A multimodal spectrometer for Raman scattering and near-infrared absorption measurement
In food and pharmaceutical industries, the compositional complexity of raw materials and products promotes the use of multiple spectroscopic techniques. Availability of complementary methods, e.g., infrared absorption and Raman, is highly desirable. A multimodal Raman/NIR spectroscopic device was proposed. The device offered fast, non-destructive measurement, and remote sensing capability. A prototype was built. Spectral range covered 300-2600 cm-1 for Raman channel. Wavelength accuracy was better than 0.57 nm. Spectral resolution was better than 14.39 cm-1 (1 nm) for Raman (NIR) channel. Spectral response linearity was better than 0.998. Instrumental stability was better than 0.72% (0.39%) for Raman (NIR) channel. The device was applied to quantify ethanol aqueous solutions (2%-10% (v/v)). Quantitation results gave a root mean squared error of prediction (RMSEP) of 0.45% (v/v). Calibration using NIR absorbance spectra produced a RMSEP value of 0.49% (v/v). Raman technique offered the most succinct calibration model using peak height at 882.29 cm-1. Quantitation of ovalbumin (8%-16% (w/v)) in aqueous solutions and denatured states was also attempted. Stepwise multiple regression was performed using Raman peaks at 1243.35, 1448.68, 1662.66, and 1667.78 cm-1, which produced RMSEPs of 1.05 % (w/v) for ovalbumin solutions and 0.74 % (w/v) for ovalbumin gels. Conformational change of ovalbumin upon thermal denaturation was studied using the height and area ratios between peaks at 1243.35 and 1662.66 cm-1. Thermal denaturation of ovalbumin occurred at 70 °C and extensive formation of anti-parallel β-sheet structure was found at 90 °C. The unique capability of this multimodal device was proven by first performing data fusion of Raman and NIR spectra for quantitation. Using the same ethanol spectra set gave a more complicated model and a higher RMSEP of 0.73% (v/v). However, important variables in fused spectra corresponded well with the spectral signatures of ethanol and water. Two-dimensional correlation spectra were calculated using concentration as the external perturbation. The autopower spectra revealed features pertinent to those of the analytes. Heterospectral correlation analysis was carried out on Raman/NIR spectra of ethanol aqueous solution. Raman peaks at 882.29 and 1454.00 cm-1 agreed with the NIR absorption feature around 906.18 nm.