ORTHOGONAL INJECTION OF MATRIX-ASSISTED-LASER-DESORPTION IONIZATION IONS INTO A TIME-OF-FLIGHT SPECTROMETER THROUGH A COLLISIONAL DAMPING INTERFACE/

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 gu ide, the ions enter an orthogonal-injection TOF mass spectrometer, alr eady used for electrospray, The collisional cooling spreads the ions o ut 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 w ith each injection pulse, and these are detected using single-ion coun ting and registered in a TDC with 0.5 ns resolution. The performance o f the instrument is similar to that obtained with an ESI source. A uni form mass resolution of about 5000 (full width at half maximum definit ion) is routinely obtained for molecular weights up to about 6000 Da, with mass accuracy around 30 ppm. The sensitivity for peptides is in t he low femtomole range. The mass range is currently limited by the low energy (5 keV) of the ions at the detector, although ions of cytochro me C (12359 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 c onditions, and a simple two-point calibration procedure. However, the most important advantages result from the nearly complete decoupling o f the ion production from the mass measurement. In the usual MALDI exp eriment the instrument must be carefully adjusted for optimum performa nce, 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 s ubstrates (including insulating substrates), and different laser wavel engths, pulse widths and fluences. Because of the decoupling, the desi gn also allows convenient use of both ESI and MALDI sources (and possi bly others) on the same spectrometer. (C) 1998 John Wiley & Sons, Ltd.