Tailing and fronting

Slant that varies with concentration. Fronting also arises when the amount of sample introduced onto a column is too large. Distortions of this kind are undesirable because they lead to poorer separations and less reproducible elution times. In the discussion that follows, tailing and fronting are assumed to be minimal. 

Figure 4.20. (a) Storage of a section of a dispersed zone of dye in one of the tubular cavities of the parallel FIA analyzer (Fig. 4.19). The zone was first loaded (Stpp/Tum 1) so that when the pump was restarted after a 4 s stop period, the outer sections of the zone were discarded (d, h, b, /, respectively). During the next stop period (Stop/Tum 2) the drum was turned back so that the stored portion of the zone was flushed out and measured (6, d, /, h, respectively). By changing the delay time, and thus the position of the dye zone in the drum, either the tail (6), the front (/i), or both tail and front portions d, f) of the dispersed zone were chopped off. The same experiment was repeated in (b), but at a lower paper speed, for a number of delay times a = 2.6, b = 2.8, c. = 3.0, d = 3.2, e - 3.4, / = 3.6, g = 3.8, h = 4.0, and / = 4.2 s), while a fixed stop period of 4 s for the load/tum cycle was preserved. The delay of 3.4 s (curve e) allows optimum repeatability of material storage and was therefore used for sample loading when running a 6 h storage experiment (c), where seven samples were loaded in parallel in seven tubular cavities (load) and measured after the storage period.

Both tailing and fronting are undesirable, as closely eluting peaks will be less well separated and retention data less reproducible. Where either occur, reducing the amount of solute chromatographed will generally improve the peak shape, but slow de-sorption may still cause some tailing. 

FIGURE 2.4 Three types of concentration profiles encountered among the thin-layer chromatographic bands (a) symmetrical (Gaussian) without tailing, (b) skewed with front tailing, and (c) skewed with back tailing [14],... 

The optimum amount of sample required to produce detectable spots with a minimum of spreading due to overloading is about 0.01 pg to 50 pg. With samples much larger than this, isotherms become nonlinear, the Iit values alter significantly with sample size and resolution suffers because of increased tailing or fronting. 

Kuge and Yoshikawa (3) related a change in the gas chromatographic peak shape to the beginning of multilayer adsorption on the surface of the solid. For small adsorbate volumes, the peak shape is symmetrical. As the adsorbate volume is increased, a sharp front, diffuse tail, and a defect at the front of the peak top is observed (Figure 11.2). It then acquires a diffuse front and sharp tail. This point corresponds to the B point of the BET Type II adsorption isotherm at which the relative surface area may be calculated. 

The success of the analysis is dependent on the selection of a suitable internal standard. Some important criteria for such selection are as follows (a) that it is not chemically reactive towards any of the components of the mixture (b) that it has a retention time which gives a base line separation from the other components, including any impurities (c) that the retention time is comparable with that of the components of the mixture (d) that the peak profile is symmetrical and therefore does not exhibit either fronting or tailing and (e) that the detector response to the internal standard is such that neither an excessive, nor a minute, weight of standard compared to the weight of mixture needs to be used. 

A line is polar if its two directions can be distinguished and a plane is polar if its two surfaces are not equivalent. This definition of polarity has, of course, nothing to do with charge separation. A polar line has a head and a tail and a polar plane has a front and a back. A vertical line on the surface of the Earth is polar with respect to gravity and a sheet of paper with one of its sides painted is polar with respect to its color. 

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