First draw samples are primarily used for testing which metals?

First draw samples are crucial in water testing, particularly when assessing certain metals. The primary focus of first draw samples is to evaluate the presence of metals that may leach from plumbing fixtures or pipes into drinking water. Copper and lead are commonly associated with residential plumbing systems, often originating from service lines or fixtures.

When water stands in pipes for extended periods prior to sampling, it can accumulate higher concentrations of these metals due to leaching from the plumbing materials. Therefore, testing first draw samples helps identify potential contamination levels for these specific metals, ensuring that water quality meets safety standards.

In contrast, the other options listed—mercury and arsenic, cadmium and zinc, nitrate and phosphate—are either not typically associated with first draw sampling or involve different testing protocols that do not specifically track leaching from plumbing materials in the same manner. Thus, the focus on copper and lead in first draw samples makes it the correct choice for this question.