ABSTRACT
The advantages of combining a vacuum MALDI source with a hybrid QIT/TOF MS are multifold. Vacuum MALDI does not suffer from low efciency in transporting ions from the atmosphere into the rst pumping stage of the instrument and should, therefore, appear as a highly sensitive ion source. The complication of the mass spectra and the reduction in ion intensities of the analyte molecules as a result of adduct formation with matrix ions is not observed at pressures below 1 Torr [8]. In parallel, the QIT/TOF MS has the potential of combining the diverse functions that can be executed with stored ions, and the high resolution, mass-accuracy, and speed of a TOF mass analyzer. The performance characteristics of a hybrid vacuum MALDI QIT/TOF MS are illustrated in the light of a number of applications, following a discussion of the obstacles encountered and a few practical solutions that emerged in developing the instrument.*
The instrument described in the present study is a modied vacuum MALDI QIT/ TOF manufactured by KRATOS Analytical (Manchester, UK), also known as the MALDI AXIMA QITTM. A description of the original system is available in the literature [9]. Here, the focus is primarily on (a) the modications made to the QIT to control variable pressure conditions; (b) ion injection efciency; (c) the restrictions imposed by the metastable character of ions generated by MALDI, and, nally; (d) ion ejection efciency and the properties of the TOF mass analyzer when fed from a QIT. A schematic diagram of the vacuum MALDI QIT/TOF MS is shown in Figure 19.1. The ion optical system for injecting ions in the QIT comprises a ‘pyramid mirror’ situated between two sets of axially symmetric electrodes, to which are applied appropriate potentials, forming two successive Einzel lenses. The pyramid mirror reects the laser beam (N2, 337 nm, 5 ns pulse) in the direction of the ion optical axis and also allows for optical access to the MALDI target by a CCD camera. The pressure in the ion source as measured by a hot cathode gauge is ca 10-4 mTorr.
The ions are focused through the electrodes using high-voltages (e.g., 5 keV). A weak electric eld is established on top of the target plate to push gently the ions through the dense MALDI plume.
