A Field-Portable, Fiber-Optic Based Near-Infrared Spectrometer and its Applications to Fuels Analysis

A fiber-optic spectrometer for measurement in the short wavelength near-infrared SW-NIR region 700-1100 nm is presented. Slightly larger than a briefcase and weighing 23 lbs., the instrument is controlled by a 12 MHz laptop computer. The optical scheme uses a concave holographic grating for wavelength dispersion, and a thermoelectrically temperature-stabilized 1024-element silicon photodiode array for detection. A computer-controlled 5.5 watt tungsten lamp, with optical feedback, is the light source. The spectral signal is digitized at a data rate of 22 Khz to 16 bits resolution. Sample measurements are made in either transmission or transflectance mode, using fiber-optic bundles as light pipes. The spectrometer has a resolution of 3 nm, stray light intensity level of 0.05, baseline noise level of 25.5 micro-absorbance units, and dynamic range of 3 orders of magnitude. The spectrometer performance is evaluated in 2 application studies. First, NIR spectroscopy is developed as a rapid method for simultaneous estimation of the freezing point and volume percentages of aromatics and saturates in JP-4 jet aviation fuel. Both multiple linear regression and partial least squares analysis methods are used to predict these physical parameters, NIR spectral analysis predictions are within published ASTM reproducibility criteria. Second, quantitative analysis of xylene mixtures is presented. Classical least squares, multiple linear regression and partial least squares analysis techniques are used to predict the volume percent of ortho-, meta-, and para-xylene in a series of xylene mixtures.