Additional Solute Property Detectors

The solute property detectors described so far are those more commonly used or, have at least at some time or other formed the basis of a frequently used, commercially available detector. There are, however, a number of detectors that have been developed that have either, not been developed into a commercial product or, have found a very limited area of application. Many of these detectors have, in fact, useful potential for specific chromatographic analyses. Some of these detectors will now be described to give the reader a broader view of solute property detectors in general, and to illustrate the wide range of solute properties that have been investigated with a view to the development of viable LC detecting systems. 

---

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. 

Linearity of Response—For quantitative accuracy, detector re nse must be proportional to the mass of hydrocarbon injected, and the response of the non-normal paraffins is assumed to be equivalent to the response of the /i-paraffin with the same carbon number. In addition, sample injection technique and sample solution properties must be such that representative sample is intr uced to the gas chromatograph without discrimination. Before use, the analysis system must be shown to conform to these requirements as specified in 9.6.1. 

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. 

The use of multiple detectors with size-exclusion chromatography (SEC) can greatly increase the information content available from a typical SEC analysis. This multidetector approach permits more accurate measurement of polymer properties than conventional SEC. The additional information, however, is obtained at the expense of an increase in the complexity of the instrumentation and data handling. In particular, a number of concerns arise in data acquisition and processing that are not present in conventional SEC. Some of these difficulties are outlined, and possible solutions are discussed. 

Because of the ubiquitous use of UV detectors in capillary electrophoresis systems, the LSER studies derived from MEKC data, a subset of solutes within approximately 100 compounds with UV-absorbing properties (mostly benzene derivatives and compounds with carbonyl moieties) is usually selected (27, 29). Interestingly, the benzene derivatives of the solute set present an additional structural feature the majority of the compounds exhibit organic multifunctionalities in the attempt to impart the necessary variability to the descriptor parameters. 

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 bottom line conclusion of this Section can be stated thus If two analytes A and B are to be chromato-graphically separated, their adjusted retention volumes V, and V,g (Equation [3.15]), and thus the corresponding retention times tj, must be sufficiently different. To separate two solutes, either their distribution coefficients and K (Equation [3.1]) must be made to differ (choose appropriate phase systems) or the volumes of stationary phase with which they interact must be made to differ (choose a stationary phase with appropriate exclusion properties based on molecular size so that the effective values of are different for different analytes) or a shrewd combination of both. It is appropriate to mention that if mass spectrometric detection is used, clean chromatographic separation of solutes in the analytical extract is not as crucial as for less selective detectors in view of the additional selectivity provided by the m/z information. 

The Crystaf apparatus shown in Figure 12 has five crystalHzation vessels that can be operated in parallel. Sample injection, dissolution, analysis, and disposal are completely automated. Small aliquots of the solution are taken through an in-line filter to avoid sampling pol5mier crystals with the polsrmer solution and sent to the on-line mass detector. The mass detector is commonly an infrared cell that is less sensitive to temperature fluctuations of the pol5mier solution. Additional detectors can also be installed on the sampling line to measm-e complementary properties, such as viscosity and copol5mier composition. 

---