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

Abstract

A compact time-of-flight mass spectrometer has been constructed with the reflecting 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 correspondingly large desorption velocity spread without the need for collisional cooling. Laser desorbed positive ions from a positively biased probe enter a field free fill region of an orthogonal accelerator, initially at ground potential. The ions are then orthogonally accelerated to reach a final potential of −20 kV and a kinetic energy of ∼21 keV. The space focus of the reflectron geometry is adjusted to lie in the ion mirror to allow easy conversion 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 1 m and, in good agreement with simulations, it provides a typical resolution of 8000 (full-width at half-maximum). Detection limits determined with ∼10–50 laser shots were in the low femtomole range up to m/z ∼ 10000. Beyond this limit, sensitivity appears to be lowered by decreasing detector efficiency and the increasing velocity spread of the desorbed ions. Mass calibration of the instrument is very simple and reproducible. Measured mass accuracy with external calibration is better than 100 ppm over several days. Limitations in mass accuracy are attributed to the drift of the power supplies and timing jitter. A focusing method for improving high mass sensitivity by a factor of typically 20 is briefly described and demonstrated with the detection of molecular ions of myoglobin (m/z ∼ 17000).