Programmable gate delayed ion mobility spectrometry-mass spectrometry: A study with low concentrations of dipropylene-glycol-monomethyl-ether in air

The concept of using a short ionisation event, in this case a pulsed corona discharge, in conjunction with programmed gate delay is described. This technique is proposed for the selective study of different ionisation processes within the reaction region of an ion mobility spectrometer. The utility of such an approach was tested in a study of the ionisation of dipropylene-glycol-monomethyl-ether (DPM); a compound commonly used to test the operation of ion mobility spectrometers. Dipropylene-glycol-monomethyl-ether at a concentration of 113 µg m−3 in air, with a water level of 75 mg m−3 in air, was analysed using a switchable, high resolution ion mobility spectrometer, operating in the positive mode at 40 °C at ambient pressure. The ion mobility spectrometer was fitted with a pulsed corona discharge ionisation source, doped with ammonia at a concentration of 1.3 mg m−3 in the reaction region, and interfaced to a mass spectrometer. Synchronisation of the ionisation event to the operation of the shutter grids for the drift region enabled different parts of the product ion population to be injected into the drift tube, and programming the gate delays produced a map of the gate delay verses drift time response surface. Ammonium bound dipropylene-glycol-monomethyl-ether was observed, [(DPM)NH4]+ (m/z 166) as well as the ammonium bound dimer [(DPM)2NH4]+ (m/z 314), the same as those observed with a 63Ni source. Two other species were also observed, but their molecular identity was not elucidated. One of them m/z 146, also observed with 63Ni, formed ammonium bound ions [(m/z 146)NH4]+ (K0 = 1.49 cm2 V−1 s−1), ammonium bound dimer ions [(m/z 146)2NH4]+ (K0 = 1.18 cm2 V−1 s−1) and a mixed cluster ion with DPM [(m/z 146)(DPM)NH4]+ (K0 = 1.18 cm2 V−1 s−1); while the other, m/z 88 a decomposition product, formed ammonium bound monomer [(m/z 88)NH4]+ (K0 = 1.68 cm2 V−1 s−1), dimer ions [(m/z 88)2NH4]+ (K0 = 1.40 cm2 V−1 s−1) and a mixed cluster ion containing DPM and ammonium, [(DPM)(m/z 88)2NH4]+ (K0 = 1.40 cm2 V−1 s−1). The assignment of responses to these ions required the additional dimensionality in the data provided from the gate delay studies. The relationships evident in the programmable gate delay data enabled these ions to be differentiated from alternative assignments of possible nitrogen clusters, formed at the interface of the mass spectrometer.