Applications of laser desorption and electrospray ionization mass spectrometry at the transition between clusters and colloids.

The development of powerful new mass spectrometric techniques such as electrospray ionization (ESI)1 and matrix-assisted laser desorption ionization (MALDI)2 has been driven by the polymer and biological sciences. Workers in other areas, for whom conventional techniques are less useful, such as cluster chemistry, are now embracing these ionization methods. The mass spectrometric characterization of high-nuclearity metal carbonyl clusters using more traditional techniques is not trivial because of the high mass of clusters, their relative involatility, and their thermal sensitivity. Recently, ESI has been applied to cluster molecules.3 This technique provides a straightforward and superior determination of molecular weights for these compounds, in contrast to laser desorption ionization (LDI), which tends to give much more complicated spectra.4 In this communication we compare LDI and ESI mass spectrometry for measuring the molecular weights of large anionic osmium clusters. It has previously been shown that the laser desorption/ionization of neutral metal carbonyl clusters can lead to the generation of high molecular weight clusters and supraclusters.4 Similar phenomena have been observed using 252Cf plasma desorption5 and electron impact Fourier transform ion cyclotron resonance mass spectrometry.6 Nevertheless, we have found that LDI mass spectrometry may be used for molecular weight determination of anionic clusters. Here, little or no supraclustering is observed and the spectra consist mainly of peaks attributable to the parent ion and to fragment ions derived from the sequential loss of carbonyl ligands. Information unavailable routinely by other techniques, i.e., the molecular weight of a cluster directly from the solid phase and, in certain cases, a count of the carbonyl groups present (vide infra), is provided. To illustrate the use of this technique, the negative-ion LDI-TOF mass spectrum of [PPN]2[Os10C(CO)24] (PPN ) Ph3PNPPh3) at moderate laser power is shown in Figure 1.8 The highest mass peak at m/z 2588 corresponds to [Os10C(CO)24], and the peaks at lower mass correspond to the ions [Os10C(CO)n] (n ) 5-23). Higher laser power caused further CO stripping to give eventually the naked metal core, [Os10C], whereas lower laser power brought about an increase in the abundance of the molecular ion. CO loss fragments are always observed, even at threshold laser power. No doubly charged ions were observed for this sample or in any of the other samples studied. Electron loss from the cluster is an entirely expected reaction in the ion plume during ablation by the laser, and in our previous LDI studies of clusters4 we have only observed singly charged ions. The ESI mass spectrum of [Os10C(CO)24] at a low cone voltage shows a single envelope of peaks due to the intact, doubly † The University of York. ‡ The University of Cambridge. § The University of Edinburgh. (1) Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. Mass Spectrom. ReV. 1990, 9, 37. (2) (a) Hillenkamp, F.; Karas, M.; Beavis, R. C.; Chait, B. T. Anal. Chem. 1991, 63, 1193. (b) Muddiman, D. C.; Bakhtiar, R.; Hofstadler, S. A.; Smith, R. D. J. Chem. Educ. 1997, 74, 1288. (3) (a) Johnson, B. F. G.; McIndoe, J. S. Coord. Chem. ReV., in press. (b) Choi, Y.-Y.; Wong, W.-T. J. Chem. Soc., Dalton Trans. 1999, 331. (c) Henderson, W.; McCaffery, L. J.; Nicholson, B. K. Polyhedron 1998, 17, 4291. (d) Bryce, D. J. F.; Dyson, P. J.; Nicholson, B. K.; Parker, D. G. Polyhedron 1998, 17, 2899. (e) Henderson, W.; McIndoe, J. S.; Nicholson, B. K.; Dyson, P. J. J. Chem. Soc., Dalton Trans. 1998, 519. (f) Henderson, W.; McIndoe, J. S.; Nicholson, B. K.; Dyson, P. J. Chem. 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Soc. 1975, 93, 4808. (7) Jackson, P. F.; Johnson, B. F. G.; Lewis, J.; Nelson, W. J. H.; McPartlin, M. J. Chem. Soc., Dalton Trans. 1982, 2099. (8) Low-resolution LDI-TOF mass spectra were obtained using a Kratos Kompact MALDI-4 instrument in linear mode. High-resolution data were recorded on [PPN]2[Os10C(CO)24] using a TOFSpec2E instrument in reflectron mode. Dilute dichloromethane solutions of the clusters were evaporated directly onto the sample slide (no matrix was used). The laser power was typically modulated to maximize the signal-to-noise ratio. In all cases the machines were run in negative-ion mode. Figure 1. Negative-ion LDI-TOF mass spectrum from [PPN]2[Os10C(CO)24]. 2430 Inorg. Chem. 2000, 39, 2430-2431