Analytica Chimica Acta 706 (2011) 128–134 Contents lists available at SciVerse ScienceDirect Analytica Chimica Acta jou rn al hom epa ge: www.elsevier.com/locate/aca Quantification of oxygenated volatile organic compounds in seawater by membrane inlet-proton transfer reaction/mass spectrometry Rachael Beale a,∗ , Peter S. Liss b , Joanna L. Dixon a , Philip D. Nightingale a a Plymouth Marine Laboratory, Prospect Place, Plymouth, Devon PL1 3DH, UK b School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK a r t i c l e i n f o Article history: Received 10 May 2011 Received in revised form 28 July 2011 Accepted 13 August 2011 Available online 22 August 2011 Keywords: Oxygenated volatile organic compounds Proton transfer reaction/mass spectrometry Membrane inlet Methanol Acetaldehyde Acetone a b s t r a c t The role of the ocean in the cycling of oxygenated volatile organic compounds (OVOCs) remains largely unanswered due to a paucity of datasets. We describe the method development of a membrane inlet- proton transfer reaction/mass spectrometer (MI-PTR/MS) as an efficient method of analysing methanol, acetaldehyde and acetone in seawater. Validation of the technique with water standards shows that the optimised responses are linear and reproducible. Limits of detection are 27 nM for methanol, 0.7 nM for acetaldehyde and 0.3 nM for acetone. Acetone and acetaldehyde concentrations generated by MI-PTR/MS are compared to a second, independent method based on purge and trap-gas chromatography/flame ionisation detection (P&T-GC/FID) and show excellent agreement. Chromatographic separation of iso- meric species acetone and propanal permits correction to mass 59 signal generated by the PTR/MS and overcomes a known uncertainty in reporting acetone concentrations via mass spectrometry. A third bioassay technique using radiolabelled acetone further supported the result generated by this method. We present the development and optimisation of the MI-PTR/MS technique as a reliable and convenient tool for analysing seawater samples for these trace gases. We compare this method with other analytical techniques and discuss its potential use in improving the current understanding of the cycling of oceanic OVOCs. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Oxygenated volatile organic compounds (OVOCs) are ubiqui- tous throughout the troposphere and play an important role in the oxidative capacity of the atmosphere [1–3] but the role of the ocean in the cycling of these compounds remains largely unanswered. The analysis of these low molecular weight alcohols, aldehydes and ketones in seawater is challenging due to their presence at trace concentrations, their reactivity and also their high solubil- ity. The current data available has been generated via various analytical systems including purge and trap-proton transfer reac- tion/mass spectrometry (PTR/MS) [4], equilibrator-inlet-PTR/MS [5], atmospheric pressure chemical ionisation mass spectrometry (API-CIMS) [6], derivatisation-high pressure liquid chromatogra- phy (HPLC) [7] and purge and trap-GC/FID [8]. However, no single technique has yet to identify all the OVOCs listed above, and use of mass spectrometry does not distinguish between isomeric species, of which acetone and propanal are considered important potential interferents. ∗ Corresponding author. Tel.: +44 01752 633100; fax: +44 01752 633101. E-mail address: rbea@pml.ac.uk (R. Beale). The high-sensitivity PTR/MS used in this method was developed and built by Ionicon, Austria. It has been widely tested over a range of applications such as in the medical and food industry [9] and on an array of atmospheric studies [for example, 10–13]. It has also been used for seawater analysis but with different methods of analyte extraction prior to detection by PTR/MS [4,5]. For a detailed review of the PTR/MS please refer to Lindinger et al. [9], only a brief description will be given here. The PTR/MS utilises water to create hydronium ions, H 3 O + , via a hollow cathode discharge source. The hydronium ions are able to undergo a non- dissociative, proton transfer reaction with any neutral compound (R) that has a proton affinity greater than that of water [9] present in the drift tube. The OVOCs undergo the proton transfer reaction, cre- ating predominantly (although not exclusively) a protonated form of the molecular ion. Methanol is the only stable alcohol that undergoes the pro- ton transfer reaction without loss of water from the functional OH group, as shown by Eq. (1): H 3 O + + CH 3 OH → CH 3 OHH + + H 2 O (1) The product is protonated methanol (CH 3 OHH + ), detected at mass 33. Ethanol also undergoes the proton transfer reaction to form the product CH 3 CH 2 OHH + of mass 47. However, due to a high background signal of mass 47 in the instrument, caused by the 0003-2670/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2011.08.023