The determination of normal paraffins in petroleum products

A normal paraffin, outside the carbon number range of the sample, is quantitatively added to the sample as an internal standard, and 10–100 μl of the mixture are injected into a small absorber unit which contains 400 mg of activated molecular sieve. The temperature of the absorber unit is raised from ambient to 300° at about 15°/min while nitrogen is passed. This elutes the non-linear hydrocarbons but leaves the normal paraffins absorbed. The sieve is removed from the absorber unit and the paraffins are released by destroying the sieve structure with hydrofluoric acid. A pellet of potassium hydroxide is then added to neutralise the excess acid and the released normal paraffins are extracted in 0.3 ml of isooctane. This isooctane solution is examined on a programmed-temperature chromatograph; the chromatogram obtained consists of a solvent peak followed by well-resolved peaks representing the normal paraffins. These are easily measured and the concentrations of the normal paraffins in the original sample are calculated against the internal standard.