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Gas Chromatograph for Oil Analysis

As a service technician, you should value the durability and reliability of your equipment. For this, knowing predictive maintenance analysis and techniques is very important. Gas chromatography is one such technique and should be taken into consideration when considering engine oil life. The following outlines the importance of gas chromatography for the condition and functionality of oil and engine life. 

What is gas chromatography 

Gas chromatography is a physical method of separating the components of a mixture through a differential interaction of its components between a STATIONARY PHASE (liquid or solid) and a MOBILE PHASE (gas).  An essential component of the unit is a long capillary tube full of very fine powder. Such a fine powder that works as a molecular sieve. If we pass two gases of different sized molecules (A and B), we will see at the other end that the smaller molecule gas (A) will come out first, and the largest (B) will come out last. A detector installed at the outlet will allow us to tell when and how much gas is coming out. The “when” is related to a specific gas. We will never see B leave before A, and each will always leave at the same time. And the “how much” is related to the concentration of these gases in the mixture. 

When engine oil volatilizes, it reduces the amount of oil film on the piston rings, cams, bearings and are thought to carry exhaust catalyst-contaminating components that can affect the systems smog-reducing components. After the smaller molecular weight compounds have evaporated out of the oil, larger molecules remain which will produce an increase in viscosity which can stress the system as well as require more energy to overcome the increased resistance. 

Engine oil volatility ASTM D 6417 is used to determine the volatility of engine oil at 371°C and in many instances are a requirement in some lubricant specifications. The results by this test method can be influenced by the presence of viscosity index improver additives or by heavier base oils.  This test method is limited to samples having a 126°C boiling point. This test is applied to base oils and finished lubricants.  Test method ASTM D 5480 provides the determination of the amount of engine oil volatilized at 700°F (371°C). This test method is applicable to engine oils containing large molecules indicative of additives and synthetics base oils.  This test method can also be used to determine the amount of oil volatilized at any temperature between 238 and 371°C. This test gives a more precise measurement of oil volatility than the traditional Noack method.  

Gas chromatographic analysis is highly effective at determining the amount of fuel is dissolved in engine oil.  Some fuel dilution of the engine oil may take place during normal operation. Fuel dilution is a concern due to a reduction in oil viscosity as well as an increase in the opportunity of oxidation.  Gas chromatographic techniques are used for analyzing the samples, by adding a known percentage of n-tetradecane as an internal standard, in order to determine the weight percent of gasoline fuel in the lubricating oil or by adding a known percentage of n-decane as an internal standard, in order to determine the mass percent of diesel fuel in the lubricating oil.  The ASTM D 3525 test method is used to determine the amount of gasoline in engine oil. There is no limitation for the determination of the dilution range, provided that the amount of sample plus internal standard is within the linear range of the gas chromatograph detector.  The ASTM D 3524-14 method is used to determine the amount of diesel fuel that may be found in diesel engine oil. The diesel fuel diluent is analyzed at concentrations up to 12%.  

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