Final answer:
The optimal time to collect a sample varies based on the substance and the test's goals. Blood samples are usually treated with methanol before analysis, while urine samples may be tested without preparation. For volatile samples, they are captured when in gaseous form and condensed for weighing.
Step-by-step explanation:
Substance testing experts believe that the best time to collect a sample relies heavily on the nature of the sample and the specific conditions required for testing. In the case of blood samples, they are typically collected into tubes containing anticoagulants such as K2EDTA and are centrifuged to obtain plasma. The plasma samples are quenched with methanol immediately before analysis to ensure stability and prevent degradation of the sample. For urine samples, immediate analysis is often preferred as these samples typically require no prior preparation before analysis. When aiming to capture volatile substances, the methodology involves removing the flask from heat at the point when all the liquid has become gas, the flask will then only contain the gaseous sample at ambient pressure, which is then sealed and later condensed to a liquid for mass determination.
In scenarios where exact particle count or the quantity of a substance required for a chemical reaction is important, precise timing and techniques must be employed to ensure accuracy. Hence, the optimal time for sample collection varies based on the test substance and the goal of the analysis, e.g., a phase change study or reaction stoichiometry.