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Abbreviated Microwave Extraction of Pesticides and PCBs in Soil
Rick McMillin, LC Miner, and Lisa Hurst
U.S. Environmental Protection Agency, Region 6 Houston Lab, 10625 Fallstone Rd., Houston, TX. 77099.
Full text can be found in: Spectroscopy; an International Journal, vol. 13, 41-53 (Feb. 1997).
Abstract:
The Houston Lab has been involved in evaluating various new types of organic extraction methods and equipment, focusing on improving productivity and reducing hazardous waste for our pollution prevention program. One such method we have investigated that holds promise is microwave assisted extraction (MAE). Our lab has "abbreviated" the MAE method by reducing or eliminating the concentration step. This greatly enhances the overall productivity and ease of use of the method while significantly reducing the amount of solvent used. This modified method is compared with sonication and automated soxhlet techniques in three matrices (sand, clay, & sediment).
Results
Recoveries: Microwave recoveries were typically higher than those for both Soxtherm and sonication at high and low level spikes. Chart #1 indicates the average low level recoveries for each analyte. These results were averaged from all seven replicates and all three matrices. Chart #2 indicates the overall recoveries for all analytes in each matrix for low level and high level, respectively. These results were averaged from all seven replicates for each matrix. The AMAE method indicates higher recoveries than the other two.
Chart #1
Chart #2
Chart #3:
Real Samples: Twelve samples from an actual site investigation that contained Aroclor 1260 were extracted by all three methods we evaluated. Results from this study are presented in chart #3. If all results were averaged and then normalized to the AMAE values, you would get: AMAE = 100%, Soxtherm = 82%, and sonication = 59% recovery.
Productivity: The main motivation for pursuing this investigation was to find a method to increase lab productivity while reducing solvent waste. Table 2 indicates the approximate time spent on the three methods evaluated. The sonication method has a time range indicated due to the fact that our lab used less solvent than the method 3550 recommendation. The times indicated are conservative estimates for 6 samples and can vary between individuals and equipment.
Table 2: ESTIMATED TIMES FOR SAMPLE PREPARATION |
|||
Function |
AMAE |
Soxtherm |
Sonication |
| Equip/Sample Prep Time | 20 |
20 |
20 |
| Time of Extraction | 20 |
150 |
45 |
| Cool-down Time | 30 |
60 |
0 |
| Concentration Time | 0 |
45* |
90 - 150* |
| Sample Cleanup Time | 50* |
20 |
20 |
| Equipment Cleanup/Store | 20 |
20 |
20 |
| Total Time: | 140 |
315 |
195 - 255 |
| Amount of Solvent Used: | 10 |
120 |
180 - 300 |
* Includes solvent switching. RapidVap N2 was used if a concentration step was required.
Questions and Conclusions:
Other reviewers of this data have raised questions at why the AMAE produced higher recoveries than the other methods. The most obvious concern was the possibility of significant solvent loss. Since a complete quantitative transfer of the entire extract is not possible with the AMAE method, the exact final volume of solvent is not known. Another possibility raised was that since there was no mechanical agitation or mixing occurring, hot spots might exist in the sample that could bias results depending on what pockets of solvent were collected for analysis. Though it may not be understood fully, the higher recoveries of the AMAE could be due to the increased solvating properties of the extraction solvent at the elevated temperature and pressure (same principle used in the ASE by Dionex). This could be more dramatic when higher molecular weight compounds are present that trap the analytes of interest (such as the oily material in the real samples of our investigation). Some type of kinetic mixing may occur, possibly by convection currents. Recoveries could also be higher because of reduced sample handling with this method.