Flow interface for charge-reduced electrospray of nanoparticle solutions.

A charge reduction (CR) interface for electrospray ionization that permits simultaneous analysis of nanoparticle solutions by multiple detection methods was characterized. In the direct infusion configuration, a constant flow of analyte solution undergoes electrospray ionization (ESI). The charged aerosol is sampled directly into the atmospheric pressure inlet of a quadrupole time-of-flight mass spectrometer and into a CR device followed by a differential mobility analyzer (DMA) and condensation particle counter (CPC). In the plug injection configuration, analyte solution is injected into a liquid chromatograph. The effluent is split to an evaporative light scattering detector (ELSD) and the ESI interface. The charged aerosol is then sampled through the CR device directly into the CPC. Performance characteristics of the two configurations were studied with sucrose and protein solutions. When a liquid flow rate in the low microliter per minute range was used, the reconstructed droplet size distribution from the ESI interface had an average diameter of 184 nm with a geometric standard deviation of 1.4. For the first configuration, the linear working range was wider for ESI-MS than the CR device-DMA-CPC. For the second configuration, the detection efficiency, defined as the fraction of molecules flowing through the ESI interface that are ultimately detected by the CPC, was on the order of 10(-6). Simultaneous measurements with the ELSD and CPC were consistent with analyte molecular size and may provide a means of estimating the size of unknown particles.