Bulk property detectors limiting sensitivity

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. 

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... 

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... 

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... 

Bulk property detectors continuously monitor some physical property of the column eluent. The presence of a solute modifies that property and provides an output that can be recorded. Bulk property detectors have limited sensitivity due to the fact that changes in ambient conditions, temperature, pressure, etc. provide signals commensurate to that from the presence of a solute. 

A eonsequence of using a bulk property for detection is that this property of the solvent must be controlled very closely the refractive index of the eluant is sensitive to fluctuations in pressure, temperature and composition. Whilst the pressure and composition can be controlled using pulse dampners and reciprocating pumps, the limits of sensitivity and stability of the RI detector are determined by temperature. The temperature must be controlled to +0.0001 K for accepted noise levels. Fluctuations in the RI caused by temperature and noise changes are compensated for by use of a reference cell. 

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