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Stable Isotope Analysis Using Tunable Diode Laser Spectroscopy

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Stable Isotope Analysis Using Tunable Diode Laser Spectroscopy San Jose State University SJSU ScholarWorks Faculty Publications Physics and Astronomy January 1992 Stable isotope analysis using tunable diode laser spectroscopy Todd B. Sauke San Jose State University, [email protected] J. F. Becker San Jose State University M. Loewenstein Ames Research Center Follow this and additional works at: https://scholarworks.sjsu.edu/physics_astron_pub Part of the Astrophysics and Astronomy Commons Recommended Citation Todd B. Sauke, J. F. Becker, and M. Loewenstein. "Stable isotope analysis using tunable diode laser spectroscopy" Applied Optics (1992): 1921-1927. This Article is brought to you for free and open access by the Physics and Astronomy at SJSU ScholarWorks. It has been accepted for inclusion in Faculty Publications by an authorized administrator of SJSU ScholarWorks. For more information, please contact [email protected]. Stable isotope analysis using tunable diode laser spectroscopy Joseph F. Becker, Todd B. Sauke, and Max Loewenstein Measurements of ratios of stable isotopes are used in such diverse fields as petroleum prospecting, medical diagnostics, and planetary exploration. The narrow emission linewidth available from tunable diode lasers permits high-resolution infrared absorption measurements of closely spaced isotopic rovibrationallines. Our dual beam spectrometer uses the sweep integration technique in a spectral region where adjacent spectral lines are of approximately equal absorbance at the expected isotopic abundances. The experimen­ tal results reported here indicate that isotopic ratios of carbon in carbon dioxide can be measured to an accuracy of better than 0.4%. This laser spectroscopic technique offers an alternative to the mass spectrometric technique for in situ isotopic analysis in field studies, as well as flight and space applications. Key words: Diode laser, isotopic ratio, 13C/ 12C, carbon dioxide, laser spectroscopy. I. Introduction ments help to trace the sources of petroleum and Variations in ratios of stable carbon isotopes have natural gases. 5 provided a wealth of information concerning the By using a tunable diode laser (TDL) spectrometer 12 13 origin and the history of carbon-containing mate­ to record absorption spectra of C02 and C02 infra­ 1 2 12 13 rial. ' Reaction rates of C differ from those of C red lines, we can measure the isotopic ratio of the owing to the kinetic isotope effect. 3 For example, carbon in a sample. The usual method of measuring when carbon dioxide is fixed during green plant 13C/ 12C ratios is by the well-accepted mass spectromet­ photosynthesis, an approximately 2%-3% decrease of ric techniqueY This method is extremely accurate 13C/ 12C occurs in most plants.4 Reaction conditions, but requires substantial sample preparation and puri­ such as temperature and pressure, can influence the fication to obtain reliable results.8 At the high spec­ 13C/ 12C ratio in the products. Also, geological process­ tral resolution provided by semiconductor diode la­ ing can mix deposits, affecting isotopic ratios. Typical sers, problems of impurity gas interference with the variations among different inorganic deposits are up measurement are essentially eliminated. The narrow to several tenths of one percent, and, among organic spectral lines of impurity gases, such as H20, CH4, 0 3, deposits, the variations are even greater.1 Thus, it is and N 0, for example, tend to lie far from the narrow possible to use the isotopic ratio measurements of 2 rovibrationallines of the C02 gas being studied. This carbon in such diverse samples as rocks, soil, and represents a major advantage over the mass spectro­ expired breath from animals to gain insight into the metric technique for measuring isotopic ratios. For origin of the carbon or the conditions under which the 13 2 example, organic material could be reacted under samples were formed. For example, Cjl C measure- either oxidizing or reducing conditions, and the result­ ing gases analyzed directly without the additional purification steps required for mass spectrometric J. F. Becker is with the Department of Physics, San Jose State analysis. This is an important advantage for in situ University, 1 Washington Square, San Jose, California 95192- analysis and remote applications such as planetary 0106. T. B. Sauke is with the Solar System Exploration Branch, exploration missions. Tunable diode lasers are well Ames Research Center, National Aeronautics and Space Adminis­ suited in general for space and flight applications and tration, Moffett Field, California 94035. M. Loewenstein is with the are currently in use in several flight instruments, for Atmospheric Chemistry and Dynamics Branch, Ames Research example, the airborne tunable laser absorption spec­ Center, National Aeronautics and Space Administration, Moffett 9 Field, California 94035. trometer (ATLAS). Received 8 January 