Oil and grease is defined as any substance recovered from an acidified sample by extracting it into a designated solvent and which is not volatilized during the analysis.
After the sample extracted, the extract may be measured either gravimetrically or by infrared spectroscopy.
The solvents used have the ability to dissolve not only oil and grease but also other organic substances. Some non oil and grease materials commonly included in the determination of oil and grease are certain sulfur compunds, chlorophyl and some organic dyes.
The total oil and grease designation includes oils and fats of biological origin (animal and vegetable fats and oils) as well as mineral oils (petroleum products). Silica gel has the ability to sorb certain organic compounds know as polar compounds. Since petroleum hydrocarbons are mostly nonpolar and most hydrocarbons of biological origin are polar, silica gel is used to separated these different types of hydrocarbons. If a solution of non polar and polar hydrocarbons is mixed with silica gel, the polar hydrocarbons such as fatty acids are removed selectively from the solution. The materials not eliminated by silica gel adsorption are designated as “petroleum hydrocarbon” by this test. Normally, total oil and grease is measured first and then petroleum hydrocarbon are determinate after the animal and vegetable fats are removed by silica gel adsorption. The difference between total oil and grease and petroleum hydrocarbons is designated as biologically derived hydrocarbons.
Eugene R. Weiner. Application of Environmental Aquatic Chemistry. A Practical Guide. Third edition. CRC Press