A tandem DMA system for detailed balance studies of cluster decomposition kinetics: Experimental setup and quantification of spectra

We describe an experimental apparatus able to study the decay rate of small clusters Cn+→Cn-1++C with single molecule resolution. It is based on a tandem differential mobility analyzer (TDMA) including two high-resolution planar DMAs separated by a heated tubular reactor. The downstream electrode of the second DMA (DMA2) has 256 electrical detectors rather than a single outlet slit, such that high resolution full DMA2 mobility spectra may be obtained in real time as DMA1 is scanned. The high transmission of planar DMAs permits operating at sample flow rates as small as 0.6 L/min, resulting in fully developed flow over most of the reaction tube, enabling efficient theoretical conversion of experimental parent/product abundance ratios into reaction rates. The operation of the system is illustrated with electrosprayed salt clusters [(C+A−)nC+z]+z, which naturally decay through either neutral evaporation: [(C+A−)nC+z]+z→[(C+A−)n-1C+z]+z + [C+A−], or ion evaporation: [(C+A−)nC+z]+z→[(C+A−)n-pC+z-1]+z-1+ [C+A−]pC+ (p = 0 or 1). Quantification of the surviving parents is complicated by the fact that their mobilities Z1 and Z2 before and after the reaction tube are the same, while the diagonal line Z1=Z2 in a TDMA spectrum is often contaminated with doubly charged clusters. A quantification approach for this diagonal region is nevertheless demonstrated in the case of charge-reduced clusters for which z = 1 species dominate. The considerable drift of the zero of the 256 electrometer circuit is reduced by periodically gating the aerosol exiting the reactor, resulting in noise levels below 1 fA.Copyright © 2025 American Association for Aerosol Research