Which principle does a spectrophotometer rely on to determine concentration?

• A. Emission of light at a single wavelength for detection.
• B. Mass-to-charge ratio analysis.
• C. Measurement of refractive index changes.
• D. Absorption of light at specific wavelengths proportional to concentration.

Answer: (D)

Light absorption by a solute is the basis for determining concentration with a spectrophotometer. A beam of light at a chosen wavelength passes through the sample, and the detector measures how much light is transmitted. The amount absorbed is related to concentration through Beer-Lambert law: A = εlc. Here, A is the absorbance the instrument reports, ε is the molar absorptivity of the solute at that wavelength, l is the path length the light travels through the sample, and c is the concentration. For a fixed wavelength and path length, higher concentration yields higher absorbance, so you can build a calibration curve from standards and read the unknown concentration from its absorbance. The other approaches describe detecting emitted light, analyzing mass, or measuring refractive changes, which are not how concentration is determined in a standard spectrophotometric method.