A fuel analysis system and method wherein an x-ray source emits x-rays at an energy level below but proximate the absorption edge of sulfur. A monochromator is in the optical path between the source and a fuel sample for directing x-rays at a single energy level at the fuel sample to limit excitation of any sulfur in the fuel sample. A detector is responsive to x-rays emitted by the sample and an analyzer is responsive to the detector and configured to determine the amount of silicon and aluminum in the sample.