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A continuous wave CO, laser has been used for the production of gaseous ions from organic sample layers. The long desorption times of a few seconds enabled a quadrupole mass spectrometer to be used for recording the mass spectra by signal accumulation. The laser desorption spectra of several thermally labile compounds are reported and discussed. The level of fragmentation is rather low. Molecular and fragment ions are formed almost exclusively by alkali ion attachment, even with acids. A particularly long desorption time of more than one minute has been observed with quarternary ammonium salts.The production of gaseous quasimolecular ions by exposure of organic samples to short laser pulses has been reported by several Lasers of different wavelengths from the ultraviolet to the infrared (IR) have been applied. The most striking results were obtained by Posthumus et d 3 using C 0 2 laser pulses of about 1 MW cm-': The laser desorption (LD) mass spectra of a number of thermally labile compounds, such as oligosaccharides, glucosides, peptides, etc., were found to exhibit strong [M+Na]+ or [M-tK]' ion signals. The level of fragmentation was rather low and comparable with that of field desorption (FD) mass spectra. In these experiments mass analysis was carried out by magnetic sector type instruments allowing integrated recording of a complete mass spectrum simultaneously. In addition, time of flight mass spectrometers are used for LD.In this paper it is shown that even a continuous wave (CW) CO, laser of low intensity ( 2 20 W cm-') generates gaseous ions of thermally labile compounds via alkali ion attachment. For desorption times in the order of a few seconds, mass spectrometers having a rapid mass scan such as quadrupole mass filters can be employed. The LD behaviour of several compounds is tested in order to evaluate the potentialities of this new mode of laser desorption. EXPERIMENTALA schematic diagram of the experimental set-up, a laser desorption ion source combined with a quadrupole mass spectrometer is shown in Fig. 1. The sample was deposited on a metal sheet (sample holder) made of copper which was connected to the pushrod of a FD ion source allowing rapid replacement of the sample via a vacuum lock system. The position of the sample with respect to the ion optics of the mass analyser could be adjusted by micromanipulators.The samples were deposited onto the metal sheet from solution. The thickness of the sample layer after evaporation of the solvent was about 0.5 mm or less.The samples were irradiated by a CW CO, laser (GTE-Sylvania, model 941 P) operating at a wavelength of 1 0 . 6 p m ( 2 948cm-I). The output power was 3 W, which results in a power density of about 20 W cmp2 for the unfocused beam for a beam diameter of 4mm. The laser beam could be focused up to about 7 kW cmp2 by a Ge lens (f = 128 mm). The position of the laser beam with respect to the sample could be varied by a micromanipulator system. A metal foil was used to interrupt the laser beam by hand before and after exposure.The ion beam ...

In general cationised molecules are more stable than radical molecular ions or protonated molecules. The fragmentation of the polar molecules resulting from a cationisation by for instance alkali ions, has a higher activation energy than splitting off functional groups after a protonation. Cationisation is therefore an interesting tool for the determination of molecular weights. Three different methods for achieving a cationisation are described and discussed: (1) field-induced cationisation at low anode temperatures using a heterogeneous reaction of the molecules in the gas phase with molecules of a salt in the adsorption layer on the field anode: (2) a largely thermallyinduced cationisation at high emitter temperatures and low electric fields: (3) a cationisation, causing less thermal excitation to the molecules, using the field desorption technique. ONE OF the important aims of analytical organic mass spectrometry is the determination of molecular weights. The achievement of this objective by the detection of the [MI+. ion is not always easy, however, and in many cases completely impossible, since radical molecular ions are generally very unstable. The activation energy for the decomposition of molecular ions is particularly small in those cases where a rearrangement reaction via a cyclic transition state is possible. The difficulties in detecting the molecular ion increase with rising evaporation temperature of the test substance because of the increased thermal activation of the ions produced.The generation of nonradical [M + HI+ ions by chemical ionisation (CI), field

Simple untreated wire emitters can be used for field desorption of electrolytic solutions, e.g. acids or substances containing alkali salts. The ionization is achieved by the attachment of protons or alkali ions to molecules in the condensed phase which are subsequently desolvated by the action of high electric fields. This field desorption mode is demonstrated and discussed for a few characteristic examples. It is shown that the addition of substances of high viscosity and low vapour pressure in the liquid state favours the field desorption behaviour of many compounds. In general the mass spectra of untreated wires are less complicated than those of activated emitters since field ionization processes occurring at the tip of the field-enhancing microneedles are avoided.