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Derivatization chemically converts an analyte into a related product whose properties are more suitable for a measurement or separation. A useful change might improve fluorescence detection or alter chromatographic behavior. It is a chemical transformation of the analyte, rather than merely diluting the sample or increasing the detector gain.

A primary capillary electrophoresis experiment by James W. Jorgenson and Krynn DeArman Lukacs used a fluorescence detector for separated amino acids, peptides and urinary amines; its abstract explicitly states that fluorescent derivatives were necessary for detection.^[ref] The observation demonstrates the dependence of a successful separation-and-detection system on the chosen chemical and optical response, not universal fluorescence of the unmodified analytes.

The order of reaction and separation matters. IUPAC defines post-column derivatization as reaction of separated components after they leave the column and before detection, and contrasts this with derivatization before the sample enters the separation medium.^[ref]

A simple mass-balance model exposes derivatization yield bias. Let $n$ be the initial analyte amount and let a 1:1 reaction convert a fraction $y$ to a detectable product. With background-subtracted signal $S=kyn$, a calibration made at yield $y_c$ givesFor stipulated sample yield $y=0.80$ and calibration yield $y_c=1.00$, an actual $10\mathrm{nmol}$ gives an apparent $8\mathrm{nmol}$. A perfectly repeatable detector can therefore support a biased original-analyte result. This model assumes one detected product per converted molecule and equal detector response for that product.

Reagent blanks, product identity, competing reactions and the sample matrix address different parts of that inference. Measuring the derivative accurately does not by itself prove complete conversion or establish the amount originally present.