Which method is used for quantitative measurement of absorbance in a solution?

Spectrophotometry is the method used for quantitative measurement of absorbance in a solution. This technique involves measuring how much light a substance absorbs at specific wavelengths, providing important information about the concentration of solutes in a solution. The key principle behind spectrophotometry is the relationship described by Beer-Lambert Law, which states that absorbance is directly proportional to concentration, allowing for quantification.

In contrast, the other methods listed serve different purposes. Gel electrophoresis is primarily used for separating nucleic acids or proteins based on size and charge, rather than measuring absorbance. Activity assays are designed to measure the biological activity of enzymes or other molecules, often relying on colorimetric changes or other indicators rather than direct optical absorbance. ELISA (Enzyme-Linked Immunosorbent Assay) is a technique commonly used for detecting and quantifying proteins or antibodies; while it may use absorbance measurements to interpret results, its primary focus is on the binding interactions rather than direct quantification of absorbance in a solution.