Low-Volume Diffuse-Transmittance Fiber-Optic Near-IR Spectrophotometer

Abstract

A simple, inexpensive near-IR spectrophotometer is described that employs single fiber-optic strands in a diffuse-transmission geometry. The instrument can examine unusually small sample volumes and offers sufficient throughput to perform multicomponent analyses on diffusely transmitting powdered samples. Control of the effective optical pathlength is found to be critical in obtaining reproducible results when scattering samples are to be analyzed. A pseudo-double-beam arrangement is employed to reduce instrument noise and to set the optical pathlength independently for each sample. Sample sizes of only a few tens of milligrams are required for analysis. The resulting performance is nearly equivalent to that of a commercial reflectance instrument. An average standard error of prediction of 2.77% w/w was obtained for four-component mixtures of benzoic acid derivatives. The single-fiber transmission geometry is shown to be useful also in the near-IR analysis of microliter volumes of liquids. A standard error of cross-validation of 17 mM sodium chloride dissolved in water was achieved.