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A capillary electrophoresis/electrospray ionisation mass spectrometry (CE/ESI-MS) method was developed for the determination of 38 organic acids in atmospheric particles and cloud water. The target analytes include many functionalised carboxylic acids, such as carboxylic acids with additional oxo-, hydroxy- or nitro-groups. These compounds are of large interest as their determination might give new insights into the atmospheric multiphase chemistry. OASIS HLB sorbent material (Waters) was used to extract and enrich polar carboxylic acids from aqueous solutions with recoveries greater than 80% for most anallytes. Relative standard deviations in the range of 4-20% for peak areas (n = 5), including the SPE step, and 0.2-0.5% (n = 8) for migration times were found. The limits of detection (S/N = 3) ranged from 0.005 to 0.6 mu mol l(-1) for an ion-trap mass spectrometer and from 0.0004 to 0.08 mu mol l(-1) for a time-of-flight mass spectrometer. These detection limits translate into atmospheric concentrations in the low pg m(-3) range based on the experimental conditions in this study. Severe matrix effects were observed for real samples, arising from complex co-extracted organic material. However, using the method of standard addition, most of the analytes could successfully be quantified in samples of ambient particles and cloud water with concentrations in the low ng m(-3) to high pg m(-3) range. These results demonstrate the suitability of the proposed method for the determination of a wide range of polar carboxylic acids at low concentrations in complex samples of different atmospheric phases. (c) 2007 Elsevier B.V. All rights reserved.