A microfabricated drift tube for differential mobility spectrometry (DMS) was used with pyrolysis-gas chromatography (py-GC) to chemically characterize bacteria through three-dimensional plots of ion intensity, compensation voltage from differential mobility spectra, and chromatographic retention time. The DMS analyzer provided chemical information for positive and negative ions simultaneously from chemical reactions between pyrolysis products in the GC effluent and reactant ions of H+(H2O)n and O2-(H2O)n in air at ambient pressure. Authentic standards for chemicals formed in the pyrolysis of bacteria showed favorable matches with plots from py-GC/DMS analysis and were supported by py-GC/MS results. These and other yet-unidentified constituents provided a means to distinguish Escherichia coli from Micrococcus luteus. A Gram-positive spore former (Bacillus megaterium) was distinguished by an abundant peak for crotonic acid evident in positive and negative ions and not observed with M. luteus. In contrast, plots from py-GC/DMS of lipid A and lipoteichoic acid showed poor matches to plots for a Gram-negative (E. coli) bacterium and a Gram-positive (M. luteus) bacterium and the differences were attributed to differences in genus sources of the biopolymers. A significant percentage of the chemical information available in py-GC/DMS is unidentified, and the analytical utility must be established. Precision in the chemical measurement was determined as +/- 0.2 V, 10% relative standard deviation (RSD), and +/- 0.05 min for compensation voltage, peak intensity, and retention time, respectively. The minimum number of total bacteria (cell forming units) detected was 6000 though detection limits and resolution could be varied by the magnitude of the separation voltage in the differential mobility spectrometer.

The performance of a planar differential mobility spectrometer (DMS) is investigated when operated in air at ambient pressure and driven by a rectangular asymmetric waveform, limited to frequencies of <1.2 MHz and voltage pulse amplitudes of <1 kV with steep rise times of the order of approximately 15 ns. Independent control of frequency, voltage pulse amplitude, and duty cycle allow for characterizing the DMS in terms of transmission, resolution and separation. The tradeoff between sensitivity and resolution and the effect of duty cycle on instrument performance are demonstrated experimentally. The dependence of ion mobility on the magnitude of the electric field determines the displacement of ions measured by the DC compensation voltage as a function of the duty cycle. Optimum values for the duty cycle exist for the separation of A- and C-type ions, while, B-type ions exhibit a more complex behavior. An analytical expression for describing the effect of duty cycle on the separation of the ions, determined by variations in the compensation voltage, is developed and compared to experimental results obtained in air below 75 Td using estimated alpha parameters for a set of ketones. In this context, errors associated with the calculation of alpha parameters using polynomials of even powers are highlighted.

No abstract

No abstract