When working with XRF, it is common for elements to interfere or "mask" each other during analysis. When analyzing multiple elements at the same time via XRF, be sure to calibrate with standards that have the concentrations of each element randomized unlike the wear metal dilution series standards you may be used to. This will help account for any interference you may expect or not expect. With more complex formulations, be sure to shake vigorously prior to sampling for analysis to ensure a good mix. If you see sedimentation at the bottom after shaking, something is coming out of solution and you should give us a call.