Detection indirect

Indirect detection is possible with all selective detection principles, e.g. with fluorescence detection (if the mobile phase itself is fluorescent) or electrochemical detection (if the mobile phase can act as an electrochemical reaction partner). Even indirect detection with atomic absorption spectrometry has been described, the mobile phase containing lithium or copper and the spectrometer being used with a lithium or copper lamp.  

System peaks are always obseiwed with indirect detection and need attention (see Section 19.9). In quantitative analysis it is important to note that peak areas are not only a function of sample mass but also of k values (in relation to the k value of the system peak) and of the concentration of the detectable component in the mobile phase. The effects were explained by Sehili and Crommen. 

To optimize sensitivity in indirect detection, the concentration of the mobile phase additive should be kept as low as possible and the ratio of background signal to background noise (dynamic reserve) should be as large as possible. Another important factor is the transfer ratio, the number of molecules of the background buffer additive that are displaced by a molecule of analyte. For the best sensitivity, this ratio should be large. 

Indirect detection is universal, although limited to ionic compounds however, its nonspecificity can also become a limitation, because of ionic components of a mixture can potentially interfere with the analytes of interest. Thus, lacking detection selectivity, the whole selectivity of the method relies on separation. Other drawbacks of this detection mode are inferior sensitivity (almost constantly lower than with the corresponding direct mode) and a rather narrow range of linearity. 

These techniques imply the existence of scalar coupling between the metal atom and a spin-1/2 nucleus which is easy to observe, usually H, or P, but also [10,11] (see also Chapters 2 and 3). 

More modem one-dimensional (ID) sequences such as INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) or DEPT (Distortionless Enhancement by Polarization Transfer) [6,13-15] have not been used extensively [16]. This type of experiment seems to be more powerful than the old INDOR technique. 

Usually the NMR spectra of metal atoms in clusters are simple, except when they are coupled to each other. Spectra may then be complex and require simulation when second order effects are involved. The effect of isotopic abundance will be discussed later (see Section 2.3). 

In some cases, when a spin-1/2 nucleus such as P, or is coupled to a quadrupolar nucleus, the simulation by a computer program of the unusual lineshape of the spin-1/2 may provide information about the chemical shift anisotropy, the quadrupole coupling constant and the indirect scalar spin-spin coupling constant involving the metal atom. This method has been applied to the P spectra of phosphines bound to cobalt in heteronuclear clusters [17]. 

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Ohta and Tanaka reported a method for the simultaneous analysis of several inorganic anions and the cations Mg + and Ca + in water by ion-exchange chromatography. The mobile phase includes 1,2,4-benzenetricarboxylate, which absorbs strongly at 270 nm. Indirect detection of the analytes is possible because their presence in the detector leads to a decrease in absorbance. Unfortunately, Ca + and Mg +, which are present at high concentrations in many environmental waters, form stable complexes with 1,2,4-benzenetricarboxylate that interfere with the analysis. 

Indirect detection of an intermediate. The overall reaction of hydroxylamine with a carboxylic acid derivative yields a hydroxamic acid as the product, Eq. (3-176). 

In October 2006, a research team of scientists from the Lawrence Livermore National Laboratory in California, USA, and the Joint Institute of Nuclear Research in Dubna, Russia, reported the indirect detection of Uuo-294 (Element 118). It is reported to be produced by the following collisions. 

Tran, C. D., Huang, G., and Grishko, V. I., Direct and indirect detection of liquid chromatography by infrared thermal lens spectrometry, Anal. Chim. Acta, 299, 361, 1995. 

Perhaps the most revolutionary development has been the application of on-line mass spectroscopic detection for compositional analysis. Polymer composition can be inferred from column retention time or from viscometric and other indirect detection methods, but mass spectroscopy has reduced much of the ambiguity associated with that process. Quantitation of end groups and of co-polymer composition can now be accomplished directly through mass spectroscopy. Mass spectroscopy is particularly well suited as an on-line GPC technique, since common GPC solvents interfere with other on-line detectors, including UV-VIS absorbance, nuclear magnetic resonance and infrared spectroscopic detectors. By contrast, common GPC solvents are readily adaptable to mass spectroscopic interfaces. No detection technique offers a combination of universality of analyte detection, specificity of information, and ease of use comparable to that of mass spectroscopy. 

