A charge detector for time-of-flight mass analysis of high mass ions produced by matrix-assisted laser desorption/ionization (MALDI)

A novel detector for ion detection in MALDI time-of-flight instruments is described. It comprises a hat 18 mm diameter metal electrode as a Faraday charge collector followed by a fast, low noise, charge-sensitive FET impedance converter/amplifier The resulting staircase signals are differentiated by a bandpass filter which can be optimized with respect to rise/fall time and electronic noise. A net charge, equivalent to 1.8 x 10(4) elementary charges, is required for a signal-to-noise ratio of 2. In the low mass range the detector requires a threshold laser intensity for the detection of MALDI analyte ions of about a factor of 1.5 above that needed if a standard secondary electron detector is used. Above an ion mass of approximately 30 kDa, its sensitivity is equal to that of the other detectors. With a rise time of 25 ns, the mass resolution for MALDI ion detection in the mid and high mass range is not limited by the detector. The influence of secondary ion generation at the collector electrode is also reported.