Calibration-free? Reference methods and normalisation, honestly
Every method that skips the fitted curve trades it for an assumption. Normalisation, reference methods and pre-loaded factors each have one; name it, check it, and keep calibrating the instrument.
By Medrado Analytical Innovations · Foundations, No. 21 · for the working analyst
"No calibration needed" is one of the most attractive phrases in analytical chemistry, and one of the least precise. Some methods really do skip the fitted curve: they read concentration from a physical law, a ratio, or a share of the total. That is a genuine advantage. But every one of them trades the curve for an assumption, and the assumption has to be true. This piece is about those trades: what each kind of "calibration-free" method actually relies on, and what still has to be calibrated underneath it.
What "calibration-free" replaces
A calibration-free method replaces the fitted curve with a relationship it trusts; it does not remove the need for a calibrated instrument and traceable reference data. The first piece in this series made the point for reference methods: density read through a traceable density–concentration table, absorbance read through Beer–Lambert with a known absorptivity. Neither builds a fresh curve each run, and both still stand on a calibrated instrument (the density meter's daily air and water check, the photometer's own standards) and on reference data that must be traceable and right for the matrix.1 What you save is re-fitting the curve, not the calibration.
There are three common ways to skip the curve. Each has its own assumption.
Part of the GC course
This lesson belongs to From signal to result, a course on the gas chromatograph from detector to reported number, coming soon. The rest of the lesson, its questions and the course run will be part of it.
Want to hear when it opens? Follow The Analytical Column below, or start with the free lessons: Start here →
Check yourself
Answer in your head first, then open the answer. Any question can go into your Quiz me.
What does a calibration-free method actually remove?
Show the answer
“A calibration-free method replaces the fitted curve with a relationship it trusts; it does not remove the need for a calibrated instrument and traceable reference data.”
Plain-language explanations, examples, where you would use each term, and the sources are in the full glossary.
Area-percent normalisation
Reporting each peak’s area as its share of the total; equal to amount percent only when every component responds equally.
Relative response factor (RRF)
A component’s response per unit amount, divided by a reference compound’s.
qNMR
Quantitative NMR: the signal is proportional to the number of nuclei, so a weighed standard in the same tube replaces the curve.
Reference method
A method that reads concentration through an established physical relationship instead of a fresh curve, still on a calibrated instrument.
Sources
The Analytical Column, Foundations No. 11, "The number the instrument can't give you" (reference methods).
European Pharmacopoeia, general chapter 2.2.46, Chromatographic separation techniques; the individual monograph for the response factors it specifies.
F. Malz and H. Jancke, "Validation of quantitative NMR", J. Pharm. Biomed. Anal. 38, 813–823 (2005).
BIPM, internal standard reference data for qNMR, e.g. ISRD-07 on DSS-d6 (Rapport BIPM-2019/04), bipm.org.
S. Westwood et al., "Methods for the SI-traceable value assignment of the purity of organic compounds (IUPAC Technical Report)", Pure Appl. Chem. 95(1), 1–77 (2023), doi:10.1515/pac-2020-0804.
ASTM D8071 (current edition), Standard Test Method for Determination of Hydrocarbon Group Types and Select Hydrocarbon and Oxygenate Compounds in Automotive Spark-Ignition Engine Fuel Using Gas Chromatography with Vacuum Ultraviolet Absorption Spectroscopy Detection.