Eluent, physical property measurements

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. 

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. 

In most cases, the choice of a suitable detection mode depends on the separation method and the correspopnding eluents. If detection is to be carried out by directly measuring a physical property of the solute ion (e.g. UV absorption), it must differ substantially in this property from eluent ions which are present in much higher concentration. However, eluent and solute ions often exhibit similar properties, so that direct detection is only feasible where selective detection of a limited number of solute ions is desirable. 

A much broader range of applications have detection methods, with which one measures the change in a certain physical property of the eluent (e.g. conductance) that is due to the solute ion elution. A sufficient difference between eluent and solute ions is a prerequisite in the measuring values of this property. Most of the detection methods applied to ion chromatography are based on this technique. For the ensuing discussion a further subdivision into direct and indirect methods is made. Direct detection methods are those, in which eluent ions exhibit a much smaller value than solute ions for the property to be measured. On the other hand, detection methods are called indirect, if eluent ions exhibit a much higher value for the property to be measured than do solute ions. 

In GC, normally thermally stable but volatile compounds are separated. Both uncoated and coated solid adsorbents as well as thin Hlms of liquids or polymers of high molecular weight are used as the stationary phase. Inert gases (He, Ar, Nj) and Hj are used as the mobile phase, which is normally inert compared with the separated compounds. Many GC detectors are therefore based on thermal decomposition or ionization processes which convert the eluted substances into ions. Detector systems which measure a physical property of the column eluent (mobile phase and/or compound) and the eluted substance alone are also frequently used [12, 13]. 

Non-destructive detectors usually measure a physical property of the eluent. The detected compounds are neither decomposed nor is the structure changed. Ionization processes are non-destructive as long as no dissociative reaction takes place, such as cleave-off of functional groups. Non-destructive detectors can be combined with other detectors in multi-detection systems. 

Detectors Measuring Physical Properties of the Eluent Thermal Conductivity Detector... 

Detectors which measure a physical property of the eluent such as the refractive index, rate of radioactive decay or the speed of sound waves can be used with chromatographic systems. However, in most instances the detection limits are relatively high, which reduces the application range to preparative and semi-preparative separations. 

Ibble 15-6. IVpical data for some detectors based on measuring physical properties of the eluent. 

In gradient elution, detection requires quantitative measurement of the sample components in an eluent whose composition and, hence, physical properties alter in the course of the analysis. The detection problem in gradient elution can be solved by either using a selective detector sensitive to a property of only the solute or stripping off the solvent with subsequent measurement of non-volatile residues. 

Detectors can be broadly classified into two types. Bulk Property detectors which function by measuring some bulk physical property of the column eluent (e.g. dielectric constant or refractive index) and Solute Property detectors which function by measuring a physical and/or chemical property that is characteristic of the solute only (e.g. UV absorption). This classification is not completely precise, for example, the UV detector, which is usually classed as a solute property detector, when used with an ethyl acetate-heptane solvent mixture as the mobile phase will give a constant background signal due to UV absorption by the ethyl acetate. Furthermore, any fluctuation in ethyl acetate content of the mobile phase will appear as noise on the detector output. It follows that the UV detector, although a solute property detector, behaves as a hybrid between a bulk property detector and a solute property detector under some conditions of use. 

One of the first on-line liquid chromatography detectors to be developed in the early forties was, in fact, a bulk property detector, the refractive index detector (1). Bulk property detectors continuously monitor some physical property of the column eluent and by the use of a suitable transducer provide a voltage - time output that is either proportional to the physical property being measured, or made proportional to the concentrations of the solute eluted. The properties of the mobile pheuse that are most commonly monitored in commercially available bulk property detectors are refractive index, electrical conductivity, and dielectric constant, the dielectric constant detector being the least popular of the three. 

Multi-functional detectors monitor the column eluent by the measurement of more than one physical or chemical property simultaneously, employing a single sensing cell. To date, three bifunctional detectors and one trifunctional detector have been described. The three bifunctional detectors have combined UV absorption and fluorescent detection, UV absorption and electrical conductivity detection and UV absorption and refractive index detection. The latter uniquely combines a bulk property detector with a solute property detector producing, at least in theory, the nearest approach to a universal detector. The trifunctional detector incorporates UV absorption, electrical conductivity and fluorescence functions. Multi-functional detection provides detector versatility and a means of confirmir solute identity. Such detectors have to be designed, so that the performance specifications are not seriously compromised, and the cell and eluent conduits do not contribute significantly to peak dispersion. 

Once the chromatographic separation on the column has been conducted, the composition of the eluent at the column end must be determined using a detector. In all HPLC detectors, the eluent flows through a measuring cell where the change of a physical or chemical property with elution time is detected. The most important parameter of the detector is sensitivity, which is influenced by the noise and baseline drift, the absolute detection limit of the detector, the linearity, the detector volume (band broadening), and the effects of pressure, temperature and flow (pulsation, gas bubbles). 

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