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In the used oil analysis lab, gas chromatography is becoming increasingly important for accurately determining the concentrations of certain contaminants - particularly fuel and glycol - in used oil samples.
In transformer oil analysis, the technique is used to determine the concentrations of dissolved gases within the oil sample, which can be used with gas analysis and other methods to evaluate electrical faults within a transformer or oil insulated electrical components.
Gas chromatography is one of the most widely used techniques in modern analytical chemistry. In its basic form, gas chromatography is used to separate complex mixtures of different molecules based on their physical properties, such as polarity and boiling point. It is an ideal tool to analyze gas and liquid samples containing many hundreds or even thousands of different molecules, allowing the analyst to identify both the types of molecular species present and their concentrations.
Figure 1. Gas Chromatography is Used to
Gas chromatography can be divided into two categories, gas-liquid chromatography and gas-solid chromatography. In both cases, the technique involves the separation of components of a gaseous sample, using a stationary phase, either a standard liquid in the case of gas-liquid chromatography, or a standard solid in the case of gas-solid chromatography. Because the overwhelming majority of test standards used for hydrocarbon analysis rely on gas-liquid chromatography, this article will focus exclusively on this method, although the same basic principles apply to both methods.
In gas-liquid chromatography, it is the interaction between the gaseous sample (the mobile phase) and a standard liquid (the stationary phase), which causes the separation of different molecular constituents. The stationary phase is either a polar or nonpolar liquid, which, in the case of capillary column, coats the inside of the column, or is impregnated onto an inert solid that is then packed into the gas chromatography column.
Figure 2. Gas Chromatography Instrument
A schematic layout of a gas chromatography instrument is shown in Figure 2. The basic components are an inert carrier gas, most commonly helium, nitrogen or hydrogen, a gas chromatography column packed or coated with an appropriate stationary phase, an oven that allows for precise temperature control of the column and some type of detector capable of detecting the sample as it exits or elutes from the column.
Gas-liquid chromatography works because the molecules in the samples are carried along the column in the carrier gas, but partition between the gas phase and the liquid phase. Because this partitioning is critically dependent on the solubility of the sample in the liquid phase, different molecular species travel along the column and elute at different times. Those molecules that have a greater solubility in the liquid phase take longer to elute and thus are measured at a longer interval.
Solubility is dependent on the physical and chemical properties of the solute; therefore, separation between different components of the sample occurs based on molecular properties such as relative polarity (like ethylene glycol versus base oil) and boiling point (like, fuel versus diesel engine base oil). For example, using a polar stationary phase, with a mixture of polar and nonpolar compounds will generally result in longer elution times for the polar compounds, because they will have greater solubility in the polar stationary phase.
There are many methods used to detect molecules as they elute. However, the most commonly employed method is flame ionization. In flame ionization, the eluting sample is passed through a hydrogen gas flame and the ion flux measured. As the sample passes through the flame, any molecules present are ionized, resulting in an increased ion flux. The total increase in ion flux is proportional to the amount of species present allowing the area under the increasing ion flux peak to be directly related to the concentration of the eluting species. Gas chromatography is often also coupled with Fourier Transformer Infrared (FTIR) or mass spectrometric (MS) detectors.
Several properties of used oils can be evaluated using gas chromatography. These include:
The determination of fuel dilution in engine oil samples is of prime importance because it causes a significant drop in viscosity, resulting in film strength failure at operating temperatures. Because gasoline and diesel fuel are chemically very similar to the oil itself, fuel dilution is almost impossible to quantify by conventional wet chemistry tests.
The evaluation of fuel in used engine samples by the gas chromatography method is one of the few ASTM (American Society for Testing and Materials) tests specifically designated for used oil analysis. The determination of diesel fuel dilution in used oil samples is covered under ASTM D3524, while the corresponding test for gasoline is ASTM D3525. In both cases, calibration mixtures of known dilution factors are used to calibrate the gas chromatography instrument, prior to running the test sample.
