Environ. Sci. Technol. 1991, 25, 540-546
Oxidation of Phenolic Acid Surrogates and Target Analytes during Acid Extraction of Natural Water Samples for Analysis by GWMS Using EPA Method 625 Paul H. Chen," William A. VanAusdale, and Dwight F. Roberts Analytical Services Division, Environmental Science and Engineering, Inc., P.O. Box 1703, Gainesville, Florida 32602
Zero or low phenolic acid surrogate recoveries were occasionally observed in natural water samples analyzed by EPA method 625. These low recoveries were usually caused by the oxidation of the phenolic acid surrogates during acid extraction. Three oxidation products of the phenolic acid surrogates were identified. Iodocyclohexanol, an artifact formed during extraction with cyclohexene-inhibited methylene chloride, was usually present in samples that contained oxidizing agents. Matrix effects on the recoveries of phenolic target analytes were investigated in samples having low phenolic acid surrogate recoveries. The results showed that recoveries for phenols with strong electron-withdrawing substituents, such as a nitro group, were not affected. However, decreased recoveries were observed for phenols having electron-donating substituents or weak electron-withdrawing substituents. It is demonstrated that manganese(II1,IV) oxides are most likely the oxidizing agents for the groundwater samples studied. If samples were extracted according to EPA method 625, in the presence of a large amount of manganese, Mn2+dissolved in the sample would be oxygenated to manganese(II1,IV) oxides during base extraction, which in turn would oxidize the phenols during acid extraction. The oxidation of phenols can be eliminated by adding sodium thiosulfate to the sample or extraction at acidic pH first or alone providing that manganese(II1,IV) oxides are not present in the sample before the extraction. Introduction EPA method 625 ( I , 2 ) is widely used in environmental laboratories for the analysis of semivolatile organic compounds by gas chromatography/mass spectrometry (GC/MS). In method 625, a l-L aliquot of water is extracted with methylene chloride at pH >11 and then at pH 11 and then at pH 11. Very high recoveries (>go%) were obtained if samples were extracted initially at pH >11. The low recovery is probably due to oxidation of 4chloroaniline by air during acid extraction. If manganese(II1,IV) oxides are present in the sample, they can oxidize 4-chloroaniline in acidic conditions (15). It should be noted that the 3/90 U.S. EPA CLP Statement of Work (20) uses only acid extraction a t pH 2 by continuous liquid/liquid extraction. If no manganese(II1,IV) oxides are present in the sample as suspended particles before acid extraction, any Mn2+ present in the sample would not oxidize the phenols. This is because Mn2+is not an oxidant and will not be oxygenated to manganese(II1,IV) oxides under strongly acidic conditions, i.e., pH 2 or lower. Mn2+ is known to be oxygenated to manganese(II1,IV) oxides under alkaline conditions (16,17). If manganese(II1,IV) oxides are present, the sample should be filtered through
a 0.2 pm pore diameter membrane filter before the sample is adjusted to pH 2. This filtration process should effectively remove manganese oxide particulates (10, 18). Table I1 shows that the results for the unfiltered (sample F) and the filtered (sample G) samples are very similar. This indicates that most manganese is present in the test sample as a water-soluble form (Mn2+)and not as suspended particles [manganese(III,IV) oxides]. The samples composited for the second set of test samples were all obtained from deep groundwater wells where oxygen was depleted, and the pH for the composite sample was 6.3. These conditions are not favorable for formation of manganese(II1,IV) oxides. Furthermore, the samples are transported and stored in tightly sealed filled bottles, minimizing sample contact with atmospheric oxygen. The standard procedure for EPA method 625 requires 1-L water samples to be initially extracted at pH >11with a 2-L separatory funnel. The funnel is shaken with frequent venting. Under these conditions, Mn2+dissolved in the sample will be oxidized by air to manganese(II1,IV) oxides (16,17). However, at this high pH it is unlikely that any phenols, with a possible exception of dimethylphenol, will be oxidized a t an appreciable rate. After this initial extraction, EPA method 625 requires the sample to be extracted again at pH 11and then extracted a t pH
