Chemistry
UV-Vis Spectroscopy
Quick fact
UV-Vis spectroscopy can detect concentrations as low as parts per million (ppm) and is used in everything from drug testing to water quality monitoring.
Why this is interesting
Why do leaves appear green? The answer lies in how they absorb and reflect light—a principle harnessed by UV-Vis spectroscopy to identify and measure substances.
Read the full explanation
Understanding UV-Vis Spectroscopy
Imagine shining a flashlight through a colored glass: the glass absorbs some colors and lets others pass through. UV-Vis spectroscopy works similarly, but instead of broad colors, it measures how a sample absorbs specific wavelengths of ultraviolet and visible light. A spectrophotometer sends a beam of light through the sample, and a detector measures how much light gets through. The sample's molecules absorb light only at wavelengths that match the energy difference needed to excite an electron. By measuring this absorption across a range of wavelengths, we get a spectrum—a unique fingerprint for many substances. The stronger the absorption, the more molecules are present, allowing us to calculate concentration.
A deeper explanation
The mechanism relies on electronic transitions: light's energy (E = hν) must exactly match the gap between a molecule's ground-state and excited-state orbitals. For organic molecules, this often involves π→π or n→π transitions in chromophores like C=C or C=O bonds. The Beer-Lambert Law (A = εcl) quantifies this: absorbance (A) equals the molar absorptivity (ε) times path length (l) times concentration (c). This linear relationship makes UV-Vis a powerful quantitative tool. Beyond simple identification, UV-Vis reveals conjugation (extended double bonds shift absorption to longer wavelengths), and it's crucial in kinetics (tracking reactions) and biochemistry (measuring DNA or protein concentration at 260 nm and 280 nm). Its simplicity and speed make it a cornerstone of analytical labs.