Aluminium junctions as macromolecule detectors and comparison with ionizing detectors

Cryogenic particle detectors have been proposed as alternative detectors for heavy molecule mass spectrometry [1]. The concept has been demonstrated by various groups in the last 3 years [2–10]. The single-molecule detection efficiency is independent of mass and close to the geometrical beam coverage. In addition, the charged state of the macromolecule can be determined by the pulse height of the cryodetector signal [7]. In this work, we present results from aluminium junctions (50 μm×400 μm) cooled by a dilution refrigerator. The Al-junction detectors were exposed to 6 keV X-rays from a 55Fe source and to a variety of macromolecules, including IgG and polyethylenglycols (PEGs). Identical samples have been measured with a conventional secondary electron multiplier detector, allowing a sensitivity comparison. Our data, combined with measurements from other groups, suggest that the MALDI extraction does not affect too much the sample mass distribution, and that secondary electron detector sensitivity decreases with decreasing molecule velocity as exp(−(v/v0)3.5) with v0 being of the order of 50 km/s.