internal standardisation and external standardisation

Question

What are the similarities and differences between internal standardisation and external standardisation? Explain with reference to a chromatographic technique of your choice, for example HPLC.

Answer 1

They are pretty much what they say they are.

Internal standard - the standard is added to the unknown sample.

External standard - the standard is run separately from the unknown.  It is run by itself.

 

Both types of standards (standardization) can be used to quantitate an analyte in an unknown, but the internal standard can also give information on percent recovery.  It can be added to the unknown at the beginning of any procedure, or it can be added right before performing the chromatographic method. One will give total recovery information, the other will provide information on recovery during the chromatography only.  External standards do not provide such information. 

 

Using HPLC as an example: An external standard of varying concentrations is injected and peak heights are determined. The unknown is injected, and peak height of same retention time is compared in order to quantify the unknown.

An internal standard is either added to the extraction procedure and carried through all the steps, or it is added to the sample being injected on the HPLC. Either way, it is also compared to the peak height of the external standard, so as to identify retention and quantitate the unknown. Comparing the peak height of the internal standard to that of the external standard tell you the recovery.