SIMULTANEOUS MEASUREMENT OF FLIGHT TIME AND ENERGY OF LARGE MATRIX-ASSISTED LASER-DESORPTION IONIZATION IONS WITH A SUPERCONDUCTING TUNNEL JUNCTION DETECTOR

We evaluated a cryogenically cooled superconducting Nb-Al2O3-Nb tunnel junction (STJ) for use as a molecular ion detector in a matrix-assist ed laser desorption ionization time-of-flight (MALDI-TOF) mass spectro meter. The STJ responds to ion energy and theoretically should detect large molecular ions with a velocity-independent efficiency approachin g 100%. The STJ detector produces pulses whose heights are approximate ly proportional to ion energy, thus the height of a pulse generated by tile impact of a doubly charged ion is about twice the height of a si ngly charged ion pulse. Measurements were performed by bombarding the STJ with human serum albumin (HSA) (66,000 Da) and immunoglobulin (150 ,000 Da) ions. We demonstrate that pulse height analysis of STJ signal s provides a way to distinguish with good discrimination HSA(+) from 2 HSA(2+), whose night times ape coincident. The rise time of STJ detect or pulses allows ion flight times to be determined with a precision be tter than 200 ns, which is a value smaller than the night time variati on typically observed for large isobaric MALDI ions detected with conv entional microchannel plate (MCP) detectors. Deflection plates in the flight tube of the mass spectrometer provided a way to aim ions altern atively at a MCP ion detector. (C) 1997 American Society for Mass Spec trometry.