Laser microprobe mass spectrometry: Possibilities and limitations

Abstract

Laser microprobe mass spectrometry (LMMS) offers a great potential for inorganic analysis and speciation as well as for organic structural characterization. This remarkable flexibility makes the results critically dependent on the experimental conditions, however, so an exact description is hard to achieve, since inaccessible local parameters are involved. The paper deals in detail with this aspect, which is often neglected but remains essential for making LMMS acceptable as analytical tool. Our methods of procedure are presented in the context of instrumental aspects and experimental evidence. Tentative hypotheses for desorption and ionization in LMMS provide an adequate framework for interpreting results consistently. Selected examples illustrate the potential gained from our experimental procedure as well as the limitations imposed. For organic compounds, analysis of pure products and simple mixtures, and microprobe applications are highlighted. Improved differentiation of inorganic compounds is also shown.The use of lasers in MS dates back about two decades in both inorganic and organic analysis. On the one hand, photon interaction is exploited for overall elemental determinations in non-conducting solids, for instance as an alternative to sparksource excitation [1]. On the other hand, laser desorption is widely appreciated as a so-called soft ionization method for thermolabile organics, yielding cationized molecules and the virtual absence of fragmentation [2]. These instruments have been used mainly in individually developed set-ups.

Organic and inorganic analysis with laser microprobe mass spectrometry. Part I: Instrumentation and methodology

“…The generation of elemental ions is optimized under high ir-LASER MICROPROBE MASS SPECTROMETRY I radiance; i.e., between 10' and 10" W-cm-'. Much milder conditions, typically around lo6 W. cm-', are needed for the generation of structural ions from organic compounds up to, e.g., 1 kDa (82). Speciation of inorganic substances may require a somewhat higher power density than organic molecules, but they remain far under the elemental regime (63, [82][83].…”

“…Hence, in order to relate mass spectra to the local energy regime, the experimental methodology must permit the delineation, as far as possible, of the time-domain responsible for the detected ions. Time-gating information is directly available in FTMS, whereas, in the current TOF LMMS with continuous ion extraction, mass resolution and calibration of the m/z scale can be used to assess the time-frame within which the detected ions are formed (82). Figure 3 illustrates the principle of this approach (35).…”

“…With respect to organic neutral compounds of up to 1 kDa, LMMS results generally contain some low m/z elemental ions, a series of intense organic fragments, usually accompanied by the signal of protonated or cationized molecules in the positive ion mode, and the deprotonated molecules in the negative-ion mass spectra (82,91). Depending on the structure and the locally applied energy regime, the precursor peak can be due to radical molecular ions (82, 92).…”

“…The role of thermal processes in LMMS is evident from some signals in the spectra of salts (82,(103)(104). Desorption experiments of organic compounds from a silver foil point in the same direction (105).…”

Thermal-to-plasma transitions and energy thresholds in laser ablated metals monitored by atomic emission/mass spectrometry coincidence analysis

“…Several analytical techniques make use of the lasers in such way, among which laser ionization mass spectrometry (LIMS) and laser-induced breakdown spectrometry (LIBS) are the most versatile [3,4].…”