New Ionization Method for Analysis on Atmospheric Pressure Ionization Mass Spectrometers Requiring Only Vacuum and Matrix Assistance

Matrix assisted ionization <i>vacuum</i> (MAIV) is a new ionization method that does not require high voltages, a laser beam, or applied heat and depends only the proper matrix, 3-nitrobenzonitrile (3-NBN), and the vacuum of the mass spectrometer to initiate ionization. Analyte ions of volatile as well as nonvolatile compounds are formed by simply exposing the matrix–analyte to the vacuum of a mass spectrometer. The reduced pressure at the inlet of an atmospheric pressure ionization mass spectrometer suffices to produce analyte ions, but unlike the previously reported matrix assisted ionization <i>inlet</i> method, with MAIV, heating the inlet is not necessary. Singly and multiply charged ions are formed similar to electrospray ionization but from a surface. Mass spectrometers in which a heated inlet tube is not available can be used for ionization using the 3-NBN matrix. We demonstrate rapid, high-sensitivity analyses of drugs, peptides, and proteins in the low femtomole range. The potential for high-throughput analyses is shown using multiwell plates and paper strips

Laserspray Ionization Imaging of Multiply Charged Ions Using a Commercial Vacuum MALDI Ion Source

This is the first report of imaging mass spectrometry (MS) from multiply charged ions at vacuum. Laserspray ionization (LSI) was recently extended to applications at vacuum producing electrospray ionization-like multiply charged ions directly from surfaces using a commercial intermediate pressure matrix-assisted laser desorption/ionization ion mobility spectrometry (IMS) MS instrument. Here, we developed a strategy to image multiply charged peptide ions. This is achieved by the use of 2-nitrophloroglucinol as matrix for spray deposition onto the tissue section and implementation of “soft” acquisition conditions including lower laser power and ion accelerating voltages similar to electrospray ionization-like conditions. Sufficient ion abundance is generated by the vacuum LSI method to employ IMS separation in imaging multiply charged ions obtained on a commercial mass spectrometer ion source without physical instrument modifications using the laser in the commercially available reflection geometry alignment. IMS gas-phase separation reduces the complexity of the ion signal from the tissue, especially for multiply charged relative to abundant singly charged ions from tissue lipids. We show examples of LSI tissue imaging from charge state +2 of three endogenous peptides consisting of between 1 and 16 amino acid residues from the acetylated <i>N</i>-terminal end of myelin basic protein: mass-to-charge (<i>m</i>/<i>z</i>) 795.81 (+2) molecular weight (MW) 1589.6, <i>m</i>/<i>z</i> 831.35 (+2) MW 1660.7, and <i>m</i>/<i>z</i> 917.40 (+2) MW 1832.8

Transmission Geometry Laserspray Ionization <i>Vacuum</i> Using an Atmospheric Pressure Inlet

This represents the first report of laserspray ionization <i>vacuum</i> (LSIV) with operation directly from atmospheric pressure for use in mass spectrometry. Two different types of electrospray ionization source inlets were converted to LSIV sources by equipping the entrance of the atmospheric pressure inlet aperture with a customized cone that is sealed with a removable glass plate holding the matrix/analyte sample. A laser aligned in transmission geometry (at 180° relative to the inlet) ablates the matrix/analyte sample deposited on the vacuum side of the glass slide. Laser ablation from vacuum requires lower inlet temperature relative to laser ablation at atmospheric pressure. However, higher inlet temperature is required for high-mass analytes, for example, α-chymotrypsinogen (25.6 kDa). Labile compounds such as gangliosides and cardiolipins are detected in the negative ion mode directly from mouse brain tissue as intact doubly deprotonated ions. Multiple charging enhances the ion mobility spectrometry separation of ions derived from complex tissue samples