Charge-shifting optical lock-in detection with shifted excitation Raman difference spectroscopy for the analysis of fluorescent heterogeneous samples

Shifted excitation Raman difference spectroscopy (SERDS) is a powerful tool for the investigation of fluorescent samples such as biological materials. In case of rapidly changing emission backgrounds the efficiency of SERDS can however be limited as alternating detection of spectra excited at the two shifted laser wavelengths is usually restricted to sampling rates of less than 10 Hz. To overcome this issue, a novel optical lock-in detection approach enabling rapid SERDS operation in the kilohertz range using a custom 830-nm dual-wavelength diode laser and a specialized CCD enabling charge shifting on the CCD chip is presented. As an example of fluorescent and heterogeneous natural specimens, six mineral samples were selected and moved irregularly during spectral acquisition. Compared to conventional CCD read-out (operated at 5.4 Hz) the fast charge-shifting read-out performed at 1,000 Hz demonstrated superior reproducibility between repeat spectra. Using partial least squares-discriminant analysis an improved classification performance of the charge-shifting mode (sensitivity: 99 %, specificity: 94 %) over conventional read-out (sensitivity: 90 %, specificity: 92 %) was achieved. Translating the charge-shifting concept to sub-surface analysis using spatially offset Raman spectroscopy (SORS) enabled also the successful detection of charge-shifting SERDS-SORS spectra from a polytetrafluoroethylene layer concealed behind a 0.25 mm thick opaque heterogeneous layer. Chargeshifting SERDS-SORS results demonstrate two-fold improvement in signal-to-background-noise-ratio and match reference spectra much more closely. The charge-shifting approach shows large potential when rapidly changing background interference due to sample heterogeneity, dynamically evolving systems and ambient light variations presents a major challenges, e.g. in biological and biomedical applications.