L'efficacité de l'extraction sur résine XAD-4 a été étudiée à partir de neuf composés hydrophiles non ionisables (alcools, aldéhydes, cétones, esters). Le facteur de capacité (k') est déterminé expérimentalement par élution frontale de différentes fractions volumiques de méthanol. Les rendements de récupération sont évalués pour quatre solvants. L'efficacité maximale et le facteur de concentration optimal sont définis en fonction du volume de percolation et du volume d'élution. Le rendement moyen est de 80 % après élution par l'acétone.
L'entraînement dynamique à la vapeur est par ailleurs développé pour concentrer les composés dans 200 ml de distillat à partir de 4 litres d'échantillon. Le rendement moyen est de 77 %. Ces deux techniques sont associées afin d'accroître la spécificité de la méthodologie analytique vis-à-vis des composés hydrophiles non ionisables. Après un entraînement de l'échantillon d'eau, le condensat est ensuite percolé sur une colonne de résine XAD-4 (7 cm x 1,5 cm), l'élution s'effectuant par 14 ml d'acétone. L'éluat est alors concentré, à température ambiante, par un courant d'azote jusqu'à 4 ml, puis analysé par un chromatographe en phase gazeuse. Cette méthodologie a été appliquée à un eau naturelle artificiellement polluée par 15 composés hydrophiles non ionisables. Les rendements de récupération sont compris entre 42 % et 83 % (rendement moyen de 63 %).
- Composés hydrophiles non ionisables,
- entraînement à la vapeur d'eau,
- extraction sur résine XAD-4,
Extraction of Non-Ionic Hydrophilic Compounds from Water by Steam Distillation Followed by Sorption onto XAD-4 Resin
Non-ionic hydrophilic compounds are very often used as solvents in industrial activities or for domestic use. These products are generally alcohols, aldehydes, ketones and esters. Although they are not very dangerous for the environment at low concentrations, for hazardous pollution conditions there is a need to be able to identify and quantify such compounds. Chlorination of humic substances during drinking water treatment can also generate chlorinated by-products (chlorinated ketones).
Because of their solubility in water, it is necessary to concentrate and extract these compounds before gas chromatography analysis. In this study, we describe a method where liquid/gas (steam distillation) and liquid/solid (XAD-4 resin) extractions were combined. The aim of this work was to define analytical conditions to obtain the best efficiency for the overall method.
In the first phase, we developed the liquid/solid extraction on XAD-4 resin by studying capacity factors, elution volumes and percolated water volumes. The sorption of nine non- ionic hydrophilic compounds (ethanol, butanol-1, pentanol-1, ethyl acetate, n-butyl acetate, methylisobutylketone, cyclohexanone, diisobutylketone, butanal) from water onto microparticulate XAD-4 resin, and their subsequent recovery into water-methanol mixtures, was used to estimate the capacity factor (k'water) for each analyte. According to the capacity factor values, we could define three classes of compounds:
1. highly hydrophilic compounds such as ethanol, which were not very well retained by the resin (k'water=11);
2. moderately hydrophilic compounds such as ethyl acetate, butanol-1, butanal, pentanol-1 for which capacity factors were less than 100;
3. weakly hydrophilic compounds that were well retained by the resin, such as diisobutylketone with a capacity factor equal to 1770.
Subsequently, mobilization/recovery by frontal elution into each of four different solvents (dichloromethane, acetone, acetonitrile, methanol) was studied in order to define for each of the analytes an optimal recovery procedure. Recoveries were dependent on the volume of the water sample passing through the column (100 mL, 200 mL, 400 mL) and on volume of solvent used for the elution. The best recoveries were observed when 200 mL of water were passed through the column and when the elution was performed by 14 mL of acetone. Mean recovery was 80%. Methylisobutylketone, pentanol-1 and ethyl acetate were extracted with yields higher than 80%, whereas butanal and 2,2-dichloroethanol showed a poor recovery of around 60%.
In the second phase of the study, steam distillation was also assessed as a technique for the concentration of analytes (200 mL condensate from 4 litres aqueous sample). Mean recovery was 77%. Higher recoveries were observed for ethyl acetate, butanol-1, pentanol-1 and 1,1- dichloroacetone. Dichloroethanol was not extracted with a suitable recovery (yield around 33%).
The two extraction methods were combined to increase the extraction specificity for non- ionic hydrophilic compounds. In a preliminary concentration step, the analytes were recovered by steam distillation from 4 litre water samples. Analytes were sorbed from the condensate onto a 7 cm x 1.5 cm column of XAD-4 resin, then eluted with 14 mL acetone. The column eluate was concentrated to 4 mL, at room temperature in a gentle stream of nitrogen, and analyzed by gas chromatography.
The extraction procedure was applied to 18 compounds (butanol-1, pentanol-1, hexanol-1, octanol-1, decanol, 2,2-dichloroethanol, ethyl acetate, n-butyl acetate, methyl hexanoate, methyl octanoate, methylisobutylketone, diisobutylketone, 1,1-dichloroacetone, octanone-2, butanal, hexanal, octanal, benzaldehyde) in spiked ultrapure water and also to 15 compounds (same compounds except for benzaldehyde and 2,2-dichloroethanol) in spiked natural water. Mean recovery percentages for the analytes in ultrapure water and in natural water were respectively around 60% and 63%. These results demonstrate that the ionic composition of the water medium did not interfere with the efficiency of the complete method. In the case of spiked natural water, recovery yields ranged between 42% (butanol- 1) and 83% (octanol-1).
- Non-ionic hydrophilic compounds,
- steam distillation,
- XAD-4 resin extraction,