What is an assay used for?

An assay is a scientific procedure that is specifically designed to determine the quality, quantity, presence, or activity of a particular substance within a sample. This makes it a crucial tool in various fields such as biochemistry, pharmacology, and molecular biology. Assays can be quantitative, providing exact measurements of a substance's concentration, or qualitative, indicating whether a substance is present or not.

In the context of scientific research and application, assays play a vital role in drug development, environmental monitoring, and clinical diagnostics, among other applications. For instance, in drug testing, an assay can measure how effectively a drug interacts with its target, which is critical for ensuring its efficacy and safety.

The other options suggest processes that may be a part of broader experimental techniques or methodologies but do not define the primary purpose of an assay. While inventing new laboratory techniques, purifying proteins, and sequencing DNA are significant tasks within bioscience, they do not encapsulate the function of an assay as well as the determination of quality or quantity does.