the resolution and spectral power density of diode lasers [5] with a unique dual path cell matched to the expected isotopic absorbances.

R

Manuscript received January 23. 1990; revised January 8. 1991. P. S. Lee is with the Department of Biomedical Science. General Motors Research Laboratories. Warren, MI 48090. R. F. Majkowski was with the Department of Physics, General Motors Research Laboratories. Warren, MI 48090. He is now with Lawrence Technological University, Southfield, MI 48075. T. A . Perry is with the Department of Physics, General Motors Research Laboratories. Warren, MI 48090.

This resulted in an isotope analysis system that is simple, versatile, and specific. However, the sensitivity of this approach is limited by the luminescence background and the ability to measure direct spectral transmission. To further enhance the detection sensitivity and to eliminate the small luminescent background that is present along with lasing emission, we added to this system the techniques of wavelength modulation and second-harmonic detection. This system is ideally suited for detection and measurement of small quantities of compounds present in a large background of air with minimal or no sample preparation required, since the main constituents of air (nitrogen, oxygen, and argon) are transparent to infrared radiation. This paper describes the stable isotope diode laser system and presents a sample application for the detection of background levels of isotopic carbon monoxide in exhaled human breath. METHODS A . Wavelength Modulation and Harmonic Detection

Wavelength modulation and harmonic detection provides superior signal to noise ratio compared to conventional spectroscopy for weak optical absorptions [ 6 ] ,[7]. As the laser is slowly tuned over the spectral feature of interest, the wavelength of measurement is modulated at kHz frequency and with a window of the order of the spectral bandwidth. The detector output is processed by a frequency and phase selective amplification system (such as a lock-in amplifier) which is referenced to the modulation frequency. When the detection system is tuned to the fundamental of the modulation frequency, the output is proportional to the first derivative of intensity with respect to the optical frequency. Likewise, if the system is tuned to the second harmonic of the modulation frequency the output is proportional to the second derivative of the spectral signal (the mathematical relationship is presented in the Appendix). It is important to note that the derivative spectroscopy is insensitive to any dc component of the signal, such as the broad luminescence background which may amount to 1 % of the total laser emission. This nonlasing emission would, otherwise, contribute a constant intensity background and place a limit on the accuracy of straight absorption experiments.

0018-9294/91/1000-0966$01.000 1991 IEEE

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TUNABLE DIODE LASER SPECTROSCOPI Signal

D'A

4

Lock-In

Reference I

Beam Spllner

Selectable Dual Path Cell

Modulation

Hw Ne Laser

__- -

. - - .- ... . . .-. .

Monochromator1

IChopPeri

Fig. 1. Schematic diagram of the tunable diode laser isotope analysis system. An off-axis parabola was used to collimate the IR radiation. This minimizes optical interference by preventing laser light reflection from a collimating lens. A chopper (shown in dotted line) was used for conventional absorption spectroscopy.

In the present system (Fig. l ) , the modulation is provided by the signal generator built into the current supply (SP5820 control module, Laser Photonics). A lock-in amplifier (SR5 10, Stanford Research Systems) referenced to . the modulation frequency detected the second harmonic signal.

B. Diode Laser Isotope Analysis System and Computer Interface The schematic diagram of the diode laser isotope analysis system is shown in Fig. 1. The laser was a double heterojunction growth with a PbTe active layer and PbEuSeTe confinement layers [ 5 ] , operating typically around 120 K and 0.3 A. The laser was housed in a liquid nitrogen dewar-based temperature stabilization system. The dewar housing is a Precision Cryogenic Systems model PIR 107. The laser mounting and temperature stabilization system, designed and fabricated in house, consisted of the laser diode, temperature sensing diode, control heater element, thermal link to the dewar cold stage, radiation shielding, and optical elements. At the operating temperature of the laser (typically around 120 K), the temperature could be controlled to within f 2 mK over a 2 h period as measured from a near Doppler limited spectral absorption line. The scanning of the spectrum was accomplished by fine tuning the infrared laser emission through stepwise ramping of the injection current. In the present system, the SR510 lock-in amplifier has a built in digital to analog channel with 13 bits of resolution from - 10.24 to + 10.24 V, providing a voltage increment of 2.5 mV (the increment could be made finer using a voltage divider). The transfer function of the laser current supply is 20 mA/V. Thus, a 1.5 V external input to the current supply would provide a stepwise ramping current of 30 mA and scan the spectrum in 600 steps. The laser light was collimated by a 75 mm focal length (150 mm working distance), 90" off-axis parabola (Optical Filter Corp.). The collimated light passed through the sample cell, and focused onto a liquid N, cooled InSb detector (Model 40742, Santa Bar-

