Laser desorption mass spectrometry of thermally labile compounds using a continuous wave CO₂ laser

Abstract

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 ...

Thermodynamics of Ion Formation by Fast Dissipation of Energy at Solid Surfaces

“…It has been reported [31] that a steady state irradiation of organic solids with IR laser beams may also cause quasimolecular ion formation from some thermolabile substances, whereas simple resistive heating does not. As this laser irradiation is absorbed in the uppermost layers (non-resonant irradiation does not work) one may think that excitation and desorption can be faster processes than thermal equilibration on the uppermost surface layers.…”

“…Ion formation by fast dissipation is generally proved to be kinetically a far-from-equilibrium process. for LAMMA 1000 [8], [28] UV-laser pulses > 10 17 for dust particle induced desorption [29] < 10 9 thermiionic ion formation, experimentally [31] At the surface they will be aligned due to the lowest energy constraint, i.e. sites of opposite charges will have a great chance to be neighbours.…”

“…dX may be written as Y^A max 1 tot (31) according to (16) and (17), with a total interaction probability of the high-frequent perturbation with a main frequency co" (co n oscillator frequencies (infrared) of the molecule). The probability P tot can be described as the linear response of the system of the co' n due to the perturbation F, which is given by a set of amplitudes a n (co' n ), which may be modeled by the light extinction coefficient of light frequency co' as an electric interaction strength.…”

Cationization by Alkali Ion Attachment in Laser Desorption Mass Spectrometry

“…For an absorbing substrate such as silica gel and an optically thin sucrose layer similar spectra are obtained as with electrical heating of the sample. The effect of wave length on the fragmentation of absorbing sample molecules has been discovered before [7] and was investigated in more detail. However, a discussion of the results is outside the scope of this paper.…”

“…Applying CW CO2 lasers any ionization mechanism should arise from thermal effects. The previous observation of a long lasting emission of cations from quaternary salts [7] which gave rise to speculations on nonthermal effects in LD proved to result from a pure thermal evaporation of these salts [24].…”