Improving the Sensitivity of Matrix-Assisted Ionization (MAI) Mass Spectrometry Using Ammonium Salts

Abstract

Abstract. In matrix-assisted ionization (MAI), analyte incorporated in a small molecule matrix is introduced into an aperture linking atmospheric pressure with the vacuum of a mass spectrometer. Gas-phase analyte ions are spontaneously produced without use of a laser or high voltage. Here we investigate analyte and background ion abundances upon addition of ammonium salts to various MAI matrix/analyte solutions. Regardless of the ammonium salt or matrix used, chemical background ions are suppressed and/or analyte ion abundance improved for basic small molecules, peptides, and proteins. Background ion abundances increase with increasing inlet temperature, but are suppressed with addition of any of a variety of ammonium salts without much effect on the total ion abundances of the analyte ions. However, at lower inlet temperature using the matrices 2-bromo-2-nitropropane-1,3-diol, 1,2-dicyanobenzene, and 3-nitrobenzonitrile (3-NBN), analyte ion abundance increases and any chemical background decreases upon addition of ammonium salt. The improvement in sensitivity using 3-NBN with ammonium salt allows full acquisition mass spectra consuming as little as 1 fmol of ubiquitin. More complete peptide coverage for 100 fmol of a BSA tryptic digest, and increased sensitivity of drugs spiked in urine and saliva were observed after ammonium salt addition to the 3-NBN matrix.

High Sensitivity Analysis of Nanoliter Volumes of Volatile and Nonvolatile Compounds using Matrix Assisted Ionization (MAI) Mass Spectrometry

“…Assuming minimal analyte loss by adsorption to the syringe needle walls, less than 10 fmol of the analyte is used in this analysis, producing a mass spectrum of angiotensin II with low chemical background. For comparison with methods reporting LOD values [21,22], the LOD of Method 3 for angiotensin II was determined to be < 50 amol (Supplementary Figure S2). Based on these results, the sensitivity of Method 3 is similar to n-ESI, but the MAI approach is easier and faster.…”

“…The first MAI matrix discovered that does not require a heated inlet, and still one of the most useful, is 3-nitrobenzonitrile (3-NBN) [18,19]. This matrix is employed in the studies reported herein and provides high sensitivity, especially for basic and nonvolatile compounds such as peptides and drugs [21,22]. The charge states of ions observed using MAI are similar to electrospray ionization (ESI), allowing the use of any mass spectrometer having an atmospheric pressure inlet [20].…”

“…A number of approaches have been explored for introducing the MAI matrix:analyte sample to the inlet aperture of an atmospheric pressure mass spectrometer to initiate ionization [18,21,23]. Because the inlet becomes the ion source, the commercial ESI/atmospheric pressure chemical ionization (APCI) source housing is not necessary.…”

A broad‐based study on hyphenating new ionization technologies with MS/MS for PTMs and tissue characterization

“…By selecting appropriate matrices and/or matrix solvents, analysis of different classes of compounds can be optimized to enhance a particular analysis. Matrices can be of binary composition and introduced as solid, solution, or semidry [10,11,[19][20][21].…”

The composition of liquid atmospheric pressure matrix-assisted laser desorption/ionization matrices and its effect on ionization in mass spectrometry

“…These liquid matrices typically use solid MALDI matrices (chromophores) that absorb the laser energy and are dissolved in viscous support liquids with low volatility for extended analysis time. For both LSI and liquid AP-UV-MALDI the sensitivity was shown to be in the low-femtomole range [31,32]. In the latter method this sensitivity was achieved with extremely low sample consumption allowing hours of signal acquisition and enabling novel strategies such as sample storage for prolonged periods of time and subsequent reanalysis [33].…”