Journal of Chromatography A, 824 (1998) 195–203 Concentration of small volumes of nonpolar solutions containing trace volatile compounds * ˜ ´ Vicente Ferreira , Cristina Pena, Ricardo Lopez, Cristina Crespo, Juan Cacho Department of Analytical Chemistry, Faculty of Sciences, University of Zaragoza, 50009 Zaragoza, Spain Received 27 January 1998; received in revised form 20 July 1998; accepted 20 July 1998 Abstract The analytical behaviour of the concentration of extracts under a stream of nitrogen of small volumes of hexane solutions containing volatile analytes has been studied. The proportion of analyte blown off the solution is a function of its boiling point, of its polarity, and of the final volume reached. For similar boiling points, the more polar the compound, the higher the level of losses. If the evaporation progresses to a final volume of 0.01 ml, all the compounds with boiling points below 1508C are lost in proportions higher than 50%. Compounds with boiling points between 150 and 2508C are lost in proportions ranging from 70 to 25%, and only eicosane (b.p. 3408C) is almost fully retained in the solution. The addition of small amounts (5 mg) of dehydrated silica to the evaporating solution allows the evaporation to proceed without noticeable losses of solutes more polar than hydrocarbons. After concentration, the solutes can be easily recovered by adding a small amount of ethanol or a mixture water–ether.  1998 Elsevier Science B.V. All rights reserved. Keywords: Sample handling; Evaporation loss; Volatile compounds 1. Introduction significantly. Quite a different problem is the one encountered when 1 or 2 ml of solution must be There are several steps commonly included in concentrated to a final volume of 0.1 ml or to an trace volatile analytical procedures that have re- even smaller one. In this case, the situation changes ceived rather less attention than should have been completely due to several reasons. Firstly, there is no expected. One of these steps is the concentration of standard equipment specifically designed to handle extracts through solvent evaporation. In a previous these volumes. Secondly, the mass transfer charac- paper [1], the analytical characteristics of a micro- teristics of small volumes are not the same as those Kuderna-Danish concentrator were described and the of larger ones. Thirdly, and as a consequence of conclusions reached showed that the losses become these two previous reasons, it is very difficult to critical in the last steps of the concentration of dilute perform fractional distillation. Hence, the concen- solutions. It became clear, however, that if properly tration of small volumes is usually carried out by carried out, concentration of 20–30 ml solutions up evaporating the solvent under a stream of purified to a final volume of 0.5 ml can be done without Nitrogen [2], or by simple microdistillation [3,4]. ¨ altering the analytical characteristics of the sample In a classic work, Grob and Muller [5] compared the performance of several strategies for concen- * Corresponding author. trating small volumes (from 800 to 10 ml) of pentane 0021-9673 / 98 / $ – see front matter  1998 Elsevier Science B.V. All rights reserved. PII: S0021-9673(98)00608-6