Zones, chromatographic shape

One of the major advantages of CE as a separation technique is the wide variety of separation modes available. Analytes can be separated on the basis of charge, molecular size or shape, pi, or hydrophobicity. The same CE instrument can be used for zone electrophoresis, IEF, sieving separations, isotachophoresis, and chromatographic techniques such as MEKC and capillary electrokinetic chromatography. This section provides a brief description of each separation mode. Zone electrophoresis, IEF, and sieving are the primary modes used for protein separations, and these will be discussed in detail in the following sections. 

Molecules of solute travel as a zone in the chromatographic system. Recording of molecules eluting from the column yields a chromatogram (Fig. 1), whose characteristics are peaks. When peaks are symmetrical (Gaussian shape), retention times are taken at peak height. Since k is dimensionless, one can record retention distances or retention volumes on the chromatogram and... 

Figure 4.8 The GC X GC experiment can be considered to be a series of fast second chromatograms conducted about five times faster than the widths of the peaks on the first dimension. The ID elution time is the total chromatographic run time, while the 2D time is the modulation period (e.g. 4-5 s). This figure shows two overlapping peaks A and B, with the zones of each peak collected together. When these slices are pulsed to the second column, they are resolved. Here, we show peak B eluting later on column 1, but earlier on column 2, with the 2D peak maxima tracing out a shape essentially the same as the original peak on ID.

The shape of the zone, at least a characteristic of its width, is of major concern in all chromatographic techniques. Jonsson [4] and Giddings [5] seem to discuss the case and peculiarities of open columns in more details than do other general monographs on chromatography. 

Because Eq. 4.23 does not describe chromatographic processing, its use may result in misleading conclusions. Suffice to say that the 31 values at maximums of TC zones are usually very small see Sect. 4.3.1 for the formulae. Hence, a value of az calculated from Eq. 4.25 is nearly 10 times larger than that from Eq. 4.23. This flaw limits the usefulness of this otherwise interesting pioneering attempt, which also considered influence of the temporal injection profile (5-function, short plug, exponential) on the zone shapes. 

Chapter 4 starts with some basic equations, which relate the molecular-kinetic picture of gas-solid chromatography and the experimental data. Next come some common mathematical properties of the chromatographic peak profiles. The existing attempts to find analytical formulae for the shapes of TC peaks are subject to analysis. A mathematical model of migration of molecules down the column and its Monte Carlo realization are discussed. The zone position and profile in vacuum thermochromatography are treated as chromatographic, diffusional and simulation problems. 

Harrison and co-workers [68,69] calculated, in terms of number of theoretical plates, the shapes of chromatographic zones for a more complex reversible reaction of the type A -1- C = 2B and obtained agreement with experimental results. 

The most suitable technique for chemically active and reactive trace components is the introduction of a more reactive compound into the carrier gas. This protects the trace components against moisture and trace amounts of oxygen, improves the shape of the chromatographic zones and prevents losses of the substance in the column and other units because the reactive and adsorptive component of the carrier gas poisons the adsorbing sites in the units and on the sohd support and reacts with contaminants in the carrier gas. To the best of our knowledge, one of the first applications of this method was the addition of 1% of boron trichloride to the carrier gas in the analysis of readily hydrolysable compounds. 

When on-column injection is used the end of the transfer capillary is inserted into the column inlet or retention gap where decompression of the supercritical fluid occurs. Carbon dioxide gas exits through the column and the seal made between the restrictor and septum (unless a closed injector is used). The analytes are focused by cold trapping in the stationary phase. The transfer line must be physically removed from the injector at the completion of the extraction to establish the normal carrier gas flow for the separation. Analyte transfer to the column is virtually quantitative but blockage of the restrictor is more conunon and involatile material accumulates in the injection zone eventually degrading chromatographic performance. The on-column interface is probably a better choice for trace analysis of relatively clean extracts with modest fluid flow rates than the split interface. When optimized both the on-column and split interfaces provide essentially identical peak shapes to those obtained using conventional solution injection. 

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