Coupled-column techniques

U.A.Th. Brinkman and H. Poppe (eds), Multidimensionality. Hyphenation and Coupled-Column Techniques, J. Chromatography A703 (1995). 

Coupled columns packed with different stationary phases can be used to optimize the analysis time (71, 75). In this approach the different columns are connected in a series or in parallel. liie sample mixture is first fractioned on a relatively short column. Subsequently the fractions of the partially separated mixture are separated on other columns containing the same or other stationary phases in order to obtain the individual components. Columns differing in length (number of theoretical plates), adsorptive strength or phase ratio (magnitude of specific surface area), and selectivity (nature of the stationary phase) can be employed, whereas, the eluent composition remains unchanged. Identification of the individual sample components via coupled column technique requires a careful optimization of each column and precise control of each switching step. 

E. A. Hoogendoom and P. van Zoonen, Coupled-column reversed phase liquid chromatography as a versatile technique for the determination of polar pesticides in Environmental Analysis - Techniques, Applications and quality assurance, Barcelo D (Ed.), Vol. 13, Elsevier, Amsterdam, pp. 181-196 (1993). 

Chlorophenoxy acids are relatively polar pesticides which are usually determined by LC because volatile derivatives have to be prepared for GC analysis. This group of herbicides can be detected by multiresidue methods combined with automated procedures for sample clean-up, although selectivity and sensitivity can be enhanced by coupled-column chromatographic techniques (52). The experimental conditions for Such analyses are shown in Table 13.1. 

There are many combinations of separations techniques and methods of coupling these techniques currently employed in MDLC systems. Giddings (1984) has discussed a number of the possible combinations of techniques that can be coupled to form two-dimensional systems in matrix form. This matrix includes column chromatography, field-flow fractionation (FFF), various types of electrophoresis experiments, and more. However, many of these matrix elements would be difficult if not impossible to reduce to practice. 

Phinney, K.W., Sander, L.C., Wise, S.A. (1998). Coupled achiral/chiral column techniques in suhcritical fluid chromatography for the separation of chiral and nonchiral compounds. Anal. Chem. 70, 2331-2335. 

May, W. E., Wasik, S. P., Freeman, D. H. (1978a) Determination of the aqueous solubility of polynuclear aromatic hydrocarbons by a coupled-column liquid chromatographic technique. Anal. Chem. 50, 175-179. 

Wang, X. and Song, H. Determination of octanol-water partition coefficients of organic pollutants by using the dynamic coupled-column liquid chromatographic technique, Trans. Tianjin Univ, 2(l) 59-63, 1996. 

The idea of chromatographic CF is the use of timely chromatographic apparatus and technique for this purpose. Basically, it is the application towards copolymer investigation of what is known among chromatographers as multi-dimensional or coupled column chromatography. 

For the investigation of a butadiene-styrene rubber, a set of three SEC columns in series was used 500 x 8 mm, d0 = 150 nm 500 x mm, d0 = 25 nm 800 x 8 mm, d0 = 4 nm, all three with dP = 10 pm. The flow rate was 1 ml/min. The injection amounted to 200 pi of a 2 % solution from which the carbon black had been removed. Although the additives of interest were separated from the polymer, they were still covered by an intense band from process oil. Hence, coupled-column chromatography with reversed-phase separation of SEC eluates became neccessary. 10 pi of the latter were injected into a C 8 column (250 x 2.2 mm dP = 10 pm) and analyzed at 0.5 ml/min flow rate through a water/acetonitrile gradient (rising from 20% B by 6%/ml). Here, UV detection was performed at 254 nm. The peaks of the additives could be clearly separated from the process oil band. The technique also proved useful for checking... 

The assessment of mutagenic activity in cooked foods requires tedious extraction work in order to isolate and quantify the responsible chemicals at the nanogram level. Efforts have been made to develop a rapid and efficient method to obtain chromatograms free of interfering material. Coextracted matrix components influence analyte detection limits more than does absolute detector sensitivity (182). The sample workup therefore is the most critical part of the analysis (183). Solid-phase extraction with different coupled columns provides an improvement (176) over LLE and the use of large columns filled with XAD resin. The determination of PAHs and PANHs in food has been carried out by different chromatographic techniques, including LC with fluorescence (164,171,184) and/or UV detection (185,171) and GC with FID (168,186) or MS detection (187). 

Multidimensional (or coupled) column chromatography is a technique in which fractions from a separation system are selectively transferred to one or more secondary separating systems to increase resolution and sensitivity, and/or to reduce analysis time. The application of secondary columns is illustrated schematically in Figure 8.1. The smaller the AtT value applied, then the greater is the resolution and number of runs needed to check a certain portion of the sample (5). 

A. Heim, The analysis of trace components using the total transfer technique in coupled column systems, Anal. Volatiles Methods... 

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