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

• A. Chromatography
• B. Spectrophotometer
• C. Spectrophotometry
• D. Fluorometry

Answer: (C)

Spectrophotometry identifies a substance by how it selectively absorbs light at different wavelengths. In this approach, light across a range of wavelengths is passed through a sample and the absorption at each wavelength is measured. Each substance has a characteristic absorption pattern, a spectrum that acts like a fingerprint. By comparing the sample’s spectrum to known references, you can determine what the substance is, and you can estimate concentration from the amount of light absorbed at a specific wavelength (Beer's law). The instrument used is a spectrophotometer, but the method described is spectrophotometry, not chromatography or fluorometry. Chromatography separates components rather than identifying them by their light absorption pattern, and fluorometry relies on emitted light after excitation rather than absorption across wavelengths.