What does a qualitative assay determine?

A qualitative assay is designed to determine the presence or absence of a specific substance within a sample. This type of analysis focuses on identifying what components are present, rather than quantifying how much of each component exists. This is particularly useful in scenarios where the goal is to confirm that a particular compound is in a sample, such as in drug testing, environmental monitoring, or food safety testing.

For example, in a qualitative assay, a reagent may be added to a sample, and if the substance of interest is present, a visible change (like a color change) might occur. This indicates that the substance is indeed present, but it does not provide any numerical data regarding its amount or concentration.

The other options do not accurately reflect the purpose of a qualitative assay. Concentration measurement pertains to quantitative assays, assessing purity generally involves both qualitative and quantitative aspects, while determining chemical structure is more aligned with techniques such as spectroscopy or chromatography rather than a qualitative assessment. Thus, the choice highlighting the presence or absence of a substance is the most fitting description of a qualitative assay.