bara Research). The analog signal from the detector was digitized through a 13 bit A/D converter built into the lock-in amplifier. The spectrometer system was controlled by a software package with a PC/AT which commanded the lock-in to drive the current supply, read the digital data from the lock-in, graphed the data in real time, and stored the data on the hard disk for subsequent analysis. There were several sample cells which could be inserted in the optical path. A multireflection 20 m path length White cell (Foxboro) was used for low concentration gas measurements. There was also an interconnecting dual path cell. This cell had a variable path length (0.520.0 mm) short cell and a long cell (500 mm) which also allowed for a multiple pass configuration (maximum 3.5 m). This combination of cells gives a broad range of path lengths. The absorption intensities of two isotopic species with greatly different concentrations can be made similar by using the appropriate combination of these cells. Thus, the dynamic range problem associated with processing vastly different isotopic concentrations can be eliminated if a ratio measurement is desired. A He-Ne laser, a pellicle beam splitter and a plane reflecting mirror mounted on a kinematic base were used to facilitate the alignment of IR laser radiation. The alignment was accomplished by adjustment of the optical elements so that the He-Ne laser radiation was colinear with the IR path from the laser diode to the detector. During the initial setup, a half meter grating monochromator was used to provide approximate wavelength identification and to filter out unwanted laser modes. Once the proper conditions were established, the monochromator was bypassed in all the subsequent experiments.

C. Collection and Analysis of Breath Samples Volunteer human subjects were asked to breathe into an impermeable gas sampling bag (Anspec). The contents were then purged to remove trapped air. A fresh sample was then exhaled into the bag, and introduced into a glass flask with a liquid N2 cold finger to remove moisture. This was the only sample preparation step that was performed. The purpose was to keep unnecessary moisture from the optical cells. A portion of the sample was then introduced into the optical cell for quantitative and isotopic analysis of CO. Calibration curves relating signal intensity and concentration (partial pressure in ppb to ppm in air) were obtained for isotopic spectral lines. Samples with different concentrations were prepared by mixing and dilution of 9.472 ppm total CO in air (Scott Speciality Gases EPA Protocol Gas) with compressed air (Scott Specialty Gases Hydrocarbon Free). The mixtures were prepared using two mass flow controllers with a flow range of 0-500 mL/min (Scott Specialty Gases, model 52-36AlV-50 and model 52-36E-4 control unit). Isotopic abundances for I2CI60and 13C'60were obtained from the AFCRL (Air Force Cambridge Research Laboratory) compilation [8].

