Understanding Oil and Grease Analysis

Article written by Jessica Raney, Quality Control Group Leader, Analytical Products Group

Complete Article from Edition 6 APG eNewsletter

Background
EPA Method 1664 n-Hexane Extractable Material and Silica Gel Treated Material by Extraction and Gravimetry is one of the most technique sensitive analytical methods still in use today. In 1999, EPA approved 1664 for use as a replacement for Method 413.1, the Freon-113 extraction method. When the decision to phase out Freon-113 was made, EPA faced a challenge as they tried to find a solvent that provided comparable extraction efficiency to Freon-113. After trying many alternatives, they determined that no other solvent matched the efficiency of Freon-113, however, they determined that n-hexane extracted about 96% of the material versus 100% by freon-113. This difference was deemed insignificant when compared with other errors that could occur and n-hexane was proposed as the replacement solvent.

Method 1664
The procedure specified in Method 1664 is not complex, but does require a significant amount of analytical technique in order to achieve acceptable results.

1. The sample is acidified to a pH < 2 and extracted with n-hexane by a liquid-liquid extraction.
2. The solvent layer is retained and dried over sodium sulfate to remove any water in the organic phase.
3. The solvent is distilled and the sample then dried to a constant weight.
4. The residue is weighed and this is referred to as the hexane extractable material, or HEM.
5. The HEM is redissolved in n-hexane and silica gel is added to remove the polar materials in the sample.
6. The solution is filtered to remove the silica gel, the solvent distilled, and the residue dried to a constant weight.
7. The resulting residue is the silica gel treated hexane extractable material, or SGT-HEM.

Improving Method Performance
Modifications to the method can be made to improve the method performance as long as all quality control criteria are being met and equivalency can be demonstrated. An example of this is the substitution of solid phase extraction for liquid-liquid extraction. Solid phase extraction or SPE, is a lower cost alternative to liquid-liquid extraction. Less solvent is used and the process is more automated. The automation reduces variability in the extraction process by limiting the errors that are technique related. EPA has approved SPE for use in the method without demonstrating equivalency. However, you may want to check with your accrediting authority to ensure that SPE is an acceptable alternative.

pH Adjustment
Several steps in the method are critical to success. The pH adjustment is important to ensure that the oil and grease will be extracted into the organic phase. Proper pH adjustment will also ensure that sodium sulfate will not dissolve in the organic phase.

Extraction Efficiency
Extraction efficiency is also important to the success of 1664. Liquid-liquid extractions are very technique sensitive and the key to their success is precise handling of the sample. Mechanical shakers may improve efficiency by ensuring sufficient mixing of the sample during the extraction procedure. The substitution of SPE for LLE will also improve efficiency as well as reduce cost.

Solvent Distillation
The solvent distillation is another important step in the procedure. This step should be accomplished in thirty minutes or less, but should not be done too quickly. Rapid heating to speed up the distillation can cause some of the material of interest to be evaporated from the sample.

Filter Size
The filter size used in filtering out the silica gel is important. The size of the filter should be carefully selected in order to prevent the silica gel from moving through the filter membrane and remaining in the sample during the drying process. The compounds of interest in an oil and grease sample tend to stick to everything the touch. Therefore, any surface that the sample came in contact with should be rinsed with n-hexane to make sure that no material of interest is lost in the transfer process.

Independently Prepared Quality Controls
There are many quality control procedures specified by the method. When an analytical method is as sensitive to technique as 1664, it is critical to follow the quality control guidelines in order to have confidence in data generated. Use of independently prepared quality control samples and regular participation in PT studies can help measure the performance of the method and improve the overall performance of the laboratory by helping to identify problems in the execution of the method. For more information about optimizing the performance of Method 1664, refer the EPA’s website at www.epa.gov.