Determining optical absorption of colored dissolved organic matter in seawater with a liquid capillary waveguide

Optical absorption spectra of 0.2-mu m filtered seawater samples originating from diverse oceanic and coastal waters were measured with a long pathlength capillary waveguide; results were compared with those of three different laboratory spectrophotometers. The 0.5-m-long 550-mu m (inside diameter) aqueous waveguide uses only 120 mu l of filtered seawater. making it convenient for use in flow-through cells or when sample volumes are restricted. Source light propagates inside the capillary waveguide by total internal reflection because of the lower refractive index of the waveguide walls with respect to the aqueous core. The absorption coefficient of colored dissolved organic matter (CDOM) at 355 nm and S, the slope of the log-linearized CDOM absorption spectra, were determined for all samples. The CDOM absorption spectra measured by the capillary waveguide closely matched that measured by spectrophotometers for CDOM concentrations varying over an order of magnitude. The deviations between the absorption spectra obtained with the capillary waveguide and those obtained with the standard spectrophotometers increased with decreasing total absorption and with increase in wavelength, presumably because of the greater baseline offsets observed in the capillary waveguide. The offsets are due to differences in refractive indices between the seawater samples and the freshwater reference. With a suitable reference, the capillary waveguide will be very useful for monitoring surface seawater CDOM absorption semicontinuously.