A 20 kV orthogonal acceleration time-of-flight mass spectrometer for matrix-assisted laser desorption/ionization

A compact time-of-flight mass spectrometer has been constructed with the re flecting analyzer region orientated to be orthogonal to the desorption axis of a matrix-assisted laser desorption/ionization source. The instrument is designed with a detector that is large enough to accommodate a correspondi ngly large desorption velocity spread without the need for collisional cool ing. Laser desorbed positive ions from a positively biased probe enter a fi eld free fill region of an orthogonal accelerator, initially at ground pote ntial. The ions are then orthogonally accelerated to reach a final potentia l of -20 kV and a kinetic energy of similar to 21 keV. The space focus of t he reflectron geometry is adjusted to lie in the ion mirror to allow easy c onversion to the linear mode. The detector of the current configuration is a 70 mm diameter microsphere plate. The spectrometer has a total length of less than I m and, in good agreement with simulations, it provides a typica l resolution of 8000 (full-width at half-maximum). Detection limits determi ned with similar to 10-50 laser shots were in the low femtomole range up to m/z similar to 10000. Beyond this limit, sensitivity appears to be lowered by decreasing detector efficiency and the increasing velocity spread of th e desorbed ions. Mass calibration of the instrument is very simple and repr oducible, Measured mass accuracy with external calibration is better than 1 00 ppm over several days, Limitations in mass accuracy are attributed to th e drift of the power supplies and timing jitter. A focusing method for impr oving high mass sensitivity by a factor of typically 20 is briefly describe d and demonstrated with the detection of molecular ions of myoglobin (m/z s imilar to 17000). (C) 2001 Elsevier Science B.V.