Fourier Analysis of Detector Problems

Spectroscopy. Infrared spectroscopy (48) permits stmctural definition, eg, it resolves the 2,2 - from the 2,4 -methylene units in novolak resins. However, the broad bands and severely overlapping peaks present problems. For uncured resins, nmr rather than ir spectroscopy has become the technique of choice for microstmctural information. However, Fourier transform infrared (ftir) gives useful information on curing phenoHcs (49). Nevertheless, ir spectroscopy continues to be used as one of the detectors in the analysis of phenoHcs by gpc. 

Here we have encountered the crucial, essential difficulty in X-ray diffraction analysis. It is not experimentally possible to directly measure the phase angles hki of the structure factors. The best that our sophisticated detectors can provide are the amplitudes of the structure factors Fhki but not their phases. Thus we cannot proceed directly from the measured diffraction pattern, the measured intensities, through the Fourier equation to the crystal structure. We must first find the phases of the structure factors. This central obstacle in structure analysis has the now infamous name, The Phase Problem. Virtually all of X-ray diffraction analysis, not only macromolecular but for all crystals, is focused on overcoming this problem and by some means recovering the missing phase information required to calculate the electron density. 

The problem can be circumvented by using a UV diode array detector and performing a Fourier transform analysis on an absorbance data array, for example, a range of 64 to 128 absorbance readings at evenly spaced... 

The use of infrared spectrometry for quantitive analysis became possible only in the 1980s, when affordable and user-friendly benchtop Fourier-transform spectrometers became available. The sensitivity of the FT-IR spectroscopy was, however, insufficient to meet the requirements of the immunoassay. To address this problem, an instrument equipped with a liquid nitrogen-cooled detector made from a semi-conducting material, for example MCT (mercury-cadmium-telluride) or InSb (indium antimonide), was used to increase sensitivity by a factor of 20 compared with the thermal detector DTGS found in standard FT-IR machines. Use of a light-pipe cell with a long optical path (20 mm) for a... 

Response to a Rapid Change - Analysis by Fourier Methods Fourier methods convert between functions of time and functions of frequency. This allows us to simplify (or at least mechanize) some problems that would otherwise be cumbersome. One of these is the determination of the frequency response of a detector given the frequency response of a circuit that contains a detector and electronics. We discuss that method in Chapter 15. 

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