Ionization, molecular, from high-power lasers

Laser ionization mass spectrometry or laser microprobing (LIMS) is a microanalyt-ical technique used to rapidly characterize the elemental and, sometimes, molecular composition of materials. It is based on the ability of short high-power laser pulses (-10 ns) to produce ions from solids. The ions formed in these brief pulses are analyzed using a time-of-flight mass spectrometer. The quasi-simultaneous collection of all ion masses allows the survey analysis of unknown materials. The main applications of LIMS are in failure analysis, where chemical differences between a contaminated sample and a control need to be rapidly assessed. The ability to focus the laser beam to a diameter of approximately 1 mm permits the application of this technique to the characterization of small features, for example, in integrated circuits. The LIMS detection limits for many elements are close to 10 at/cm, which makes this technique considerably more sensitive than other survey microan-alytical techniques, such as Auger Electron Spectroscopy (AES) or Electron Probe Microanalysis (EPMA). Additionally, LIMS can be used to analyze insulating sam-... 

At high laser powers only atomic (elemental) information is obtained, however, at lower power densities, close to threshold, molecular information can be obtained, although extensive fragmentation is frequently observed. By careful control of the initial conditions and laser energy this approach has been refined to allow even delicate biological molecules, such as proteins, to be ejected from a solid or liquid into the gas phase and ionized, without fragmentation. Indeed, this is the basis of the now well-established technique MALDI, which is described in further detail below. 

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