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RESULTS The diode laser emission could be broadly tuned over by varying the heat sink temperature (tuning 1-2 cm-' by rate - 5 cm-'/K), and fine tuned over varying the injection current (tuning rate 30 cm-'/A). Fig. 2(a) shows the absorption spectrum of carbon monoxide (10 torr, path length 0.5 m) scanned about a laser mode centered at about 2046 cm-I. Three isotopic lines, 13Ci60P(13) at 2045.777 cm-I, 12C180 P(12) at 2046.070 cm-I, and I2CI6OP(23) at 2046.276 cm-I, can be clearly detected. Fig. 2(b) shows the second derivative spectrum over the same region. The curvature of the background, in Fig. 2(a), induced by the laser emission is clearly absent in the second derivative spectrum, Fig. 2(b). The second derivative signals (peak to peak) for the I2CI60P(3) line at 2131.632 cm-l and the I3CI6OR(9) line at 2131.005 cm-I were determined. For constant isotopic molecular concentration (9.344 ppm '*cl60, or 0.1049 ppm 13C160)but different pressures, there is seemingly linear response at pressure below about 10 torr. At higher pressures the second harmonic signal deviates significantly from a projected linear response. The results are shown in Fig. 3(a) and (b), respectively. The nonlinear response at higher pressures is expected. As shown in the Appendix, if the conditions for an optically thin sample (i.e., weak absorption) or for a constant spectral absorption coefficient at line center (i.e., low pressure or high but constant pressure) are no longer valid, the second-harmonic signal is not linearly proportional to concentration. At a constant operating pressure of 30 torr, the relationship between second harmonic signal and isotopic molecular concentration was determined. As shown in Fig. 4, a linear response was observed for the I2Ct60P(3) line at a concentration of 1-10 ppm in air. Likewise, a linear response was also observed for the I3CI6OR(9) line at a concentration of 0.01-0.1 ppm in air. Linear regression analyses indicate correlation coefficients of 0.9994 and 0.9975, respectively. Since we could detect the presence of minute amounts of carbon monoxide in the compressed air used to blend the reference gas, the level of this impurity was corrected in all the samples. It is noted that the second-harmonic signal is linearly proportional to varying concentrations at a constant pressure of 30 torr (Fig. 4). But, at 30 torr, it clearly deviates from a linear response with varying pressures (Fig. 3). These observations indicate that a decrease in spectral absorption coefficient at line center due to pressure broadening is the main reason for the nonlinear response of signals with varying pressures (see Appendix). Consequently, a linear relationship between signal and concentration can be expected for any one of the following three experimental conditions: 1) experiments conducted at low pressure (5 torr or below), or 2) experiments conducted at a constant pressure, or 3) the line strength rather than the spectral absorption

- 300 cm-I

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q-J 0.2110

f

0.0978

30 I

0.05

(b) Laser C u m (mA)

Fig. 2 . (a) Absorption spectrum of carbon monoxide ( I O torr, 0.5 m path length) scanned by a laser mode centered at 2046 c m - ' . (b) The second derivative spectrum over the same region.

l*l@

I

0

I

I

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50 Total Pressure (Tom)

'

0.1049ppm

I

*

~

100

(b) Fig. 3. (a) Second harmonic signal as a function of total sample pressure for the "C"0 P(3) vibration rotation line. All the samples had a constant concentration of 9.344 ppm "Cl'O in air. (b) Second harmonic signal of the "C"0 R(9) line as a function of total sample pressure. The concentration of 'k"0 in all the samples was constant at 0.1049 ppm in air.

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0.00 10.0

'

"

.

I

5.0

"

10.0

'

/ /

0.01 0.00

0.05 Concentration (ppm I%%)

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Fig. 4. Second harmonic signal versus CO concentration at a constant sample pressure of 30 torr (in air) for isotopic "C"O and 'C"O.

coefficient at line center is used (i.e., integrated peak area is used). The levels and isotopic composition of carbon monox, present in exhaled breath ide, I2CI6Oand ' 3 C i 6 0naturally were determined in a group of volunteers.' The levels of I2CI6Ofall between 1.6 to 2.4 ppm for non smokers, and between 12 and 15 ppm for smokers, with measurements from a light smoker in between the two groups. Likewise, the levels of isotopic I3CI6Ofall between 0.02 and 0.03 ppm for nonsmokers, and between 0.14 and 0.18 ppm for smokers, again with light smoker's data in between the two groups. This is shown in Fig. 5. There is an overall deviation from AFCRL isotopic abundance, indicating general enrichment of the heavier I3C isotope in the carbon monoxide present in exhaled breath. Although these results are preliminary and no special efforts were made to guarantee a purely alveolar sample, they demonstrate the usefulness of the system for noninvasive biomedical testing for the following reasons: 1 ) no preparation of breath sample is needed other than removal of water vapor, and even this is not a necessity; 2) extremely high sensitivity ( - ppb) can be easily obtained with a moderately long absorption path (a 20 m multireflection path was used in the present experiment, whereas 100-1000 m folded paths are commercially available); and 3) even with a nonoptimized data handling system, the spectral information for a complete scan over a laser mode was obtained in a matter of minutes. With careful selection of the laser emission mode, high detection sensitivity down to sub-ppb levels can be easily achieved. The consistency of the measurements can be demonstrated by the absolute as well as the relative abundance of isotopic molecules. While the bulk of the data presented in this paper were obtained using an in house fabricated 10- 15 pm mesa stripe laser, these consistency ' A protocol for the experiments was approved by the General Motors Human Research Committee.

