Liquid-Phase and Evanescent-Wave Cavity Ring-Down Spectroscopy in Analytical Chemistry

Abstract

Due to its simplicity, versatility, and straightforward interpretation into absolute concentrations, molecular absorbance detection is widely used in liquidphase analytical chemistry. Because this method is inherently less sensitive than zero-background techniques such as fluorescence detection, alternative, more sensitive measurement principles are being explored. This review discusses one of these: cavity ring-down spectroscopy (CRDS). Advantages of this technique include its long measurement pathlength and its insensitivity to light-source-intensity fluctuations. CRDS is already a wellestablished technique in the gas phase, so we focus on two new modes: liquidphase CRDS and evanescent-wave (EW)-CRDS. Applications of liquidphase CRDS in analytical chemistry focus on improving the sensitivity of absorbance detection in liquid chromatography. Currently, EW-CRDS is still in early stages: It is used to study basic interactions between molecules and silica surfaces. However, in the future this method may be used to develop, for instance, biosensors with high specificity.

Total internal reflection spectroscopy for studying soft matter

“…Evanescent-wave cavity ring-down spectroscopy has recently been reviewed. 62,63 The basic experimental set-up consists of an optical cavity with highly reflective mirrors (reflectivity typically > 99.9%) containing an IRE. A laser pulse is introduced into the cavity and the exponential decay of the intensity within the cavity is measured with and without the sample present.…”

“…A big advantage of CRDS is that one measures the decay time of the pulse and not the intensity of the light, so the technique is insensitive to fluctuations in the laser power. 62 One of the main experimental challenges is to overcome the reflection losses at the entry and exit faces of the IRE. The most successful experimental design is the "folded-resonator", where the entry and exit faces of the IRE are reflectively coated and the only interface within the cavity is the one where total internal reflection takes place ( figure 13).…”

Cavity-enhanced absorption using an atomic line source: application to deep-UV measurements

“…25 The estimated limit of detection for acetone is higher than the range found in healthy breath (390 to 850 ppbv), but further improvements in sensitivity could make this approach practical for breath analysis. Other applications include detectors for chromatography, for which both CRDS 26,27 and CEAS 28 have previously been proposed. Although this work has focussed on gases, the approach is also feasible for liquid samples.…”

Probing Redox Reactions of Immobilized Cytochrome c Using Evanescent Wave Cavity Ring‐Down Spectroscopy in a Thin‐Layer Electrochemical Cell

“…To this end, we report the direct detection of the reduction of cytochrome c adsorbed to silica by a soluble redox mediator using evanescent wave cavity ringdown spectroscopy (EW-CRDS) [26,27] in a thin-layer electrochemical cell. [28] The studies provide a proof-of-principle and platform for subsequent investigations in which the complexity of the interface could be developed.…”

“…[29,30] EW-CRDS is a surface-sensitive spectroscopic technique, capable of monitoring interfacial processes in situ and with kHz temporal resolution. [26] In its simplest form, this technique involves coupling a pulsed laser beam into an optical cavity, consisting of two high-reflectivity mirrors and a fused silica prism and detecting the decay in the light level within the cavity when the laser is switched off. At the point of total internal reflection (TIR) in the prism, an evanescent field is created whose amplitude decays exponentially with distance from the prism surface.…”