Which of the following is NOT a preservation method mentioned?

The correct answer relates to preservation methods used in laboratory analysis, particularly concerning the integrity of water samples. Preservation techniques are critical to ensure the analytes in a sample remain stable until analysis.

Sodium thiosulfate is commonly used to neutralize chlorine in water samples, helping to preserve organic compounds that might be affected by chlorine. Similarly, using hydrochloric acid (HCl) to adjust the pH to less than 2 is a known method to preserve certain metal samples, preventing precipitation and ensuring stability for analysis. Amber glass is a type of container that protects samples from light degradation, particularly important for light-sensitive compounds, making it an effective preservation method.

Freezing, while it can preserve some samples by halting biological activity and chemical reactions, is not universally accepted as a standard preservation method for all analytes. Some substances can alter in composition or concentration when frozen and then thawed, which can introduce errors in analysis. Hence, freezing is not considered a formal preservation method listed in standard protocols for all types of analyses, making it the option that does not fit with the others specified in the question.