Mid-infrared laser spectroscopic determination of isotope ratios of N2O at trace levels using wavelength modulation and balanced path length detection

We present a new mid-infrared laser spectrometer for high-precision measurements of isotopic ratios of molecules at ppm concentrations. Results are discussed for nitrous oxide (N2O), where a precision of 3‰ for a single measurement and a reproducibility of 6‰ have been achieved for a concentration of 825 ppm. The room-temperature laser source employed is based on difference-frequency generation delivering a continuous-wave power up to 23 μW at wavelengths between 4.3 μm and 4.7 μm and a line width of 1 MHz. Two different measurement methods are compared; wavelength modulation with first-harmonic detection and direct absorption spectroscopy by recording the spectrum with a data-acquisition card. Two different detection schemes were employed; either all isotopomers were measured using the long path (36 m) of the multipass cell or a balanced path length detection scheme was used, where the main isotope was measured with a beam along a shorter path (40 cm) in the multipass cell. A single-pass reference cell was designed, offering two different path lengths for balanced path length detection. All combinations of measurement methods and detection schemes were tested regarding precision of a single measurement and long-term stability. The advantages and disadvantages of various measurement approaches are discussed.

[1]  S. Borri,et al.  Power-boosted difference-frequency source for high-resolution infrared spectroscopy , 2003 .

[2]  Simone Borri,et al.  A frequency-modulated quantum-cascade laser for spectroscopy of CH4 and N2O isotopomers , 2005, Isotopes in environmental and health studies.

[3]  R. Monson,et al.  Partitioning net ecosystem carbon exchange with isotopic fluxes of CO2 , 2001 .

[4]  D. Yakir,et al.  Fluxes of CO2 and water between terrestrial vegetation and the atmosphere estimated from isotope measurements , 1996, Nature.

[5]  Irina T. Sorokina,et al.  Crystalline Mid-Infrared Lasers , 2003 .

[6]  Rahn,et al.  Stable isotope enrichment in stratospheric nitrous oxide , 1997, Science.

[7]  Irina T. Sorokina,et al.  Mid-infrared coherent sources and applications , 2008 .

[8]  C. Brenninkmeijer,et al.  The isotopic fingerprint of the pre-industrial and the anthropogenic N2O source , 2002 .

[9]  G. Boyd,et al.  Parametric Interaction of Focused Gaussian Light Beams , 1968 .

[10]  L. Sternberg,et al.  The use of stable isotopes to study ecosystem gas exchange , 2000, Oecologia.

[11]  E. Kerstel,et al.  High-precision determination of the 13CO2/12CO2 isotope ratio using a portable 2.008-μm diode-laser spectrometer , 2003 .

[12]  P. Hering,et al.  Isotope-selective non-dispersive infrared spectrometry for detection of Helicobacter pylori infection with 13C-urea breath test , 1995, The Lancet.

[13]  Steven D. Sargent,et al.  Tunable diode laser absorption spectroscopy for stable isotope studies of ecosystem–atmosphere CO2 exchange , 2003 .

[14]  D. Richter,et al.  13CO2/12CO2 isotopic ratio measurements using a difference frequency-based sensor operating at 4.35 μm , 2002, Applied physics. B, Lasers and optics.

[15]  P. Crutzen,et al.  Isotopic enrichment of nitrous oxide (15N14NO, 14N15NO, 14N14N18O) in the stratosphere and in the laboratory , 2001 .

[16]  J. Faist.  Continuous-Wave, Room-Temperature Quantum Cascade Lasers , 2006 .

[17]  K. Uehara,et al.  Site-selective nitrogen isotopic ratio measurement of nitrous oxide using 2 microm diode lasers. , 2003, Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy.

[18]  H. Craig.  Isotopic standards for carbon and oxygen and correction factors for mass-spectrometric analysis of carbon dioxide , 1957 .

[19]  D. Tedesco,et al.  Chemical (He, H2, CH4, Ne, Ar, N2) and isotopic (He, Ne, Ar, C) variations at the solfatara crater (southern italy): mixing of different sources in relation to seismic activity , 1999 .

[20]  David D. Nelson,et al.  A high precision pulsed quantum cascade laser spectrometer for measurements of stable isotopes of carbon dioxide , 2005 .

[21]  H. Meijer,et al.  Simultaneous determination of the (2)h/(1)h, (17)o/(16)o, and (18)o/(16)o isotope abundance ratios in water by means of laser spectrometry. , 1999, Analytical chemistry.

[22]  K. Vodopyanov,et al.  Solid-state mid-infrared laser sources , 2003 .

[23]  W. Brand,et al.  Gas chromatography/isotope-ratio mass spectrometry method for high-precision position-dependent 15N and 18O measurements of atmospheric nitrous oxide. , 2003, Rapid communications in mass spectrometry : RCM.

[24]  G W Harris,et al.  High-precision direct measurements of (13)CH(4)/(12)CH(4) and (12)CH(3)D/(12)CH(4) ratios in atmospheric methane sources by means of a long-path tunable diode laser absorption spectrometer. , 1994, Applied optics.

[25]  E. Theocharous.  On the stability of the spectral responsivity of cryogenically cooled photoconductive HgCdTe infrared detectors , 2006 .

[26]  Intracavity single resonance optical parametric oscillator (I.S.R.O.) , 1984 .

[27]  M. Fejer,et al.  Multigrating quasi-phase-matched optical parametric oscillator in periodically poled LiNbO(3). , 1996, Optics letters.

[28]  Livio Gianfrani,et al.  Diode laser absorption spectrometry for 13CO2/12CO2 isotope ratio analysis: Investigation on precision and accuracy levels , 2005 .