High-efficiency Detection of 66 000 Da Protein Molecules Using a Cryogenic Detector in a Matrix-assisted Laser Desorption/Ionization Time-of-flight Mass Spectrometer

Abstract

We present the first experimental results obtained using a cryogenically-cooled Nb–Al2O3–Nb superconductor–insulator–superconductor (SIS) tunnel junction detector operating at 1.3 K as an ion detector in a time-of-flight mass spectrometer. As opposed to microchannel-plate ion detectors (MCPs) commonly used in such systems, cryogenic detectors such as SIS detectors offer a near 100% detection efficiency for all ions including single, very massive, slow-moving macromolecules. We describe the operating principle of an SIS detector and its use as an ion detector in our matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometer and compare its response to an MCP detector operated in the same system. To our knowledge, this is the first direct comparison of these detector types in this application. A comparison of count rates and time-of-flight spectra obtained with both detectors for human serum albumin (molecular weight 66 000 Da) indicates a two to three orders of magnitude higher detection efficiency per unit area for the SIS detector at this mass. For higher molecular masses we expect an even higher relative efficiency for cryogenic detectors since MCPs show a rapid decline in detection efficiency as ion mass increases, which is not expected to be the case for cryogenic detectors. Our results imply that time-of-flight techniques could be extended beyond the current upper mass limit if cryogenic detectors are used. Initially, cryogenic detectors will be used for the analysis of large protein molecules. If non-fragmenting ionization techniques can be perfected, cryogenic detectors will also open the possibility of the rapid analysis of large DNA molecules and perhaps intact microorganisms.