Laminar Flow Sublimation-Deposition Systems for Particle Growth with MALDI Matrices

Abstract

The aerosol time-of-flight mass spectrometer (ATOFMS), an important aerosol analysis instrument, uses laser desorption/ionization (LDI) to perform real-time single particle chemical analysis of aerosols. However, LDI is not compatible with larger biomolecules, which tend to excessively fragment when hit by the laser. A useful tool for analyzing these high molecular weight molecules is matrix assisted laser desorption/ionization (MALDI) mass spectrometry, a softer version of LDI that uses a laser-absorbing matrix to minimize fragmentation during the generation of ions. Currently, using MALDI techniques with the ATOFMS is challenging because there has not been a reliable way to coat particles with matrix material in-flight. The purpose of this work was to develop a laminar flow sublimation-deposition (S-D) system that would adequately prepare particles for the ATOFMS by coating them in matrix and growing them to optically detectable sizes. The S-D system was designed based on the theory behind condensation particle counters, where a heated region containing a packed bed of matrix (either ferulic acid or 2,4-dihydroxybenzoic acid) is followed by a cooled region, creating a zone of supersaturation where matrix molecules deposit onto single particles. To determine the thickness of the coatings, a tandem DMA system was used to analyze the size distributions of monodisperse potassium chloride (KCl) particles before and after entering the S-D system. Several variables affecting vapor uptake by the particles were examined, including the flow rate into the S-D system, the size of the particles entering the S-D system, the temperature of the saturation region, and the concentration of particles entering the S-D system. It was found that particle growth could easily be controlled by changing the temperature of the sublimation region, and at temperatures exceeding 80 °C, particles as small as 15 nm could be grown to nearly 300 nm, an optically detectable size that would increase ATOFMS ionization efficiency. Significant homogeneous nucleation of matrix vapor begins at sublimator temperatures around 90 °C, a factor that must be taken into consideration for future applications. The results of this study will enable the MALDI MS of bioaerosols using the ATOFMS, which could potentially be used for wide-ranging applications from virus detection to secondary organic aerosol analysis.