The Role of Aerosol Liquid Water in Droplet-Assisted Ionization Mass Spectrometry

Abstract

Chemical analysis of aerosols by mass spectrometry is challenging because aerosols contain little mass and have complex compositions. Consequently, relatively few approaches allow online molecular (i.e., minimal fragmentation) analysis on aerosols, particularly for ultrafine (<100 nm) particles. Droplet-assisted ionization (DAI) mass spectrometry is a promising and straightforward approach for aerosol molecular analysis. In DAI, liquid aerosol droplets are delivered directly to the mass spectrometer inlet, where the droplets break up and form molecular ions similar to those observed in electrospray ionization. This study explores how aerosol liquid water, serving as a matrix, controls ion generation in DAI for systems spanning simple binary and more complex ternary ones. The results demonstrate that ion yields are tightly coupled to the aerosol's hygroscopic response, leading to humidity- and phase-dependent ionization efficiencies. When the water to analyte molar ratio is less than ∼2, ion formation is relatively insensitive to water content, but once the matrix to analyte molar ratio exceeds ∼2, ion yield is highly sensitive to aerosol liquid water. These dependencies can be easily minimized by normalizing the environmental relative humidity experienced by the aerosol immediately before delivery to the MS inlet. This study enables a critical evaluation of the factors controlling ionization by this inlet-based approach and identifies experimental designs that will facilitate a more widespread implementation of DAI for aerosol chemical analysis in different application domains. Lastly, this work is an interesting example of how knowledge of aerosol physicochemical properties can answer questions about ionization mechanisms in mass spectrometry.

1

Experimental setup describing (a) generation of submicrometer aerosol through atomizing a solution, (b) RH manipulation of the aerosol flow, and (c) chemical and size distribution analysis of the RH-conditioned aerosol.

2

DAI-MS measured ion yields using Method I (red symbols) and Method II (blue symbols) to condition the RH, along with the corresponding MGF (lines) for a) angiotensin II, b) hydrocortisone, and c) ammonium sulfate. For angiotensin II and hydrocortisone, MGF was calculated using AIOMFAC. For ammonium sulfate, MGF was calculated using E-AIM.

3

Ion yield plotted against n _water/n _analyte for angiotensin II (blue), hydrocortisone (green), and ammonium sulfate (red). For all three systems, n _water/n _analyte was varied using Method I (i.e., a plume of liquid aerosol droplets was dried to a desired RH).

4

(a) Total ion yield and ion yields for both components of the equimolar ammonium sulfate-angiotensin II mixture aerosol as a function of RH. MGF (solid line) is plotted against the right y-axis. (b) Ion yields as a function of n _water/n _analyte for the same system. In panel (b), data for the corresponding binary systems (angiotensin II and ammonium sulfate) are also shown for comparison (open symbols).

5

Average charge state observed for angiotensin II, cytochrome C, and myoglobin in DAI mass spectra, as a function of RH.

6

Comparison between ion yields for RH-conditioned particles after supersaturation via CGC (filled symbols) and without (open symbols) for angiotensin II, hydrocortisone, ammonium sulfate, and the equimolar mixture of ammonium sulfate and angiotensin II. Error bars are smaller than the symbols.