Liquid chromatography solute property detectors

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... 

Various methods of detection are employed in chromatography. Each approach for the detection of solutes is based on their physical or chemical properties. Some of the more commonly used detectors are discussed here for liquid chromatography (LC), gas chromatography (GC), and supercritical fluid chromatography (SEC). 

Detection in liquid chromatography has long been considered one of the weakest aspects of the technique. Low concentrations of a solute dissolved in a liquid modify the properties of the liquid to a much smaller extent than low concentrations of a solute in a gas. For this reason there is no sensitive universal, or quasi-universal, detector such as the flame ionization or thermal conductivity detectors for GC. A comprehensive review of detectors has been published by Fielden (38), as well as two recent books by Scott (39) and Patonay (40). The Fundamental Review issue of Analytical Chemistry, published in even-numbered years, contains a comprehensive review of developments in instrumentation for LC, including detection techniques. 

The possibility of measuring ionic compounds without inherent detection properties as ion-pairs with a suitable counter-ion has been utilized in normal phase liquid-liquid chromatography. Picrale was used as counter-ion for acetylcholine [17] and methylguanidine [55] in extracts from biological samples. Aqueous picrate solutions at pH 6.5 were coated on microporous cellulose, and the picrate ion-pairs in the chloroalkane eluent were monitored by a UV detector. 

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. 

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. 

With the advent of advanced characterization techniques such as multiple detector liquid exclusion chromatography and - C Fourier transform nuclear magnetic resonance spectroscopy, the study of structure/property relationships in polymers has become technically feasible (l -(5). Understanding the relationship between structure and properties alone does not always allow for the solution of problems encountered in commercial polymer synthesis. Certain processes, of which emulsion polymerization is one, are controlled by variables which exert a large influence on polymer infrastructure (sequence distribution, tacticity, branching, enchainment) and hence properties. In addition, because the emulsion polymerization takes place in an heterophase system and because the product is an aqueous dispersion, it is important to understand which performance characteristics are influended by the colloidal state, (i.e., particle size and size distribution) and which by the polymer infrastructure. 

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