Detectors bulk-property

By far the most used detector is the thermal conductivity detector (TCD). Detectors like the TCD are called bulk-property detectors, in that the response is to a property of the overall material flowing through the detector, in this case the thermal conductivity of the stream, which includes the carrier gas (mobile phase) and any material that may be traveling with it. The principle behind a TCD is that a hot body loses heat at a rate that depends on the... 

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. 

Refractive index detectors. These bulk property detectors are based on the change of refractive index of the eluant from the column with respect to pure mobile phase. Although they are widely used, the refractive index detectors suffer from several disadvantages — lack of high sensitivity, lack of suitability for gradient elution, and the need for strict temperature control ( + 0.001 °C) to operate at their highest sensitivity. A pulseless pump, or a reciprocating pump equipped with a pulse dampener, must also be employed. The effect of these limitations may to some extent be overcome by the use of differential systems in which the column eluant is compared with a reference flow of pure mobile phase. The two chief types of RI detector are as follows. 

The second most widely used detector in HPLC is the differential refractometer (RI). Being a bulk property detector, the RI responds to all substances. As noted in Table 3 the detection limits are several orders of magnitude higher than obtained with the UV detector. Thus, one turns to the RI detector in those cases in which substances are non-UV active, e.g. lipids, prostaglandins. In addition, the RI detector finds use in preparative scale operation. Finally, relative to the UV detector, the RI is significantly more temperature and flow sensitive and cannot be used in gradient elution. 

HPLC is extremely useful in monitoring and optimizing industrial processes. Conventional process monitors measure only bulk properties, such as the temperature and pressure of a reactor, while HPLC permits continuous realtime monitoring of consumption of starting materials, product composition, and impurity profile. There are a number of new initiatives relevant to HPLC for process monitoring, including sample preparation, automation, miniaturization, and specialized detectors. 

Bulk property detectors function by measuring some bulk physical property of the mobile phase, e.g., thermal conductivity or refractive index. As a bulk property is being measured, the detector responses are very susceptible to changes in the mobile phase composition or temperature these devices cannot be used for gradient elution in LC. They are also very sensitive to the operating conditions of the chromatograph (pressure, flow-rate) [31]. Detectors such as TCD, while approaching universality in detection, suffer from limited sensitivity and inability to characterise eluate species. 

KBr) databases. Quantitative analysis by GC-FUR is complicated by many uncertainties associated with both the chromatography and spectroscopy [196]. Bulk property detectors (e.g. TCD, FID, etc.) can be used for quantitative analysis when mixture components are known, but provide little structural information for unknown mixture components. Both integrated absorbance and Gram-Schmidt vector methods have been used for the quantitative analysis of mixture components in GC-FTIR. 

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. 

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). 

Bulk-property detectors They specifically measure the difference in some physical property of the solute present in the mobile-phase in comparison to the individual mobile-phase, for instance ... 

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 thermal conductivity (TC) detector is the best known of a class of detectors known as bulk property detectors. These are sensitive to some overall property of the effluent. Often the measured property is a physical parameter, rather than a chemical one. The distinguishing characteristic of a bulk property detector is that there is a significant response in the absence of sample. A bulk property detector, when sample comes through, measures a change of property from the baseline value, A to A+... 

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. 

Refractive Index. Commercial LC refractometers are based on one of two designs, deflection or Fresnel, as shown in Figure 9.28. Both types require reference and sample cells typical of bulk property detectors. 

The electron capture detector (ECD) was invented by Lovelock in 1961 and is probably the third most used detector. As its name implies, it is selective for materials that capture electrons—halogen- and nitrogen-containing compounds such as pesticides and unsaturated compounds such as the polynuclear aromatics. It is an ionization detector, but unlike the FID it is a concentration type and a bulk property type detector. As such it is an exception to our generalization that bulk property detectors are not very sensitive. 

Generally, HPLC detectors are classified as either of two types (1) Those which monitor bulk properties of the mobile phase and (2) Those which are sensitive to solute properties. 

Bulk property detectors, which are universal detectors, operate by comparing a property of uncontaminated mobile phase with the corresponding property of solute-containing column effluent. 

To many analysts the major limitation of electrochemical detection for liquid chromatography (LCEC) is its limited applicability to gradient elution techniques. Amperometric electrochemical detectors exhibit both the best and the worst characteristics of solute property and bulk property detectors. While the Faradaic current arises only from the solute, the non-Faradaic current arises from... 

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... 

This type of bulk property detector monitors the conductivity of the eluent. All ions from the analyte and from the buffer contribute to produce a signal. Detector response is linear over a wide range. Cell resistance is inversely proportional to electrolyte concentration. Since AC voltages must be used to avoid polarization of the sensing electrodes, the physical quantity measured is impedance, not resistance. 

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. 

The pressure sensitivity of a detector is extremely important as it is one of the detector parameters that determines both the long term noise and the drift. As it influences long term noise, it will also have a direct impact on detector sensitivity or minimum detectable concentration together with those other characteristics that depend on detector sensitivity. Certain detectors are more sensitive to changes in pressure than others. The katherometer detector, which is used frequently for the detection of permanent gases in GC, can be very pressure sensitive as can the LC refractive index detector. Careful design can minimize the effect of pressure but all bulk property detectors will tend to be pressure sensitive. 

The conductivity detector is a bulk property detector and, as such, responds to any electrolytes present in the mobile phase e.g. buffers etc.) as well as the solutes. It follows that the mobile phase must be arranged to be either non-conducting, which in many cases is difficult if not impossible to achieve, or the mobile phase buffer electrolytes must be removed prior to the detector. This technique of buffer ion removal is commonly called ion suppression. The first type of ion... 

Bulk property detectors, and in particular, the refractive index detector, have an inherently limited sensitivity irrespective of the instmmental technique that is used. Consider a hypothetical bulk property detector that monitors, for example, the density of the eluent leaving the column. Assume it is required to detect the concentration of a dense material, such as carbon tetrachloride (specific gravity 1.595), at a level of 1 pg/ml in w-heptane (specific gravity 0.684). 

This situation is typical for a bulk property detector and the sample chosen will be particularly favorable for this hypothetical detector, as the solute to be sensed exhibits a large difference in density from that of the mobile phase. 

In a similar way, the density of the contents of the cell will change with pressure and, if there is a significant pressure drop across the cell, also with flow rate. These arguments apply to all bulk property detectors and it must therefore be concluded that all bulk property detectors will have a limited sensitivity (probably, on average, and for most compounds, the maximum sensitivity that could be expected would be about 10 g/ml). Furthermore, to realize this sensitivity, the... 

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