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Introduc)on Raman spectroscopy is typically performed using CCD cameras in conjunction with mirror-based Czerny-Turner (C-T) spectrographs or lens-based instruments using volume phase holographic gratings (VPHG). Each design comes with significant tradeoffs. While C-T instruments offer scanning capability and flexible spectral coverage from the VUV to the mid-IR region, aberrations inherent in the off-axis C-T design reduce the resolution, particularly at focal plane locations distant from the center of the CCD array. Astigmatism and coma are the most significant of these aberrations. VPHG spectrographs image well, but lack flexibility, cannot achieve high resolution, and are subject to slit image curvature. A new family of spectrographs from Princeton Instruments, the IsoPlane SCT-320 and 160, do away with tradeoffs, providing flexible scanning with multiple grating turrets while increasing spectral resolution and signal-to- background ratios with their low-aberration, high fluence designs. Spectral resolution in dispersive CCD-based systems is determined primarily by the focal length, grating groove density, entrance slit width and imaging quality of the spectrograph. Reducing the camera pixel pitch cannot improve resolution when the spectrograph point spread function (PSF) diameter is significantly larger than the pixel size. The IsoPlanes, unlike shorter focal length C-T instruments, do produce very small focused spots, thereby achieving higher spectral and spatial resolution than would normally be expected for compact mirror-based spectrographs. This poster shows that the well-known Raman spectra of carbon tetrachloride (CCl 4 ) and cyclohexane (C 6 H 12 ) are obtained with higher resolution and signal-to-background ratios with the IsoPlane spectrographs than with a conventional 300 mm C-T instrument. Raman spectroscopy setup Laser: DPSS doubled YAG laser @ 532 nm (Impex), ca. 50 mW. Optics: beam expander, 20X objective, dichroic Raman mirror and RazorEdge filter (Semrock, U grade). Excitation and light collection: Neat liquid samples in NMR tube, 180° backscattering geometry, focus into fiber bundle (19 x 200 or 128 x 50 µm cores), linearized end at entrance slit (10-20 µm). No polarization control. Spectral images were acquired with several minutes of exposure time to reduce noise, followed by software binning. Spectrographs (coverage @ 600 nm center λ): • IsoPlane SCT-320/600 gr/mm grating, spectral coverage 3580 cm -1 . • IsoPlane 160/1200 or 1800 gr/mm, coverage 2530 or 1400 cm -1 . • Acton SP-2356/1200 or 1800 gr/mm, coverage 1655 or 900 cm -1 . Cameras: • Pixis 400 BR, 1340 x 400 20 µm pixels (w/ Iso-320). • Pixis 2KBUV, 2048 x 512 13.5 µm pixels (w/ Iso-320/SP-2356). Acknowledgments & further informa)on We thank the organizers of the 11 th WITec Symposium on Confocal Raman Imaging for the opportunity to present our work. For more information: [email protected]. IsoPlane 160 and SP2356 spectra Figure 2. Raman spectral images of cyclohexane acquired with IsoPlane 160 (L) and SP-2356 (R). Figure 3. Fingerprint region Raman spectra. Figure 4. 801 cm -1 cyclohexane peak. IsoPlane dispersion is 33% less than SP-2356. Figure 5. 801 cm -1 cyclohexane peak at comparable dispersion. Figure 6. 460 cm -1 CCl 4 peak. Isotopic splitting is barely resolvable. Figure 7. After deconvolution, isotopic splitting is clearly visible. Conclusions Fig. 2 shows dispersed fiber images. The blurring caused by aberrations in the C-T instrument is clearly visible. Figs. 3-5 compare spectra obtained at different dispersions. The IsoPlane 160 focal length is 2/3 that of the SP-2356, so with the same grating its dispersion is 2/3 as great. Figs. 3 and 4 show that the IsoPlane 160 achieves narrower linewidths and greater peak height even at reduced dispersion. Fig. 5 shows a further increase in resolution at comparable dispersions. The spectral coverage of this configuration is ~1400 cm -1 , enough to cover the fingerprint region with an 1800 gr/mm grating. Fig. 6 shows that neither instrument has the resolution necessary to resolve isotopic splitting in CCl 4 . However, the IsoPlane, as a linear shift-invariant imaging spectrograph, lends itself to deconvolution by standard techniques (Lucy-Richardson algorithm found in the Matlab Image Processing Toolbox). Results are shown in Fig. 7, along with a spectrum obtained with a high resolution instrument (Trivista triple Raman spectrometer in additive mode, with 8X the dispersion). In summary, the IsoPlane imaging spectrographs show clear improvements relative to Czerny- Turner instruments in spectral coverage, resolution and peak height. Edward Gooding and Jason McClure Princeton Instruments IsoPlane SCT320 spectrum The IsoPlane SCT-320 with 600 gr/mm grating has only half the dispersion of the SP-2356 with 1200 gr/mm grating, yet the two instruments have comparable resolution. At right, a high resolution Raman spectrum of cyclohexane is shown (Fig. 1). The entire Raman region is covered in a single acquisition.
