When is including the substrate in a sample not allowed?

Including the substrate in a sample is not allowed when measuring by percentage by weight because the goal is to identify the lead content in the paint or coating itself, excluding any weight contributions from the underlying material. This ensures that the measurement reflects only the concentration of lead in the paint rather than a potentially misleading value that includes the substrate's weight. Accurate assessment of lead levels is critical for risk assessment and regulatory compliance, thus requiring a pure sample of the lead-containing material.

In contexts such as testing for color accuracy, assessing surface appearance, or evaluating the adhesion of paint, including the substrate may be relevant for those specific assessments, as they pertain to the qualities and performance of the paint itself, where the underlying material’s influence can be significant. However, when trying to ascertain the lead concentration specifically, the purity of the sample is paramount, which is why the substrate is excluded in those scenarios.