Ion detection with a cryogenic detector compared to a microchannel plate detector in MALDI TOF-MS

Detection of molecular ions in mass spectrometry is typically accomplished by an ion colliding with a surface and then amplifying the emitted secondary electrons. It is well established that the secondary electron yield decreases as the mass of the primary ion increases [1-3], thus limiting the detection efficiency of large molecular ions. One way around this limitation is to use secondary ion detectors because the emission efficiency of secondary ions does not seem to decrease for increasing primary ion mass [1]. However this technique has limitations in timing resolution because of the mass spread of the emitted secondary ions. To find other ways around high mass detection limitations it is important to understand existing mechanisms of detection and to explore alternative detector types. To this end, a superconducting tunnel junction (STJ) detector was used in measuring the secondary electron emission efficiency, se, for a MCP detector. STJ detectors are energy sensitive and do not rely on secondary emission to produce a signal. Using a linear MALDI-TOF mass spectrometer, a STJ detector is mounted directly behind the hole in an annular MCP detector. This mounting arrangement allows ions to be detected simultaneously by each detector. The STJ detector sits in a liquid helium cryostat and is operated at 1.3 K to minimize thermal noise (see [4,5] for more details). Primary ions passing through the center hole of the MCP detector collide with the 0.04 mm{sup 2} STJ surface and generate a detector-pulse that is approximately proportional to the ion's total energy. A mask with a small hole in it was placed in front of the MCP detector so that the MCP and STJ detectors have approximately the same effective active areas. The ion beam diameter near the MCP is over 2.5 cm (measured with a MCP-phosphorus screen detector) and the axial separation of the two detectors is about 4 mm. Both detectors were operated in pulse-counting mode and set to have the same effective deadtime