Fourier Transform Spectrophotometers in the Near-Infrared
Near-infrared (NIR) spectroscopy has an established pedigree. When Herschel studied NIR energy in 1800, it was the first region of the electromagnetic spectrum to be discovered (after the visible region). Almost a hundred years later the Michelson interferometer is often given as the historical beginning of Fourier Transform (FT) spectroscopy. Almost a hundred years ago William Coblentz measured infrared (IR) spectra of organics. Almost 50 years ago, Karl Norris used NIR to characterize agricultural products using the new field of chemometrics. Over 25 years ago, the advent of minicomputers allowed Fourier Transform Infrared (FTIR) spectroscopy to dominate IR identification, but the chemometrics techniques were slow to be adopted in FTIR. One reason is that many spectra are required for chemometrics and a FTIR spectrum has thousands of data points compared to the hundreds of spectral data points in a dispersive NIR spectrum. The arrival of increasingly powerful computers has renewed interest in chemometrics for FT spectrometers and allowed the Fourier Transform Near-Infrared (FT-NIR) spectrophotometer industry to grow. As of today there are many companies producing FT-NIR spectrophotometers to exploit the inherent advantages of the NIR region: Analect/Orbital, Bio-Rad, Bomem, Bran + Luebbe, Bruker, Buehler, Nicolet, and Perkin-Elmer. The FT-NIR spectrophotometer shown in Figure 5.1 displays many of the common
DBURNS: “7393_c005” — 2007/7/27 — 17:11 — page 80 — #2
sampling options used in the NIR. Integrating spheres or other types of diffuse reflectance accessories are utilized heavily in the NIR. Fiber-optic probes can measure remote samples in diffuse reflectance or transmission. Transmission accessories or cells are often temperature controlled or heated.