Ions are produced from a conventional matrix-assisted laser desorption/ionization (MALDI) target by irradiation with a nitrogen laser pulsed at 20 Hz. After being cooled by collisions in an RF-quadrupole ion guide, the ions enter an orthogonal-injection TOF mass spectrometer, already used for electrospray. The collisional cooling spreads the ions out along the axis of the quadrupole to produce a quasi-continuous beam, which is then pulsed into the mass spectrometer at a repetition rate of about 4 kHz. Approximately five ions enter the mass spectrometer with each injection pulse, and these are detected using single-ion counting and registered in a TDC with 0.5 ns resolution.
The performance of the instrument is similar to that obtained with an ESI source. A uniform mass resolution of about 5000 (full width at half maximum definition) is routinely obtained for molecular weights up to about 6000 Da, with mass accuracy around 30 ppm. The sensitivity for peptides is in the low femtomole range. The mass range is currently limited by the low energy (5 keV) of the ions at the detector, although ions of cytochrome C (12 359 Da) have been detected.
The performance of the instrument for peptides is competitive with delayed-extraction MALDI in the usual axial geometry, but with the advantage of mass-independent focusing conditions, and a simple twopoint calibration procedure. However, the most important advantages result from the nearly complete decoupling of the ion production from the mass measurement. In the usual MALDI experiment the instrument must be carefully adjusted for optimum performance, and the optimum parameters depend on the matrix and the method of sample preparation. As a result of the decoupling, the performance of the instrument is independent of source conditions. This allows much greater flexibility to experiment with different matrices, different substrates (including insulating substrates), and different laser wavelengths, pulse widths and fluences.
Because of the decoupling, the design also allows convenient use of both ESI and MALDI sources (and possibly others) on the same spectrometer.