Tandem differential mobility spectrometry with ion dissociation in air at ambient pressure and temperature.

Proton-bound dimers were dissociated to protonated monomers in air at ambient pressure and temperature using electric fields of ultrahigh Field Asymmetric Ion Mobility Spectrometry (ultraFAIMS) with the onset of dissociation for ethyl acetate as 96 Td and for dimethyl methyl phosphonate as 170 Td. Ions then were measured by differential mobility spectrometry (DMS). Fragment ions were formed with propyl acetate at electric fields of 90 Td or greater. The dissociation in ultraFAIMS of ions, with compensation fields near zero, to form smaller ions with new compensation fields, provided a method to improve peak capacity in DMS without gas modifiers. These findings also lay the foundation for a triple stage DMS with a centre stage for ion dissociation or fragmentation.

Characterization of ion processes in a GC/DMS air quality monitor by integration of the instrument to a mass spectrometer.

The air quality monitor (AQM), which included a portable gas chromatograph (GC) and a detector was interfaced to a mass spectrometer (MS) by introducing flow from the GC detector to the atmospheric pressure ion source of the MS. This small GC system, with a gas recirculation loop for carrier and detector make-up gases, comprised an inlet to preconcentrate volatile organic compounds (VOCs) in air, a thermal desorber before the GC column, a differential mobility spectrometer (DMS), and another DMS as an atmospheric pressure ionization source for the MS. Return flow to the internally recirculated air system of the AQM's DMS was replenished using purified air. Although ions and unreacted neutral vapors flowed from the detector through Viton® tubing into the source of the MS, ions were not detected in the MS without the auxillary ion source, (63)Ni as in the mobility detector. The GC-DMS-MS instrument provided a 3-D measurement platform (GC, DMS, and MS analysis) to explore the gas composition inside the GC-DMS recirculation loop and provide DMS-MS measurement of the components of a complex VOC mixture with performance significantly enhanced by mass-analysis, either with mass spectral scans or with an extracted ion chromatogram. This combination of a mobility spectrometer and a mass spectrometer was possible as vapors and ions are carried together through the DMS analyzer, thereby preserving the chromatographic separation efficiency. The critical benefit of this instrument concept is that all flows in and through the thoroughly integrated GC-DMS analyzer are kept intact allowing a full measure of the ion and vapor composition in the complete system. Performance has been evaluated using a synthetic air sample and a sample of airborne vapors in a laboratory. Capabilities and performance values are described using results from AQM-MS analysis of purified air, ambient air from a research laboratory in a chemistry building, and a sample of synthetic air of known composition. Quantitative measures of a stand-alone AQM are disclosed for VOCs in the ppb to ppm levels with an average precision of 5.8% RSD and accuracy from 4% to 28% error against a standard method.