Limnol. Oceanogr., 44(4), 1999, 1142–1148

Optical absorption spectra of 0.2-mm 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-mm (inside diameter) aqueous waveguide uses only 120 ml 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. Colored dissolved organic matter (CDOM) plays an important role in many oceanic processes. CDOM strongly absorbs light, particularly the biologically damaging ultraviolet (UV) B wavelengths (280–320 nm), thus protecting phytoplankton and other biota (Blough and Green 1995; Arrigo and Brown 1996). It can also reduce the photosynthetically active radiation available to phytoplankton, thus decreasing primary production. High CDOM absorption in the blue region of the spectrum can also degrade the accuracy of satellite-derived phytoplankton chlorophyll estimates (Yentsch 1983; Carder et al. 1991). Knowledge of CDOM distributions, the processes controlling CDOM, and its influence on optical properties are limited by the methods currently used for measurement. Spectrophotometers with 5and 10-cm optical cells can measure CDOM absorption with sufficient sensitivity in the UV and visible wavebands for many coastal and shelf waters. However, in oligotrophic waters, the levels of CDOM absorption approach the detection limit of these instruments. Measuring CDOM absorption spectra in these waters requires long pathlength cells (Bricaud et al. 1981; Peacock et al. 1994) or sample concentration (Carder et al. 1989). These approaches are time consuming. Alternative methods have used fluorescence measurements to retrieve CDOM absorption coefficients (Hoge et al. 1993; Green and Blough 1994). One way of increasing sensitivity in spectrophotometry is to increase the length of the cuvette. Usually this increase is accompanied by an increase in sample volume. Various types of fluid-filled light waveguide capillary tubing that increase effective pathlength and reduce sample volume for spectral absorption measurements have been proposed (Dasgupta 1984; Tsunoda et al. 1989). To attain total reflection of source light inside a glass capillary cell, Fujiwara and Fuwa (1985) used carbon disulfide as a solvent in the spectrophotometric detection of iodine. Because carbon disulfide has a refractive index greater than that of the glass capillary cell, light from the source propagates inside the capillary cell via total reflection at the inner cell wall as in a solid optical fiber. Tsunoda et al. (1989) used the capillary glass– air interface as the total reflecting surface without any coating. Although in principle this apparatus can function as an effective fluid-filled waveguide, its performance will degrade if there is any contamination of its reflecting surface. Recently, the availability of an amorphous fluorocarbon material with a refractive index less than that of water has been used to create a practical aqueous waveguide (Liu, U.S. patent 5570447) for spectral absorption measurements of aqueous fluid samples, including seawater (D’Sa et al. 1998). The World Precision Instruments capillary waveguide has the liquid forming the optical core contained by a rigid quartz capillary tubing that is coated by an amorphous polymer optical cladding with a refractive index less than that of an aqueous solution (Fig. 1). Source light that is axially introduced into the waveguide via an optical fiber is transmitted and constrained within the capillary cell by total internal reflection because of the higher refractive index of the seawater in relation to the cell wall. At the opposite end of the waveguide, a detection fiber conducts the light that is not absorbed by the aqueous medium to a fiber-optics-based spectrometer that uses a diffraction grating to disperse the transmitted light into a CCD detector array. The 0.5-m-long waveguide is coiled into a 10-cm-diameter coil and has standard ST fiber optic connectors that attach to external optical fibers. There is an inlet or outlet connection at each end of the waveguide for injecting filtered seawater samples or any other aqueous solution. A deuterium lamp was used as a light source for UV wavelengths, and a halogen lamp provided visible wavelengths. Using electronically controlled shutters, source light from either of the lamps was coupled into the waveguide using an optical fiber that was attached to the ST connector. The option of combining the UV and visible waveband spectra at a particular wavelength was provided through software. The performance of the capillary waveguide to measure CDOM absorption spectra of diverse water types was evaluated by comparing the absorption spectra of 0.2-mm-filtered seawater samples to that obtained with commercial spectrophotometers. Seawater samples for this study were obtained from (1) a transect in the Middle Atlantic Bight (MAB) from the Delaware Bay southeast to the Gulf Stream, (2) the Gulf of