Uncertainty in chemical analysis for the example of determination of caffeine in coffee

The comprehensive quantifying of uncertainty for the determination of caffeine in coffee is given according to the EURACHEM/CITAC Guide. The cause-effect diagram is very useful for the identification of the sources of uncertainty and the relationship between them. In this case the uncertainty caused by the pipette, the stock solution and the calibration curve are the three main sources. Concerning the concentration of the stock solution the purity of the caffeine was most important. The combined uncertainty showed that none of the three parts (stock solution, pipette, calibration curve) can be neglected. The expanded uncertainty (coverage factor k=2) is about 5.4%.