Instant acquisition of high resolution mobility spectra in a differential mobility analyzer with 100 independent ion collectors: Instrument calibration
Luis J. Perez-Lorenzo, Raymond O’Mahony, Mario Amo‐González, Juan Fernández de la Mora
Abstract
A parallel plate differential mobility analyzer (DMA) having 100 independent current collectors is calibrated to relate the axial distances Ln 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 In versus collector position Ln, In(Ln), yield the mean value Lo of the collector position maximizing the signal for a given ion. The many (Z,V,Lo) triads obtained at given Q from many different DMA voltages and standard mobilities collapse into a single 1/(ZiVj) vs Lo curve when slight adjustments are made to the Zi. For different flow rates, Q/(ZiVj) vs. Lo 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/(ZiVj) vs. Lo 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.Copyright © 2020 American Association for Aerosol Research