Tandem DMA Generation of Strictly Monomobile 1–3.5 nm Particle Standards

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Abstract
Generation of monomobile molecular standards by electrospray (ES) followed by classification in a differential mobility analyzer (DMA) fails at diameters above ∼2 nm because many clusters in different charge states z crowd in a narrow mobility range. Use of a second DMA (DMA2) in series (tandem) with DMA1 is very helpful because, unexpectedly, many multiply charged ions selected in DMA1 undergo spontaneous transitions, appearing as pure species at different mobilities in DMA2. Remarkably, for salt clusters of composition (CA) n (C+ ) z carrying z elementary charges and n neutral ion pairs, (i) ion evaporation (CA) n (C+ ) z →(CA) n –1(C+ ) z– 1+(CA)C+ and (ii) neutral evaporation transitions (CA) n (C+ ) z →(CA) n –1(C+ ) z+CA affect a substantial fraction of the clusters. Neutral evaporation (fueled by the Kelvin effect) is effective in isolating singly charged clusters, yielding mobility standards easily exceeding 2 nm. Ion evaporation (fueled by large electric fields) produces even larger well-resolved standards. Singly charged clusters of up to 2.5 nm rising in isolation result from metastable doubly charged parent ions (z = 2→1 transition). Isolated doubly charged ions of up to 3.5 nm arise from the z = 3→2 transition, but are harder to resolve from the products of higher initial charge states. We report tandem DMA measurements for electrosprayed nanodrops of two ionic liquids: EMI-Im and EMI-Methide, both based on the small cation EMI+ (1-Ethyl-3-methylimidazolium+) and two relatively large anions: Im− = (CF3SO2)2N−; Methide− = (CF3SO2)3C−. Some exploration on the effect of actively reducing the charge on the clusters as they pass between both analyzers is also included. Copyright 2013 American Association for Aerosol Research

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