Selection and generation of waveforms for differential mobility spectrometry

Abstract

Devices based on differential mobility spectrometry (DMS) are used in a number of ways, including applications as ion prefilters for API-MS systems, as detectors or selectors in hybrid instruments (GC-DMS, DMS-IMS), and in standalone systems for chemical detection and identification. DMS ion separation is based on the relative difference between high field and low field ion mobility known as the alpha dependence, and requires the application of an intense asymmetric electric field known as the DMS separation field, typically in the megahertz frequency range. DMS performance depends on the waveform and on the magnitude of this separation field. In this paper, we analyze the relationship between separation waveform and DMS resolution and consider feasible separation field generators. We examine ideal and practical DMS separation field waveforms and discuss separation field generator circuit types and their implementations. To facilitate optimization of the generator designs, we present a set of relations that connect ion alpha dependence to DMS separation fields. Using these relationships we evaluate the DMS separation power of common generator types as a function of their waveform parameters. Optimal waveforms for the major types of DMS separation generators are determined for ions with various alpha dependences. These calculations are validated by comparison with experimental data.

Figure 1

Implementation of the SV generators. (1) Pulse amplifier, PA; (2) Flyback generator, FB; (3) Two-harmonics generator, H2 (a) inductive coupling and (b) capacitive coupling.

Figure 2

Coupling coefficient as a function of the initial resonant frequencies ratio.

Figure 3

Variation of the waveform shape with form parameter for different SV generators (a) ideal rectangular, IR; (b) flyback, FB; (c) two harmonics, H2, (zero phase shift); (d) H2, phase shift (0.66 form parameter).

Figure 4

Examples of the three main types of alpha functions behavior with field: monotonic increasing alpha-A (positive reactant ion); monotonic decreasing alpha-C (estradiol sulfate), and max-type alpha-B (glutamic acid).

Figure 5

Dispersion plots, C(S), for different SV generators and alpha functions: (a) monotonic increasing alpha-A (positive reactant ion); (b) max-type alpha-B (glutamic acid) and (c) monotonic decreasing alpha-C (estradiol sulfate).

Figure 6

Compensation field vs form parameter for different SV generators and alpha functions: (a) monotonic increasing alpha-A (positive reactant ion); (b) max-type alpha-B (glutamic acid) and (c) monotonic decreasing alpha-C (estradiol sulfate).

Figure 7

DMS peak position vs form parameter for different alpha functions and SV generators: (a) IR; (b) FB; (c) H2.

Figure 8

Dependence of positive reactant ion peak position on FB waveform form parameter. DMS spectra are in the upper insertion and DMS waveforms are in the lower insertion.

Figure 9

Compensation voltage for two pentanone plotted against duty cycle for a real PA generator. Experimental data are from Ref. . Theoretical curve obtained with the algorithm proposed in this article.

Figure 10

Experimental data and theory for (a) CV vs H2 harmonics ratio; (b) CV vs H2 harmonics phase shift.