Direct, rapid quantitative analyses of BVOCs using SIFT-MS and PTR- MS obviating sample collection David Smith, Patrik S ˇ pane ˇl The purpose of this short review is to describe the origins and the principles of operation of selected-ion flow-tube mass spectrometry (SIFT-MS) and proton-transfer-reaction mass spectrometry (PTR-MS), and their application to the analysis of bio- genic volatile organic compounds (BVOCs) in ambient air, the humid air (headspace) above biological samples, and other samples. We briefly review the ion chemistry that underpins these analytical methods, which allows accurate analyses. We pay attention to the inherently uncomplicated sampling methodologies that allow on-line, real-time analyses, obviating sample collection into bags or onto traps, which can compromise samples. Whilst these techniques have been applied successfully to the analysis of a wide variety of media, we give just a few examples of data, including for the analysis of BVOCs that are present in tropospheric air and those emitted by plants, in exhaled breath and in the headspace above cell and bacterial cultures (which assist clinical diagnosis and therapeutic monitoring), and the products of combustion. The very wide dynamic ranges of real-time analyses of BVOCs in air achieved by SIFT-MS and PTR-MS – from sub- ppbv to tens of ppmv – ensure that these analytical methods will be applied to many other media, especially when combined with gas-chromatography methods, as recently trialed. ª 2011 Elsevier Ltd. All rights reserved. Keywords: Absolute quantification; Ambient air; BVOC; Chemical ionization; Gas chromatography; Headspace analysis; Mass spectrometry; PTR- MS; Sample collection; SIFT-MS Abbreviations: BVOC, Biogenic volatile organic compound; m/z, Mass-to-charge ratio; ppbv, Parts per billion by volume; ppmv, Parts per million by volume; PTR-MS, Proton-transfer-reaction mass spectrometry; SIFT-MS, Selected-ion flow-tube mass spectrometry David Smith Institute for Science and Technology in Medicine, School of Medicine, Keele University, Thornburrow Drive, Hartshill, Stoke-on-Trent ST4 7QB, UK Patrik S ˇ pane ˇl* J. Heyrovsky ´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejs ˇkova 3, 182 23, Prague 8, Czech Republic 1. Introduction and overview As discussed in other contributions to this Special Issue, the standard approach to the analyses of biogenic volatile organic compounds (BVOCs) in air usually in- volves the collection of samples into vessels or onto traps, followed by extrac- tion of the BVOCs and their separation by a GC column [1]. With proper use of standards and careful calibration, this approach provides reliable results, but currently cannot deliver absolute quanti- fication in real time. Selected-ion flow- tube mass spectrometry (SIFT-MS) and proton-transfer-reaction mass spectrome- try (PTR-MS) were thus conceived primarily to meet the challenge of the immediate absolute quantification of the trace gases in ambient air and in humid exhaled breath. Both SIFT-MS [2,3] and PTR-MS [4,5] involve efficient selected chemical ioniza- tion (CI) of the trace gases present in air samples largely to the exclusion of major components N 2 ,O 2 , Ar, H 2 O and CO 2 (in exhaled breath) using judiciously chosen reagent ions [5,6]. SIFT-MS exploits a flow tube as a thermal energy ion-chemical reactor that allows accurate, real-time, trace-gas analysis based on well-under- stood ion/molecule kinetics, whilst PTR- MS exploits a flow/drift tube for the same purpose. Details of these two techniques are given in the next section, which fo- cuses on their relative strengths and weaknesses, and their special, unique features for trace-gas analysis, especially BVOC analysis. We show that real-time analyses of ambient air, including humid samples (e.g., exhaled breath and the headspace above biological samples) can * Corresponding author. E-mail: spanel@jh-inst.cas.cz Trends in Analytical Chemistry, Vol. 30, No. 7, 2011 Trends 0165-9936/$ - see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.trac.2011.05.001 945