Comparison of methods used for pre-concentrating small volumes of organic volatile solutions

Eight pre-concentration techniques were compared for their capacity to retain volatile and semi-volatile solutes during evaporation of solvent (dichloromethane). The 2-ml test-samples containing 0.2 ppm or 2 ppm (v/v) of volatile and semi-volatile solutes were concentrated to a final volume of 1 ml, 200 μl and 50 μl, respectively. When pre-concentrating to 50 μl, the highest recoveries for both the diluted (0.2 ppm) and concentrated (2 ppm) solutions were found by passive evaporation in a test tube at 22 °C. The pre-concentration time from 2 ml to 50 μl by this method was 19–20 h. Heating the test tube to 47 °C yielded lower recoveries in dilute samples, but the recoveries of concentrated samples were only slightly lower than the recoveries obtained by passive evaporation. The evaporation time was decreased to 1–2 h. The recoveries and the reproducibility of these methods were superior to the other pre-concentration methods tested. Loss of solute was apparently mainly caused by the fast vapour streams created when speeding up the process of evaporation by heating or by introducing a gas stream into the tube. This increased co-evaporation and thereby solute loss. The capacity of the methods to trap the escaping vapours and create a reflux determined the capacity of the methods to recover the solutes. The experiments demonstrated that more solute is lost during the pre-concentration of dilute samples compared to more concentrated solutions.