Active pixel sensors for mass spectrometry

Active pixel sensors (APS) are micro-fabricated CMOS amplifier arrays that are rapidly replacing CCD devices in many electronic imaging applications. Unlike the pixels of a CCD device, the sensing elements of the APS will respond to locally situated electrostatic charge, owing to the amplifier present in each pixel. We have built two small test arrays with microscopic aluminum electrodes integrated onto standard APS readout circuitry for the purpose of detecting low-energy gas-phase ions in mass spectrometers and other analytical instruments. The devices exhibit a near-linear dynamic range greater than four orders of magnitude, and a noise level of less than 100 electrons at room temperature. Data are presented for the response of the APS detectors to small ions in a miniature magnetic sector mass spectrometer and in an atmospheric pressure jet of helium. Data for individual highly-charged electrospray droplets are presented as well. Anticipated improvements suggest that in the near future APS ion detectors will posses noise levels approaching 10 electrons and will have a useful dynamic range over six orders of magnitude. (Int J Mass Spectrom 215 (2002) 101-111) (C) 2002 Elsevier Science B.V. All rights reserved.