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INTRODUCTION According to the Convention On the Marking of Plastic Explosives for the Purposes of Detection of the International Civil Aviation Organization, signed in 1991, nitrocompounds 4-nitrotoluene (NT) and 2,3-dimethyl-2,3-dinitrobutane [...], Results of the study of the conditions of detection of traces of marking agents added to industrial plastic explosives in air using preconcentration and subsequent analysis on a ECHO-V-IDVS modernized gas chromatograph with a polycapillary column, an ionization detector of a varying selectivity (IDVS), and atmospheric air as a carrier gas are presented. A significant effect of air humidity on the limits of detection for 4-nitrotoluene and 2,3-dimethyl-2,3-dinitrobutane is demonstrated; the highest IDVS sensitivity was observed at the humidity of the carrier gas less than 8.5 mg/[m.sup.3]. For the preconcentration of marking agents, concentrators based on Tenax adsorbent or those produced as metal wire grids coated with a stationary phase are most efficient. The limits of detection for vapors of marking agents in sampling 1 L of air to a concentrator are 22 pg/[cm.sup.3] for 4-nitrotoluene and 0.1 pg/[cm.sup.3] for 2,3-dimethyl-2,3-dinitrobutane. Keywords: marking agents, 4-nitrotoluene, 2,3-dimethyl-2,3-dinitrobutane, ionization detector of a varying selectivity, polycapillary column, preconcentration DOI: 10.1134/S1061934814120053