Detectors indirect detection

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. 

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. 

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... 

Many applications for ion analysis use a UV detector with indirect detection, though other electrochemical, laser-induced fluorescence (LIE), or mass spectrometry detectors have been described. The main advantage of UV detection is its availability on commercial instruments and that both UV-absorbing and non-UV-absorbing analytes may be detected. Nowadays, electrochemical detectors are also available specific background electrolytes (BGEs) must be used and the detector has to be adapted to existing CE instruments. 

Most CE applications are based on a UV detector using direct and indirect detection, though other detectors have been developed. 

Laser-based refractive index detector, Cuprammonium reagent,4-Aminobenzoic acid reagent, Indirect detection methods for cyclodex-trins, and sugar phosphates Reversible derivatization using 2-amino-pyridine ... 

Figure 26-30 shows the principle of indirect detection,M which applies to fluorescence, absorbance, amperometry, conductivity, and other forms of detection. A substance with a steady background signal is added to the background electrolyte. In the analyte band, analyte molecules displace the chromophoric substance, so the detector signal decreases when analyte passes by. Figure 26-31 shows an impressive separation of Cl isotopes with indirect... 

Arora, A., Eijkel, J.C.T., Morf, W.E., Manz, A., A wireless electrochemiluminescence detector applied to direct and indirect detection for electrophoresis on a microfabricated glass device. Anal. Chem. 2001, 73, 3282-3288. 

If a solute of interest does not contain a chromophore, it may be detected by indirect UV detection. Indirect detection is a technically simple and sensitive method for the detection of compounds with little inherent detector response. Indirect UV detection is a nondestructive technique, in which the analyte is characterized in native form. Indirect detection is a universal detection mode, with few requirements as to the exact nature of the analyte. The properties of indirect detection have been reviewed by Yeung.22 Indirect detection is particularly attractive for the analysis of biological compounds. Optical systems are the same for direct or indirect detection the only difference is that, in indirect detection, the mobile phase, rather than the analyte, contains a UV chromophore. 

Indirect detection does require more steps, but oftentimes yields amplified signals relative to direct methods because layering of bridging molecules may increase the number of detector molecules per probe molecule. It is probably this bridging/amplification technique that has allowed current enzyme detection systems to approach the sensitivity of radiolabeled systems. The use of these indirect methods reduces steric problems that might arise from having enzyme molecules directly bound to probe molecules. 

In many cases the inorganic chemist will be interested in the NMR spectrum of a nuclide of low sensitivity (denoted S or indirectly detected spin) in a compound containing a high-sensitivity nucleus (denoted I or detector spin). If scalar coupling exists between the two nuclides, then the 2D HMQC and HSQC techniques offer an alternative to direct observation. The distinction between HMQC and HSQC is that TS -magnetization is stored as either multiple... 

Nuclear track detectors can also be used to indirectly detect fast neutrons. Fast neutrons interact with the base material of a special film and cause recoil protons to be released. These protons then cause damage tracks in the film which can be made visible and counted as described above. The number of tracks can be used to determine the neutron dose. 

Often it is required to detect compounds with no or only very weak chromophores such as sugars and amino acids. Refractive index detectors and mass sensitive detectors can be used but they are relatively insensitive in the context of biological sample concentrations. Indirect detection using a UV or fluorescent eluent can also be employed. However, the most common approach is the use of derivatisation. Derivatisation of some chemically reactive moiety on the analyte can be performed in two modes. In post-column derivatisation the sample is separated first and then reacted with a flowing stream of derivatising reagent being pumped into... 

As discussed earlier, a chromatogram containing n -b 1 peaks is recorded upon injection of a sample containing n nondetectable components in a chromatographic system using a binary mobile phase with a detectable additive [8,18,20]. Of these peaks, the n analyte system peaks, appear at the retention times of the nondetectable analytes. The extra system peak is characteristic of the additive. Equation 13.22ab shows that the area of an analyte system peak is proportional to the size of the perturbation, ie., to the amoimt of the corresponding component in the sample injected. The response factor in the case of an additive used for indirect detection can be derived from Eq. 13.22b. In indirect detection, the detector responds to... 

Indirect detection A method which permits the quantitation of components with a detector which does not respond to their concentration changes (e.g., analysis of carbohydrates with a UV detector). A component to which the detector responds and which is retained on the column is added to the mobile phase in constant concentration. The system peak associated to each component peak can be detected and recorded (see Chapter 13, Section 13.1.3). 

As a general rule the mobile phase should not be detector-active, i.e. it should not have a property which is used for detection (exception indirect detection, see Section 6.9). Otherwise it is very possible that unwanted baseline effects and extra peaks will show up in the chromatogram. However, this recommendation cannot be followed in the case of bulk-property detectors such as the refractive index detector. 

In IC, the detector must be able to pick out or see sample ions in the presence of the eluent ions. There are several methods that can be employed to make this possible. One is to choose a detector that will response only to the sample ions of interest, but not the eluent ions. Another method is to use indirect detection (sometimes called replacement detection). This is where the eluent has a background signed and the presence of samples ions cause a decrease in eluent ions through a replacement process (see Chapter 7). The detector looks for the absence of eluent ions when the sample ion peak elutes and a decreasing signal is detected. 

BGE 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 proton-ated amine cation, such as benzylamine, for detection of cations. 

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