ASTM D7691 Standard Test Method for Multielement Analysis of Crude Oils Using Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES)
4. Summary of Test Method
4.1 This test method usually requires several minutes per sample. A weighed portion of a thoroughly homogenized crude oil is diluted tenfold by weight with mixed xylenes, kerosene, or other suitable solvent. Standards are prepared in the same manner. Amandatory internal standard is added to the solutions to compensate for variations in test specimen introduction efficiency. The solutions are introduced to the ICP instrument by a peristaltic pump. By comparing emission intensities of elements in the test specimen with emission intensities measured with the standards, the concentrations of elements in the test specimen are calculable.
5. Significance and Use
5.1 Most often determined trace elements in crude oils are nickel and vanadium, which are usually the most abundant; however, as many as 45 elements in crude oils have been reported. Knowledge of trace elements in crude oil is important because they can have an adverse effect on petroleum refining and product quality. These effects can include catalyst poisoning in the refinery and excessive atmospheric emission in combustion of fuels. Trace element concentrations are also useful in correlating production from different wells and horizons in a field. Elements such as iron, arsenic, and lead are catalyst poisons. Vanadium compounds can cause refractory damage in furnaces, and sodium compounds have been found to cause superficial fusion on fire brick. Some organometallic compounds are volatile which can lead to the contamination of distillate fractions, and a reduction in their stability or malfunctions of equipment when they are combusted.
5.2 The value of crude oil can be determined, in part, by the concentrations of nickel, vanadium, and iron.
5.3 Inductively coupled plasma-atomic emission spectrometry (ICP-AES) is a widely used technique in the oil industry. Its advantages over traditional atomic absorption spectrometry (AAS) include greater sensitivity, freedom from molecular interferences, wide dynamic range, and multi-element capability. See Practice D7260.
6. Interferences
6.1 Spectral - There are no known spectral interferences between elements covered by this test method when using the spectral lines listed in Table 1. However, if spectral interferences exist because of other interfering elements or selection of other spectral lines, correct for the interference using the technique described in Test Method D5185.
6.2 Check all spectral interferences expected from the elements listed in Table 1. Follow the manufacturer's operating guide to develop and apply correction factors to compensate for the interferences. To apply interference corrections, all concentrations shall be within the previously established linear response range of each element listed in Table 1. (Warning - Correct profiling is important to reveal spectral interferences from high concentrations of some elements on the spectral lines used for determining trace metals.)
6.2.1 Spectral interferences can usually be avoided by judicious choice of analytical wavelengths. When spectral interferences cannot be avoided, the necessary corrections should be made using the computer software supplied by the instrument manufacturer or the empirical method described below. Details of the empirical method are given in Test Method C1109 and by Boumans. This empirical correction method cannot be used with scanning spectrometer systems when both the analytical and interfering lines cannot be located precisely and reproducibly. With any instrument, the analyst shall always be alert to the possible presence of unexpected elements producing interfering spectral lines.
6.2.2 The empirical method of spectral interference correction uses interference correction factors. These factors are determined by analyzing the single-element, high purity solutions under conditions matching as closely as possible those used for test specimen analysis. Unless plasma conditions can be accurately reproduced from day to day, or for longer periods, interference correction factors found to affect the results significantly shall be redetermined each time specimens are analyzed.
6.2.3 Interference correction factors can be negative if off-peak background correction is employed for element, i. A negative Kia correction factor can result when an interfering line is encountered at the background correction wavelength rather than at the peak wavelength.
6.3 Viscosity Effects - Differences in the viscosities of test specimen solutions and standard solutions can cause differences in the uptake rates. These differences can adversely affect the accuracy of the analysis. The effects can be reduced by using a peristaltic pump to deliver solutions to the nebulizer or by the use of internal standardization, or both. When severe viscosity effects are encountered, dilute the test specimen and standard twentyfold rather than tenfold while maintaining the same concentration of the internal standard. See Table 2.
6.4 Particulates - Particulates can plug the nebulizer thereby causing low results. Use of a Babington type high-solids nebulizer helps to minimize this effect. Also, the specimen introduction system can limit the transport of particulates, and the plasma can incompletely atomize particulates, thereby causing low results.
