Liquid-Phase Broadband Cavity-Enhanced Absorption Spectroscopy Measurements in a 2 mm Cuvette

Abstract

A novel implementation of broadband cavity enhanced absorption spectroscopy (BBCEAS) has been used to perform sensitive visible wavelength measurements on liquid-phase solutions in a 2 mm cuvette placed at normal incidence to the cavity mirrors. The overall experimental methodology was simple, low cost, and similar to conventional ultraviolet-visible absorption spectroscopy. The cavity was formed by two concave high reflectivity mirrors. Three mirror sets with nominal reflectivities (R) of R = 0.99, 0.9945, and 0.999 were used. The light source consisted of a high intensity red, green, blue, or white LED. The detector was a compact charge-coupled device (CCD) spectrograph. Measurements were made on the representative analytes, Ho³⁺, and the dyes brilliant blue-R, sudan black, and coumarin 334 in appropriate solvents. Cavity enhancement factors (CEF) of up to 104 passes for the high reflectivity mirrors were obtained. The number of passes was limited by relatively high scattering and absorption losses in the cavity, of ∼1 × 10⁻² per pass. Measurements over a wide wavelength range (420–670 nm) were also obtained in a single experiment with the white LED and the R = 0.99 mirror set for Ho³⁺ and sudan black. The sensitivity of the experimental setup could be determined by calculating the minimum detectable change in the absorption coefficient α_min. The values ranged from 5.1 × 10⁻⁵ to 1.2 × 10⁻³ cm⁻¹. The limit of detection (LOD) for the strong absorber brilliant blue-R was 620 pM. A linear dynamic range of measurements of concentration over about two orders of magnitude was demonstrated. The overall sensitivity of the experimental setup compared very favorably with previous generally more experimentally complex and expensive liquid-phase cavity studies. Possible improvements to the technique and its applicability as an analytical tool are discussed.