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7.2.1 Example 7.1: Soxhlet Extraction of Polycyclic Aromatic Hydrocarbons from Contaminated Soil
220.127.116.11 Extraction Conditions These were as follows:
• Sample: 10 g, plus 10 g anhydrous sodium sulfate
• Solvent: 150 ml dichloromethane
• Extraction time: 24 h
Comments The sample was heated by using an isomantle. Typically, refluxing of the solvent occurred at the rate of four cycles per hour. Extracts were concentrated to 10 ml using a rotary evaporator and then diluted twofold before addition of the internal standards.
18.104.22.168 Analysis of Extracts by GC-MS
Separation and identification of the individual polycyclic (polynuclear) aromatic hydrocarbons (PAHs) was carried out on an HP 5890 Series II Plus gas chromatograph, fitted to an HP 5972A mass spectrometer. A 30 m x 0.25 mm id x 0.25 Rm film thickness DB-5 capillary column was used, with temperature programming from an initial temperature of 85°C held for 2 min, before commencing a 6°Cmin-1 rise to 300°C, to give a final time of 7 min. The split/splitless injector was held at 300°C and operated in the splitless mode with the split valve closed for 1 min following sample injection. The split flow was set at 40 ml min-1, and the mass spectrometer transfer line was maintained at 270°C. Electron impact ionization at 70 eV, with the electron multiplier voltage set at 1500 V, was used, while operating in the single-ion monitoring (SIM) mode.
22.214.171.124 Typical Results
These are shown in Figure 7.4 .
Methods for Environmental Trace Analysis
200 150 -41
i 100 4
i i i i i i i i i i i i i 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Polycyclic aromatic hydrocarbon
Figure 7.4 Results obtained for the Soxhlet extraction of various polycyclic aromatic hydrocarbons from contaminated soil: 1, naphthalene; 2, acenaphthylene; 3, acenaphthene; 4, fluorene; 5, phenanthene; 6, anthracene; 7, fluoranthene; 8, pyrene; 9, benz[a]anthracene; 10, chrysene; 11, benzo[fc,?]fluoranthene; 12, benzo[a]pyrene; 13, indeno[1,2,3-cd]pyrene; 14, benzo[ghi]pyrene  (cf. DQ 7.2).
Comment on the results obtained in this study (see Figure 7.4).
It can be seen that Soxhlet extraction is able to extract a whole range of polycyclic aromatic hydrocarbons effectively and with reasonable precision.
7.3 Shake-Flask Extraction
Conventional liquid-solid extraction, in the form of shake-flask extraction, is carried out by placing a soil sample into a suitable glass container, adding a suitable organic solvent, and then agitating or shaking.
What agitating or shaking actions do you think are possible?
The most common actions are rocking, a circular action or an ‘end-over-end’ motion.
Agitating or shaking is carried out for a pre-specified time-period. After extraction, the solvent containing the analyte needs to be separated from the matrix by means of centrifugation and/or filtration. In some instances, it may be advisable to repeat the process several times with fresh solvent and then combine all
Figure 7.5 Typical procedure used for the shake-flask extraction of solids.
of the extracts. A typical procedure used for shake-flask extraction is shown in Figure 7.5.
7.3.1 Example 7.2: Shake-Flask Extraction of Phenols from Contaminated Soil
126.96.36.199 Extraction Conditions These were as follows:
• Sample: 1 g
• Solvent: 50 ml methanol-water (60-40 vol%)
• Extraction time: 30 min
Comments The sample and solvent were placed in a 100 ml screw-capped bottle and extracted on a rotating-disc Warburg mixer. The resultant sample/solvent mixture was filtered under vacuum, and the extracted sample then filtered through a 0.45 ^m membrane ‘Acrodisk’ prior to analysis.
188.8.131.52 Analysis by HPLC
Separation and quantitation was achieved by using a 25 cm x 4.6 mm id ODS2 column with UV detection at 275 nm. The mobile phase was acetonitrile-H2O-
Methods for Environmental Trace Analysis
§ 40 ?c
Figure 7.6 Results obtained for the shake-flask extraction of various phenols from contaminated soil and ‘Celite’: A, phenol; B, 3-cresol; C, 4-ethylphenol; D, 1-naphthol: ?, ‘Celite’; ?, soil  (cf. DQ 7.4).
acetic acid (40 : 591), operating under isocratic conditions, at a flow rate of 1 mlmin-1. A 20 Rl ‘Rheodyne’ injection loop was used to introduce samples and standards onto the column (at 30oC).
184.108.40.206 Typical Results
These are shown in Figure 7.6 .
Comment on the results obtained in this study (see Figure 7.6).
It can be noted that, in most cases, lower recoveries are obtained when shake-flask extraction is used to remove phenols from soil when compared to ‘Celite’ (an inert siliceous matrix). This effect is most pronounced with 1-naphthol.
7.4 Ultrasonic Extraction
Sonication involves the use of sound waves to agitate a sample immersed in an organic solvent. The preferred approach is to use a sonic probe, although a sonic bath can also be used. The sample is placed in a suitable glass container and enough organic solvent is then added to cover the sample. The system is then sonicated for a short time-period, typically 3 min, using the sonic bath or probe. After extraction, the solvent containing the analyte is separated by centrifugation and/or filtration and fresh solvent added. The whole process is repeated three times and all of the solvent extracts are then combined.