The Analytical Column

A friend of mine oversees several laboratories that all run ASTM D4815, oxygenates in gasoline, on the same chromatography data system. He found that some sites typed the weight of the gasoline into the sample amount field and others typed the total weight in the vial: the gasoline plus the internal standard. The field's label did not settle it, and neither did the vendor. This lesson works the method's calculation from first principles, shows where the sample weight belongs, how large the error is if it is wrong, and how to prove which entry your own software setup needs.

What the method calculates

Read the method itself, not the software, to find out what the answer is supposed to be. In D4815 the calibration standards are made up by weighing each oxygenate and the internal standard, 1,2-dimethoxyethane (DME), into a flask and diluting with oxygenate-free gasoline.¹ Each oxygenate is calibrated by plotting its area ratio to the internal standard against its mass ratio to the internal standard, and the line is not forced through zero. To run a sample, a measured volume of DME is weighed into a small flask, the flask is filled with the gasoline, and the gasoline's net mass is recorded separately. The result is the oxygenate's mass, recovered from the calibration line and the weighed DME, divided by the net mass of gasoline and multiplied by 100.¹

In plain terms, with m the slope and b the intercept of the calibration line:

Two things follow. The mass of oxygenate comes out of the ratio with no sample weight in it at all. And the sample weight enters once, as the denominator, and the method's denominator is the gasoline alone, not the gasoline plus DME.