chromatography • spectroscopy 17
Measurement of pharmaceutical active ingredients and excipients
Nick Barnett outlines developments in Raman spectroscopy.
L
ow Resolution Raman Spectroscopy (LRRS) is a very useful technique for
non-destructive chemical analysis and has gained increased attention and acceptance in the analytical laboratory as an affordable, robust and convenient alternative to cumbersome wet chemical analysis or expensive FT-Raman and FT-IR spectrometers.
Unlike high-resolution Raman spectrometers that are often capable of resolving peaks of less than three or four wave numbers apart, the new miniaturized systems relinquish resolution in favour of emphasizing basic identifying spectral features, or fingerprints, that are often distinct and clearly separated. Recent advances allow some of the high-end LRRS systems to compete around the 4cm-1 resolution.
Te introduction of the silicon-
charged coupled device (CCD) detector was the first technological development to enable LRRS. Te CCD sensor has replaced the more light-sensitive single-element detectors such as the photomultiplier tube and avalanche photodiode. Integrated into a well-designed optical bench, the CCD can allow for resolution as fine as 4cm-1. A back-thinned CCD detector, which is more sensitive than a standard CCD, has been integrated into Raman spectrometers allowing for 6cm-1 resolution, increased quantum efficiency and high signal to noise. Advances in these devices have improved wavelength stability and increased S:N ratio, both crucial factors in building libraries and performing unambiguous spectral identification of raw materials.
Ocean Optics offer different Raman Spectroscopy options for their customers. Our approach
to this market is based on OOI sales philosophy of applications customisation sales and service. Some of the available Raman analysis options include Ocean Optics spectrometer-based systems (modular) using the QE65000 as the detector platform with accessories (lasers and probes) provided by partner companies. Turnkey Raman Systems which uses OOI spectrometers as their detection devices provide the next option. Tese ready to use systems offer different resolution, sensitivity and laser wavelength options, including a low-resolution hand held device and desktop units. Te main advantage of these instruments is their software, which better focuses on applications and includes the capability of creating and maintaining a spectral library.
Raman spectroscopy offers a number of benefits for testing and characterisation. Like regular IR spectroscopy, it is rapid and non- destructive. Unlike IR spectroscopy, which falls spectrally within the water window, Raman spectroscopy can be used to capture data on aqueous samples or samples with high moisture content.
So, how to select between a Raman
Fig. 1. The Ocean Optics QE65000 Raman-configured spectrometer incorporates a back-thinned, cooled detector array.
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