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12cl@ in Exhaled Breath (ppm)

Fig. 5 . Isotopic carbon monoxide naturally present in exhaled human breath. Deviation from projected AFCRL isotopic abundance (dotted line) indicates general enrichment of the "C isotope which was especially noticeable in the smokers.

measurements were obtained using a commercial 4 pm buried layer laser (Laser Photonics). At a total CO concentration of 2.37 ppm in air (total sample pressure 10 torr), the standard deviation for the relative abundance of I2CI8Oto I2CI6Omeasured over a period of 2-3 h was found to be 0.5 %. The standard deviation for the absolute concentration of 12C'80at 4.92 ppb was 1.5%.There was no difference in the standard deviation for the relative abundance of two isotopic molecules measured either within a single laser emission mode or across two emission modes. This is consistent with the specified accuracy of k l % gain accuracy for the lock-in amplifier used in the present experiment. We have demonstrated the detection of isotopic carbon monoxide naturally present in exhaled human breath at levels far below those reported in the literature [9]-[ 1 11. The ease with which such measurements can be made suggests that new fundamental physiological information can be derived from non-invasive testing using only a breath sample. Rigorous efforts are under way to improve and expand these measurements by: 1) collecting more consistent end tidal air samples such as those collected with a Rahn End Tidal Breath Sampler [12]; and 2) collaborating with medical institutions to obtain samples from subgroups of the general population for possible identification of medical disorders.

DISCUSSION A . Biomedical Applications It is conceivable that a number of simple, noninvasive and nonradioactive biomedical, clinical, and field diagnostic applications can be developed using this system. A few examples are: diabetes, liver function, alcoholic cirrhosis, malnutrition, lean body mass, total body water,

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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING. VOL. 38. NO. IO, OCTOBER 1991

fat malabsorption, blood disorders, jaundice, ileal dysfunction, drug metabolism, lung function, energy expenditure (caloric requirement), protein turnover/synthesis, lactase deficiency (milk intolerance), small intestine bacterial overgrowth, inborn errors of metabolism. Some potential diagnostic procedures have already been demonstrated in clinical research using either radioactive isotopes or stable isotope/mass spectrometry [ 131, [ 141. However, because of the radiation safety restrictions or the technical difficulties, even those diagnostic techniques already demonstrated to be feasible remain generally unavailable as routine procedures. We have demonstrated the measurement of isotopic carbon monoxide in exhaled breath. Such measurements should be useful in the following studies: I ) Study related to the catabolism of heme proteinCarbon monoxide is endogenously produced along with billirubin from the catabolism of heme protein on a mole per mole basis [ 15]-[ 181. Since hemoglobin comprises over 90% of the total body heme, monitoring of endogenously produced CO in subjects who refrain from smoking should be a good indicator for a number of blood disorders that result in the alteration of red blood cell catabolism rate [ 191-[21]. 2) Study that may be related to the catabolism of heme protein-Jaundice is caused by high levels of billirubin in circulating blood. An increased concentration of billirubin can be caused either by the excessive destruction of red blood cells, or by interference with the mechanism of billirubin excretion through the bile. A noninvasive test based on the detection of endogenously produced CO could be developed for infant jaundice. 3) Study related to the formation of hemoglobinHemoglobin, specifically the a-methene bridge carbon atom of the heme ring [20], [22], is the precursor of CO. It is synthesized in living cells from starting materials derived from food sources. The isotopic composition for a given element in various food sources is known to have natural variations that reflect the isotopic fractionation through different biochemical pathways [23], [24]. Indeed, isotopic variations were observed for I3C in expired CO2 among different human individuals [25], and the preferential use of I6O in consumed oxygen associated with respiration has been reported [26]. The detection of less abundant isotopic specie in the endogeneously produced CO could provide unique information on the dietary history of the subjects. Other molecules such as carbon dioxide, water, ammonia, formaldehyde, hydrogen peroxide etc, can be detected and measured noninvasively in the breath. Indeed, the applicability of the system can be extended to almost any infrared active molecules. This is because of the extremely high spectral resolution and spectral power density of the individual diode laser, and the broad range of wavelengths for which diode lasers are available. The sensitivities required for most diagnostic tests would be much less demanding than the ppb levels demonstrated

