Which analytical method identifies a substance by its selective absorption of different wavelengths of light?

• A. Spectrophotometry
• B. Chromatography
• C. Mass Spectrometry
• D. Fluorometry

Answer: (A)

Identifying a substance by how it absorbs light at specific wavelengths. Each compound has a characteristic way it absorbs different wavelengths, creating a unique absorption pattern that can be used to recognize what’s present and estimate how much is there.

In spectrophotometry, light at chosen wavelengths is passed through a sample, and the instrument measures how much light is absorbed (or transmitted). The amount absorbed at particular wavelengths corresponds to the substance’s electronic transitions, producing peaks at characteristic wavelengths. By comparing the measured absorption with known spectra or using a calibration curve, you can identify the substance and determine its concentration. This method relies on selective absorption, making it a direct way to both identify and quantify a sample.

Other methods don’t use this absorption-based identification: chromatography separates components but doesn’t rely on light absorption for identification; mass spectrometry identifies substances by their mass-to-charge ratios; fluorometry detects light emitted after excitation rather than absorbed light.