a quartz lens is appropriately positioned in the chamber for focusing the sample luminescence onto the entrance slit of a grating spectrometer the x ray source can be a commercially available tungsten target x ray tube designed to operate at an electrical potential of 50 kv the spectrometer can be of 25 cm focal length with a plane diffraction grating and a fused silica window photomultiplier e g rca 7268 the entrance and exit slits of the spectrometer are set at 100 microns and the spectrum may be scanned at the rate of about 200 angstroms per minute as the phosphor is irradiated the resulting photocurrent is amplified and recorded on a suitable strip chart recorder the method of correlating experimental and calibration data in terms of an internal standard is applicable in the instant case a known but small amount of a lanthanide such as erbium is added along with the y 2 o 3 or other rare earth oxide the concentrations of other lanthanides in the sample are correlated in a graph or other means to the optical intensity ratio of the lanthanide fluorescence to erbium fluorescence at characteristic peaks of each element by treating data in this manner many effects which would ordinarily influence luminescence such as time and temperature of heating residual impurities and x ray power drift are satisfactorily diminished the following examples are offered to illustrate the phosphor composition of the present invention its preparation and the method of its use in determining lanthanides in transition elements two 34 milligram samples of 303 stainless steel c 0 15 max mn 2.