Cross-polarisation (CP) in the rotating frame has been introduced as a means of transferring polarisation between different nuclear species in solids [168], and has become of central importance for obtaining spectra of rare spins with low gyromagnetic ratios such as 13C, since a significant sensitivity enhancement may be achieved. Cross-polarisation can be used either for direct observation of low-sensitivity nuclei or for indirect detection of such nuclear species via high-sensitivity nuclei such as protons [169]. 

Indirect detection Method for the observation of an insensitive nucleus (e.g., 13C) by the transfer of magnetisation from an abundant nucleus (e.g., 1H). This method of detection offers great improvements in the sensitivity of proton-carbon correlated techniques. 

Inverse geometry Term used to describe the construction of a probe that has the 1H receiver coils as close to the sample as possible and the X nucleus coils outside these 1H coils. Such probes tend to give excellent sensitivity for 1H spectra at the expense of X nucleus sensitivity in 1-D techniques. They offer a lot of compensation in terms of sensitivity of indirectly detected experiments. 

Optical fiber sensors that use enzymes can operate in the direct or indirect detection mode. In the first case, the optical properties of the reactives, intermediates or products of the biocatalyzed reaction can be monitored using the optical fibers. In the second type, an optochemical transducer generates the optical changes. 

There are mainly three types of transducers used in immunosensors electrochemical, optical, and microgravimetric transducers. The immunosensors may operate either as direct immunosensors or as indirect ones. For direct immunosensors, the transducers directly detect the physical or chemical effects resulting from the immunocomplex formation at the interfaces, with no additional labels used. The direct immunosensors detect the analytes in real time. For indirect immunosensors, one or multiple labeled bio-reagents are commonly used during the detection processes, and the transducers should detect the signals from the labels. These indirect detections used to need several washing and separation steps and are sometimes called immunoassays. Compared with the direct immunosensors, the indirect immunosensors may have higher sensitivity and better ability to defend interference from non-specific adsorption. 

Another detection mode, commonly used in LC and in FIA and recently adapted to CE separations, is indirect detection, based on the detection of a nonchemilumi-... 

Although virtually all drug targets are protein based, the inference that protein expression levels can be accurately (if indirectly) detected/measured via DNA array technology is a false one, as ... 

Many transition metal complexes have been considered as synzymes for superoxide anion dismutation and activity as SOD mimics. The stability and toxicity of any metal complex intended for pharmaceutical application is of paramount concern, and the complex must also be determined to be truly catalytic for superoxide ion dismutation. Because the catalytic activity of SOD1, for instance, is essentially diffusion-controlled with rates of 2 x 1 () M 1 s 1, fast analytic techniques must be used to directly measure the decay of superoxide anion in testing complexes as SOD mimics. One needs to distinguish between the uncatalyzed stoichiometric decay of the superoxide anion (second-order kinetic behavior) and true catalytic SOD dismutation (first-order behavior with [O ] [synzyme] and many turnovers of SOD mimic catalytic behavior). Indirect detection methods such as those in which a steady-state concentration of superoxide anion is generated from a xanthine/xanthine oxidase system will not measure catalytic synzyme behavior but instead will evaluate the potential SOD mimic as a stoichiometric superoxide scavenger. Two methodologies, stopped-flow kinetic analysis and pulse radiolysis, are fast methods that will measure SOD mimic catalytic behavior. These methods are briefly described in reference 11 and in Section 3.7.2 of Chapter 3. 

As was mentioned previously, photometric detection is the most frequently applied detection technique. Most of the commercial CE-systems are equipped with at least a UV detector. Some compounds, such as low molecular weight organic and inorganic ions [57-60], do not produce a direct analytical signal. In such cases indirect detection, by indirect UV or fluorescence [59-64] is applied. Besides photometric detection, an application of indirect amperometric [65] detection was also reported. When the analytical signal results from a decrease in... 

Fig. 17.9. Indirect detection of analytes by the replacement of the probe (co-ion or background electrolyte) ions that are responsible for a constant high-background analytical signal.

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