Near-infrared spectroscopy using a supercontinuum laser - application to long-wavelength transmission spectra of barley seeds

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The supercontinuum laser is a new type of light source which combines the coherent light properties of a laser with a broad spectral region.[1] One of its main advantages is a very high brightness compared to traditional light sources used for near-infrared spectroscopy. This makes it possible to focus the light onto small samples without losing intensity and facilitate either rapid or high-precision measurements.
Single seed analysis is one area that might benefit from the supercontinuum light source. The aim of the present study was to establish an apparatus that provided advantageous working conditions for the supercontinuum laser and at the same time to investigate its capability to measure the barley endosperm by transmission experiments in the long-wavelength region. An experimental spectrometer was therefore developed consisting of a supercontinuum laser source, a dispersive monochromator and a PbSe detector. Barley seeds were applied to the instrument as 1 mm slices and extracted oils. To optimize the signal-to-noise ratio, pulse-to-pulse normalization was applied and the scan rate set such that each wavelength point in the barley slice and oil measurement was an average of 105 and 35 ms laser light, respectively. Spectral accumulations of five slice scans and six oil scans gave a total measurement time of 60 s. Effects from light scattering and diffuse transmission were minimized by applying different pre-processing methods to the spectra.[2]
Barley and wheat seeds have only been studied by near-infrared transmission in the short wavelength region up to 1100 nm.[3] However, the long-wavelength region from 2260-2380 nm have previously shown to be particularly useful in differentiating barley phenotypes by using near-infrared spectroscopy in reflectance mode.[4] In the present study, five barley genotypes in 1 mm slices from 70 seeds of each genotype were measured by near-infrared transmission from 2235-2381 nm and the oils were measured in a cuvette with a 1 mm path length from 2003-2497 nm. In order to substantiate the spectral findings the content of ß-glucan, starch, protein and oil was determined for each barley genotype and the fatty acid composition were found by GC-MS for the five barley oils. These values were used for the generation of covarygrams which show the correlation between the long-wavelength NIR spectra and the chemical composition of the five different barley types.
[1] J. M. Dudley, G. Genty & S. Coen, Supercontinuum generation in photonic crystal fiber, Reviews of Modern Physics (2006), 78, 1135–1184.
[2] Å. Rinnan, F. van den Berg & S.B. Engelsen, Review of the most common pre-processing techniques for near-infrared spectra, TRAC-Trends in Analytical Chemistry (2009), 28(10), 1201-1222.
[3] D.K. Pedersen, H. Martens, J.P. Nielsen & S.B. Engelsen, Near-Infrared Absorption and Scattering Separated by Extended Inverted Signal Correction (EISC): Analysis of Near-Infrared Transmittance Spectra of Single Wheat Seeds, Applied Spectroscopy (2002), 56(9), 1206-1214.
[4] H.F. Seefeldt, A. Blennow, B.P. Møller, B. Wollenweber & S.B. Engelsen, Accumulation of mixed linkage (1→3)(1→4)-D-β-glucan during grain filling in barley - A vibrational spectroscopy study, Journal of Cereal Science (2009), 49(1), 24-31.