ASTM D5185 for additive elements, wear metals, contaminants and selected elements
ASTM D5185 Standard Test Method for Determination of Additive Elements, Wear Metals, and Contaminants in Used Lubricating Oils and Determination of Selected Elements in Base Oils by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES)
6. Interferences
6.1 Spectral - 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 must 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 additive elements on the spectral lines used for determining wear metals.)
6.1.1 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 C 1109 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 must always be alert to the possible presence of unexpected elements producing interfering spectral lines.
6.1.2 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 must be redetermined each time specimens are analyzed.
6.1.3 Interference correction factors, Kia, are defined as follows:
For analyte a, we have:
Ca = Ia/Ha
where:
Ca = concentration of analyte a,
Ia = net line intensity (that is, background corrected) of analyte a, and
Ha = sensitivity.
6.1.3.1 Similarly, for an interferent i at the same wavelength:
Ci = Ii/Hi
where:
Ii = contribution from the peak or wing of the interferent line to the peak intensity of the analyte a.
6.1.3.2 The correction factor, Kia is defined as:
Kia = Hi/Ha = Ii/(Ci x Ha)
6.1.3.3 Analysis of high-purity stock solutions with a calibrated instrument gives Ii/Ha, the concentration error that results when analyzing a solution containing an interferent of concentration Ci. Dividing by Ci gives the dimensionless correction factor Kia. To apply these correction factors:
Ca, apparent = (Ia + Ii)/Ha
Ca, apparent = Ca + Ii/Ha
Ca = Ca, apparent - Ii/Ha
Ca = Ca, apparent - Kia * Ci
and, for more than one interferent:
Ca = Ca, apparent - K1a x C1 - K2a x C2 - ...
6.1.4 Interference correction factors can be negative if off-peak background correction is employed for element i. A negative Kia can result when an interfering line is encountered at the background correction wavelength rather than at the peak wavelength.
6.2 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 twenty-fold rather than tenfold while maintaining the same concentration of the internal standard.
6.3 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.
