Low-Frequency Raman Spectra of Amino Acids Measured with an Astigmatism-Free Schmidt-Czerny-Turner Spectrograph: Discovery of a Second Fingerprint Region

A novel astigmatism-free spectrograph design, the Princeton Instruments SCT 320 IsoPlane Schmidt-Czerny-Turner (SCT) spectrograph, is shown to give Raman spectra with better resolution and signal-to-noise ratios than traditional Czerny-Turner (CT) spectrographs. A single-stage SCT spectrograph has been interfaced to a new low-frequency Raman spectroscopy module that uses volume phase holographic gratings as Rayleigh line filters. The system has been used to measure the low-frequency Raman spectra of several crystalline amino acids. A spectrum of l-cystine shows that peaks as close as 10 cm to the Rayleigh line are measureable. The spectra show complex patterns of peaks that can be used to easily distinguish the acids from each other, resulting in what may best be called a “second fingerprint region.” While useful for low-frequency Raman work, the system described here is useful for examining higher-frequency modes of samples as well. Introduction The Czerny-Turner (CT) spectrograph has been used to measure Raman spectra of samples for decades. Inherent in the design of CT spectrographs are optical aberrations, including astigmatism, coma, and spherical aberration. Astigmatism occurs when mirrors are used to focus a source off axis, giving distorted images and spectral peaks that are broadened, thus degrading spectral resolution. Coma occurs when mirrors are used to image a source off axis, and gives rise to images with a comet-like tail and spectra with asymmetrically broadened peaks. Spherical aberration is caused by using spherical mirrors to focus an image and causes a symmetrical blur in images and broadening in spectra. These optical aberrations are a result of the laws of physics and are present in CT spectrographs regardless of their manufacturer. The net result of the astigmatism, coma, and spherical aberration inherent in the CT design are Raman peaks of poor quality. The 802 cm Raman shift peak of cyclohexane measured with a CT spectrograph seen in Figure 1 has an asymmetric lineshape, is broadened so it has degraded spectral resolution, and is short, thus lowering the signal-to-noise ratio (SNR). In response to the problems with the CT spectrograph, Princeton Instruments has developed the Schmidt-Czerny-Turner (SCT) spectrograph, or IsoPlane SCT 320 spectrograph. Compared to the CT design, the SCT spectrograph has zero astigmatism at all wavelengths across the entire focal plane, as well as reduced levels of coma and spherical aberration. Figure 1 shows a comparison of the 802 cm Raman shift peak of cyclohexane measured with