A chromatogram gives you peak areas. Nobody reports an area. Every number a GC lab reports is an area turned into an amount, and the tool that does the turning is a response factor. How you measure that factor, and what you divide by, decides whether your result depends on things you cannot control, such as exactly how much liquid the syringe delivered. This lesson works through the arithmetic of external and internal standard quantitation, on a volume basis and on a mass basis, so you can check any result your software gives you.
An area is not an amount
A detector's signal grows with the amount of analyte reaching it. Within the linear range the relationship is a straight line; whether it also passes through zero is something you check, not assume. When it does, one number turns areas into amounts. EPA calls it the calibration factor:1
Notice what the detector actually sees: the mass that reached it, which is the concentration times the volume injected. Concentration stands in for mass only while every injection delivers the same volume.
Different compounds give different responses for the same amount. A flame ionization detector responds roughly to the carbon mass in a molecule, so equal masses of hydrocarbons give similar areas and oxygenated compounds usually give less; a mass spectrometer's response is specific to each compound and each ion. So every compound needs its own response factor, measured on your instrument.
One warning before any arithmetic: some software stores these factors as area over amount, some as amount over area. EPA defines them as response over amount.1 Read your software's definition, write it in your procedure, and check one result by hand.
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.
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