Orthogonal injection of matrix-assisted laser desorption/ionization ions into a time-of-flight spectrometer through a collisional damping interface

Abstract

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. # 1998 John Wiley & Sons, Ltd. Received 26 February 1998; Revised 16 March 1998; Accepted 17 March 1998 Over the last ten years mass spectrometry has emerged as a major analytical tool in biotechnology and biochemistry. This is mainly a result of the invention of two new ionization techniques for large and/or thermally labile biomolecules: electrospray ionization (ESI) and matrixassisted laser desorption/ionization (MALDI).1 The techniques have complementary advantages and disadvantages, so it is useful to have both available. However, the ion beams produced by the two methods have very different characteristics, and consequently it has been necessary to examine them in different types of mass analyzer. ESI produces a continuous beam of ions. Like other continuous ion sources, it is more compatible with mass spectrometers that operate in a similarly continuous fashion, such as quadrupole mass filters. On the other ha...

Ganglioside analysis by thin-layer chromatography matrix-assisted laser desorption/ionization orthogonal time-of-flight mass spectrometry

“…In addition to the pressure of the background gas and extraction field strength in the MALDI technique, the major factors affecting metastable fragmentation are laser wavelength/pulse width and the choice of the MALDI matrix [6,7]. In addition, the presence of electrons may play a role in analyte fragmentation and may cause suppression of multiply charged ions in a spectrum [8,9].Control of the metastable fragmentation in MALDI by increasing the pressure in the ionization source and reduction of the ions internal energy was first introduced by Loboda et al on orthogonal time-of-flight (oTOF) instruments [10] and later extended to Fourier transform mass spectrometry (FTMS) [11] and to the operation at atmospheric pressure on various types of instruments [12]. These methods have since been used for stabilization of glycolipids [13,14] and peptides and proteins [11,15].…”

“…Control of the metastable fragmentation in MALDI by increasing the pressure in the ionization source and reduction of the ions internal energy was first introduced by Loboda et al on orthogonal time-of-flight (oTOF) instruments [10] and later extended to Fourier transform mass spectrometry (FTMS) [11] and to the operation at atmospheric pressure on various types of instruments [12]. These methods have since been used for stabilization of glycolipids [13,14] and peptides and proteins [11,15].…”

“…Introduction of the collisional cooling source in the prOTOF 2000 instrument allowed for stabilization of the fragile molecules on desorption from the MALDI target [10,21,22]. The time the ions spend in the instrument between desorption from the target and detection is typically several milliseconds, which is several orders of magnitude longer than the time of TOF analysis, and, therefore, metastable fragmentation under conditions of low ion source gas pressure is apparent, and stabilization of the ions becomes essential for their detection.…”

Vacuum compatible sample positioning device for matrix assisted laser desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry imaging

“…Although 0.1 μm positioning accuracy might seem somewhat excessive for the purposes, recent developments in laser optics within mass spectrometry community [12][13][14][15][16] allows acquiring MALDI with submicron laser spot diameters. Clearly, for such small areas, laser power must be significantly increased causing excessive fragmentation, yet vibrational cooling [32][33][34][35][36][37][38] should prove itself useful to reduce such fragmentation.…”

A novel high-capacity ion trap-quadrupole tandem mass spectrometer

“…As ions collide with molecules of buffer gas they gradually loose their kinetic energy and accumulate at the bottom of the potential well [59,60]. Since the force along the z-coordinate is negligible at the bottom of the well, ions will eventually spread along the entire z-axis under the influence of the repulsive space charge forces between ions in the trap.…”