Abstract: An ion mobility spectrometer system with improved specificity. The ion mobility spectrometer has a reaction chamber and a drift chamber, an ionization source disposed in the reaction chamber, an injection grid separating said reaction and drift chambers, a collector electrode disposed in the drift chamber and a sample input injecting the sample and carrier into the reaction chamber.A source injects acetone and/or carbon tetrachloride as reagents into the carrier gas prior to entering the sample input. The acetone reactant forms a dimer ion in the positive ion spectrum which impedes the clustering of water and provides a narrow fixed peak in the ion spectrum which serves as a reference point in the ion spectrum for the algorithm used in the electronic control unit generating an output characteristic of the sample. The carbon tetrachloride reactant forms a (H.sub.2 O).sub.n Cl.sup.- ion which provides a reference in the negative ion spectrum for the algorithm used in the electronic control unit.