Absorption mechanism detectors

Detectors Based on Light Emission/Absorption Mechanisms. 835... 

A problem encountered with atomic absorption is that emission from the flame may fall on the detector and be registered as negative absorption. This can be eliminated by modulating the light source, either mechanically or electronically, and using an a.c. detector tuned to the frequency of modulation of the source. D. C. radiation, such as emission from the flame, will then not be detected. A high intensity of emission, however, may overload the detector, causing noise fluctuations. 

Energy from a light beam is absorbed by molecules with a chromophore. An absorption spectrophotometer uses this mechanism, and the energy loss depends on the concentration and molecular absorption constant of analyte molecules and the wavelength of the light. The most popular detector, the... 

Infrared spectroscopy is often used for qualitative analysis, and its powerful selectivity means that it can be used as a detector. However, the absorption of the eluent molecules, particularly in reversed-phase separations, often interferes with the detection of analytes. The infrared absorption detector therefore requires mechanical assistance to eliminate the solvent or needs powerful computer assistance to eliminate the background signal. 

Si(Li) detectors without Be windows ("windowless") or with thin metal-coated polymer films (Ultra-Thin Window UTW) have become an important peripheral to modern-day AEMs for the qualitative detection of elements with 5vacuum requirements because the removal of the Be window increases the probability of detector contamination (from the specimen or column environment) and consequent degradation of performance [12]. Windowless and UTW Si(Li) detectors are commonly installed with additional airlock mechanisms and only on instruments with acceptable levels of vacuum cleanliness. Thus, design constraints on modern AEMs preclude placement of the UTW detector close to the sample. In addition, loss of detection efficiency at low energies (light-element K-lines with the L-lines of transition metals all conspire to limit windowless or UTW EDS analysis of minerals to a qualitative basis only. 

Film and Photomultiplier Detector Systems. In the original ultracentrifuge built by Svedberg and co-workers (3,4) and also in the commercial instrument, the light intensity pattern was recorded on film, requiring a densitometer to obtain the absorption profile. Schachman and co-workers ( 5,6) constructed a mechanically scanned photomultiplier system in the early 1960 s. In addition to a commercial version now available (7), a number of other systems with computer controlled gathering of data have been constructed (8, , 10, 11, 12). 

The use of chiral mobile phases has both advantages and disadvantages. For example, the multiple equilibria occurring in the mobile phase and in the stationary phase complicates elucidation of the separation mechanism. The presence of the chiral mobile phase additive can also complicate detection. For instance, additives with relatively high UV absorbance decrease the detection limit of the separated enantiomers when using UV detection. Furthermore, resolved enantiomers enter in the detector cell in the form of complexes with the chiral resolving ligand. These complexes are diastereomers and therefore may differ in molecular absorptivity, as well as other properties. As a consequence, it is necessary to have a separate calibration curve for each enantiomer. 

Quenching of Cgo by electron donors occurs efficiently, and the mechanism is primarily electron transfer, as shown by the formation of the well-known transient absorptions (350 to 800 nm) of aromatic amine radical cations [64]. Because of the broad visible absorption of these radical cations, it is difficult to confidently assign visible absorptions to the C o radical anion. However, with an infrared-sensitive germanium detector, a prominent transient with maxima at 950 and 1075 nm appears assigned to the Cgo radical anion, [64] in good agreement with simultaneous and later measurements of others [17, 56,65-67]. 

A typical y-ray spectrum taken with a Ge(Li) detector is shown in Fig. 7.15. Because of the different mechanisms of y-ray absorption (section 6.4) y-ray spectra... 

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