Diagnostic and normalization techniques for laser-generated plumes based on beam deflection and photoacoustic wave measurements

摘要

Because of its excellent spatial resolution and sensitivity, the laser microprobe analyzer (LMA) has become an indispensable tool for direct solid analysis. The laser microprobe has been hyphenated to a variety of analytical techniques, in particular, optical spectroscopy and mass spectrometry. However, despite its widespread use, it suffers from relatively poor precision and accuracy;To better understand the laser-solid interactions, especially for those neutral and nonemitting plume species, a new universal detector is developed. The density gradient associated with the transient atomization event is intercepted by a probe laser beam, resulting in beam deflection (BD) in far field. Both the shape and magnitude of the BD signal agree well with the predictions based on a plume model with a radially linear density profile. The probe laser beam can be well focused on the plume, generating a BD signal in a single pass, so both spatial and temporal resolutions are excellent. The plume dynamics (expansion and drift), the spatial density profile, and the amount of evaporated material can be derived with a limit of detection of 1 ng. This technique compares favorably with interferometry for diagnostics of transient atomization events and will find widespread applications;The correlation between atomic emission and photoacoustic wave associated with laser-generated plumes is investigated. Over a widely varied vaporization conditions including laser power, focusing, surface treatment and, to a limited extent, chemical compositions, the amplitude of the photoacoustic wave is linearly related to the atomic emission intensities of both major and minor components. This implies that the photoacoustic signal can be used as an internal standard for the quantitation of laser microprobe analysis.