detector
/ dih-TEK-ter /
Think of a doorbell that chimes whenever someone walks past, and chimes louder for a bigger group. As compounds leave a chromatography column one after another, the detector is that doorbell — a device that notices something passing in the stream and turns it into an electrical signal the recorder can plot.
Formally, a detector is the part of a chromatographic or electrophoretic instrument that senses the components as they elute and converts that sensing into a measurable signal. Different detectors respond to different properties — light absorption, fluorescence, ions produced in a flame, changes in electrical conductivity, or mass — and the strength of the signal is read as a peak in the chromatogram, ideally rising and falling in proportion to how much of the component is present.
It matters because the detector decides what the whole method can and cannot see: a compound invisible to the chosen detector simply leaves no peak, no matter how cleanly it was separated. Its honest caveats are selectivity and range — a detector blind to certain compounds will miss them, and one pushed past its linear range will report peaks that no longer grow in proportion to amount, quietly distorting quantitation.
On an HPLC running a drug assay, a UV detector set to 254 nanometres flashes a signal each time a light-absorbing compound passes its tiny flow cell, drawing the peaks the analyst will later integrate.
The instrument's eye: it senses each eluting component and turns it into a peak.
Detectors are broadly either universal, responding to almost everything that passes, or selective, responding strongly to a particular class — each choice trades breadth for either sensitivity or freedom from interference.