Accurate Elemental Analysis of Multigrade Lubricating Oils by ICP Method: Effect of Viscosity Modifiers

Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP) is an industry accepted method for elemental analysis of lubricating oils. High sensitivity and multi-element measurement capability are some of the reasons for its popularity. The technique has been developed into a formal ASTM procedure, D4951. In many instances, however, the ICP method has been found to yield unreliable results. In particular, elemental analyses of multigrade oils have been frequently observed to be in error by as much as 20%.

A study of oil formulation components identified viscosity modifiers (VMs) as having the greatest impact on the accuracy of this technique. The magnitude of this effect depends, among other things, on VM concentration in the lubricant. It has been found that viscosity modifier polymers interfere with the formation of aerosol, a critical step in the ICP analytical procedure, thus affecting the sample delivery to the plasma torch. Modifications of the ASTM D4951 ICP-AES procedure have been developed to eliminate biased results due to the presence of viscosity modifiers.