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

Abstract

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.

Shifted excitation Raman difference spectroscopy as enabling technique for the analysis of animal feedstuff

“…A possible solution to overcome that issue would be the fast alternating acquisition of spectra at both wavelengths in the kilohertz range using a recently demonstrated charge-shifting concept. [38,39] A comparison with SERDS spectra recorded on chemical-grade monocalcium phosphate (data not shown) confirms the spectral signature with the presence of all characteristic Raman bands but the abovementioned luminescence bands are completely absent in this case. The cause of the luminescence features is thus likely associated with some (yet unknown) impurities in the food-grade monocalcium phosphate that are not present in its chemical-grade counterpart.…”

“…A possible solution to overcome that issue would be the fast alternating acquisition of spectra at both wavelengths in the kilohertz range using a recently demonstrated charge‐shifting concept. [ 38,39 ]…”

Parallelized shifted‐excitation Raman difference spectroscopy for fluorescence rejection in a temporary varying system

“…We note that, recently, the concept of parallelization of SERDS has also been demonstrated by Sowoidnich et al [27,28] independently, based on a totally different implementation from the present study. They modified the detector circuits to electrically manipulate the accumulated charges on the detector.…”

“…Restricting the observation ROIs to the central part of the imaging plane, where the aberration is at most corrected, helped to reduce the distortion. Alternatively, the lock-in CCD-based SERDS technique recently proposed [28] circumvent the aberration issues at the expense of reduced effective CCD pixel (row) number.…”

6.4 - Bildgebende Differenz-Raman-Spektroskopie mit "Nod-and-Shuffle"-Technik

“…Je kürzer die Einzelmesszeiten sind, desto geringer machen sich Signalfluktuationen bemerkbar. An einem Raman-Einkanal-Spektrometer mit CCD-Detektor konnte bereits gezeigt werden, dass schnelles Hin-und Herschalten auch für SERDS vorteilhaft ist [9]. Diese Technik wurde nun auf einem IFS-Spektrographen mit großflächigem CCD-Chip implementiert, um damit bildgebende SERDS bei sich verändernden Proben und Umgebungsbedingungen zu ermöglichen.…”