Thermal aspects of laser desorption mass spectrometry (LDMS)

Lasers and Mass Spectrometry

“…Figure 8 shows that the desorption of quaternary ammonium ions from tetramethylammonium chloride continues for some time (microseconds) following the laser pulse but that the desorption of these "molecular ions" is enhanced at the early stages, while decomposition is generated in a longer time frame (66). A thermal model described by Kistemaker (68,69), in which the substrate continues to rise in temperature following the absorption of laser radiation and then dissipates the heat over several microseconds, is consistent with the extended desorption profile. While the long desorption profiles can pose some problems for TOF analysis, a draw-out pulse for focusing and establishing a "start" time (27) can be used to produce a spectrum.…”

Laser desorption mass spectrometry of nonvolatiles under shock wave conditions

“…For this purpose we used various oligosaccharide samples to get information on the thermal stress exerted on the desorbed molecules. Oligosaccharides are known to be thermally labile-stachyose more than raffinose and sucrose-and have been used repeatedly for similar applications (7)(8)(9)(10)(11)(12)18,25). We found direct evidence for the existence of at least two different desorption modes, thermal and nonthermal, depending on laser power density and sample thickness, which may even coexist under particular conditions.…”

“…Thousands of gas chromatograph/ mass spectrometers (GC/MS) are now used daily worldwide (J). A major application is the identification of unknown compounds, which in many laboratories results in the production of hundreds of unknown mass spectra per day, making obvious the need for computerized identification systems (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16). For samples representing complex mixtures, incomplete GC separation is unavoidable (17,18)•, for the resulting spectra which represent more than one component reverse searching (only requiring the peaks of the reference to be in the unknown) improves retrieval performance (4)(5)(6)(7).…”

“…But keeping in mind the above outlined situation concerning the diversity of instrumentation and sample handling, it is not to be expected that there will by the one model describing all experimental conditions. Kistemaker (7)(8)(9)(10), Rpllgen (11,12), Cotter (13)(14)(15)(16)(17), and Heresch (18) favor a thermal mode for their experimental conditions: sample on bulk substrates and irradiation with moderately focused laser beams at moderate laser power densities at the sample side. However, as is pointed out in some cases (11,18,19), thermal processes cannot explain sufficiently all observed phenomena.…”

Laser desorption time-of-flight mass spectrometry of high mass molecules

“…There are no noticeable thermal effects, since the energy is quickly dissipated by the ablation (vaporization) of a small spot on the foil which leaves a small, micrometer-sized hole (7). On the other hand, for methods which use larger amounts of sample on a thicker support, i.e., bulk analysis instruments (6, 7), the laser beam is less focused (8) and the desorbed ions are the result of a basically thermal process (9). The latter process is evidenced by the extended desorption 1 Present address: Ecole Polytechnique, Laboratoire de Synthese Organique, 91128 Palaiseau, France.…”