Potential

Employed in fields ranging from forensics to astrophysics to industrial chemistry, Fourier transform infrared spectroscopy (FT-IR) systems vary considerably in price, performance, and flexibility. Before investing in one of these instruments, scientists should define how they plan to use the technology and which materials they will examine with the equipment. Manufacturers say that basic infrared spectroscopy technology has neared maturity; as a result, most standard units produced by different manufacturers are likely to perform equally well. The sophistica- tion of FT-IR accessories, however, is constantly advancing, so scientists should seek systems with flexibility to accommodate additions as the technology evolves. FT-IR spectroscopy uses the Fourier transform mathematical process to obtain the spectra of samples irradiated by an infrared spectrometer. The spectrometer is composed of three basic parts—an optical head or bench, a computer, and the electronics required to conned the two. The quality of these components varies according to the planned application. The optical head of the spectrometer, which includes a source, an interferometer, a sampling compartment, and a detector, is available in a variety of configurations. For example, there are several types of detectors to choose from, ranging from triglycine sulfate detectors—the least sensitive and least expensive detectors—to mercury cadmium telluride detectors, which offer more sensitive measurements in a shorter period of time. Different sampling systems, used according to the material to be examined, include transmission, attenuated total reflectance, diffuse reflectance, or photoacoustic measurement. Other specifications to consider include the size of the sample compartment, the interferometer’s scanning speed (scans in second), and the source’s intensity and stability. Dr. Peter Griffiths, a chemistry professor at the University of California, Riverside, who has worked extensively in the FT-IR spectroscopy field, suggests that the most important specifications to consider in an optical unit before pur chasing an FT-IR spectrometer are resolution, signal-to-noise ratio, and photometric accuracy. Resolution is often considered the most critical specification relating to the optical head, but, according to Griffiths, it shouldn’t always be. While researchers studying certain substances need spectrometers with high resolution—examining gas spectra, for instance, requires a resolution of 0.1 wave numbers or better—most condensed materials require resolution of only four, or perhaps two, wave numbers. Photometric accuracy, or the degree to which band intensity matches its actual value, is extremely important, according to Griffiths. While there is no agreed-upon method for specifying this value, Grifliths recommends testing an FT-IR spectrometer before buying it by measuring a 50-micron-thick piece of polystyrene film with triangular anodization. Dave Compton, an applications manager with the Digilab division of Bio RAD Laboratories in Boston, recommends that scientists take a critical attitude toward specifications in general. A manufacturer might claim its interferometer can do 50 scans per second, while neglecting to mention that the accompanying noise would make the data “essentially useless.” The computer components of FT-IR spectrometers, however, offer a fairly safe area of comparison. The number of bits in the A/D converter, memory capacity, and communication networks vary considerably and influence the price of the instruments. (See chart.) Compton and other manufacturers agree that basic FT-IR spectroscopy technology is fairly mature. Current advances are focused on accessories for FT-IR spectrometers. Canadian-based Bomen Inc., for example, recently developed an -accessory that converts an FT-JR spectrometer into a Raman spectrometer. On a less elaborate scale, Nicolet Instruments’ basic FT-JR spectrometer systems offer a variety of options and accessories including an automatic sample shuttle, purge shutters, a microscope accessory, and a laser CD disk drive. Barbara Allgaeir, a product specialist with Analect Instruments of New York, notes that the demand for flexibility in FT-JR spectroscopy is part of a trend toward “hyphenating,” or combining technologies to provide researchers with a fuller picture. To meet this trend, her company. has recently developed the Optibus optical conduit that allows scientists to connect several different sampling systems to a single optical head. Miles Weiss is a freelance writer based in Washington, D.C.

Potential

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