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Sensitive real-time detection of vapors produced by the precursors, reagents and solvents used in the illegal drugs manufacture represents a priority nowadays. Acetic anhydride (AA) is the key chemical used as acetylation agent in producing the illegal drugs heroin and methaqualone. This study was directed towards quick detection and quantification of AA in air, using two fast and very sensitive analytical techniques: photoionization detection (PID) and ion mobility spectrometry (IMS). Results obtained indicated that both PID and IMS can sense AA at ultra-trace levels in air, but while PID produces a non-selective response, IMS offers richer information. Ion mobility spectrometric response in the positive ion mode presented one product ion, at reduced ion mobility K0 of 1.89 cm2 V−1 s−1 (almost overlapped with positive reactant ion peak), while in the negative ion mode two well separated product ions, with K0 of 1.90 and 1.71 cm2 V−1 s−1, were noticed. Our study showed that by using a portable, commercial IMS system (model Mini IMS, I.U.T. GmbH Berlin) AA can be easily measured at concentrations of 0.05 ppmv (0.2 mg m−3) in negative ion mode. Best selectivity and sensitivity of the IMS response were therefore achieved in the negative operation mode. [ABSTRACT FROM AUTHOR]