title: Stable isotopic analysis of atmospheric methane by infrared spectroscopy by use of diode laser difference-frequency generation.
abstract: An infrared absorption spectrometer has been constructed to measure the stable isotopic composition of atmospheric methane samples. The spectrometer employs periodically poled lithium niobate to generate 15 microW of tunable difference-frequency radiation from two near-infrared diode lasers that probe the nu3 rotational-vibrational band of methane at 3.4 microm. To enhance the signal, methane is extracted from 25 l of air by use of a cryogenic chromatographic column and is expanded into the multipass cell for analysis. A measurement precision of 12 per thousand is demonstrated for both delta13C and deltaD.
authors.affiliation: Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Hilo, Hawaii 96720, USA. michael.trudeau@noaa.gov
authors.forename: Michael E
authors.initials: ME
authors.lastname: Trudeau
authors.forename: Pin
authors.initials: P
authors.lastname: Chen
authors.forename: Guilherme de Andrade
authors.initials: Gde A
authors.lastname: Garcia
authors.forename: Leo W
authors.initials: LW
authors.lastname: Hollberg
authors.forename: Pieter P
authors.initials: PP
authors.lastname: Tans