Detector solute property

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 

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Solute-property detector A detector which monitors a property of the analyte, e.g. the UV detector. 

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 ... 

In contrast, solute property detectors directly measure some physico-chemical property of the eluate species... 

The function of the detector in hplc is to monitor the mobile phase emerging from the column. The output of the detector is an electrical signal that is proportional to some property of the mobile phase and/or the solutes. Refractive index, for example, is a property of both the solutes and the mobile phase. A detector that measures such a property is called a bulk property detector. Alternatively, if the property is possessed essentially by the solute, such as absorption of uv/visible radiation or electrochemical activity, the detector is called a solute property detector. Quite a large number of devices, some of them rather complicated and tempremental, have been used as hplc detectors, but only a few have become generally useful, and we will examine five such types. Before doing this, it is helpful to have an idea of the sort of characteristics that are required of a detector. 

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. 

Fluorescence detection, because of the limited number of molecules that fluoresce under specific excitation and emission wavelengths, is a reasonable alternative if the analyte fluoresces. Likewise, amperometric detection can provide greater selectivity and very good sensitivity if the analyte is readily electrochemically oxidized or reduced. Brunt (37) recently reviewed a wide variety of electrochemical detectors for HPLC. Bulk-property detectors (i.e., conductometric and capacitance detectors) and solute-property detectors (i.e., amperometric, coulo-metric, polarographic, and potentiometric detectors) were discussed. Many flow-cell designs were diagrammed, and commercial systems were discussed. 

The detector converts a change in the column effluent into an electrical signal that is recorded by the data system. Detectors are classified as selective or universal depending on the property measured. Selective (solute property) detectors, such as fluorescence detectors, measure a physical or chemical property that is characteristic of the solute(s) in the mixture only those components which possess that characteristic will be detected. Universal (bulk property) detectors measure a physical property of the eluent. Thus, with refractive index (RI) detectors, for example, all the solutes which possess a refractive index different from that of the eluent will be detected. Selective detectors tend to be more sensitive than universal detectors, and they are much more widely used. Universal detectors are more commonly used in preparative chromatography, where a universal response is desired and sample size is large. 

By comparison, a specific property type produces no signal (or perhaps only a very small signal) when there is no sample present. The appearance of a sample in the detector introduces a new type of signal and thus produces a relatively large signal (compared to zero signal for the baseline). This type is also called an analyte property or solute property detector. Some examples are given in Table 2. They are inherently the more sensitive type. 

These detectors are less sensitive than solute property detectors with a maximum sensitivity of 1 in 106. Examples of this type of detector include those which monitor refractive index (RI), dielectric constant, or eluant density. The latter two are relatively insensitive and not generally used. 

Solute property detectors measure a characteristic of the solute alone. These detectors are generally more sensitive yielding a detectable signal for nanogram quantities of solute. Representative detectors of this type include, for example, ultra-violet (UV), solute transport, fluorescence, and conductivity monitors. Other less frequently employed detectors of this nature are those based on radioactivity, polarography, and... 

Current IPC detectors are on-stream monitors. HPLC detectors range from (1) non selective or universal (bulk property detectors such as the refractive index (RI) detector), characterized by limited sensitivity, (2) selective (discriminating solute property detectors such as UV-Vis detectors) to (3) specific (specific solute property detectors such as fluorescence detectors). Traditional detection techniques are based on analyte architecture that gives rise to high absorbance, fluorescence, or electrochemical activity. Mass spectrometry (MS) and evaporative light scattering detectors (ELSDs), can be considered universal types in their own right... 

One secondary method of classification, and one that is probably the most frequently used, is based on a more rational differentiation and defines a detector as either a bulk property detector or a solute property detector. This secondary classification applies to GC, LC and TLC detectors. 

In contrast, the electrochemical detector responds only to substances that can be oxidized or reduced and thus, providing the mobile phase is free of such materials, it will only detect oxidizable or reducible substances when they are eluted. It follows that this detector is not only a solute property detector but is also a specific detector. The electrical conductivity detector is a non-specific detector and used widely in ion chromatography where it occupies a unique and almost exclusive position. In contrast, the electrochemical detector, in its... 

Bulk property detectors generally have neither the sensitivity nor the linear dynamic range of solute property detectors and, as a consequence, are less frequently used in modem LC analyses. Furthermore, none can be used with gradient elution, flow programming or temperature programming and so they place considerable restrictions on the choice of chromatographic system. They do, however, have certain unique areas of application, some of which have already been mentioned, but their use probably represents less than 5% of all LC analyses. 

Another bifunctional detector that was developed by Knauer and made commercially available in Europe operated on the simultaneous measurement of UV absorption and refractive index. This detector is particularly interesting as it combines a solute property detector with a bulk property detector and consequently should have a wide field of application. 

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 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. 

Postcolumn reaction detection was reported used by 9% of the respondents to the detector survey (47). The most popular LC detectors are solute property detectors. From a cursory glance at the popular detection techniques already discussed, it is apparent there are many classes of important compounds for which there is no sensitive solute-property detector. For this reason, many types of postcolumn chemistries have been devised to derivatize separated solutes to form a detectable species. Postcolumn reaction detection has been thoroughly reviewed (68,69). 

Two general type of detection devices are available. These are bulk property detectors and solute property detectors. The bulk property detector measures a change in some overall physical property in the mobile phase as it emerges from the column. Two typical examples are measurement of refractive index and conductance. The solute property detector is sensitive to changes in a physical property of the solute as it emerges from the column in the mobile phase a typical example is the measurement of ultraviolet and / or visible absorption. In general,... 

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