FAQ series: This series of publications explores some of the most frequently asked questions regarding Timegated® spectrometers and measurements without having you read page after page of text.
Both Raman and NIR (Near-infrared) spectroscopies are non-destructive optical vibrational techniques that can provide molecular and structural sample information. These two techniques are often seen as complementary because many vibrations that produce strong Raman responses produce low-intensity IR responses and vice versa. For example, many polar bonds (e.g. O-H) are weak Raman scatterers but strong NIR absorbers and this is the reason NIR techniques often have difficulties with samples with high water concentrations. Water may cause very strong interfering responses in NIR whereas the water responses are still observable but not in any way limiting with Raman spectroscopy. Consequently, NIR spectroscopy is better suited for detecting small water concentrations and Raman can even be used to measure different kinds of water-based solutions such as upstream process cell culture media in bioreactors.
Raman and NIR spectroscopies often have different strengths and weaknesses with different sample types. One challenge Raman spectroscopy has traditionally faced has been fluorescence interference. Time-gating and other fluorescence suppression techniques have broadened the types of applications where Raman spectroscopy can be used while retaining the technique’s flexibility.
Raman spectroscopy usually produces spectra with sharper and more well-defined peaks whereas NIR responses are often broader and overlapping. The sharper bands enable easier and more accurate use of spectral libraries for identification purposes. Because of the overlapping and less-defined peaks, analyte quantification using NIR usually relies on the effective use of more advanced multivariate chemometric techniques. These techniques generally also produce promising results with Raman spectroscopy, but they are often not required, and the necessary calibration sample sets may be smaller because of this.