A method and system for on-line measurements of mineral additives in or on a paper web utilizing shaped spectrum x-rays and solid-state PIN detectors. Each of three detectors are cover by a filter used to shape the spectrums of x-rays received by the detectors. The filters are selected to maximize sensitivity differences between detectors for the desired detectable mineral additives. A computer processes signals from the detectors and from basis weight and moisture measuring instruments to determine total mineral content, and the individual amounts of mineral additives in or on (e.g. coating) the paper web.