Industry Development and Support

What is infrared spectroscopy?

Infrared radiation is the region of the electromagnetic spectrum between the visible and the microwave wavelengths. The nominal range of wavelengths for near-infrared (NIR) is between 750 and 2,500 nm (13,400 to 4,000 cm^-1), while for the mid-infrared (MIR), the spectral range is from 2,500 to 25,000 nm (4,000 to 400 cm^-1). Solid, liquid or gaseous samples can absorb some of the incoming infrared radiation at specific wavelengths resulting in a 'fingerprint' or spectrum. Spectral 'signatures' in the MIR result from the fundamental stretching, bending, and rotating vibrations of the sample molecules, whilst NIR spectra result from complex overtone and frequency combinations at the shorter wavelengths. Although NIR intensities are 10-1000 times lower than for the MIR range, and the peaks concomitantly smaller, highly sensitive spectrometers can be built through several means including the use of efficient detectors and brighter light sources. This allows concentrated bulk or even aqueous materials to be scanned and analysed quickly and easily.

Although spectral peaks in the MIR frequencies are often sharper and better resolved than in the NIR, all the higher overtones (1^st through 6^th) of the OH, NH, CH, and SH bands from the MIR wavelengths are still observed in the NIR region, although much more weakly. This, in addition to the existence of combination bands (e.g. CO stretch and NH bend in protein), gives rise to a crowded NIR spectrum with severely overlapping bands. A major disadvantage of this characteristic overlap and complexity in NIR spectra has been the difficulty of quantification and interpretation from NIR spectra. On the other hand, the broad overlapping bands can diminish the need for using a large number of wavelengths in calibration and analysis routines. In recent years, new instrumentation and computer algorithms have taken advantage of this complexity and have made the technique much more powerful and simple to use. Multivariate chemometrics such as partial least-squares (PLS) regression analysis in particular have revolutionised the ability of NIR and MIR to be used for quantitative analysis.