A new external ionization multisample MALDI source for Fourier transform mass spectrometry
Introduction
Matrix-assisted laser desorption/ionization (MALDI) [1] has proven to be a sensitive and reliable method for producing gas-phase oligosaccharide ions [2]. Neutral oligosaccharides typically coordinate alkali metal ions when ionized by MALDI to produce quasimolecular ions [2], [3]. The respective alkali-metal-coordinated species are readily produced by the addition of the appropriate alkali-metal chloride to the oligosaccharide solution. This method has been extensively used in this laboratory and elsewhere to produce various ionic species and to study the effect of alkali metals on fragmentation during ionization [2], [3]. It has also been used to obtain relative dissociation thresholds of alkali-metal ions coordinated to oligosaccharides [4], [5]. The method is facilitated by MALDI, which is not as sensitive to salts as, say for example, electrospray ionization.
Fourier transform mass spectrometry (FTMS) has long been an important tool for studying ion/molecule chemistry and specifically metal ion chemistry. The external source represented a major innovation in the advancement of (FTMS) as it allowed the coupling of various ionization sources including MALDI [6], [7], [8] to FTMS. When MALDI was coupled as an external source for FTMS, it provided a versatile tool for producing and analyzing metal coordinated species [8].
The determination of intrinsic properties such as the binding energies of metal ions coordinated to oligosaccharides is difficult due to the complexity of the system. For this reason, relative values are often obtained. However, reproducibility is often a problem with MALDI as various factors including, for example, the rate of crystallization, can affect the intensities of the ions. For this reason it is preferable to maintain specific conditions such as those pertaining to ionization and detection. The development of a multisample probe for external MALDI source would be of value in this regard as it allows several samples to be produced at once and introduced into the MALDI source simultaneously. This greatly facilitates the analysis and helps maintain the experimental conditions through the course of several experiments.
Most external source FTMS instruments rely on ion guides to transfer ions produced in a low vacuum source to an ultrahigh vacuum analyzer chamber to gain the high-resolution and high-mass-accuracy capabilities of FTMS. However, the position of the ion guide poses several technical problems. Although multipole ion guides such as quadrupole and octapole have large angular acceptance, MALDI produces sufficiently divergent beams that the entrance of the guide must be directly in front of the MALDI probe (Scheme 1). For multipole ion guides, the simplest and most effective orientation is to position the laser and the ion guide 90° relative to each other. The probe surface is made accessible to the laser beam by orienting it 45° to the incidence beam and 45° to the extractor electrode. Because of the relative orientation of the laser and the ion guide, the number of samples that can be accommodated in this design is severely limited. Time-of-flight (TOF) instruments do not need highly collimated ion beams. For this reason, the development of the multisample MALDI source for TOF instruments was relatively simple. Commercial TOF-MS instruments routinely come equipped with multisample probes. Recently, FTMS manufacturers have began offering multisample probes for external MALDI sources but these arguably lack the flexibility or the performance of the TOF sample probes.
In this article, we present a versatile design for an external ionization source that will accommodate multisample probes and even microchip-based separation devices. This design readily allows a large number of sample wells to be incorporated into a single probe. It will also be useful for analyzing separation devices that spread the sample over a relatively large surface area such as thin layer chromatography or gel electrophoresis that are currently not amenable to direct analysis. The multisample probe will not only benefit the study of metal coordinated species, but will generally allow the analyses of multiple samples to meet other needs such as the analysis of products from combinatorial libraries where dozens or hundreds of samples need to be analyzed in a relatively short period of time.
Section snippets
Experimental
The oligosaccharides and the MALDI matrix, 2,5-dihydroxybenzoic acid, were obtained from Sigma-Aldrich (St. Louis, MO). All compounds were used without additional purifications. Experiments were performed using a homebuilt external source FTMS described in detail in earlier publications [9], [10]. The instrument consists of two vacuum chambers (a dual chamber) with one dedicated to MALDI and another dedicated to electrospray ionization. The two chambers share a 5.2 T Oxford superconducting
Construction of new multisample ionization source for MALDI/FTMS
The MALDI source region of the instrument was modified from a single sample to a multiple sample configuration. The previous MALDI source consisted of an array of stainless steel lenses fashioned into a cube with the sample probe positioned in the middle of the cube and in front of the extractor plate (Scheme 1). The entire assembly was mounted on a six-inch flange. The flange was bolted to a six-inch vacuum cube. The probe was made from a stainless steel shaft and is removed through a
Conclusion
A simple and easily implemented multisample MALDI external source is described for a Fourier transform mass spectrometer. The ionization source is versatile and does not diminish the performance of the instrument relative to the single-sample device. On the contrary, the multisample probe adds additional capabilities that are well suited for ion/molecule reactions. Moreover, the probes will find wide applications in the analysis of biomolecules such as oligosaccharides. The probe heads are
Acknowledgements
Funding provided by the National Institute of Health and the National Science Foundation are gratefully acknowledged.
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