Atmospheric pressure chemical ionization-tandem mass spectrometry (APCI-MS/MS) was applied for the first time to the direct analysis of octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) in gaseous matrixes without extraction or prior chromatographic separation. Mass spectrometric characteristics of both compounds under APCI conditions and their fragmentation behavior in MS/MS were investigated. Unlike the classical gas chromatography/mass spectrometry (GC/MS), which involves solvent extraction before gas chromatography, the proposed approach prevents contamination from GC system components and provides unambiguous structural assignments. The method performs well achieving good linearity (R(2) > 0.997), low limits of detection (4-6 microg/m(3)), good precision (RSD < 10%) and accuracy (>93%), and a wide dynamic range. Its applicability to real-world samples was evaluated through measurements of D4 and D5 concentrations in air and biogas samples. The high sensitivity, selectivity, and reliability of this method render our approach a good alternative to the commonly used GC/MS method.

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Real-time monitoring of benzene, toluene, ethylbenzene, and xylenes (BTEX) in ambient air is essential for the early warning detection associated with the release of these hazardous chemicals and in estimating the potential exposure risks to humans and the environment. We have developed a tandem mass spectrometry (MS/MS) method for continuous real-time determination of ambient trace levels of BTEX. The technique is based on the sampling of air via an atmospheric pressure inlet directly into the atmospheric pressure chemical ionization (APCI) source. The method is linear over four orders of magnitude, with correlation coefficients greater than 0.996. Low limits of detection in the range 1-2 g/m 3 are achieved for BTEX. The reliability of the method was confirmed through the evaluation of quality parameters such as repeatability and reproducibility (relative standard deviation below 8% and 10%, respectively) and accuracy (over 95%). The applicability of this method to real-world samples was evaluated through measurements of BTEX levels in real ambient air samples and results were compared with a reference GC-FID method. This direct APCI-MS/MS method is suitable for real-time analysis of BTEX in ambient air during regulation surveys as well as for the monitoring of industrial processes or emergency situations. B enzene, toluene, ethylbenzene, and isomers of xylene, commonly called BTEX, are volatile organic compounds (VOCs) widely present in petroleum products and added to fuels to increase their octane number [1]. They have toxic health effects depending on duration and levels of exposure [2][3][4][5]. In addition, BTEX in urban air contribute significantly to the smog photochemical reactions leading to ozone formation in the troposphere [6].Several analytical methods have been developed to perform real-time measurements of such compounds in ambient air, and recently, we have published a comprehensive review of the most commonly used on-site methods for continuous real-time monitoring of airborne VOCs [7]. Differential optical absorption spectroscopy (DOAS) has been applied widely for near real-time monitoring of BTEX in ambient air [8,9], but the presence of oxygen, ozone, and several hydrocarbons with similar spectra gives rise to severe interference effects.Proton-transfer reaction/mass spectrometry (PTR-MS) has been used in several field studies for real-time monitoring of atmospheric BTEX [10,11], but there are certain isobaric interferences in the PTR-MS measurements of these hydrocarbons (e.g., notably benzaldehyde).Other direct-air sampling (DS)-MS methods widely used for real-time determination of BTEX in ambient air are low-pressure chemical ionization/tandem mass spectrometry (LPCI-MS/MS) [12,13] and atmospheric pressure chemical ionization/tandem mass spectrometry (APCI-MS/MS) [14,15]. However, they are not yet very conclusive for measuring BTEX in ambient air at low-g/m 3 concentrations [7].The work described in the present article demonstrates the application of a direct APCI-MS/MS system for real-ti...