hollow-cathode lamp
/ HOL-oh KATH-ode lamp /
A hollow-cathode lamp is the special light source of an absorption instrument — a lamp that emits exactly the colour the target element will drink up, and almost nothing else. The trick is to build the lamp partly out of the very element you want to measure, so it shines that element's own private set of colours.
More precisely, the lamp contains a small cup-shaped cathode made from (or coated with) the analyte element, sealed in a glass tube filled with low-pressure argon or neon. A voltage makes the gas glow and sputter metal atoms off the cathode; those atoms get excited and emit the element's characteristic sharp lines. Because the line is so narrow, it overlaps the absorbing atoms' line almost perfectly.
It matters because this perfect colour match is what gives AAS its sensitivity and freedom from interference — only the lamp's own line is absorbed. The practical caveat is that you generally need a separate lamp for each element, so a lab measuring many metals owns a drawer full of them.
To measure lead, you slot a lead hollow-cathode lamp into the AAS; it emits lead's 283.3 nm line. Swap to copper tomorrow and you swap in a copper lamp, because the lead lamp cannot probe copper.
A lamp made of the element emits exactly the line that element absorbs.
Some lamps are multi-element, combining a few metals in one cathode, but resolution and intensity often suffer. The hollow-cathode lamp is a feature of absorption (AAS); emission and ICP methods need no such lamp, because the hot source makes the atoms shine on their own.