Determination of Calcium in Powdered Milk via X-ray Fluorescence Using External Standard and Standard Addition Based Methods

A handheld energy-dispersive X-ray fluorescence (XRF) analyzer was used to determine calcium in powdered milk. Quantification was performed using two different methods (external standards and the method of standard additions) to illustrate a matrix effect as well as a means for compensating for it. Both methods require calibration of the XRF analyzer using authentic standards prepared by mixing known masses of calcium carbonate into known masses of cellulose or dry milk. The use of XRF for this application requires analysis times on the order of 1 min per sample, provides linear calibration curves, and gives good precision with %RSDs of 4% or less. External standard based calibration gave erroneously low results due to the attenuation of calcium fluorescence by potassium in the sample, whereas the method of standard additions gave 1.29% calcium, which is very close to the manufacturer’s equivalent concentration of 1.3%. This experiment is well suited for an analytical chemistry course and provides an excellent example of the advantages and limitations of these two calibration methods for addressing matrix effects and deriving accurate quantitative results.