Routine spectrochemical analysis of used oil samples with rotating disc electrode (RDE) or inductively coupled plasma (ICP) emission spectrometers has become standard practice in machine condition monitoring programs to determine elemental content. Unfortunately, both of these techniques become less responsive to particles as they increase in size. To overcome this limitation, several complementary methods and analytical techniques to detect large particles in used oil have been applied, including acid digestion, ferrography, and X-ray analysis. Unfortunately, these techniques are not practical or economical to implement in a high sample throughput machine condition-monitoring program. One technique that has demonstrated success as a complementary method for detecting large particles is rotrode filter spectroscopy (RFS). RFS extends the capability of rotating disc electrode (RDE) spectrometers to detect particles greater than 10 μm in size.