Topics - Vacancy

Vacancy The term vacancy in chromatography arises from the technique of ’Vacancy Chromatography’. Just as a positive concentration of a solute is eluted through a column and each solute is retained to different extents so can a negative concentration of solutes be developed in the same way. If the mobile phase contains a small percentage of a particular solute in a given solvent and a sample of pure solvent, completely free of the solute, is injected onto the column then, in effect, a negative concentration of solute will be placed as a sample on the column. This negative concentration of solute will be eluted through the column and have exactly the same retention time as that of a positive sample placed on the column with pure solvent being used as the mobile phase. This technique has been called Vacancy Chromatography. Both the plate and rate theories apply to vacancy chromatography except concentrations must be made negative. The peaks produced in vacancy chromatography (unless electronically reversed in sign) will be negative and under these circumstances are called vacancy peaks. Vacancy chromatography has not been used extensively for analytical purposes but could be very useful in product control such as a pharmaceutical mixture. If the mobile phase contains a small concentration of a reference pharmaceutical product (having the correct proportions of each constituent) and a sample of a new batch is made up in the same concentration as that in the mobile phase and injected onto the column, if the composition of the new product is correct, no peaks will appear on the chromatogram. However, if any components in the sample mixture are below specifications they will show as a negative peak and any components in excess of the specifications will show as a positive peak.