An optical absorbance cell that offers a selection of multiple optical path lengths has been developed as part of a portable spectrometric instrument that measures absorption spectra of small samples of water and that costs less than does a conventional, non-portable laboratory spectrometer. The instrument is intended, more specifically, for use in studying colored dissolved organic matter (CDOM) in seawater, especially in coastal regions. Accurate characterization of CDOM is necessary for building bio-optical mathematical models of seawater. The multiple path lengths of the absorption cell afford a wide range of sensitivity needed for measuring the optical absorbances associated with the wide range of concentrations of CDOM observed in nature.
The instrument operates in the wavelength range of 370 to 725 nm. The major subsystems of the instrument (see figure) include a color-balanced light source; the absorption cell; a peristaltic pump; a high-precision, low-noise fiber-optic spectrometer; and a laptop or other personal computer. A fiber-optic cable transmits light from the source to the absorption cell. Other optical fibers transmit light from the absorption cell to the spectrometer, as described below.
The absorption cell is of a type known in the art as a liquid-core waveguide: the liquid to be characterized is contained in a narrow tube. The material of the tube or a material coating the outside of the tube is chosen to be of an index of refraction less than that of the liquid so that light propagating inside the liquid remains confined by total internal reflection. Thus, the liquid and tube constitute an optical waveguide or optical fiber.
The tube has an inner diameter of 2 mm and is coiled for compactness. The peristaltic pump draws the sample water from a beaker into and along the tube and discharges the water into another beaker. Light that has propagated through the liquid in the tube is collected by an optical fiber at path lengths of 2, 10, 50, and 200 cm from the light-input end. By means of a fiber-optic switch, the light from one of these optical path lengths is selected for transmission to the spectrometer via a fiber-optic cable. The spectrometer readings are transmitted to the computer for analysis.
In tests using phenol red secondary standards, the instrument exhibited a baseline repeatability in absorbance of better than 1 mAU. During absorption measurements of natural CDOM samples, the instrument exhibited a sensitivity of about 0.002 m–1 at a wavelength of 370 nm. On the other hand, it was found that by using the 2-cm path length cell, one could measure CDOM absorbance as high as 200 m–1 at 370 nm. Hence, the instrument has an exceptional dynamic range, as needed to measure a wide range of CDOM concentrations.
This work was done by Richard L. Miller of Stennis Space Center and Mathias Belz and Su Yi Liu of World Precision Instruments Inc.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to World Precision Instruments Inc. 175 Sarasota Center Boulevard Sarasota, FL 34240 Refer to SSC-00143, volume and number of this NASA Tech Briefs issue, and the page number.