Selection of Separation and Detection Systems

As previously mentioned, a wealth of different separation techniques could be described under the term ion chromatography. Therefore, what follows is a survey of the criteria for selecting stationary phases and detectors being suitable for solving a specific separation problem. 

The analyst usually has some information regarding the nature of the ion to be analyzed (inorganic or organic), its surface activity, its valency, and its acidity or basicity, respectively. With this information and on the basis of the selection criteria outlined schematically in Table 1-1, it should not be difficult for the analytical chemist to select a suitable stationary phase and detection mode. In many cases, several procedures are feasible for solving a specific separation problem. In these cases, the choice of the analytical procedure is determined by the type of matrix, the simplicity of the procedure, and, increasingly, by financial aspects. Two examples illustrate this  

The representation of chromatographic signals in the form of a chromatogram generally has the appearance similar to that in Fig. 2-1  

Two different components are separated in a chromatographic column only if they spend different times in or at the stationary phase, respectively. The time in which the components do not travel along the column, is called the solute retention time, f5. The column dead time, tm, is defined as the time necessary for a non-retained component to pass through the column. The gross retention time, ms, is calculated from the solute retention time and the column dead time  

The chromatographic terms for the characterization of a separator column can be inferred from Fig. 2-1. 

Inorganic anions Carboxylic acids Sulfonic acids Fatty acids C, 

Formic acid Acetic acid Citric acid 

---

Selection of Separation and Detection Systems 111 Table 1.1 Schematic representation of selection criteria for separation and detection modes. 

The most usual method of element speciation, however, is the utilization of combined and hyphenated systems. Here, species are first (as selectively as possible) separated, and subsequently the elements in the species are detected. For increased quality control, molecule selective detection is performed with separation devices. An overview of typical combinations of separation and detection systems is provided in Figure 3.4. 

The most suitable analytical methodology should be selected based on the required performance characteristics. A sound literature search is always of great help with respect to known methods for the respective analyte and matrices. In most cases the search results will not directly provide the method wanted but will allow the most likely successful analytical approach to be set up. In this context, pre-considerations should address the most appropriate sample work-up procedure as well as the suitable analytical separation and detection system. The question of direct analysis of the analyte or a derivate formed after chemical reaction should be clarified. And finally, some thoughts should already be given to the question of chemical stability of the analyte in the given matrices under the applied conditions. 

The purpose of sample preparation is to create a processed sample that leads to better analytical results compared with the initial sample. The prepared sample should be an aliquot relatively free of interferences that is compatible with the HPLC method and that will not damage the column. The whole advanced analytical process can be wasted if an unsuitable preparation method has been employed before the sample reaches the chromatograph. Specifically, analytical work with samples from fermentation processes require a sample pre-treatment that eliminates the fermentation broth before the analytes can be injected into the chromatographic columns. This is primarily to remove macromolecular sample constituents, which easily clog the columns. Complex matrices often require a more selective sample preparation than for instance pharmaceutical solutions. In practice the choice of sample-preparation procedure is dependent on both the nature and size of the sample and on the selectivity of the separation and detection systems employed. Sample pre-treatment may includes a large number of methodologies. Ideally, sample preparation techniques should be fast, easy to use and inexpensive. In papers I and II careful sample pre-treatment was performed before all injections. 

SIA system integrating processes of separation and detection by optical fibers, incorporated in the diffusion cell for ammoniacal nitrogen determination. HC holding coil RC reaction coil SV selection valve. 

The real-time analytical systems are all similar in the basic overall design. Each has an interface, linking the ionization region to the sample (outside world) an ionization region and a mass separation and detection system (Fig. 1). Selectivity can potentially be obtained at each of the key points in... 

Another way to improve the analysis of complex matrices can be the combination of a multidimensional system with information-rich spectral detection (31). The analysis of eucalyptus and cascarilla bark essential oils has been carried out with an MDGC instrument, coupling a fast second chromatograph with a matrix isolation infrared spectrometer. Eluents from the first column were heart-cut and transferred to a cryogenically cooled trap. The trap is then heated to re-inject the components into an analytical column of different selectivity for separation and subsequent detection. The problem of the mismatch between the speed of fast separation and the... 

Analysis of methyl parathion in sediments, soils, foods, and plant and animal tissues poses problems with extraction from the sample matrix, cleanup of samples, and selective detection. Sediments and soils have been analyzed primarily by GC/ECD or GC/FPD. Food, plant, and animal tissues have been analyzed primarily by GC/thermionic detector or GC/FPD, the recommended methods of the Association of Official Analytical Chemists (AOAC). Various extraction and cleanup methods (AOAC 1984 Belisle and Swineford 1988 Capriel et al. 1986 Kadoum 1968) and separation and detection techniques (Alak and Vo-Dinh 1987 Betowski and Jones 1988 Clark et al. 1985 Gillespie and Walters 1986 Koen and Huber 1970 Stan 1989 Stan and Mrowetz 1983 Udaya and Nanda 1981) have been used in an attempt to simplify sample preparation and improve sensitivity, reliability, and selectivity. A detection limit in the low-ppb range and recoveries of 100% were achieved in soil and plant and animal tissue by Kadoum (1968). GC/ECD analysis following extraction, cleanup, and partitioning with a hexane-acetonitrile system was used. 

---