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Performance Evaluation and Method Equivalency Study for Automated Electrically Assisted Liquid-Liquid Extraction


Oral Presentation at the 1996 Pittsburgh Conference, Chicago, Ill., paper #1269

Rick McMillin, Diane Gregg, Lisa Hurst, U.S. Environmental Protection Agency,

10625 Fallstone Road, Houston, TX 77099

Kevin P. Kelly, Ph.D., David L. Stalling, Ph.D., OI Analytical Sample Preparation Products Group, 555 Vandiver Drive, Columbia, MO 65202

ABSTRACT

Electrically assisted liquid-liquid extraction (EAE) employs high voltage fields to disperse aqueous sample droplets into methylene chloride (MeCl), producing rapid extraction and also preventing emulsions. Automated EAE is simple and provides rapid throughput (2 hours per batch) with less labor consumption than other methods. EAE yields good precision with adequate recoveries when compared with traditional extraction methods such as separatory funnel (SF) or continuous liquid-liquid (CL) extraction of reagent water, ground water, wastewater, and TCLP buffer. Because EAE doesn't change the extraction chemistry, it can be an acceptable extraction method for RCRA analysis. This is allowed under the SW-846 test methods disclaimer that allows for "equivalent or superior method performance, or performance appropriate for its intended application". Because the mechanism for extraction with EAE is very similar to other liquid/liquid extraction techniques, an ATP is not necessary.

Since the last study performed on this device was made by this lab1, a modification was made to the EAE device (ExCell) in which a motorized stirring mechanism was added. Initial results with a single prototype showed promising increases in analyte recoveries in some compounds not well extracted in the last study. For seven reagent water replicates extracted at a pH < 2, recoveries of 52 analytes spiked at 10ug/L were recovered at a mean of 74.6%. This compares favorably with results obtained from separatory funnel, 69.1%, and the Accelerated One-Step (AOS), 72.9%. The unmodified ExCell had a recovery of 60.5%. This study will compare the newly modified version of the ExCell with the AOS using an acid and base extraction of 65 analytes (including 8 surrogates) at a low level of 10 ug/L and a high level of 300 ug/L. Analysis will be performed by method 8270.


RESULTS

It was discovered that the use of methylene Chloride as the spiking solvent could cause some unexpected spiking biases. Methanol was selected as a more appropriate choice, and a comparison study was performed. Results are listed in table 1 below.


Table 1:

Average % Recoveries for All Compounds and All Replicates -- Low Level

Matrix

ExCell / MeCl ExCell / MeOH AOS / MeCl AOS/ MeOH
DI Water

61

77

72

72

Ground Water

55

75

61

63

Waste Water

59

67

75

65

 


 

Surprisingly the AOS results were not as affected as the EAE results by the use of MeCl. It was assumed that the MeCl would fall straight through the AOS apparatus and never reach equilibrium with the water sample. Apparently equilibrium is reached, though not by a mechanism we have positively identified. The negative bias of the EAE could be due to what is referred to as a "wall effect", in which the MeCl adheres to the walls of the sample container used in the EAE. Since the walls are not mechanically rinsed, this could leave the less water soluble analytes behind. Using MeOH ensures the thorough mixing of the spiking solution with the water sample, thus giving the more representative sample. A more thorough study of spiking bias was conducted as a result of this study6.


Conclusions

Upon use of MeOH as the spiking solvent, the recoveries of the modified ExCell produced recoveries that were comparable to the AOS. In addition, the modifications reduced the amount of extraction solvent and time involved for the ExCell.

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