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 ...

The thermal desorption of [M+Alkali]' quasimolecular ions from a heated metal surface is reported for soine alkali salts of carboxylic acids and mixtures of alkali halides with a crown ether, glucose and adenosine. No quasimolecular ion could be detected from sucrose. With benzo[l5]crown-5 the desorption of [M+Na]' ions takes place even below the threshold temperature for themionic emission of alkali ions. In addition, the desorption of intact [B(C&l,)J ions from a layer of NaB(C&l,), is reported.

Evidence is presented for a purely thermal evaporation of intact positive ions of quaternary ammonium and phosphonium salts.

Thermal ionization on the surface of a heated wire is applied to the volatilization products of alkali salts of carboxylic and sulfonic acids, and to quaternary ammonium salts. The mass spectra of the alkali salts exhibit almost exclusively cationized molecules. They provide evidence for the evaporation of intact clustered molecules even under conditions of a slow rate of evaporation of the salts. The method appears to be of interest for selective detection of alkali salts from complex mixtures. With the ammonium salts [&N]+ ions are formed by thermal surface ionization of intact salt molecules. The evaporation of these molecules wold be detected at rather low temperatures.

The formation of quasimolecular ions by alkali ion attachment in laser desorption mass spectrometry has been investigated by using a CW CO2 laser. For molecules such as sucrose the desorption of preformed ions could be excluded. Evidence is presented for the ionization by alkali ion attachment in the gas phase. Quasimolecular ions of organic salts were found to result from cationization reactions in the gas phase and from surface reactions. Very fast cation exchange reactions have been discovered for salts and acids impinging on alkali ion emitting surfaces.