Parts-per-trillion by volume

A high-speed sensor for the assay of dimethyl sulfide in the marine troposphere based on its CL reaction with F2 was recently reported [18]. Sample air and F2 in He were introduced at opposite ends of a reaction cell with a window at one end. The production of vibrationally excited HF and electronically excited fluorohydrocarbon (FHC) produced CL emission in the wavelength range 450-650 nm, which was monitored via photon counting. Dimethyl sulfide could be determined in the 0-1200 pptv (parts per trillion by volume) concentration range, with a 4-pptv detection limit. 

Sampling is difficult because the vapor pressures for most explosives are very low. For example, the room temperature equilibrium headspace concentration of RDX in air is about 10 pptv (parts per trillion by volume). Collection of vapor is further compounded for explosives that are bound in matrices and wrappers and/or are concealed in wrappings or baggage. The prospects for trace detection of explosives are considered to be better when sampling objects for explosives contamination in the form of particles and residue. 

The nitrogen species enter the atmosphere from a variety of natural and anthropogenic sources (7). The largest sources are concentrated in urban and industrialized areas. The levels of the species in the atmosphere vary from hundreds of parts per billion by volume (ppbv, that is, 10 9 mole fraction) in these source regions to below one part per trillion by volume (pptrv, 10"12 mole fraction) in remote areas. Even at the pptrv level, these species can play significant roles in atmospheric chemistry, and measurements of species at the sub-pptrv level can yield useful information concerning atmospheric photochemistry. 

FTIR, TDLAS, and LIF are in situ techniques, whereas DOAS is a long-path method that gives only a path-integrated result. NO, N02, NO,3, and HONO have been successfully measured with DOAS even in rural and remote regions. PAN, HN03, and NH3 have been measured with FTIR in urban areas, but its sensitivity at present is not adequate for levels below a few parts per billion by volume. NO, N02, PAN, HN03, and NH3 have been measured with TDLAS down to sub-ppbv levels. A review with references for applications of these three methods is available (13). The LIF method has been more recently developed, has been applied to the measurement of NO, N02, NH3, and HONO (see reference 14 for an example), and offers sensitivity down to the parts per trillion by volume level. 

A proton transfer reaction-mass spectrometer (PTR-MS) system has been developed which allows on-line measurement of VOCs with concentrations as low as a few pptv (parts per trillion by volume) (Hansel et al., 1998). The acute measurement sensitivity to VOCs and real-time characteristics make it a very powerful tool in both indoor and outdoor environmental research. 

In this work all concentrations are reported in units of nanomole per cubic meter (nmol nr3) at 20°C and 1013 hPa, and/or as parts per trillion by volume (pptv) with 1 nmol m 3 corresponding to 24 pptv. 

TABLE 1. Tropospheric concentrations (parts per trillion by volume) of principal organohalide source gases... 

The beam enters the 1.75 m Teflon-lined White cell containing a corner cube reflector (3) and undergoes 102 passes before exiting to the detector. Absorptions at least as low as 10 s can be measured which, for a total path length of 150 m corresponds to detection limits in the range 25 to 100 parts per trillion by volume for most atmospheric gases. 

Fig. 13. Measurements of atmospheric CFCI3 (CF ll), and CF2CI2 (CFS-12), in parts per trillion by volume between 1977 and 1982. Also shown are measurements of atmospheric nitrous oxide (N2O) in parts per billion. (From U.S. Government, 1982).

Note Surface samples were collected with a stainless steel bucket from the top meter of the ocean. Air and seawater concentrations are reported as parts per trillion by volume. For seawater this is milliliters of gas at 25 °C per 10 2 mL of seawater. This unit is used to facilitate the air-sea exchange model. + indicates a peak was detected but not quantitated —, no peak was detected and NM, not measured. 

Bandy AR, Tucker BJ, Maroulis PJ. 1985. Determination of part-per-trillion by volume levels of atmospheric carbon disulfide by gas chromatography/mass spectrometry. Anal Chem 57 1310-1314. 

The development of proton-transfer-reaction mass spectrometry (PTR-MS) as a tool for the analysis of volatile organic compounds (VOCs) is described. PTR-MS is based on the rapid, non-dissociative transfer of protons from H30 to most common VOCs, but not to the principal gases in the air sample. Recent developments in the design of PTR-MS instruments allow detection of some VOCs in the parts per trillion by volume range. This sensitivity and the capability of PTR-MS instruments to be operated for extended periods in both laboratory and field settings has allowed exploration of many aspects of VOC analysis in environmental, food and medical applications. 

Mixing ratios of the individual NMHCs vary greatly, from several parts per trillion by volume (pptv) to several parts per billion by volume (ppbv). Variations in the reactivities of NMHCs are also substantial for example, isoprene (2-methyl butadiene), which is emitted by deciduous vegetation, has an atmospheric lifetime with respect to oxidation by OH of about 20 min in polluted air ([OH] = 10 radicals cm ). Monoterpenes (CiqHis), which are emitted from coniferous trees, react extremely rapidly with OH and O3 and have lifetimes of minutes. The atmospheric lifetimes of 2-methylpropene, 2-methylbutane, and the xylenes, which are found in vehicle emissions, are approximately 30 min, 7 h, and 1.5 h, respectively. Oxidation of the terpenes... 

Tanimoto, H., Hirokawa, J., Kajii, Y., and Akimoto, H., A new measurement technique of PAN at parts per trillion by volume levels gas chromatography/negative ion chemical ionization mass spectrometry, J. Geophys. Res., 104, 21343-21354, 1999. 

Nitric oxide (NO) plays a central role in atmospheric chemistry, influencing both ozone cycling and the tropospheric oxidation capacity through reactions with hydroperoxy- and organic peroxy-radicals. When the NO concentration exceeds 40 pptv (pptv = parts per trillion by volume) it catalyzes the production of ozone (O3) ... 

Proton transfer reaction mass spectrometry (PTR-MS) was first developed at the Institute of Ion Physics of Innsbruck University in the 1990s. Nowadays, PTR-MS is a well-developed and commercially available technique for the on-line monitoring of trace volatile organic compounds (VOCs) down to parts per trillion by volume (ppt) level. PTR-MS has some advantages such as rapid response, soft chemical ionization (Cl), absolute quantification, and high sensitivity. In general, a standard PTR-MS instrument consists of external ion source, drift tube, and mass analysis detection system. Figure... 

By combining the principles of PICI with an apparatus derived from selected-ion flow tube (SIFT) experiments, the technique of proton transfer reaction mass spectrometry (PTR-MS) has been developed as a dedicated tool for the analysis of volatile organic compounds (VOCs) at parts-per-trillion by volume (pptv) level in air [51,52]. PTR-MS is highly useful as a fast and quantitative method for the determination of VOCs as proven by numerous applications in food control, envi-... 

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