Interferometric spectrophotometers
Technical details
• The IR source is a Globar or similar proprietary ‘hot’ element that operates at approximately 1300 K. The detector for FTIR needs to have a fast response and low inherent noise. The TGS detector remains the most widely employed for routine use. Pyroelectric devices based upon lithium tantalite are becoming popular as they are less expensive, have greater ordinate linearity and present better temperature stability (TGS linearity falls off above approximately 32C). For high precision work with lower noise, liquid nitrogen-cooled semiconductor detectors are available based on indium–antimonide (In–Sb), indium–gallium–arsenide (In–Ga–As) ormercury–cadmium–telluride (Hg–Cd–Te).
• FTIR spectrometers have advantages over dispersive instruments. The interferometer offers greater light collection and throughput. More light means less noise and greater sensitivity. The multiplex advantage concerns the very nature of the interferometric measure ment. All wavelengths are observed for a single scan at the same time. In a dispersive instrument, only one wavelength is detected at any one time. In the interferometer, more time is spent effectively measuring each wave length, even though the total scan time may be the same. More time to measure each wavelength means lower noise and greater sensitivity.
• Two options are available to the FT technique: either a spectrum can be scanned much faster than a dispersive instrument, or the same time as for the dispersive instrument can be spent measuring a spectrum to present lower noise results. In practice, a single mirror oscillation in the interferometer produces a spectrum scan in a fraction of a second. A dispersive instrument scan can take up to 10 minutes. Taking 10 minutes over an FTIR measurement allows the averaging of very many scans. The signal-to-noise (S/N) ratio in an IR spectrometer is proportional to (number of scans). Accumulating and aver aging 1, 4, 16, 64, 256 or 1024 scans produces a signal-to-noise improvement of 1-, 2-, 4-, 8-, 16- or 32-fold, respectively.
• The level of stray light associated with FTIR spectroscopy is low, typically less than 0.02%, because the technique is devoid of imperfect gratings and the signals selected are associated only with the oscillating mirror move ment. Absorbance values remain linear up to two absorbance units, which in turn leads to more accurate quantitative measurements, even with strongly absorbing bands.
• Resolution is generally excellent over the whole spectrum, with an effective measure ment spectral bandwidth of 1cm1 being achieved readily. The increased sensitivity and scanning speed of FTIR spectrometers and associated sensitive cooled semiconductor detectors enables IR microscopes and gas chromatographs to be coupled to FTIR spectrometers, thereby expanding the types of samples that can be analysed by FTIR. The ability to couple a micro scope opens up possibilities for examining minute quantities of a contaminant, for example in tablets or other pharmaceutical preparations; mapping the homogeneity/inhomogeneity of tablets; and identifying individual layers in laminate polymers.
