absorption spectrum
/ ab-SORP-shun SPEK-trum /
Shine every colour of the rainbow through a solution one at a time and note how much of each it swallows. Plot those results and you get a curve with humps where the substance drinks light eagerly and valleys where it lets light pass. That curve is the absorption spectrum — a fingerprint of how a substance interacts with light.
Technically, an absorption spectrum is a graph of absorbance against wavelength. Its peaks mark the wavelengths where the substance's chromophores absorb most strongly, and their positions and shapes are characteristic of the substance, much like a barcode.
The spectrum serves two roles. Its peak positions help identify what a substance is (qualitative analysis), while the height of a chosen peak, read against a calibration, tells how much is present (quantitative analysis). Choosing where on the spectrum to measure is the key first step of any UV-Vis method.
Recording the absorption spectrum of a food dye reveals a single broad peak at 630 nm; the analyst then fixes the instrument there to measure the dye's concentration in soft drinks.
The spectrum first identifies the peak, then quantitation happens there.
UV-Vis absorption peaks are usually broad and overlapping, unlike the sharp lines of atomic spectra, so they identify substances less precisely on their own.