The analysis of oil and grease has long presented a challenge to the analytical community. The actual compounds of interest range from vegetable oils to animal fat to petroleum-based products. While it would be possible to develop methods that could identify each and every compound, it would be cost and time prohibitive. The sheer number of compounds for which you would have to calibrate would easily overwhelm anyone with more than one sample or a deadline to meet.
This led to the development of methods to measure oil and grease as an aggregate analyte. Because oil and grease is a rather non-specific class of compounds it is nigh unto impossible to develop a test that is selective only to a sum of the combined oil and grease. Currently n-hexane is the accepted solvent of choice for the analysis of oil and grease compounds. Because of the solvent choice and the lack of a good working definition for oil and grease, the term hexane extractable material (HEM) is becoming more widely used. Hexane is a fairly selective solvent but does have some limitations to be aware of. Among the non-oil and grease compounds that will dissolve in hexane are elemental sulfur, some aromatic compounds, and some organic dyes.
Liquid–liquid extraction and solid phase extraction
There are two common methods that are used for oil and grease determination: liquid–liquid extraction (LLE) and solid phase extraction (SPE).
Typical Filtration Time of SPE Discs*
They both follow the same basic chemistry—dissolution in hexane followed by gravimetric determination of the dissolved compounds. The main difference is in the extraction phase. In LLE the sample and solvent are vigorously mixed together and then allowed to separate. This allows the hexane to extract the appropriate material and then be removed from the aqueous phase. In SPE a disc, bed, or cartridge of resin, typically C18, is used. The oil and grease components, as well as many others, have a higher affinity for this resin than they do for water. By passing the sample through the resin all the material of interest is removed from the aqueous phase and adsorbed onto the solid phase. The desired materials have an even higher affinity for hexane. Thus, it is a simple matter of passing hexane through the resin to remove the analytes of interest. Whichever initial extraction is used, the final step is to evaporate the hexane and weigh the residue. The final result is commonly expressed as mg HEM/L.
Due to the physical characteristics of oil and grease, there are certain procedures that should be followed in order to obtain an accurate analysis. Some of these apply to both LLE and SPE while some are method specific.
- Always use whole volume samples when analyzing for HEM. Most of the materials that are detected by this test are, at best, sparingly soluble in an aqueous phase. This means that most of the interaction is taking place at the boundary layer between phases. Almost all of the material will either be at the surface of the liquid or on the walls of the container. This makes it very difficult to pull a representative subsample.
- Repeated extractions with hexane are the best way to optimize your recovery of HEM. Three extractions of one volume will extract more efficiently than one extraction of three volumes. This takes full advantage of concentration equilibrium while still keeping the analysis time down.
- When using SPE the activation or conditioning of the resin with solvent is perhaps the most important step. If the extraction resin is not activated prior to use the overall extraction efficiency will decrease and entire categories of compounds will not be adsorbed as normal. Additionally, the amount of time required to pull the aqueous sample through the resin will greatly increase. Proper conditioning consists of adding enough solvent to completely immerse the extraction resin. You must then allow the resin to remain in contact with the solvent before removing it. With HEM analysis you will want to replace the solvent with DI water while keeping the resin damp. If the resin is allowed to dry prior to adding the sample, you will deactivate the resin and be forced to start the conditioning process over. Resins do exist that do not require activation prior to use. These may be helpful if you find that you get low recoveries and are not sure if the activation step is being carried out properly.
- If the sample contains a significant quantity of suspended solids, a prefilter will help to improve the flow rate through the SPE resin. Make sure that the solids collected on the prefilter are rinsed with hexane along with the resin.
- Vigorous mixing of samples in LLE often causes emulsions to form. There are a few options for breaking emulsions. The easiest but often most time consuming is to let the sample sit. Usually the emulsion will break on its own. Adding a salt, such as NaCl, can help to break emulsions. If time is of the essence, adding the emulsion layer along with 10–15 mL of solvent, to a centrifuge tube and spinning at approximately 2400 RPM for 5 minutes will usually achieve the desired results. Utilizing phase separation paper will eliminate the need for subsequent transfer back to a separatory funnel.
While these tips are by no means the only things to keep in mind, they will help in your initial work with HEM as well as in making a decision on which method to pursue. When making the ultimate determination on how to analyze HEM in your lab, remember that each alternative has its own pluses and minuses. Keep these in mind and remember that there is no single best solution that fits every circumstance.
David Smith is a Technical Specialist with Environmental Express, Inc. David has a BS in Chemistry from The University of South Carolina as well as 8 years of commercial laboratory experience; tel.: 843-881-6560; e-mail: DavidS@envexp.com; www.envexp.com.