Shifted Excitation Raman Difference Spectroscopy with Charge-Shifting Charge-Coupled Device (CCD) Lock-In Detection.

Shifted excitation Raman difference spectroscopy (SERDS) can provide effective, chemically specific information on fluorescent samples. However, the restricted ability for fast alternating detection (usually < 10 Hz) of spectra excited at two shifted laser wavelengths can limit its effectiveness when rapidly varying emission backgrounds are present. This paper presents a novel charge-shifting lock-in approach permitting fast SERDS operation (exemplarily demonstrated at 1000 Hz) using a specialized dual-wavelength diode laser (emitting at 829.40 nm and 828.85 nm) and a custom-built charge-coupled device (CCD) enabling charge retention and shifting back and forth on the CCD chip. For six selected mineral samples (moved irregularly during spectral acquisition), results demonstrate superior reproducibility of the fast charge-shifting read-out over the conventional read-out (operated at 5.4 Hz). Partial least squares discriminant analysis revealed improved classification performance of charge-shifting (four latent variables, sensitivity: 99%, specificity: 94%) versus conventional read-out (six latent variables, sensitivity: 90%, specificity: 92%). The charge-shifting concept was also successfully translated to sub-surface analysis using spatially offset Raman spectroscopy (SORS). Charge-shifting SERDS-SORS spectra recorded from a polytetrafluoroethylene layer, concealed behind a 0.25 mm thick, opaque, heterogeneous layer, matched reference spectra much more closely and exhibited a signal-to-background-noise (S/NB) ratio two times higher than that achieved with conventional CCD read-out SERDS-SORS. The novel approach overcomes fundamental limitations of conventional CCDs. In conjunction with the inherent capability of the charge-shifting lock-in technique to suppress rapidly varying ambient light interference demonstrated by us earlier it is expected to be particularly beneficial with heterogeneous fluorescent samples in field applications.