Spectrochimica Acta Part A 58 (2002) 2465 – 2479 Infrared laser spectrometer with balanced absorption for measurement of isotopic ratios of carbon gases J. Barry McManus *, Mark S. Zahniser, David D. Nelson, Leah R. Williams, Charles E. Kolb Aerodyne Research Inc., 45 Manning Road, Billerica, MA 01821, USA Abstract Measurement of the isotopic compositions of carbon dioxide and methane is a powerful tool for quantifying their atmospheric sources and sinks, which is especially important considering the dramatic increase in these greenhouse gases during the industrial era. Laser absorption spectroscopy is a technique which has demonstrated the high sensitivity needed for isotopic measurement. A significant problem in the spectroscopic measurement of isotopic abundances is the large difference in concentrations of the major and minor isotopic constituents. The measurement of two isotopic species using lines of similar strength but very unequal concentrations leads to low precision, with either the minor constituent having too small an absorption depth, or the major constituent having too great an absorption depth. If lines with unequal strength are chosen to compensate for the absorption depth imbalance, then precision tends to suffer due to the greater temperature sensitivity of the weaker line strength. We describe the development of a compact instrument for isotopic analysis CO 2 and CH 4 using tunable infrared laser absorption spectroscopy which combines novel optical design and signal processing methods to address this problem. The design compensates for the large difference in concentration between major and minor isotopes by measuring them with pathlengths which differ by a factor of 72 within the same multipass cell. We have demonstrated the basic optical design and signal processing by determining  13 C (CO 2 ) isotopic ratios with precision as small as 0.2 ‰ using a lead salt diode laser based spectroscopic instrument. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Infrared laser absorption spectroscopy isotope measurement; Dual length absorption cell; Balanced adsorption depth www.elsevier.com/locate/saa 1. Introduction Measurement of the isotopic composition of atmospheric carbon dioxide and methane is a valuable tool for developing an understanding of the sources and sinks of these greenhouse gases. Both CO 2 and CH 4 have a wide range of sources and sinks with both natural and anthropogenic components [1], and there are remaining uncer- tainties in their magnitudes. Isotopic measure- ments are useful because there are characteristic isotopic enrichments or depletions generated by the chemical kinetics or biochemistry of source and sink processes. The spatial and temporal vari- ations in the isotopic concentrations of CO 2 and CH 4 can provide constraints on the strengths of the fluxes controlling their atmospheric abun- dances [2]. For instance, measurements of 13 C/ 12 C * Corresponding author 1386-1425/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S1386-1425(02)00064-1