Instant acquisition of high resolution mobility spectra in a differential mobility analyzer with 100 independent ion collectors: Instrument calibration

A parallel plate differential mobility analyzer (DMA) having 100 independent current collectors is calibrated to relate the axial distances L_n between the inlet slit and the detector position to the particle mobility Z at given voltage difference V and sheath gas flow rate Q. Calibrating species are tetraheptylammonium bromide clusters (THABr) and polyethylene glycol (PEG35k, 5 nm in diameter), generated by a bipolar electrospray source, and purified in a cylindrical DMA. Gaussian fitting of the raw discrete mobility spectra in the form of ion current I_n versus collector position L_n, I_n(L_n), yield the mean value L_o of the collector position maximizing the signal for a given ion. The many (Z,V,L_o) triads obtained at given Q from many different DMA voltages and standard mobilities collapse into a single 1/(Z_iV_j) vs L_o curve when slight adjustments are made to the Z_i. For different flow rates, Q/(Z_iV_j) vs. L_o curves collapse also, as long as the peaks are moderately narrow. However, for sufficiently small Q/Z, the THABr cluster peaks become broad, and the curves Q/(Z_iV_j) vs. L_o cease to collapse precisely. In contrast, the data for PEG show that this behavior is not a low-Q (Reynolds number) effect from the growth of the two lateral boundary layers, but is rather due to the broad and non-Gaussian peak shapes obtained at low Q or high Z. The calibration is accordingly unaffected by the Reynolds number. This simplicity was unexpected, given the three-dimensional flow in this DMA with growing lateral boundary layers.

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