Specific property detectors

1 Ultraviolet detectors. Ultraviolet detectors function by monitoring the light absorbed by the solute molecules from the incident beam. Ultraviolet detectors are the most commonly used type with LC systems, they are not appreciably flow or temperature sensitive, have a good dynamic linear range, but are, however, selective. The absorbance is proportional to concentration and obeys the Beer-Lambert Law which is defined as follows  

Liquid lens effect caused by change in refractive index within cell 

Adapted from Snyder and Kirkland (1979) Introduction to Modern Liquid Chromatography (2nd edn). Wiley-Science, New York. 

A consequence of the high sensitivities demanded by LC down to 0.0005 AUFS deflection is the need to use greater band widths with variable wavelength detectors ( 5nm being typical), otherwise the resulting noise would render the detector unusable. However, as deviations from Beer-Lambert s Law can arise due to the use of non-monochromatic radiation, it is important to check the linearity of response of the sample components 

Reference cells filled with the eluant being used for analysis were incorporated into early instrumentation. However, this served no useful purpose and in fact caused additional noise due to the difficulty in exactly matching the solvent flows, a problem especially in gradient work. 

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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). 

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. 

Physical detection methods are based on inclusion of substance-specific properties. The most commonly employed are the absorption or emission of electromagnetic radiation, which is detected by suitable detectors (the eye, photomultiplier). The / -radiation of radioactively labelled substances can also be detected directly. These nondestructive detection methods allow subsequent micropreparative manipulation of the substances concerned. They can also be followed by microchemical and/or biological-physiological detection methods. 

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 ... 

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 ... 

The most commonly used detector is the differential refractometer. For polymers, the variation in the refractive index is usually independent of molecular mass. Other detectors, like photometric detectors in the UV or IR range, can also be used besides the refractometer to measure specific properties of macromolecular solutions (Fig. 7.4). 

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 nature of the detector has a major effect on the results obtained. The choice is made according to the requirements of the analysis, and depends on whether the aim is to detect all compounds present, or only those with specific properties, such as, for example, those with an ultraviolet (UV) absorbance at a specific frequency, or those containing a particular metal. 

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. 

Silicon oils do not break down on contact with air while hot, but on the other hand they can polymerize and develop an insulating film on electronics. Thus, their use is not acceptable for instruments such as mass spectrometers (including He leak detectors). Some specific properties of a spectrum of diffusion pump fluids are shown in Table 7.9. 

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... 

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, 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). 

The identification of the chemical forms of an element has become an important and challenging research area in environmental and biomedical studies. Two complementary techniques are necessary for trace element speciation. One provides an efficient and reliable separation procedure, and the other provides adequate detection and quantitation [4]. In its various analytical manifestations, chromatography is a powerful tool for the separation of a vast variety of chemical species. Some popular chromatographic detectors, such flame ionization (FID) and thermal conductivity (TCD) detectors are bulk-property detectors, responding to changes produced by eluates in a characteristic mobile-phase physical property [5]. These detectors are effectively universal, but they provide little specific information about the nature of the separated chemical species. Atomic spectroscopy offers the possibility of selectively detecting a wide rang of metals and nonmetals. The use of detectors responsive only to selected elements in a multicomponent mixture drastically reduces the constraints placed on the separation step, as only those components in the mixture which contain the element of interest will be detected... 

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. 

Nonselective detectors react to the bulk property of the solution passing through. A refractive index detector monitors the refractive index of the eluate. The pure mobile phase has a specific refractive index which changes when any compound is eluted. The detector senses this difference and records all peaks hence the term nonselective or bulk-property detector. This is why the refractive index as well as the conductivity detector is not suited for gradient elution. 

The response to the carrier gas forms the baseline signal and any change in composition of the eluant will produce a change in the overall physical property being monitored by the detector, and hence a change in the detector signal. The detector is also sensitive to variations in parameters that affect the specific property being measured, for example, temperature, flow-rate, pressure and carrier gas purity. 

It is convenient to use continuous monitoring detectors located at the column exit. Various detector designs have been used and these may be classified as those which monitor a specific property of the solute or those which detect changes in a bulk property of the column effluent. 

An example of the former is the ultraviolet spectrophotometer which may be of fixed wavelength (usually 254 or 280 nm) or variable wavelength design. The detector functions by monitoring the change in absorbance as the solute passes through the detector flow cell, i.e. it utilises the specific property of the solute to absorb ultraviolet radiation. 

Detectors which monitor a specific property of the solute which is not shared with the solvent, e.g. ultraviolet absorbance and fluorescence. Possession of such a property by the solute affords its detection in the effluent. 

Initial application of ion exchange to modern LC depends on the analyte having a specific property such as ultraviolet absorbance, fluorescence or radioactivity. As many ion exchange methods require the presence of com-plexing agents (EDTA, citrate) and various electrolyte additions to achieve the required resolution, conductivity detectors could not be used without modification of the technique, since this parameter is a universal property of ionic species in solution. 

The FID is the most widely used GC detector, and is an example of the ionization detectors invented specifically for GC. The column effluent is burned in a small oxy-hydrogen fiame producing some ions in the process. These ions are collected and form a small current that becomes the signal. When no sample is being burned, there should be little ionization, the small current (10 a) arising from impurities in the hydrogen and air supplies. Thus, the FID is a specific property-type detector with characteristic high sensitivity. 

Specific properties of superconductors at 77 K may be used for electronic applications, e.g., for passive microwave devices such as transmission lines and high-quality resonators. YBCO is widely used in active devices such as SQUIDs and detectors based on Joseph-son and quasi-particle tunnelling. A further field of... 

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