Diode laser microwave induced plasma cavity ringdown spectrometer: Performance and perspective

Abstract

Recent studies combining an atmospheric-pressure plasma source (inductively coupled plasma or microwave induced plasma) with cavity ringdown spectroscopy (plasma-CRDS) have indicated significant promise for ultra-sensitive elemental measurements. Initial plasma-CRDS efforts employed an inductively coupled plasma as the atomization source and a pulsed laser system as the light source. In an effort to improve the portability and reduce the cost of the system for application purposes, we have modified our approach to include a compact microwave induced plasma and a continuous wave diode laser. A technique for controlling the coupling of the continuous wave laser to the ringdown cavity has been implemented using a standard power combiner. No acouto-optic modulator or cavity modulation is required. To test the system performance, diluted standard solutions of strontium (Sr) were introduced into the plasma by an in-house fabricated sampling device combined with an ultrasonic nebulizer. SrOH radicals were generated in the plasma and detected using both a pulsed laser system and a diode laser via a narrow band transition near 680 nm. The experimental results obtained using both light sources are compared and used for system characterization. The ringdown baseline noise and the detection limit for Sr are determined for the current experimental configuration. The results indicate that a plasma-CRDS instrument constructed using diode lasers and a compact microwave induced plasma can serve as a small, portable, and sensitive analytical tool.