Peak capacity in differential mobility spectrometry: effects of transport gas and gas modifiers

Differential mobility spectrometry (DMS) can be used as a pre-filter to improve selectivity prior to mass spectral analysis, because it provides an additional mobility based separation to mass analysis. Ions within a differential mobility spectrometer are separated based upon differences in mobility behavior, which are manifested by applying a strong high frequency asymmetric RF waveform. Currently it is well documented that DMS separation power (peak capacity), can be substantially augmented by adding chemical modifiers to the transport gas. This process is generally referred to as the dynamic cluster/decluster model, and it occurs due to gas phase processes when analyte ions are clustered with modifier molecules in the lower field portion of the RF pulses and declustered during the higher field portion of the waveform. Previous publications have demonstrated that the planar electrode geometry is key to realizing improved performance of DMS sensors with high concentrations of chemical modifiers. The clustering modifier is provided with sufficient concentration to reach conditions where separation peak capacity can be substantially augmented (typically around 1.5 %). The present paper expands on these previous findings by mapping out the behavior of structurally similar compounds, including structural isomers, with a series of different transport gas modifiers, including isopropanol, methanol, acetonitrile, acetone, and ethylacetate. The results highlight the importance of analyte/modifier interactions, demonstrating the variability that can be achieved when different modifiers are used.