Mass spectrometry is a major growth area in scientific instrumentation utilized in the chemical, biomedical, materials and earth sciences. In general, samples for a mass spectrometer have to be vaporized to allow ionization to take place. 1-5 Kistemaker and co-workers, were the first to demonstrate, using a laser microprobe. the usefulness of laser desorption as an ionization technique for thermally labile high-mass compounds.Laser desorption has been differentiated into the separate ionization techniques referred to as matrix-assisted laser desorytion and ionization (MALDI),' laser desorption with post-ionization,' substrate-assisted laser d e s~r p t i o n ,~ surfaceenhanced neat desorption (SEND)" and ultrafine metal powder laser desorption.' ' Other techniques, often using timeof-flight mass spectrometry (TOF-MS), that have been used to generate high-mass ions are plasma desorption," secondary ion mass ~pectrometry,'~ field i o n i z a t i~n '~ and fast atom bombardment. 'The MALDI technique pioneered by Hillenkanip and coworkers'~' ,-19 allows the use of a time-of-flight mass spectrometer to mass analyse gas-phase protein ions. In MALDI the sample is diluted in a matrix, often an aromatic acid, and the mixture is irradiated with a nanosecond laser pulse with a sufficiently high energy density to release sample ions from the matrix. Ideally, the wavelength of the laser is chosen in an absorption band of the matrix. In the mass spectrum, multiply charged ions and clusters ions from the sample are observed. Using this technique ions can be generated from molecules with masses of up Lo 500000.'9 'The greatest analytical strength of the technique seems to be that it can be applied to any protein, with a practical sensitivity of -10 fniol of the sample. MALDI constitutes a major development in the investigation of bioorganic molecules.We describe here the performance of the fullerene C,, in assisting laser desorption-ionization mass spectrometry of insulin (RMM 57341, cytochrome c ( I 2 384) and bovine serum albumin (66 OOO).In the MALDI technique, the analyte and matrix are both dissolved in the same solvent, and the resulting sample conpirlsed laset beam 337 nrn f E sists of the analyte dispersed in th.e matrix. C,, is insoluble in aqueous solutions and so this method cannot be used for many analytes. We used instead a layering technique, as shown in Fig. 1. The method may be considered to be similar to the SEND technique, in which the C,, layer can be used to modify the substrate surface."The linear tandem time-of-flight mass spectrometer used for the investigations reported here was built at the University of New South Wales. A schematic diagram is shown in Fig. 2 and more specific details have been described elsewhere.20-22The nitrogen laser (VSL-337; LSI Laser Science) has a wavelength of 337 nni and an output pulse length of 3 ns; the maximum pulse energy is specified as 175 pJ. The laser beam has a rectangular output shape of approximately 3 mm x 8 mm. The angle of incidence of the laser beam on the sample...

An application of C@-assisted laser desorptionhonization mass spectrometry to a heteropoly inorganic system: phosphotungstic acid, HQO,(WO,),,, is presented. A nitrogen laser (337 nm) on a linear tandem time-of-flight mass spectrometer is used for the observation of the laser-power dependence of mass spectra of this material.

A tandem time-of-flight mass spectrometer incorporating a fast atom bombardment source has been designed and constructed for the purpose of studying collision-induced decomposition spectra. A deflection of the secondary ion beam is used to form the ion packets for time-of-flight analysis. A floating collision cell that represents a potential barrier to ions transversing the time-of-flight tube enables ionic and neutral fragments from collision-induced decomposition reactions to be separated in time from the parent ion. The instrument operating in the tandem mode is capable of performing charge exchange and collision-induced decomposition studies of both positive and negative incident ions.

The photoelectron spectrum of sulphur trioxide has been recorded at high resolution. The interpretation, particularly of the vibrational structure, leads to a different assignment of the electronic states of SO; from those recently published. Two recently published He(1) photoelectron spectra of SO3were analysed to give different assignments to the electronic states of SO;. assigned a medium resolution spectrum mainly by analogy with the photoelectron spectra of BF3 and BC13.4 Mines and Thomas obtained considerably higher resolution, and presented alternative assignments which they justified on the basis of an analysis of the extensive vibrational structure which they observed. In particular, this analysis changed the assignment of the ground state of SO; from 2A2) to 2E". We have also recorded this spectrum at high resolution and, although our spectra are essentially the same as those of Mines and Thomas, we present arguments in favour of assignments which differ from both those published, while giving a 2Ai ground state for SO;.

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