Solvent-property detectors

Solvent-property detector A detector which monitors a property of the HPLC mobile phase which is perturbed when an analyte elutes from the chromatographic column. 

Another classification of detector is the bulk-property detector, one that measures a change in some overall property of the system of mobile phase plus sample. The most commonly used bulk-property detector is the refractive-index (RI) detector. The RI detector, the closest thing to a universal detector in lc, monitors the difference between the refractive index of the effluent from the column and pure solvent. These detectors are not very good for detection of materials at low concentrations. Moreover, they are sensitive to fluctuations in temperature. 

Solute property detectors, such as spectroscopic andj electrochemical detectors, respond to a physical or chemical] property characteristic of the solute which, ideally, is] independent of the mobile phase. Althou this criterion is rarely met in practice, the signal discrimination is usually sufficient to permit operation with solvent changes (e.g., flow programming, gradient elution, etc.) and to provide high sensitivity with aj wide linear response range. Table 5.4. Solute-specific detectors complement ulk property detectors as they provide high ... 

Solute-property detectors. They critically respond to a particular physical or chemical characteristic of the solute (in question), which should be ideally and absolutely independent of the mobile-phase being used. But complete independence of the mobile-phase is hardly to be seen, however, signal discrimination is good enough to enable distinctly measurable experimental procedures with solvent changes, such as gradient-elution. 

Transport detectors are a unique type of solute property detector in that the signal from the sensor is entirely independent of the solvent that is used as the mobile phase. Various forms of transport detectors have been commercially available over the years past but, due to certain deficiencies in the early models, they did not become popular and (to the author s knowledge) none are currently being manufactured. Nevertheless, the transport detector has the potential qualities that are inherent in the ideal detector, i.e. universal detection, high sensitivity,... 

The other two principles that detect changes in the solvent properties are conductivity and density measurements. Conductivity as a detection principle can only be used for ionic substances. The detection range is quite high, but the detector is sensitive to changes in solvent composition and shows a baseline shift if gradient elution is applied. 

The UV detector is the most widely used detector for LC. It is a solute property detector that is suitable for those solute compounds that absorb radiation in the UV range ( 190-400 nm). Ultraviolet-photometric detectors are relatively insensitive to temperature and flow rate fluctuations. The sensitivity to solute detection is high (noise equivalent concentration 10 °g/ml).f Ultraviolet-photometric detectors are also well suited to applications that use gradient elution, given that many common LC solvents have low UV absorptivities. 

Bulk property detectors they make use of the variations in the characteristic properties of the bulk solution (the solvent or solvent mixture). Hence variations are the direct result of the elution of the solute. The most widely spread of these detectors is the refractive index (RI) detector. 

Detectors can be broadly classified into two types. Bulk Property detectors which function by measuring some bulk physical property of the column eluent (e.g. dielectric constant or refractive index) and Solute Property detectors which function by measuring a physical and/or chemical property that is characteristic of the solute only (e.g. UV absorption). This classification is not completely precise, for example, the UV detector, which is usually classed as a solute property detector, when used with an ethyl acetate-heptane solvent mixture as the mobile phase will give a constant background signal due to UV absorption by the ethyl acetate. Furthermore, any fluctuation in ethyl acetate content of the mobile phase will appear as noise on the detector output. It follows that the UV detector, although a solute property detector, behaves as a hybrid between a bulk property detector and a solute property detector under some conditions of use. 

Solute property detectors can place stringent demands on the purity of the solvent employed as the mobile phase. For example the UV detector often operates at sensitivities of 10 to 10 g/ml and thus the mobile phase must be free from UV absorbing materials at these concentration levels. This is necessary because trace impurities often have significantly differing polarities from that of the bulk solvent and thus it may take a... 

Transport detectors are a unique type of solute property detector. In most solute property detectors, the sensing system monitors some property of the solute that is not shared by the solvent or that the solvent has to markedly less extent. It follows that such detecting systems are,to some extent, selective in their detecting capabilities and further restricts the choice of solvents to those that do not possess the property being measured. Thus, the choice of mobile phase is limited, and this can be particularly disadvantageous when employing gradient elution development. The transport type of detector was developed to overcome these limitations. 

Solute property detectors measure some property that is exclusive to the solute only and not to the column eluent. Among them are the most sensitive, most specific and those having the widest linear dynamic range. In general, they require to be used with very pure mobile phases or solvents free from substances that possess the property being measured. 

Other properties of solvents which need to be considered are boiling point, viscosity (lower viscosity generally gives greater chromatographic efficiency), detector compatibility, flammability, and toxicity. Many of the common solvents used in HPLC are flammable and some are toxic and it is therefore advisable for HPLC instrumentation to be used in a well-ventilated laboratory, if possible under an extraction duct or hood. 

This classification is concerned with whether the detector monitors a property of the solute (analyte), e.g. the UV detector, or a change in some property of the solvent (mobile phase) caused by the presence of an analyte, e.g. the refractive index detector. 

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