TWO-STEP LASER MASS SPECTROMETRY
Two-Step Laser Mass Spectrometry (L2MS) is a new mass spectrometric method where the two essential steps of any mass spectrometric analysis, vaporization and ionization, are performed by lasers. In the first step, the output of a pulsed CO2 laser is focused onto a thin film of sample deposited on a chemically inert substrate, such as glass, quartz or ceramics. This causes rapid heating of the substrate and results in desorption of intact, neutral analyte molecules from its surface. In a second step, a pulsed ultraviolet laser causes 1 + 1 Resonance-Enhanced Multiphoton Ionization (REMPI) of the desorbed neutrals. The ions are then mass-analyzed in a reflectron-type time-of-flight mass spectrometer. Under suitable experimental conditions, fragmentation in both the desorption and ionization steps can often be avoided. The method is well suited for the analysis of nonvolatile, polar, and thermally labile species up to high molecular weight. Because of the optical selectivity of the REMPI ionization process, L2MS is also an ideal methodology for the analysis of complex mixtures.
In this article, the process of laser-induced thermal desorption is considered both from an analytical and surface science perspective. First, experimental findings from the literature and models to explain the process are contrasted. Second, experimental and theoretical results on laser surface heating of dielectric materials are presented. Third, preliminary results on kinetic energy distributions of desorbed molecules are discussed.
A number of analytical applications of L2MS are presented. They include a study of terpenoid compounds in amber samples, an investigation of polycyclic aromatic hydrocarbons (PAHs) in meteorite samples, and a spatially resolved organic analysis of the Allende meteorite, which again focuses on PAHs. The most important impact of these studies is that they document the possibility to analyze trace amounts of organics in small quantities of complex mixtures without prior sample preparation, extraction, purification and separation steps. This not only decreases analysis time, but also greatly reduces the danger of contamination inherent in trace analysis. Modern, laser-based methods such as L2MS have great potential for studying very small quantities of precious samples and for analyzing the distribution of organic material in a wide variety of samples with unprecedented spatial resolution.
