What preservation method is required if oil and grease sample analysis is delayed?

Lowering the pH to less than 2 is an effective preservation method for oil and grease samples when analysis is delayed. This acidification helps to stabilize the compounds of interest and prevents microbial growth that could potentially alter the composition of the sample. By reducing the pH, you are hindering the degradation processes that could lead to loss of volatile components and changes in the chemical structure of the oil and grease, thereby ensuring more reliable analytical results when the sample is eventually analyzed.

In contrast, maintaining samples at room temperature or using preservatives not specifically designed for oil and grease may not adequately prevent changes or degradation in the sample. Likewise, avoiding mixing the samples doesn't directly address the issue of preservation and maintaining the integrity of the sample over time. Hence, lowering the pH to less than 2 is the most effective and widely accepted method for preserving the integrity of oil and grease samples during delays in analysis.