Rotating Disc Electrode Optical Emission Spectroscopy (RDEOES)

Spectrometers that look at the multitude of spectral lines from a heated (or “excited”) sample are called optical emission spectrometers. All optical emission spectrometers consist of three main components, these components are:

1. Excitation Source - introduces energy to the sample.
2. Optical System - separates and resolves the resulting emission from that excitation into
its component wavelengths.
3. Readout System - detects and measures the light that has been separated into its component wavelengths by the optical system and presents this information to the operator in a usable fashion.

One typical method used in the excitation source in modern spectrometers is an electric discharge. The source is designed to impart the energy generated in an arc or spark to the sample. For oil analysis spectrometers, a large electric potential is set up between a disc and rod electrode with the oil sample in the gap between them. An electric charge stored by a capacitor is discharged across this gap creating a high temperature electric arc which vaporizes a portion of the sample forming plasma. A plasma is a hot, highly ionized gas which emits intense light. The light given off as a result of this process contains emissions from all the elements present in the sample. These emissions can now be separated into individual wavelengths and measured using a properly designed optical system.  Temperatures in the 5000 to 6000 °C range are achieved and hard to excite elements emitted enough light to be readily detected.

Since the early days of spectroscopic oil analysis, oil has been sparked (or “burned”) between a rotating carbon disc electrode and a carbon rod electrode. The sample is placed in a sample cap, the disc is partially immersed in the oil sample and the disc rotates as the burn proceeds. This requires about 2 or 3 ml of sample depending on the exact cap being used. A fresh disc and a newly sharpened rod are required for each sample to eliminate sample carryover. This method is called RDE optical emission spectroscopy (OES), or combining the two, RDEOES. Alternatively, one often sees it referred to as RDE¬AES, for rotating disc electrode atomic emission spectroscopy.