Absorption detectors indirect detection

Any of the methods of detection used in liquid chromatography can be used in IC, though some are more useful than others. If the eluent does not affect the detector the need for a suppressor disappears. Common means of detection in IC are ultraviolet (UV) absorption, including indirect absorption electrochemical, especially amperometric and pulsed amperometric and postcolumn derivatization. Detectors atomic absorption spectrometry, chemiluminescence, fluorescence, atomic spectroscopic, refractive index, electrochemical (besides conductivity) including amperometric, coulometric, potentiometric, polaro-graphic, pulsed amperometric, inductively coupled plasma emission spectrometry, ion-selective electrode, inductively coupled plasma mass spectrometry, bulk acoustic wave sensor, and evaporative light-scattering detection. 

Sample ions that absorb sufficiently in the UV or visible spectral region may be detected by direct spectrophotometry. Indirect spectrophotometric detection is commonly used for ions that do not absorb. An absorptive reagent is added to the BGE, and this gives a peak in the direction of reduced absorbance when a sample ion passes through the detector. The absorbing reagent, which is sometimes called a visualization reagent, should have a mobility that matches those of the sample ions as closely as possible. Chromate is often used for the indirect detection of anions and a protonated amine cation, such as benzylamine, for detection of cations. 

Detector Several options are available. Direct UV-vis is most common for ions with sufficient absorption. Indirect detection with an added visualization reagent may also be used. 

Molecules that do not possess the appropriate chromophore may be detected when derivatized with a chromophore with high extinction coefficients at peaks in the visible spectrum to facilitate detection. For example, carboxylic acids can be derivatized with 7-methoxy-4-bromomethylcoumarin to give them a chromophore. Specialized books may be consulted for further information concerning derivatiza-tion. Other techniques include resorting to wavelengths below 200 nm under special conditions. In some cases, indirect detection can be used based on the presence of a background UV absorber in the mobile phase. The analyte displaces some of this absorber under certain conditions, resulting in a decrease in the absorption level in the detector cell which can be measured in the same manner as an increase in absorption. Further information on this technique can be obtained from specialized references on the subject. 

Several kinds of detection systems have been applied to CE [1,2,43]. Based on their specificity, they can be divided into bulk property and specific property detectors [43]. Bulk-property detectors measure the difference in a physical property of a solute relative to the background. Examples of such detectors are conductivity, refractive index, indirect methods, etc. The specific-property detectors measure a physico-chemical property, which is inherent to the solutes, e.g. UV absorption, fluorescence emission, mass spectrum, electrochemical, etc. These detectors usually minimize background signals, have wider linear ranges and are more sensitive. In Table 17.3, a general overview is given of the detection methods that are employed in CE with their detection limits (absolute and relative). 

In addition to direct detection in photon-detected absorption or emission spectra, interference effects may be detected indirectly, for example by monitoring photofragment atoms or ions. Walter, et al., (2000) recorded spectra of the N2 (b Ej c1] caE+) <— a E v" = 0) transition. A molecular beam of the metastable N2 a// LE+ v" = 0) state is excited by a tunable laser to the b c c perturbation complex and transitions are detected by a decrease in the a"1 Eg (v" — 0) beam flux and by the coincidence detection of two N atoms on a time and position sensitive detector. The coincidence detection scheme... 

Since neutrons do not directly ionize atoms, they are detected indirectly upon producing a charged particle or a photon, which is then recorded with the help of an appropriate detector. The charged particle or the photon is the result of a neutron interaction with a nucleus. If the mechanism of the interaction is known, information about the neutron can be extracted by studying the products of the reaction. Many types of interactions are used, divided into absorptive and scattering reactions. 

Other detectors are possible in ion chromatography, in addition to the measurement of conductivity. UV detection enables anions absorbent in the UV range (e.g. NO2 , N03 ) to be detected specifically, and also makes it possible to work with a UV-absorbent eluent. In the process, the background absorption is reduced by the non-absorbing ions. With this indirect UV absorption, therefore, negative peaks are obtained. 

LIF detectors are more sensitive and selective than the UV-visible absorption ones, but only a few different laser sources (488 nm Ar ion, 442 nm He-Cd, and 324 nm He-Ca) are available. Direct detection of native fluorescence compounds separated by CE has been demonstrated for some fluorescent dyes. Fluorescence detection of other inorganic and organic contaminants is also achieved by indirect methods - either direct fluorescence by the formation of complexes or derivatives, or by incorporating a fluorophor into the BGE. 

UV photometric. Such detectors are applicable only to molecules with a UV chromophore. They can either use a filter to select a particular wavelength or be tunable to any wavelength. Radiation is absorbed at that wavelength to an extent determined by the extinction coefficient of the molecule and thus calibration is required for individual molecules. By using highly purified solvents, it is possible to extend their use down to 210 nm or below, where many molecules with no chromophore absorb weakly. An alternative approach is to incorporate a chromophore as part of the solvent and monitor a reduction in absorption (indirect photometric detection). 

UV detection may also be performed indirectly. This method is called indirect photometric chromatography (IPC). Introduced independently by Small et al. [65] and by Cochrane and Hillman [66] in 1982, a UV-absorbing eluant is utilized for the determination of UV-transparent ions. In case of anion analysis, the anion exchanger being used is equilibrated with a UV-active eluant Na E . According to Fig. 7-35 (A), a constant signal at the detector ouflet is observed at a constant flow rate and an appropriate absorption wavelength. If a sample Na S ... 

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