A line-scan hyperspectral Raman system for spatially offset Raman spectroscopy

Authors: Qin, Jianwei - Tony Qin; Kim, Moon; Schmidt, Walter; CHO, BYOUNG-KWAN - Chungnam National University; PENG, YANKUN - China Agricultural University; Chao, Kuanglin - Kevin Chao

Submitted to: Journal of Raman Spectroscopy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/5/2015
Publication Date: 4/16/2016
Citation: Qin, J., Kim, M.S., Schmidt, W.F., Cho, B., Peng, Y., Chao, K. 2016. A line-scan hyperspectral Raman system for spatially offset Raman spectroscopy. Journal of Raman Spectroscopy. 47(4):437-443.

Interpretive Summary: Spatially offset Raman spectroscopy (SORS) is a useful noninvasive method of chemical-specific evaluation of subsurface materials. A series of Raman spectral measurements on the surface of a sample, spatially offset from the point of laser excitation, can provide information on the internal composition of biological samples or sealed packaged products. These measurements are easier and less costly than some other detection methods such as magnetic resonance imaging. As source-detector distance increases for the series of measurements, the contribution of Raman signals from deeper internal layers gradually outweighs that from the top layer. The series of offset measurements is conventionally performed either by moving a single fiber optic probe step by step away from the excitation point, or by using an array of fiber optic probes arranged at fixed intervals along a predetermined maximum distance from the excitation source. This study investigated the use of a line-scan hyperspectral Raman imaging system to simultaneously collect a series of offset measurements in a shorter time, and with greater options for the offset range (distance from excitation) and the offset intervals (narrow or wide). The spatial profiles extracted from the Raman scattering images were utilized to determine or refine the optimal spatial range used for SORS data analysis. This new line-scan SORS measurement technique was demonstrated by measuring the Raman signals of melamine powder placed under layers of butter that ranged between 1 and 10 mm in thickness. The method shows promise as a more rapid and less costly method for SORS evaluation of packaged food stuffs and for complex sample materials, and will benefit food processors and regulatory agencies.

Technical Abstract: Conventional methods of spatially offset Raman spectroscopy (SORS) typically use single-fiber optical measurement probes to slowly and incrementally collect a series of spatially offset point measurements moving away from the laser excitation point on the sample surface, or arrays of multiple fiber optic probes that are fixed in number and arrangement. This study investigates the use of a line-scan hyperspectral Raman imaging system to conduct SORS measurements, using a 785 nm point laser for the excitation source and an imaging spectrograph and CCD camera for acquisition of linear SORS data in the spectral region of -592 to 3015 cm-1. With the laser excitation point centered at the midpoint of the linear field of view, a single scan instantly collects a series of Raman spectra offset up to 36 mm on either side of the laser excitation point and with a narrow interval, e.g., 0.07 mm. Four layered samples were created by placing butter slices with thicknesses of 1, 4, 7, and 10 mm on top of melamine powder, providing different individual Raman characteristics to test the line-scan SORS technique. Self-modeling mixture analysis (SMA) was used to analyze the SORS data. Raman spectra from butter and melamine were successfully retrieved for all four butter-on-melamine samples using the SMA method. The line-scan SORS measurement technique provides a flexible and efficient method for noninvasive subsurface evaluation of internally complex or packaged materials, including food ingredients for food safety and quality inspection.