The Analytical Column

Sometimes the sample itself changes how much signal the analyte gives. In headspace GC this is the rule rather than the exception: how much of a volatile compound leaves the liquid and reaches the gas above it depends on everything else dissolved in that liquid. A standard made in clean solvent then sees a different world from the sample, and an external standard result is biased however carefully it is made. Standard addition solves that by calibrating inside the sample. This lesson works through its arithmetic in GC, and then a second piece of arithmetic every GC lab needs: turning a mass result into a volume result.

Calibrating inside the sample

Matrix here means everything else in the sample, which can change how much signal the same amount of analyte gives. Split the sample into equal portions. Spike all but one with known, increasing amounts of the analyte, and bring every portion, the unspiked one included, to the same volume with the same solvent. Prepare and run the set. Because the added analyte and the analyte already there sit in the same matrix, the matrix affects both the same way, and the slope of area against amount added is the response in that matrix.

Snow, in LCGC, describes it the same way: the amount in the sample is found by extrapolating the line to the x-axis.¹