A white light-emitting diode (LED) with emission between 420 and 700 nm and a supercontinuum (SC) source with emission between 450 and 2500 nm have been compared for use in evanescent wave broadband cavity-enhanced absorption spectroscopy (EW-BB-CEAS). The method is calibrated using a dye with known absorbance. While the LED is more economic as an excitation source, the SC source is superior both in terms of baseline noise (noise equivalent absorbances lower than 10(-5) compared to 10(-4) absorbance units (a.u.)) and accuracy of the measurement; these baseline noise levels are comparable to evanescent wave cavity ringdown spectroscopy (EW-CRDS) studies while the accessible spectral region of EW-BB-CEAS is much larger (420-750 nm in this study, compared to several tens of nanometres for EW-CRDS). The improvements afforded by the use of an SC source in combination with a high sensitivity detector are demonstrated in the broadband detection of electrogenerated Ir(IV) complexes in a thin-layer electrochemical cell arrangement. Excellent signal to noise is achieved with 10 micros signal accumulation times at a repetition rate of 600 Hz, easily fast enough to follow, in real time, solution kinetics and interfacial processes.

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.

A novel method for applying cavity ring-down spectroscopy in the liquid phase, compatible with LC analyses, is presented. The core of the setup is a home-built cavity ring-down flow cell (cell volume 12 µL) that is constructed using a silicon rubber spacer, which is clamped leak-tight between two high-reflectivity mirrors. The mirrors are in direct contact with the liquid flow, which provides for a small path length and short ring-down times. Inside the cavity there are no windows, reflection losses, or Brewster angles to be considered. Due to the small size of the presented cavity geometry, the setup can be implemented in conventional-size LC apparatuses. With a flow injection setup, a detection limit of 2.5 nM was obtained for Crystal Violet in ethanol, and the linear dynamic range of the system is at least 2 orders of magnitude. The method has the potential to become a powerful alternative for commercial LC UV/visible absorbance detectors.In liquid chromatography (LC), direct optical absorption is the generic and commonly used method to detect absorbing analytes and their concentration in eluting compounds. Almost any analyte absorbs light in the visible or ultraviolet range, and standard absorption detection shares the advantages of simplicity and versatility. Since direct absorption techniques are based on monitoring attenuation of light intensity, they are fundamentally limited by the intensity fluctuations of the light sources employed, where currently ∆I/I values of 5 × 10 -5 are reached. 1 An option for improving the limit of detection (LOD) in LC lies in the increase of the optical path length without altering the detector cell volume, and therefore, Z-shaped cells of typically 8-mm optical path length and 1-mm diameter are commonly used. With the use of liquid core waveguide flow cells, optical path lengths as long as 30-50 cm can be obtained, 2,3 but the number of applications is still limited. Considering the broad applicability of absorption detection in LC, it is highly relevant to improve sensitivity and detection limits, but at the same time, the chromatographic restrictions of LC should be complied with. In view of chromatographic band-broadening effects, even in conventional-size LC the detector cell volume should be limited to typically 20 µL, while for micro-and especially nano-LC, the constraints on detection volumes are obviously much more serious.The invention of the cavity ring-down (CRD) detection technique 4 has introduced an elegant manner for improving sensitivity in direct absorption. CRD is based on the injection of a short laser pulse into an optical resonator with high-reflectivity mirrors, followed by the detection of the light leaking away through a mirror. Since its principle is based on measuring the rate of decay of an optical cavity, CRD in effect circumvents the common limitations of absorption measurements: the decay time is independent of the intensity of the light pulse exciting the optical resonator, and hence, pulsed laser sources with pulse-to-pulse intensity fluctua...

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