Chemical effects in the separation process of a differential mobility/mass spectrometer system.

In differential mobility spectrometry (also referred to as high-field asymmetric waveform ion mobility spectrometry), ions are separated on the basis of the difference in their mobility under high and low electric fields. The addition of polar modifiers to the gas transporting the ions through a differential mobility spectrometer enhances the formation of clusters in a field-dependent way and thus amplifies the high- and low-field mobility difference, resulting in increased peak capacity and separation power. Observations of the increase in mobility field dependence are consistent with a cluster formation model, also referred to as the dynamic cluster-decluster model. The uniqueness of chemical interactions that occur between an ion and cluster-forming neutrals increases the selectivity of the separation, and the depression of low-field mobility relative to high-field mobility increases the compensation voltage and peak capacity. The effect of a polar modifier on the peak capacity across a broad range of chemicals has been investigated. We discuss the theoretical underpinnings which explain the observed effects. In contrast to the result with a polar modifier, we find that using mixtures of inert gases as the transport gas improves the resolution by reducing the peak width but has very little effect on the peak capacity or selectivity. The inert gas helium does not cluster and thus does not reduce low-field mobility relative to high-field mobility. The observed changes in the differential mobility alpha parameter exhibited by different classes of compounds when the transport gas contains a polar modifier or has a significant fraction of inert gas can be explained on the basis of the physical mechanisms involved in the separation processes.