ICP (Inductively Coupled Plasma) spectrometry is designed to measure elements in solution, not particles in suspension. Droplets larger than about 4.5 microns destabilize the plasma. The proper operation of the instrument depends on the spray chamber removing "particles" larger than the 4.5-5 micron target range. The finer the droplets the nebulizer produces and the higher the efficiency of the spray chamber system at removing the larger droplets, the better the detection limit and relative standard deviation of the instrument. Manufacturers go to great lengths to send those 4.5+ micron drops down the drain tube instead of into the plasma.
The size of the particle is it's aerodynamic diameter, which is affected by density and shape. The 4.5 micron size is based on spherical water droplets. Due to the density of metallic wear particles, the largest that should get passed to the plasma is 1-3 microns, depending on the specific metal.
The first 5 pages of the attached article is a good explanation of aerodynamic diameter.
Most ICP operators don't have a clue has to how the instrument works or what they are actually measuring. Some of Blackstone's comments are a prime example of this. The older technology DCP(Direct Current Plasma) spectrometry that was commonly used for oil analysis before ICP was common had a more robust plasma and was more tolerant of larger particles. Their sample induction systems would pass particles in the 10 micron range. I'm guessing that there was some small carryover of particles in the 20 micron range. This may be where the idea that current oil analysis sees 10-20 micron particles comes from.
Ed