Mass overload Overloading

A chromatographic separation can be developed in three ways, by displacement development, by frontal analysis, and by elution development, the last being almost universally used in all analytical chromatography. Nevertheless, for the sake of completeness, and because in preparative chromatography (under certain conditions of mass overload) displacement effects occur to varying extents, all three development processes will be described. 

The major cause of peak asymmetry in GC is sample overload and this occurs mostly in preparative and semi-preparative separations. There are two forms of sample overload, volume overload and mass overload. 

Volume overload results from too large a volume of sample being placed on the column, and this effect will be discussed later. It will be seen that volume overload does not, in itself, produce asymmetric peaks unless accompanied by mass overload, but it does broaden the peak. Mass overload, however, frequently results in a nonlinear adsorption isotherm. However, the isotherm is quite different from the Langmuir isotherm and is caused by an entirely different phenomenon. 

Figure 6. An Experimental Example of the Mass Overload of Benzene...

The problem is made more difficult because these different dispersion processes are interactive and the extent to which one process affects the peak shape is modified by the presence of another. It follows if the processes that causes dispersion in mass overload are not random, but interactive, the normal procedures for mathematically analyzing peak dispersion can not be applied. These complex interacting effects can, however, be demonstrated experimentally, if not by rigorous theoretical treatment, and examples of mass overload were included in the work of Scott and Kucera [1]. The authors employed the same chromatographic system that they used to examine volume overload, but they employed two mobile phases of different polarity. In the first experiments, the mobile phase n-heptane was used and the sample volume was kept constant at 200 pi. The masses of naphthalene and anthracene were kept... 

Figure 9. The Effect of Mass Overload of Benzyl Acetate...

Preparative chromatography involves the collection of individual solutes as they are eluted from the column for further use, but does not necessarily entail the separation of large samples. Special columns can be designed and fabricated for preparative use, but for small samples the analytical column can often be overloaded for preparative purposes. Columns can be either volume overloaded or mass overloaded. Volume overload causes the peak to broaden, but the retention time of the front of the peak... 

From a practical point of view it is better to use the maximum volume of sample possible, assuming there is no mass overload. This will allow the detector to be operated at the lowest possible sensitivity and in doing so provide the greatest detector stability and, as a consequence, the highest accuracy. 

The easiest way for an analyst to obtain small quantities of a component of a mixture is to overload an analytical column. In order to exercise this technique, the solute of interest must be well separated from its closest neighbor. The column can then be overloaded with sample until the peak dispersion resulting from the overload, causes the two peaks to touch at their base. There are two types of column overload, volume overload and mass overload. In practice, it is often advantageous to employ a combination of both methods and a simple procedure for doing this will be given overleaf. 

Volume overload can be treated in a simple way by the plate theory (8,9). In contrast, the theory of mass overload is complicated (10-12) and requires a considerable amount of basic physical chemical data, such as the adsorption isotherms of the solutes, before it can be applied to a practical problem. Volume overload is useful where the solutes of interest are relatively insoluble in the mobile phase and thus, to apply a sample of sufficient size onto the column, a large sample volume is necessary. If the sample is very soluble in the mobile phase then mass overload might be appropriate. 

Volume overload employing a solution of the material in the mobile phase at a level of about 5% w/v is a recommended method of sampling for preparative columns if the system is not optimized. However, a combination of volume overload and mass overload has also been suggested as an alternative procedure by Knox (13). 

Knox and Piper (13) assumed that the majority of the adsorption isotherms were, indeed, Langmuir in form and then postulated that all the peaks that were mass overloaded would be approximately triangular in shape. As a consequence, Knox and Piper proposed that mass overload could be treated in a similar manner to volume overload. Whether all solute/stationary phase isotherms are Langmuir in type is a moot point and the assumption should be taken with some caution. Knox and Piper then suggested that the best compromise was to utilize about half the maximum sample volume as defined by equation (15), which would then reduce the distance between the peaks by half. They then recommended that the concentration of the solute should be increased until dispersion due to mass overload just caused the two peaks to touch. 

Finally, an officially updated definition of the retardation factor, R, issued by lUPAC is important to the whole field of planar chromatography (the linear and the nonlinear TLC mode included). The importance of such a definition has two reasons. First, it is promoted by the growing access of planar chromatography users for densitometric evaluation of their chromatograms and second, by the vagueness of the present definition in the case of skewed concentration profiles with the samples developed under mass overload conditions. 

FIGURE 12.5 Human serum tryptic digest analysis. Fractionation in the first LC dimension was performed using a C18 column at pH 10. Fractions were analyzed using NanoEase 0.3 x 150 mm Atlantis d18 column. Approximately 66 lg (400 pmole of semm albumin peptides) was injected on column. Arrow points to a selected albumin peptide illustrating a local column mass overloading. Ten-5mm wide fractions were collected in 1st LC dimension. 

In practice the value of (w) will vary between about 2 and 5 ( i.e sample concentrations will lie between 2%w/v and 5%w/v) before mass overload becomes a significant factor In band dispersion. A numerical value for (g>) of 5 will be taken In subsequent calculations. The correct value of ( ), for the particular solute concerned, can be experimentally determined on an analytical column carrying the same phase system If so required. 

Band dispersion from sample mass overload is a direct result of the chromatographic process proceeding under conditions, where the adsorption isotherm of the solute on the stationary phase, is no longer linear. The development of an equation that describes the extent, of band spreadinn as a function of mass of sample placed on the column, is complex. This problem has been elegantly approached by 6uiochon and his co-workers (15-18) from the basis of the adsorption isotherm of the solute on the stationary phase. 

Often a large sample injection (e.g., 200 juJL) will experience only a little additional band broadening over that which would occur for a small injection (e.g., 10 (jlL). Thus, the large injection can result in a higher concentration for detection with no loss in resolution. (This assumes there is no mass overload, which is often a valid assumption in situations where insufficient sensitivity occurs due to very small concentrations of the sought-after substance. However, experimentally this assumption should be verified for each situation.) Also, improved chromatographic precision can result if there is smaller uncertainty of measurement with the larger volume injection. 

Mass overload occurs when the stationary phase does not have the capacity to retain the amount of sample injected. This can occur even for small injection volumes if the concentration of sample is high enough. This results in a characteristic shark-fin peak shape, where peak tailing starts from the peak s apex. For example, in order to obtain sufficient sensitivity, analytes with weak UV molar absorptivity may require a large enough amount of sample to be injected that the stationary phase becomes overloaded. Injecting less amount of sample, either by a smaller injection volume or by diluting the sample, can solve the problem of mass overload. However, sensitivity will decrease in this case. 

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