1991. In this paper we describe an optical technique to 13 12 0003-6935/92/121921-07$05.00/0. measure C/ C ratios in C02 by TDL laser spectros­ © 1992 Optical Society of America. copy. The technique, as well as the optical system, 20 April 1992 I Vol. 31, No. 12 I APPLIED OPTICS 1921 data acquisition, and data analysis are described and The TDL spectrometer used in our laboratory is a discussed. Preliminary experimental results are also modified Model LS-3 (Laser Photonics, Analytics presented. Division, Bedford, Mass. 01730) and is shown schemat­ ically in Fig. 2. The diode lasers (model L5621, Laser II. Experimental Methods Photonics, Analytics Division, Bedford, Mass. 01730) Several approaches that use (TDL's) for carbon isoto­ used in the spectrometer were of the buried hetero­ 1 13 pic analysis have been reported. 0- The present structure type fabricated by the molecular beam research uses a technique proposed in 1981 by Wallet epitaxy technique and were composition tuned to 12 1 al., which involves the use of absorption lines of operate in the region 2290-2305 cm- • These diodes 12 13 C02 and C02 in the 4.3-J.Lm vibrational bands. In have only recently become commercially available the spectral region where the v3 bands of the two and are an important component in achieving highly 12 isotopes overlap, certain closely spaced C02 and accurate measurements. They operate at tempera­ 13 C02 rovibrational spectral lines have approximately tures above 77 K, eliminating the need for a closed equal absorbance at the anticipated -1:90 isotopic cycle helium refrigerator with its associated mechani­ ratio, as shown in Fig. 1. Several isotopic line pairs 13 cal vibrations, and they have improved single-mode are suitable, such as the R(lO) line of C02 at 1 12 emission characteristics compared with previously 2291.681 cm- and theP(60) line of C02 at 2291.542 available diodes. The operating frequency of the laser em-\ both of the v3 fundamental band of C02• These 1 output is tuned by varying the diode temperature and two lines are separated by only 0.139 cm- but can be the injection current. The laser beam is passed well resolved by using a TDL. Comparing the absorp­ through a 0.5-m Czerny-Turner grating monochro­ tions of such a line pair yields a measure of the mator to ensure isolation of a single laser mode. The isotopic ratio in the sample gas. It is important for the laser beam is split with a pellicle beam splitter, and isotopic lines to have approximately equal absor­ the two beams are directed through identical sample bances so that measurement errors can be mini­ and reference gas cells, each having a 16-cm path mized. length and barium fluoride windows. The liquid­ nitrogen cooled indium antimonide detectors (Infra­ Red Associates, Cranbury, N.J. 08512) are de coupled -18 into a transimpedance preamplifier. The amplified -19 signals from the detectors are fed into a 12-bit, 100-kHz, analog-to-digital converter board (DAS- 16F, Metrabyte Corp., Taunton, Mass. 02780) in­ -21 stalled in an AT&T personal computer Model 6300, -22 which is IBM PC compatible. The laser temperature is adjusted to select an ., -23 L~~22~ll. ~ 2200 2250 2300 2350 2400 appropriate lasing mode, and the injection current is ..c Frequency (cm-1) ramped so that the laser output scans across the ] ----.1 ~ spectral region that includes the absorption lines of I'll interest. We achieved continuous frequency tuning in .3"" -19 ,.... a single mode over a frequency interval greater than 5 em -I, as shown in Fig. 3. This interval includes a line pair that is appropriate for isotopic ratio determi­ -20 : nation. Verification of spectroscopic line identity was made by comparing measured spectral parameters with corresponding parameters from the HITRAN data­ base.14 -21 li I I '• 2280 2285 2290 2295 2300 At the beginning of the current ramp, a blanking Frequency (cm-1) current pulse briefly turns off the laser and provides a Fig. 1. Upper graph,"' integrated absorbances [cm- 1/(molecule zero signal level. The blanking pulse and the current 2 13 ramp are repeated at a fixed frequency, typically cm- )] of individual rovibrationallines from the v3 bands of C02 12 between 20 and 50 Hz. The detected signal from each (dashed lines) and C02 (solid lines). The relative absorbance of the lines is indicated for an isotopic ratio (i3C/ 12C) of -1:90 as is the sweep is digitized and repeated sweeps are co-added to case for Earth samples. The bands overlap in such a way as to have increase signal-to-noise ratio. This technique is known approximately equal absorbances for the two isotopes in the region as sweep integration. 15 Rapid data acquisition, at 100 1 from -2280 to -2300 cm- • Lower graph, expanded plot of the k samples/s, is controlled by the built-in timing and overlap region of the v bands of 13C0 (dashed lines) and 12C0 3 2 2 the direct memory access circuitry on the data acqui­ (solid lines). A particularly suitable pair of 13C/ 12C lines is indicated 13 12 sition board, proceeding as a hardware-controlled by an asterisk.
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