What should be done to preserve a sample intended for oil and grease analysis?

To preserve a sample intended for oil and grease analysis, it is important to cool and acidify the sample. This preservation technique helps to prevent the separation of oil and grease fraction from the water phase and inhibits microbial growth, which could otherwise alter the composition of the sample before testing.

Cooling the sample slows down any biological activity and chemical reactions that could lead to sample degradation or changes in the concentration of constituents. Acidifying the sample plays a critical role in stabilizing the constituents of the oil and grease, ensuring that they are maintained in their original state until analysis can be conducted.

The other options are not appropriate for preserving samples for oil and grease analysis. Heating the sample would increase the volatility of certain compounds and lead to loss of some components. Allowing the sample to sit at room temperature could allow for unwanted biological activity and chemical reactions, changing the sample's integrity. Lastly, stating that no preservation is required does not recognize the importance of maintaining the sample's condition prior to analysis, which could compromise the results.