Chemistry
Spectrophotometry
Quick fact
Spectrophotometry can detect substances at concentrations as low as one part per million, making it essential for quality control in pharmaceuticals and environmental monitoring.
Why this is interesting
How can scientists determine the exact amount of a drug in a blood sample or measure pollutants in water—without ever touching the substance? The answer lies in a simple yet powerful trick using light.
Read the full explanation
Understanding Spectrophotometry
Imagine shining a flashlight through a glass of coloured drink. The more dye present, the darker the colour and the less light reaches the other side. Spectrophotometry works the same way. A device called a spectrophotometer sends a beam of light of a specific colour (wavelength) through a sample. The sample absorbs some of that light. By measuring how much light passes through (transmittance) and comparing it to a blank reference, we calculate the absorbance. For many substances, absorbance is directly proportional to concentration—this relationship is the Beer-Lambert law. Scientists choose the wavelength where the substance absorbs most strongly for the most sensitive measurement.
A deeper explanation
The core mechanism is light absorption by molecules. When light of the right energy hits a molecule, it can promote an electron from a lower-energy orbital to a higher-energy one—an electronic transition. Each substance has a unique pattern of such transitions, creating a characteristic absorption spectrum. Spectrophotometry exploits this by isolating a narrow band of wavelengths using a monochromator (e.g., a prism or grating) and measuring the intensity loss. The Beer-Lambert law, A = εlc, mathematically links absorbance (A) to the substance's molar absorptivity (ε), path length (l), and concentration (c). This linear relationship is why spectrophotometry is a cornerstone of quantitative analysis, allowing rapid, non-destructive measurements in laboratories worldwide—from checking chlorine levels in swimming pools to monitoring DNA purity in molecular biology.