The Analytical Column

Years ago, someone told me that no two gas chromatographs will ever read the same number. Different syringes, different inlets, different columns, different detectors: there is no way to make them agree. My immediate question was, then what is the purpose of calibration?¹

The belief behind that claim is common, even among experienced analysts. Two instruments are never identical. But identical hardware was never the point. This piece is about what calibration actually does, why it lets two different instruments agree, and what it can and cannot fix.

The idea in plain words

An instrument's raw output is a signal: a peak area, an absorbance, a current. It is not a concentration. (Some instruments show a concentration on the screen, but only because they convert the signal internally, and that conversion is itself a calibration.) In a chromatograph, the peak area tracks how much analyte reached the detector; the concentration in the vial is worked out from that amount and from how the sample was introduced. How big the signal is for a given amount depends on that particular system: its syringe, its inlet, its column, its detector, even the day.

Calibration is how you find out. You run standards whose values you know, record the signal each one gives on this system, and use that relationship to turn every later signal into a result. Calibration ties an instrument's signal to standards of known value, so that the reference, not the instrument, decides the number. A complete calibration also says how unsure that relationship is.