Comparison of the peak capacity for DMS filters with various gap height: experimental and simulations results

Abstract

Differential Mobility Spectrometry (DMS) devices have been integrated with mass spectrometers to provide an orthogonal pre-separation for targeted quantitation. The distance between the DMS electrodes, referred to as the gap height, is a critical parameter for optimization of DMS performance. The literature includes descriptions of various atmospheric pressure DMS devices with gap heights spanning a range of approximately 0.035 to 2 mm. However, despite the presence of numerous publications describing different gap height devices, there is no systematic investigation that describes the effect of DMS gap height on separation peak capacity. This paper summarizes the results from experimental investigations of the effect of gap height on analytical performance (peak capacity and speed) for DMS devices with gap heights that range from 100 μm to 1.5 mm. In addition, this manuscript provides computer simulations of ion motion that shed some light on the relationship between DMS gap height, waveform frequency, and ion transmission. In order to provide accurate and precise experimental data for use in the simulations and development of models, a custom waveform generator has been designed to maintain separation waveform shape regardless of the capacitive load of the DMS cell and the separation waveform amplitude. The differential mobility function (alpha) has been calculated from experimentally measured compensation fields with each of the cells. When the experimental conditions were constant for a given compound, all tested cells provided similar alpha functions, confirming that similar separation mechanisms are observed regardless of gap height. On the basis of these comprehensive experimental data, it is suggested that a phenomenological model can be used for prediction of peak position for particular ion species in planar DMS devices with different gap height. A number of examples will be presented illustrating peak capacity and speed for separations at fixed E/N ratios, as well as at the maximum E/N achievable for each differential mobility filter.