Matrix assisted laser desorption/ionization orthogonal acceleration time-of-flight mass spectrometry: development and characterization of a new instrument

The performance of a linear matrix assisted laser desorption/ionization massspectrometer (MALDI-oa-TOFMS) was improved with more reproducible samplepreparation methods, a higher rate digitiser for integrating signals and customisablecomputer control, data acquisition and analysis in the LabVIEW?programmingenvironment. This resulted in a ~20% improvement in resolution (up to 4,400) andenabled measurement of desorption velocities of 1,000 - 1,800 ms-1 for analytes withm/z 615 ?1,350 Da, with matrix ion velocities being 4,000 ?4,800 ms?. Detectorlimitations and restrictions on source axis energy (and hence velocity) required for theanalysis of ions prevented detection of other species with this instrument.A 20 kV reflecting geometry MALDI-oa-TOFMS was constructed to overcome theselimitations and extend the mass range. This mass spectrometer was able to analyse ionsdesorbed with a wide range of energies (32 ?197 eV). The resolution was found to be8,000 -10,000. Best mass accuracy was 15-80 ppm (internal standards ). Externalcalibration gave larger mass errors, mostly due to timing jitter, but the mass axis wasstable for &lt2 weeks. Mass accuracy was independent of the analyte and matrix used.Ions with m/z of ~10,000 - 20,000 Da were observable with the use of a pulsed lens inthe target region. This lens increased signal approximately 20 times, but degradedresolution. The detection limit of the instrument (sample consumed) was estimated tovary from 10 ?90 fmol, by extrapolation, with more moles required at higher m/z.The microsphere plate (MSP) electron multiplier used in the reflecting instrument wasfound to have a temporal response of &gt1 ns FWHM, but with a low secondary electronconversion efficiency, making it unsuitable for high m/z species. Experiments were alsoperformed with a novel rectangular mesh grid, which (in correct orientation) providedsimilar resolution to conventional square mesh grids, but with significantly improvedtransmission and hence sensitivity