Extending the Laserspray Ionization Concept to Produce Highly Charged Ions at High Vacuum on a Time-of-Flight Mass Analyzer

Abstract

A new matrix compound, 2-nitrophloroglucinol, is reported which not only produces highly charged ions similar to electrospray ionization (ESI) under atmospheric pressure (AP) and intermediate pressure (IP) laserspray ionization (LSI) conditions but also the most highly charged ions so far observed for small proteins in mass spectrometry (MS) under high vacuum (HV) conditions. This new matrix extends the compounds that can successfully be employed as matrixes with LSI, as demonstrated on an LTQ Velos (Thermo) at AP, a matrix-assisted laser desorption/ionization (MALDI)-ion mobility spectrometry (IMS) time-of-flight (TOF) SYNAPT G2 (Waters) at IP, and MALDI-TOF Ultraflex, UltrafleXtreme, and Autoflex Speed (Bruker) mass spectrometers at HV. Measurements show that stable multiple charged molecular ions of proteins are formed under all pressure conditions indicating softer ionization than MALDI, which suffers a high degree of metastable fragmentation when multiply charged ions are produced. An important analytical advantage of this new LSI matrix are the potential for high sensitivity equivalent or better than AP-LSI and vacuum MALDI and the potential for enhanced mass selected fragmentation of the abundant highly charged protein ions. A second new LSI matrix, 4,6-dinitropyrogallol, produces abundant multiply charged ions at AP but not under HV conditions. The differences in these similar compounds ability to produce multiply charged ions under HV conditions is believed to be related to their relative ability to evaporate from charged matrix/analyte clusters.