IR vs. UV-Vis Spectroscopy: Key Differences & When to Use
IR spectroscopy detects molecular vibrations by measuring infrared light absorption, revealing functional groups like C=O or O-H. UV-Vis spectroscopy tracks electronic transitions when molecules absorb ultraviolet or visible light, quantifying conjugated systems and transition-metal complexes.
Lab rookies swap the two because both spit out colorful plots and promise “molecular ID.” The confusion peaks when a brown liquid shows up: “Do I IR it for C=O or UV-Vis it for color?” Same bench, different question.
Key Differences
IR uses 2.5–25 µm light to map bond stretches, giving fingerprint peaks for solids, liquids, and gases. UV-Vis uses 200–800 nm light to probe π-electrons and d-d transitions, delivering concentration data in clear solutions. IR excels at structure ID; UV-Vis excels at quantification.
Which One Should You Choose?
Pick IR to confirm a suspected ester or amide in your powder. Reach for UV-Vis when you need to know how much dye is in river water or track a reaction by color change. If both structure and concentration matter, run them back-to-back.
Can I run UV-Vis on a cloudy sample?
No—turbidity scatters light and skews absorbance; filter or dilute first.
Is IR safe for aqueous samples?
Yes, but water strongly absorbs IR; use ATR accessories and short path lengths.