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retention volumes (in chromatography)

Retention measurements (and measurements of hold-up volume and peak

width) may be made in terms of times or chart distances as well as volumes.

If flow and recorder speeds are constant, the volumes are directly proportional to the times and

chart distances. The following definitions are drawn

up in terms of volume, and it is recommended that theoretical discussion

should be couched in the same terms wherever possible.

Retention VolumeThe retention volume is the volume of mobile phase passed through thecolumn between the injection point and the peak maximum. In liquid-chromatography, the

compressibility of the mobile phase is very small, so the retention volume can be taken as the

product of the time interval between the injection point and the peak maximum and the mobile

phase flow rate. In gas-chromatography, however the compressibility of the mobile phase is very

significant, so the corrected retention volume must be taken as the product of the time interval

between the injection point and the peak maximum and the mobile phase flow rate (measured at

the column exit) corrected for the compressibility of the gas.

Specific Retention VolumeThe specific retention volume of a solute is the corrected retentionvolume of the solute per unit mass of stationary phase. The corrected retention volume is

obtained from a column carrying a known weight of stationary phase and is measured at a

carefully controlled, accurately known temperature. The corrected retention volume is taken as

the difference between the solute retention volume and the dead volume. The retention volume is

taken as the volume of mobile phase that passes through the column from the time of injection to

the elution time of the peak maximum. The dead volume is taken as the volume of mobile phase

that passes through the column from the time of injection to the elution time of the peak

maximum of a completely untretained solute. If the mobile phase is a liquid, no pressure

correction is usually necessary, if the mobile phase is a gas, then all measured volumes must be

corrected for the compressibility of the gas. If the specific retention volume is divided by the

density of the stationary phase, the corrected retention per ml of stationary phase can be

calculated, the logarithm of which gives the standard energy of distribution. If the standard

energy of distribution is determined over a range of temperatures, then, by plotting the corrected

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retention volume per ml of stationary phase against the reciprocal of the temperature, the

standard enthalpy and standard entropy of the distribution can be calculated.

Retention indices are relative retention times normalised to closely eluting n-alkanes. Retention

indices are system independent and long-term reproducible, even after many years and in

different laboratories around the world. (2) Identifying peaks by library searches should not

solely focus on mass spectral similarity, but also include retention indices in order to optimise

the quality and reliablitity of library hits. Many isomeric compounds like sesquiterpene

hydrocarbons can only be identified if taking both mass spectra and retention indices into

account.

The retention time is the total time that a compound spends in both the mobile

phase and stationary phase. Retention time is generally reported in minutes