On-line measurements of a-pinene ozonolysis products using an atmospheric pressure chemical ionisation ion-trap mass spectrometer

An on-line technique to investigate complex organic oxidation reactions in environmental chamber experiments is presented. The method is based on the direct introduction of the chamber air into an atmospheric pressure ion sou rce of a commercial ion-trap mass spectrometer. To demonstrate the analytic al potential of the method (atmospheric pressure chemical ionisation/mass s pectrometry, APCI/MS), the ozonolysis of cc-pinene was investigated in a se ries of experiments performed in various sized reaction chambers at atmosph eric pressure and 296 K in synthetic air. Investigations were focussed on t he influence of the water vapour concentration on the formation of the pred ominant oxidation product, pinonaldehyde, derived from the alpha -pinene/oz one reaction. Quantification of pinonaldehyde was achieved by conducting a standard addition technique. The molar yield of pinonaldehyde was found to depend strongly on the actual water vapour concentration between <1 and 80% relative humidity. Starting with an average yield of 0.23 +/- 0.05 at dry conditions, pinonaldehyde formation was approximately doubled by reaching a yield of 0.53 +/- 0.05 at a relative humidity of around 60%. Furthermore, the formation mechanism of pinonaldehyde was investigated in greater detail using isotopically labelled water. Applying on-line APCI/MS, pinonaldehyde formation under incorporation of O-18 was observed, strongly supporting th e reaction of the stabilised Criegee radical with water in the gas phase as suggested by Alvarado et al. (Journal of Geophysical Research 103 (1998) 2 5541-25551). Furthermore, the mass spectra recorded on-line were used to pe rform a semi-quantitative estimation of the decomposition pathway of the pr imary ozonide, indicating a branching ratio of 0.35/0.65. (C) 2001 Elsevier Science Ltd. All rights reserved.