Capillary Chromatography - Heart Cutting 1
Heart cutting is term given to the technique of isolating of a pair (or more) of unresolved solutes from one separation and placing the mixture on another column which does resolve them. This technique is one that is usually employed in the separation of multi-component mixtures where a change in stationary phase merely results in a peak reshuffle, the solute of interest is changed from being eluted in conjunction with one component with that of another. One example of the use of heart cutting is shown in figure 22.
In this example, the technique is used as a form of enrichment process where the solute of interest is eluted in the tail of a major component, the compound of interest being completely obscured by the tail of the overloaded peak. If this sample was separated under non-overload conditions the two solutes would be eluted discretely and the quantity of the minor component present easily estimated. If the portion of the column eluent, eluting between the boundaries marked in green in figure 22, is collected the amount of the major component present in the fraction is now very much smaller than in the original sample and therefore can be injecting onto another column without any overload. This would result in the second separation shown in figure 22. A realistic practical example of this type of heart cutting procedure would be in the determination of thiophene in benzene. Thiophene is a natural contaminant of benzene obtained as a by-product from the pyrolysis of coal (i.e., in the production of coke many important aromatic hydrocarbons are extracted from the gaseous products).
The thiophene is a trace contaminant that can be a useful product in its own right or considered as a serious sulfur containing contaminant of the product benzene. In either way its assay is important and heart cutting in the manner described could afford an elegant solution to the analytical problem. Another example of heart cutting is shown in figure 23.
Figure 22. Heart Cutting for Sample Enrichment