Concentration of Sample Extracts

The analytes are often diluted in the presence of a large volume of solvents used in the extraction. This is particularly true when the analysis is being done at the trace level. An additional concentration step is necessary to increase the concentration in the extract. If the amount of solvent to be removed is not very large and the analyte is nonvolatile, the solvent can be vaporized by a gentle stream of nitrogen gas flowing either across the surface or through the solution. This is shown in Figure 1.1.

Figure 1.1. Evaporation of solvent by nitrogen.

Care should be taken that the solvent is lost only by evaporation. If small solution droplets are lost as aerosol, there is the possibility of losing analytes along with it. If large volume reduction is needed, this method is not efficient, and a rotary vacuum evaporator is used instead. In this case, the sample is placed in a roundbottomed flask in a heated water bath. A water cooled condenser is attached at the top, and the flask is rotated continually to expose maximum liquid surface to evaporation. Using a small pump or a water aspirator, the pressure inside the flask is reduced. The mild warming, along with the lowered pressure, removes the solvent e‰ciently, and the condensed solvent distills into a separate flask. Evaporation should stop before the sample reaches dryness.