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Relative Retention Time (RRT) or Relative Retention

"Relative retention" and "relative retention time" are used interchangeably to describe the retention-time relationship between two peaks. Usually one of the peaks is a known standard, the other one is an unknown. RRT among other experimental data can be used for peak identification, although RRT by itself is usually insufficient. The exact RRT calculation has been redefined in European Pharmacopoeia since 2003. However, it may be unclear to chromatographers which method exactly was used even in current literatures.

Relative Retention Time Calculation - method 1

This is the current method used by the United States Pharmacopeia and the European Pharmacopeia.
RRT = (t2 - t0) / (t1 - t0)

Relative Retention Time Calculation - method 2

This is an older version used by European Pharmacopeia up to 2002. It can occasionally be found in current literatures. In this method, the void time is not taken into account for the RRT calculation. You should be aware of this method, but it should not be used.
RRT = t2 / t1

Regardless of which method was used, if the RRT is greater than one, the unknown elutes later than the standard. If the RRT is smaller than one, the unknown elutes before the standard. If the RRT is one, the unknown and the standard elute at the same time.

Related Terms and Additional Information

Liquid Chromatography

Glossary

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