ASTM D7455 Standard Practice for Sample Preparation of Petroleum and Lubricant Products for Elemental Analysis
14. Special Issues
14.1 In the atomic spectroscopic methods it has been shown that the viscosity index (VI) improvers added to lubricating oils give low results when calibrations are performed using standards that do not contain VI improvers. Examples of test methods requiring such considerations in sample preparation include Test Methods D4628 AAS determination of additive elements in lubricating oils; Test Methods D4951, D5185, and D7040 for determination of additive elements in lubricating oils; D5600 for elements in coke; and D7111 for trace elements in mid-distillate fuels, all by ICP-AES.
14.2 To overcome this interference, the standards need to be matched with samples in their VI improver content, ifavailable in analyte-free form and of the same type VI improver as the one being used in the sample, or using the specified solvent-to-sample dilution and an internal standard (IS) such as metal sulfonates of beryllium, cadmium, cobalt (most commonly used), lanthanum, lead, manganese, or silver.
14.2.1 The internal standard shall be added in a stable form to the sample matrix; it should pose no spectral interferences for analyte(s) lines used; should not react or precipitate with the sample matrix or analyte; and must ideally not be present in any of the samples being analyzed or if present, should be at much less than the doping level used (a maximum <1 % of dopant level).
14.3 Viscosity Effects - Differences in the viscosities of test specimen solutions and standard solutions can cause differences in the sample uptake rate in ICP-AES measurements. 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 observed, the test specimen and the standard should be diluted with appropriate solvent to up to twentyfold.
14.4 In several XRF test methods, an internal standard is added to the sample to compensate for the interelement effects of X-ray excitation and fluorescence so that the analytical line is subjected to matrix effects of similar magnitude as the analyte's radiation; thereby rendering the need for interelement correction (other than by using the internal standard) unnecessary. The use of internal standard is to avoid having to use more complicated methods for interelement corrections. If the interelement correction for the element is not employed, the results can be low due to absorption, or high due to enhancement.
14.4.1 These samples need to be well blended with the added internal standards such as organic sulfonates of nickel, lead, tin, titanium, or zirconium. Examples of such use include Test Method D4927 for additive elements in lubricating oils.
14.5 One needs to consider how small a sample can be taken and still remain a representative of the bulk sample. For example because of the very low detection limits of ICP-AES test methods (such as Test Methods D4951, D5185, etc.) only a few drops of oil sample are necessary for analysis. Even if the weight dilution is accurate using a 3 or 4 place balance, the elemental composition of each drop may not. Thus, if small sample sizes (perhaps < 10 drops) are used, the analyst needs to confirm through multiple measurements that the small sample sizes are giving repeatable results consistent with the precision stated in the test method.
14.6 End Measurements - Protocols for spectroscopic measurements for elemental analysis are given in Practice D7260 (ICP-AES), Practice D7343 (XRF), and Practice D7740 (AAS).
15. Keywords
15.1 acid decomposition; ash; atomic absorption spectrometry; bomb combustion; ED-XRF; ICP-AES; lubricants; microwave heating; petroleum products; SASH; sample preparation; WD-XRF
