Dual column applications

As pointed out above, IC is a well-established method for the analysis of inorganic anions and has become the method of choice in many application areas. Many techniques are available using singlecolumn [46] or dual-column systems with various detection modes. IC can be used both for analytical and preparative purposes. Large sample volumes, up to 1300 pul, can be injected to determine trace anions and cations and to attain detection limits of 10-400 ng/1. For determinations at a pig/1 to mg/1 level, a sample size of 10-50 xl is sufficient. Preconcentration is necessary for lower concentrations (an additional column, a sample pump, an extra valve and an extra time are the disadvantages of this approach [47]). With an IEC column and isocratic... 

Samples are injected onto the turbulent-flow column similar to single-column methods. The analytes of interest are retained in the turbulent-flow column while the large macromolecules are eluted to waste. Once the analytes are separated from the matrix, the samples are then eluted into the analytical column. The characteristics of the analytical column determine the peak shape and separation seen at the MS detector. Flow rates which are compatible with the mass spectrometer can then be used and the chromatograms are based on conventional HPLC parameters. The key to dual-column methods is that the retentive properties of the analytical column must be sufficiently stronger than that of the turbulent-flow column the dual-column approach is performed in such a manner so that the mobile-phase composition needed to elute the analyte from the turbulent-flow column does not elute the analyte from the analytical column. The sample is then focused at the head of the analytical column until the mobile-phase conditions are changed to elute the analyte. The choice of columns is critical to the success of dual-column methods. Table 10.2 lists some of the applications of dual-column methods found in the literature. 

TABLE 10.2. Selected Applications Using Dual-Column TFLC and LC-MS/MS Detection (Zeng et al., 2002)... 

LC pumps. While the sample is running on the analytical separation column, the extraction column is in use with the next sample. Many applications have been reported with TFC in both the single-column mode [107,108] and the dual-column mode [109-111]. 

Duebelbeis DO, Pieczonka G, Kapila S, et al. 1989. Application of a dual column reaction chromatography system for confirmatory analysis of polychlorinated biphenyl congeners. Chemosphere 19 143-148. 

Dual columns — The introduction of capillary columns brought about a revolution in the level of resolution, and these columns are now almost universally used. Nonetheless, no single column is able, for example, to resolve all of the 209 PCB congeners so that the application of two columns in tandem with separate ovens and detector systems has been advocated. Fraction cuts from one column are quantitiatively transferred to the other (Duinker et al. 1988). A few examples are given as illustration. 

Dual column analysis involves the simultaneous analysis of a sample using two columns in parallel having stationary phases with widely differing polarities such as a dimethylsilicone and Carbowax (Figure 5.1(c)). The log-log plot of retention time on the two columns for the components to be identified are constructed, thus enabling complex mixtures to be analysed when there are problems of achieving good separation. Experiment 20, Chapter 9 illustrates this application. 

Cotton fibers are single cells composed primarily ( 96%) of the polymer cellulose. In our laboratory (5), cotton fibers were dissolved directly in the solvent DMAC-LiCl. This procedure solubilizes fiber cell wall components directly without prior extraction or derivatization, processes that could lead to degradation of high MW components. MW determinations have been carried out by a size-exclusion chromatography (SEC) system using commercial columns and instrumentation with DMAC-LiCl as the mobile phase. Incorporation of viscometry and refractive index (RI) detectors (6) allowed application of the universal calibration concept (7) to obtain MW distributions (MWDs) based on well-characterized narrow-distribution polystyrene standards (5). The universal calibration concept used by incorporation of dual detectors bypasses the need for cellulose standards. There are no cellulose standards available. Polystyrene standards for a wide range of MWs dissolved readily in DMAC-0.5% LiCl with no activation necessary. 

The application of dual detection [UV and refractive index (RI)] to the SEC analysis of polystyrene-poly(methyl methacrylate) (PS-PMMA) has already been studied in this laboratory (2). Both MWD and CCD were determined using a methodology outlined by Runyon et al. (3). This approach relies on SEC column calibration with narrow polydis-persity standards for each of the homopolymers as well as a measure of the detector response factors for each homopolymer to produce a copolymer MWD. In the case of PS and PMMA this is feasible, but in other block copolymer systems the availability of suitable molecular weight standards may be more limited. In addition, this procedure does rely on true SEC and is not valid for block copolymers for which the universal calibration does not hold true for both blocks in a given solvent system. 

In this paper we describe the application of an adaptive network based fuzzy inference system (ANFIS) predictor to the estimation of the product compositions in a binary methanol-water continuous distillation column from available on-line temperature measurements. This soft sensor is then applied to train an ANFIS model so that a GA performs the searching for the optimal dual control law applied to the distillation column. The performance of the developed ANFIS estimator is further tested by observing the performance of the ANFIS based control system for both set point tracking and disturbance rejection cases. 

The Karr reciprocating-plate column (Fig. 15-50) is a popular example. It uses dual-flow plates with 50 to 60 percent open area and has no downcomers [Karr, AlChE J., 5(4), pp. 446-452 (1959) Karr and Lo, Chem. Eng. Prog., 72(11), pp. 68-70 (1976) and Karr, AIChE J., 31(4), pp. 690-692 (1985)]. Because of the high open area, a Karr column may be operated with relatively high throughput compared to other types of agitated columns, up to about 1000 gaF(h ft ) [40 m /(h m )] depending upon the application. The plates are mounted... 

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