D/H ratios of atmospheric H2 in urban air: results using new methods for analysis of nano-molar H2 samples

• Published in: Geochimica et cosmochimica acta 2002-07, Vol.66 (14), p.2475-2481
• Main Authors: Rahn, Thom, Kitchen, Nami, Eiler, John
• Format: Article
• Language: English
• ISSN: 0016-7037
• DOI: 10.1016/S0016-7037(02)00858-X

We present the results of a study of the concentration and D/H ratio of molecular hydrogen from air in the Los Angeles Basin and adjacent San Gabriel Mountains. These data define a mixing relationship in dimensions of D/H ratio vs 1/(H2) which constrains the delta-D(VSMOW) of unpolluted winter air in this region to be ca. +100 to +125 per mil and that of urban H2 sources to be ca. -270 per mil. This study makes use of a new method for measuring the deuterium content of molecular hydrogen in small samples (about 100 to 500 cc) of air, which we describe in detail. The method consists of an off-line separation of H2 followed by introduction to the mass spectrometer in a continuous flow of He. Off-line, all components of an atmospheric gas sample, with the exception of He, H2, and Ne, are condensed by exposure to a cold-trap held at 30 Kelvin. This separation is followed by cryo-transfer of non-condensable gases to a small volume molecular sieve finger, with the aid of a mercury piston pump. At the mass spectrometer, the sample is put in line with a continuous flow of He, where it is focused on to an additional column of a molecular sieve before subsequent introduction into the ion source. Analyses of DH/H2 ratio have accuracy and precision of +/- 4 to 7 per mil. Comparison of sample peak area to peak areas of standards of known size allows for determination of H2 concentration with accuracy and precision of +/- 5 percent, relative. The method reduces sample size and processing time by several orders of magnitude compared to previous methods, allowing for sampling at proportionately higher spatial and temporal resolution. (Author)