Advantages of Fourier-Transform Near-Infrared Spectroscopy

Introduction A near-infrared spectrum (12000 – 4000 cm-1) is composed of combination and overtone bands that are related to absorption frequencies in the mid-infrared region. These combination and overtone bands correspond to the frequencies of vibrations between the bonds of the atoms making up the material. Because each different material is a unique combination of atoms, no two compounds produce the exact same near-infrared spectrum. Therefore, near-infrared spectroscopy can result in a positive identification (qualitative analysis) of each different material. In addition, the size of the peaks in the spectrum is a direct indication of the amount of material present. With modern software algorithms and statistical treatments, NIR spectroscopy is an excellent tool for quantitative analysis, offering a practical alternative to time-consuming wetchemical methods and liquid chromatographic techniques. NIR has become a versatile technique with no sample preparation, decreased costs and analysis time, and the ability to sample through glass and packaging materials. Fourier transform near-infrared (FT-NIR) spectroscopy was developed in order to overcome the limitations encountered with dispersive NIR instrumentation. Using a prism or moving grating, the dispersive instrument separates the individual frequencies emitted from the nearinfrared source. A detector then measures the amount of energy at each frequency that has passed through the sample. To reduce the analysis time per sample, a method was needed to measure all of the infrared frequencies simultaneously, rather than individually. The FT-NIR spectrometer produces a unique type of signal called an interferogram that has all of the infrared frequencies “encoded” into it. The signal can be measured very quickly, with the collection of multiple scans per second. This increased speed of measurement is known as the Felgett Advantage. Via a well-known mathematical technique called Fourier transformation, the interferogram is transformed or “decoded” into a spectrum of frequency verses intensity. Analysis Process