Recent studies combining an atmospheric-pressure plasma source ͑inductively coupled plasma or microwave induced plasma͒ with cavity ringdown spectroscopy ͑plasma-CRDS͒ 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 ͑Sr͒ 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.

Cavity ringdown spectroscopy is applied to line-of-sight measurements of OH radicals in an atmospheric-pressure argon inductively coupled plasma, operating at low power (200 W) and low gas flows (approximately 18 liters/min). Density populations of the single S21(1) rotational line in the OH A2sigma(+)-X2Pi (0-0) band are extracted from the measured line-of-sight absorbance. Plasma gas kinetic temperatures, derived from the recorded line shapes of the S21(1) line, ranged from 1858 to 2000 K with an average uncertainty of 10%. Assuming local thermodynamic equilibrium, an assumption supported by the comparison of the experimental and simulated spectra, the spatially averaged total OH number density at different observation heights was determined to be in the range of 1.7 x 10(20)-8.5 x 10(20) (m(-3)) with the highest OH density in the plasma tail. This work demonstrates that ringdown spectra of the OH radical may be used both as a thermometer for high-temperature environments and as a diagnostic tool to probe the thermodynamic properties of plasmas.

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