Chromatographic methods for analyzing petroleum dates to the early twentieth century. Gas and liquid chromatography are widely used today by laboratories to separate and identify organic and inorganic constituents of petroleum products. In the 1940’s paper chromatography was first used for the analysis of aqueous liquids and was later applied to lubricating oils in the 1950’s.
The method, commonly called the blotter spot test, still has widespread application - both in the field and oil analysis laboratories. The typical procedure involves placing a drop of lubricant on an absorbent filter paper or by permitting the oil to absorb vertically into a strip of the paper. Rings or bands are interpreted in terms of various components in the oil.
A variation of paper chromatography is Thin Layer Chromatography (TLC). In this procedure, the oil is introduced to a stationary material. The stationary phase can be cellulose, silica or other suitable material that is coated on an inert plate of glass, plastic or metal. The samples are spotted or placed as streaks on the plate. The sample travels across the plate in a mobile phase, propelled by capillary action.
Separation of components occurs through absorption, partition, exclusion or ion-exchange processes, or a combination of these. In some procedures, the fluid travels vertically up a strip of the material. Solvents and chemicals are occasionally used to help facilitate the separation of target components such as oxides and contaminants in the oil.
Radial Planar Chromatography
In oil analysis, a quick and effective technique for measuring and identifying various lubricant components is Radial Planar Chromatography (RPC). The term “radial” refers to the circle formation of the oil after the sample has eluted (dispersed) on a TLC substrate, when left on a horizontal plain. Thus the term: Radial Planar Chromatography.
When interpreting the oil components, the position of the resultant bands or zones, after development (dispersion), is analyzed by appropriate methods. Because of its convenience and simplicity, sharpness of separations, high sensitivity, speed of separation and ease of recovery of the sample components, the method finds many applications.
When used as an oil analysis tool various machine and oil combinations will show unique trends during the life of the machine and oil. These characteristics are seen as bands or zones of different colors, densities and even unwanted wear metals and debris.
Ideally, a reference oil is tested to establish a baseline of fresh, clean new oil. Subsequently, used samples from a machine are spotted on the chromatography substrate at regular, time-based intervals. Changes in the appearance of the zones/bands are a clear indication that something in the lubricant has changed. As with most analytical methods, RPC is not a predictor of future performance, rather a measurement of the situation at the time of sampling.
A closer look at the zones, their unique formation, and the debris field contained therein will reveal high particle counts that can be correlated to ISO Code, water contamination and even wear debris. This can be done with the unaided eye if the situation is severe or by using a 10 power microscope in cleaner systems.
As shown in the gear oil examples in Figure 1, the changes in the oil from a clear, clean, new oil, to a dark, oxidized-looking, used oil were confirmed by sophisticated laboratory analysis methods. In fact, the level of oxidation of the #1 sample was not detected by the Acid Number, but was suspect in the chromatogram and confirmed by Fourier Transform Infrared Analysis (FTIR) as having higher oxidation products than the fresh oil (Figure 2).
The gear oils show ever increasing signs of oxidation as the color of the center zone and the density of the dark outer zone indicate. The acid numbers (TAN) and infrared analysis correspond to the results. In this case there was an additional sample that came in with this set and is shown after the FTIR scans. It was obvious that the sample was not the same oil and the FTIR and Inductively Couple Plasma Spectrometer (ICP) confirmed the conclusion (Figure 3).
Numerous industry sources have noted 60% to 65% of all machine problems are lubricant related and that an equal number of problems are due to contamination of the lubricant. This being the case, the method of analysis described here can have an important and timely impact of the cost of oil analysis, maintenance, oil consumption and overall operational costs.
It is a simple process of establishing a new oil baseline for each machine, sample and test the machines’ used lubricant on a regular basis and then looking for changes in the results. When changes are observed, further laboratory analysis may be warranted to understand the source of the abnormal observation. Once this is done, it may not be necessary to send laboratory samples out again for this machine and lubricant combination. Instead, simply record the finding and use your plant’s Radial Planar Chromatography analysis tool to monitor the equipment.
Reference:
Altgelt, Klaus H. and T. H. Gouw, Chromatography in Petroleum Analysis, Marcel Dekker, 1977.
