Keepers are substances (typically solvents, but sometimes adsorbent solids) added in relatively small quantities during an evaporative procedure in analytical chemistry, such as concentration of an analyte-solvent mixture by rotary evaporation.
The purpose of a keeper is to reduce losses of a target analyte during the procedure.
In the case of non-volatile target analytes, the presence of the keeper solvent or solid is intended to prevent all the solvent from being evaporated off, thereby preventing the loss of analytes which might irreversibly adsorb to the container walls when completely dried, or if it is totally dried (in the case of a solid keeper), provide a surface where the analyte can be reversibly rather than irreversibly adsorbed.
[1] A solid keeper of sodium sulfate has been shown to be effective for reducing losses of polycyclic aromatic hydrocarbons (PAHs) in evaporative procedures.
[2] The following solvents are commonly used as keepers:[1] This article about analytical chemistry is a stub.