Bulk property detectors sensitivity

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. 

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. 

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). 

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. 

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. 

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... 

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. 

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). 

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... 

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. 

The peak represented 3 pm of material and, taking the concentration at the peak maximum as twice the peak average concentration, the peak maximum concentration was about 8 pm/ml. The peak height appears to be about three times the noise and so the sensitivity (that concentration that will give a signal equivalent to twice the noise) is about 5.3 pm/ml or in more standard terms 5.3 x 10 g/ml. As the chiral detector is a bulk property detector, a sensitivity of 5.3 x 10 g/ml seems more realistic. Nevertheless, this sensitivity is a great improvement on many chiral detectors previously described. 

An example of the separation of a mixture of tert-Boc-valine and phenylanaline methyl ester from a Sephadex LH-20 column monitored by the density detector is shown in figure 16. Each peak represents 50 mg of solute and thus the sensitivity is extremely low. Although it is a bulk property detector, and thus will detect all substances that have a density that differs from that of the mobile phase, it will obviously not tolerate gradient elution. Density measurement may be a basis for LC detection, and, in fact, this work has proved its validity. Nevertheless,... 

The advantage of the suppressor technique is its higher sensitivity. In addition, the specificity of the method is also increased, since the chemical modification of eluent and sample in the suppressor system converts the conductivity detector from a bulk property detector into a solute specific detector [52]. Thus, exchanging eluent and sample cations with protons means that only the sample anions to be analyzed are detected by the conductivity detector and appear in the resulting chromatogram. 

Refractive index detectors continuously monitor the difference in bulk RI of the mobile phase and that of a reference mobile phase containing no solute. As such, RI is a bulk property detector. Unlike UV detectors, RI detectors are highly sensitive to temperature fluctuations and somewhat sensitive to flow rate fluctuations. Temperature should be maintained within 0.001°C for high-sensitivity measurements. Noise equivalent concentration for RI is IQ- g/ml. ... 

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,... 

An example of the second type of detector is the refractive index monitor which functions by recording the refractive changes in the eluant as the solute passes through the detector cell. Bulk property detectors, though more versatile, are generally several orders of magnitude less sensitive than specific property detectors and the choice for a particular application is often dictated by solute characteristics. 

Hyphenation refers to the online combination of a separation technique and a spectroscopic detection method that provides structural information on the analytes concerned. Liquid chromatography (LC), mass spectrometry (MS), and gas chromatography (GC) are the most popular hyphenated techniques in use today. The choice of detection is important to the overall scheme of LC make up and is contingent upon criteria such as the noise, sensitivity, and linearity. Of the two basic categories of detectors, viz., solute and bulk property detectors, UV detection belongs to the former category. 

The slow development of LC from the time of Tswett, to the late 1950 s was entirely due to the lack of high sensitivity on-line detectors. Since, the inception of effective LC detectors there has been a continuous synergistic interaction between column development and detector development which has resulted in the present highly sophisticated LC systems of today. There are a number of ways of classifying LC detectors, specific and non-specific detectors, mass and concentration sensitive detectors and finally bulk property and solute property detectors. The classification of detectors as bulk property and solute property detectors is recommended. Bulk property detectors respond to a change in some overall property of the eluent such as refractive index or dielectric constant whereas solute property detectors respond to some property that is unique to the solute alone. In practice solute property detectors are rarely ideal and many respond, at least weakly, to the same property of the mobile phase as well as the solute. 

Bulk property detectors such as the refractive index detector or the dielectric constant detector are often particularly pressure sensitive and for that reason wide diameter exit tubes are strongly recommended, however, such detectors are not useful for multidimensional analyses. It follows that the pressure sensitivity of the detector should be specified by the manufacturer. The pressure response Dp should be given as the output in mV for unit pressure change in ihe detector cell. The pressure response should be given in both mV/psi and mV/kg/m2, It is also recommended that the pressure noise is given in terms of that pressure change which would provide a signal equivalent to the noise, i.e. 

In a similar manner the density of the contents of the detector cell will change with pressure. It follows that the detector would respond to fluctuations in pump pressure or indirectly to fluctuations in mobile phase flow rate if there was a significant flow resistance subsequent to the detector as in the case of multidimensional column systems. It follows that a bulk property detector, functioning on the measurement of density could only be employed with pumps having very constant flow rates and pressure control. The above argument applies equally to other bulk property detectors which monitor refractive index or dielectric constant and it can therefore be concluded that all bulk property detectors will have a limited sensitivity and have to be employed with very well controlled mobile phase supply systems. 

As a result of the limited sensitivity of bulk property detectors, they have also a very limited linear dynamic range, usually about three orders of magnitude and sometimes considerably... 

As a bulk property detector, the conductivity detector has limited sensitivity as a result of trace impurties, ionic in nature, in the mobile phase. Disolved carbon dioxide and ammonia can be a particular problem. The detector is not, however, very... 

The cell can be made a few microlitres in volume and thus would be suitable for use with small bore columns. A sensitivity of 10 AU was claimed for the detector and a linear dynamic range of about three orders of magnitude (although by now this may have been increased). As with other bulk property detectors, the thermal lens detector would not be suitable for use with gradient elution. The use of lasers make the detector extremely expensive, however, the availability of a UV laser, should it be developed, might make the device more useful. 

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