for the present system. This would be especially true if the particular isotopic specie comes from the intake of labelled compounds, for example, the detection of excess expired 13C02 from oral administration of I3C enriched glucose [27]. Most screening procedures using natural levels of endogenously produced molecules would also require lower sensitivities, since only the most abundant isotopic specie need to be detected. The higher sensitivities (ppb or sub-ppb levels) would only be required in the detection of less abundant isotopic specie and in the assay of biological tissues where isotope analysis can be made on CO (oxygen isotope) or CO2 (carbon isotope) resulting from pyrolysis or combustion of the sample. A simple, compact and versatile tunable diode laser isotope analysis system would meet the requirements for the development of diagnostic procedures and functional tests for the general population under free living conditions. These procedures would be noninvasive, are nonradioactive, require no hospitalization, and provide dynamic evaluations. B. Isotopic Primary Standards The precise analysis of isotopic composition is usually accomplished in mass spectrometry through ratio measurements of isotopic ion beams. The ratio of the isotopic ion beams (current or voltage) is proportional to the true isotopic abundance ratio with the proportionality constant dependent upon a number of factors [28]: a fractionation factor due to capillary constriction of viscous into molecular flow during gas inlet; a factor due to difference in ionization cross section of isotopic molecules; a factor to account for the collection yields (such as focusing conditions) of the isotopic ion beams; a factor due to unequal gains (dynamic range) of amplifiers, etc. For most work, the proportionality constant is difficult to determine. Furthermore, long term stability in ion beam ratios is difficult to maintain. Consequently, the concept of differential comparison and the del (%o) notation was introduced, and the isotopic abundance is expressed as the relative difference in ratio between a sample and a standard [29]. In the del expression, the proportionality constant is canceled out, and the difference in long term drift between the sample and the standard remains most likely the same. For this purpose, primary standards such as SMOW (Standard Mean Ocean Water) [30], and V-SMOW [31] for hydrogen and oxygen, or PDB (PeeDee Belemnite) for carbon [32] were defined. Reference standards, typically CO2 (there is none for CO at the current time), were then derived from primary standards and used in the differential comparison measurements. However, uncertainties due to undetermined equations arising from overlapping isotopic ions of the same nominal mass [28] may still persist even in the standards. Because of the high spectral resolution of the diode lasers, the problem of un-

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determined equations is eliminated if a diode laser based system is used. It should also be noted that the necessity of having to determine the isotopic abundance as a relative difference in ratio is not critical here, since none of the fractionation problems mentioned above exist in this type of high resolution spectroscopy.

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APPENDIX The laser intensity I(v) can be expanded about frequency is, as is is slowly tuned by ramping the diode laser current

y

CU

I(v)

(n)

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n.

-

For small modulation amplitude a , the intensity at 2w is proportional to the second derivative of I(v) with respect to v. The incident laser intensity IJv) is related to the transmitted intensity I(v) by the Beer-Lambert law

(v - V ) "

I(v)

where I ( " ) ( i s )is the nth derivative of Z(v) with respect to v evaluated at is. To generate the harmonic signal, a small sinusoidal modulation of amplitude a and angular frequency w is superimposed on the diode laser ramping current,* resulting in

+ a sin ut

v =V

Substituting (4) and (5) into (3) and rearranging terms, the signal with angular frequency 2w has the form

= Zo(v) exp [-a(v)

*

I p]

(7)

where a(v) is the spectral absorption coefficient, 1 and p are the path length and pressure (concentration), respectively. For an optically thin sample (i.e., a(v) * 1 p