Topics - Standard
The term standard
in chromatography is employed in two ways. It can be used to describe a reference substance, the retention time of which is compared with the retention time of an unknown substance for identification purposes. Alternatively, it can be used to provide a reference peak height or peak area which can be compared with the peak heights or areas of the substances of interest to provide quantitative information. A standard employed in either application can be used in two ways, either as an internal standard
or as an external standard
. An internal standard
is added as a known quantity to the sample itself, but must be chosen so that it is adequately resolved from its neighbors so that accurate measurements can be made. By using a synthetic sample mixture the response factors of the solutes of interest and the internal standard can be determined. Then, from the retention of the standard and the retention ratios of the solutes of interest to the standard, the identity of the components of interest can be confirmed. From the peak heights or areas of the standard and those of the solutes of interest the amount present of each solute of interest can also be assessed. The external standard
is used when a suitable internal standard that can be separated from the components of the mixture can not be selected. In this case the external standard is run as a separate chromatogram under exactly the same conditions. The properties of the standard from the separate chromatogram are then compared with the properties of the solutes in the chromatogram of the mixture. In general, analyses obtained by employing an internal standard provide more accurate results than those employing an external standard.