Atmospheric Pressure Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry

TLDR

The oaTOF mass spectrometer struggles to evaluate ionization of larger proteins due to limitations with singly charged high‑mass ions. The study introduces a novel ionization source that merges atmospheric pressure ionization with MALDI for biological mass spectrometry. PA‑AP MALDI pneumatically transfers ions from atmospheric pressure to high vacuum using a nitrogen stream, is interchangeable with electrospray on an oaTOF MS, employs standard MALDI matrices, and its performance is benchmarked against conventional vacuum MALDI‑TOF. PA‑AP MALDI detects low femtomole peptides with good signal‑to‑noise, produces minimal metastable fragmentation, yields diagnostic fragments for oligosaccharides, consumes more sample due to inefficient ion transfer, and generates protonated small protein ions that cluster with matrix.

Abstract

A novel ionization source for biological mass spectrometry is described that combines atmospheric pressure (AP) ionization and matrix-assisted laser desorption/ionization (MALDI). The transfer of the ions from the atmospheric pressure ionization region to the high vacuum is pneumatically assisted (PA) by a stream of nitrogen, hence the acronym PA-AP MALDI. PA-AP MALDI is readily interchangeable with electrospray ionization on an orthogonal acceleration time-of-flight (oaTOF) mass spectrometer. Sample preparation is identical to that for conventional vacuum MALDI and uses the same matrix compounds, such as α-cyano-4-hydroxycinnamic acid. The performance of this ion source on the oaTOF mass spectrometer is compared with that of conventional vacuum MALDI-TOF for the analysis of peptides. PA-AP MALDI can detect low femtomole amounts of peptides in mixtures with good signal-to-noise ratio and with less discrimination for the detection of individual peptides in a protein digest. Peptide ions produced by this method generally exhibit no metastable fragmentation, whereas an oligosaccharide ionized by PA-AP MALDI shows several structurally diagnostic fragment ions. Total sample consumption is higher for PA-AP MALDI than for vacuum MALDI, as the transfer of ions into the vacuum system is relatively inefficient. This ionization method is able to produce protonated molecular ions for small proteins such as insulin, but these tend to form clusters with the matrix material. Limitations of the oaTOF mass spectrometer for singly charged high-mass ions make it difficult to evaluate the ionization of larger proteins.