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-sensiti ve FET impedance converter/amplifier The resulting staircase signals a re differentiated by a bandpass filter which can be optimized with res pect 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 r atio of 2. In the low mass range the detector requires a threshold las er 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 e qual to that of the other detectors. With a rise time of 25 ns, the ma ss resolution for MALDI ion detection in the mid and high mass range i s not limited by the detector. The influence of secondary ion generati on at the collector electrode is also reported.