Calcium K<sub>β1</sub> and K<sub>β5</sub> both have second order (II) lines that interfere spectrally with the phosphorus K<sub>α1</sub> first order (I) analytical line in the x-ray fluorescence determination. By combining maximum pulse height discrimination with a mathematical correction for peak overlap, phosphorus can be accurately determined in a wide variety of geologic samples.
Brent P. Fabbi, "Rapid X-Ray Fluorescence Determination of Phosphorus in Geologic Samples," Appl. Spectrosc. 25, 41-43 (1971)