Development of field-portable NIR absorption spectrometry system by using glass phosphor combined with LED

Near-infrared (NIR) wideband light sources are used in an absorption spectrometry for agricultural applications. In general, the NIR absorption spectroscopy is well known as quick and non-destructive measurements. Halogen lamps or light-emitting diodes (LEDs) are usually used as light-sources for the absorption spectrometry. However, halogen lamps are big and the lifetime is short. On the other hand, LEDs are small and have a long lifetime, but have a narrow spectral width. Therefore, we have proposed a new-type wideband NIR light-source for the absorption spectroscopy by combining a wideband NIR phosphor with an LED in one package. We successfully expanded the spectrum as wide as from 760 nm to 1100 nm by Pr3+, Sm3+ co-doped glass phosphor [1]. By using this glass phosphor, we have developed a measurement system for a detection of pesticides [2]. However, this system was large (W500 ×H170 ×D300 mm), it was difficult to using on-site. In this study, we developed a field-portable NIR absorption spectrometry system by using the glass phosphor LED and a Fourier Transform (FT) spectrometer.
The size of field-portable NIR absorption spectrometry system was W120 ×H171 ×D240 mm. Therefore, we succeeded to decrease the size of measurement system by 1/5. Moreover, diluted Cu solutions were measured for preliminary experiments of Cu based pesticides. Samples were prepared by diluting the 1000 ppm Cu standard solution with HNO3. Cu concentration was set to 0.1 to 1.0 ppm, and HNO3 was used as a zero ppm sample. NIR absorption spectra of these samples were measured by using a 10 mm quarts cell. An integration time of FT spectrometer was set to 20 msec, and absorption spectra were averaged by 50 times. Therefore, total measurement time for one spectrum was 1000 msec.
For a partial least squares (PLS) analysis, spectra were treated by a smoothing and a second derivative. Moreover, a regression coefficient spectrum was calculated from the PLS analysis. In the coefficient spectrum, a strong correlation was observed around 820 nm. It is due to a d-d transition of Cu [3]. A relationship between nominal and predicted Cu solution concentrations by PLS analysis was plotted. The PLS analysis showed the lower limit of Cu concentration was 0.1 ppm. The values of R2 and RMSEP are 0.988 and 0.034 ppm, respectively. These values indicate the high correlativity and accuracy.
In summary, the field-portable NIR absorption spectrometry system by using our novel light source and FT spectrometer was developed and verified by using Cu diluted solutions. Results of PLS analysis for Cu diluted solutions indicated the field-portable NIR absorption spectrometry system has enough lower limit of concentration and accuracy. From these results, it is concluded that our field-portable NIR absorption spectrometry system should be useful for practical on-site applications.

[1] S. Fuchi, et al., Applied Physics Express, 7 (2014) 072601.
[2] H. Uemura, et al., Journal of Physics: Conference Series, Accepted for publication.
[3] M. W. Pantoliano et al., J. Am. Chem. Soc., 104 (1982) 6310.