Flow Interface for Charge-Reduced Electrospray of Nanoparticle Solutions

Abstract

A charge reduction (CR) interface for electrospray ionization was characterized that permits simultaneous analysis of nanoparticle solutions by multiple detection methods. 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 (QTOF) and to 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 µL/min 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 CR-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 ELSD and CPC were consistent with analyte molecular size and may provide a means of estimating the size of unknown particles.

Targeted Absolute Quantification of Intact Proteins by Reversed Phase Liquid Chromatography-Mass Spectrometry, Charge Reduced Electrospray, and Condensation Particle Counting

“…Beyond the upper point of linearity, most of the aerosol particles detected by the CPC contain more than one molecule (association of molecules in the liquid phase at high concentrations). Compared to our previous results, the observed linear range is smaller. One possible explanation could be based on the spray solvent used here.…”

“…For each solution, an aliquot of 10 μL was injected and analyzed under the conditions described in the Experimental Section. The time-dependent CPC raw data were processed as described before . From a plot of molecules detected as a function of cytochrome C concentration, it was found that CPC response was linear from 0.04 to 0.8 μM.…”

“…The entire HPLC effluent, at a rate of 40 μL/min, was directed to the ESI source. The same ESI aerosol was sampled simultaneously by both analytical instruments as described previously . Aerosol sampling into the CPC was performed approximately 5 mm from the axis of the spray tip and 15 mm orthogonally to the aerosol plume.…”

“…On the basis of these results, the authors disqualified the CPC as a good quantitative detector for RPLC analysis of protein mixtures. While the work described here also involves the use of RPLC, CRES, and CPC, it does heavily rely on MS-generated molecular data and the concept of CPC detection efficiency . From a set of model proteins, it is shown that, under CRES conditions, CPC detection efficiency is linearly related to the square root of protein molecular weight and that CPC-generated number concentrations may be used for the absolute quantification of nearly any protein in liquid solutions.…”

“…While the work described here also involves the use of RPLC, CRES, and CPC, it does heavily rely on MS-generated molecular data and the concept of CPC detection efficiency. 33 From a set of model proteins, it is shown that, under CRES conditions, CPC detection efficiency is linearly related to the square root of protein molecular weight and that CPC-generated number concentrations may be used for the absolute quantification of nearly any protein in liquid solutions. For globular proteins, application of this procedure to the absolute quantification of cytochrome C in a commercial HPLC protein mixture is demonstrated.…”