<title>Sensitive absorbance measurement for gas-phase analytes based on multiwave-mixing spectroscopy</title>

Abstract

Multi-wave mixing spectroscopy is presented as a simple and sensitive laser method for elemental analysis at trace- concentration levels with sub-Doppler spectral resolution. Since the signal is a coherent beam, virtually 100% of the generated signal can be directed into a photodetector. The nonlinear signal has a cubic dependence on excitation intensity, and hence, laser power requirements are low, and mW-level continuous-wave lasers and nJ-level pulsed dye lasers can be used for this multi-photon spectroscopic setup. The optical alignment and beam quality requirements are relatively less demanding compared to other multi-photon methods, and hence, compact inexpensive lasers, including laser diodes, can be used. Since the coherence time and coherence length requirements are also relatively low, many types of laser sources can be used quite conveniently in this nonlinear spectroscopic setup. Parts-per-trillion level detection sensitivity can be obtained for a wide range of fluorescing and non-fluorescing analytes and matrices using various analytical atomizers. The graphite furnace atomizer offers many advantages, including convenient sample introduction for solid, liquid or gas analytes, high atomization temperature, clean atomization environment, and minimum source and chemical interferences, resulting in lower atomizer background noise. Taking advantage of the unique features of this multi-wave mixing optical method and those of a graphite furnace atomizer, one can obtain both excellent spectral resolution and detection sensitivity.