Abstract: Spectrometric analysis for determining the elemental concentration of various wear metals, contaminants and additives present in a fluid sample for condition monitoring and preventive maintenance. RDE spectroscopy method and apparatus includes use of rotrode disk as a filter which captures large particles of the surface of the disk, which particles are then subjected to RDE spectroscopy for evaluation. In one method, the rotrode with the captured large-particle filtrate is subjected to a solvent wash, and the washed sample is then submitted to RDE spectroscopy to obtain a highly accurate multi-elemental determination of concentration of large particles. In another method, conventional RDE spectroscopy is used to characterize the small particles in a used oil sample, and this is combined with the obtained large particle data for highly accurate multi-elemental determination of concentration of wear debris, contaminants and additives in the fluid sample.