While other physical tests such as changes in viscosity, FTIR and reduced flash point are all used to determine the presence of fuel in a lubricating oil; gas chromatography offers a more precise and reliable means of determining fuel dilution. This is particularly true for changes in viscosity, which can be offset by soot loading, and FTIR, which at best is capable of detecting fuel dilution only when it reaches two percent due to new oil reference mismatches and variable aromatic content in diesel and gasoline fuels.
Just like fuel dilution, ASTM D4291 covers the use of gas chromatography to detect trace amounts of ethylene glycol due to coolant leaks; another method specifically developed for used oil analysis. In the case of ethylene glycol, the method works by first extracting the glycol using water (because ethylene glycol is a polar molecule, it is easily extracted using water) and injecting it into a calibrated gas chromatography column.
Figure 3. Typical Chromatogram
Fuel and ethylene glycol content can be quantified from the gas chromatogram by determining the expected response of the gas chromatography to different concentration of fuel or glycol, using standard calibration mixtures to create a calibration curve (Figure 3). From this curve, the area under the fuel or glycol peak in the gas chromatogram from the unknown sample can be converted to a percent by volume, allowing precise quantification of the fuel or glycol content.
For heat transfer fluids and radiological samples (those that have been exposed to gamma-radiation) one of the major areas of concern is the possibility of cracking due to either extreme temperatures in the case of heat transfer fluids, or the effects of the radiation for nuclear samples. Cracking is a process by which base oil hydrocarbon molecules are broken into smaller fragments. Thermal cracking of heat transfer fluids has traditionally been determined by direct distillation. Under this method, the sample is slowly heated and the boiling point range of the sample determined. Because cracking results in smaller hydrocarbon molecules, the boiling point range for severely cracked oil will be significantly lower than the new oil.
Gas chromatography offers a simpler, more convenient means of determining a reduction in boiling point range. In this method, the gas chromatography instrument is used in the temperature programmable mode. In this mode, the sample is slowly heated by increasing the temperature linearly over time. As the temperature rises, the boiling point range of the sample can be determined by measuring the sample as it elutes, often using a flame ionization source, as a function of the temperature. This method, which is commonly used in petroleum research and QA labs, is often referred to as a simulated distillation. The simulated distillation of a petroleum oil, appropriate to determining cracking of used oil sample is covered under ASTM D2887.
For gas compressors, there is often a need to determine the amount of dissolved process gas present in an oil sample. In this instance, gas chromatography can be used in an analogous way to dissolved gas analysis on transformer oil samples, to determine the presence and concentration of these gases.
While the use of gas chromatography to detect known contaminants such as fuel and ethylene glycol works because the contaminant has a known elution time, under carefully controlled column conditions. Often, there is a need to determine the presence of an unknown contaminant, which may be producing an unusual color or a strange odor. Under these circumstances, gas chromatography can often be used to separate different molecular species in the oil sample, prior to further analysis.
While the gas chromatography serves to reduce the number of different molecular species present, through molecular speciation by replacing the flame ionization detector with another analytical instrument such as an FTIR instrument or a mass spectrometer, the exact nature of any unknown contaminant can often be determined. In the case of FTIR, the technique is identical to the infrared analysis of bulk oil samples.
In the case of MS (mass spectrometry), the eluting species are introduced direct from the gas chromatography column into the MS instrument. A mass spectrometer works by atomizing and ionizing the sample into its constituent elements, or molecular fragments of the parent molecule using a high-energy source, commonly a high-energy electron beam. The ions are then separated by the MS based on the ratio their mass-charge (m/z) ratio. (MS will be discussed in a later issue of Practicing Oil Analysis magazine.)
While gas chromatography may not be appropriate for every sample, when accurate concentrations of a known contaminant such as fuel, glycol or dissolved gases are required, or there’s a need to diagnose a specific problem such as thermal or radiological cracking or the identity of an unknown contaminant, gas chromatography is an effective, versatile, indispensable and under-utilized used oil analysis tool.
“Oil Analysis 101: Fourier Transform Infrared Spectroscopy,” Practicing Oil Analysis magazine, March-April, 2002.