ASTM D7260 Standard Practice for Optimization, Calibration, and Validation of Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) for Elemental Analysis of Petroleum Products and Lubricants
10. Procedure
10.1 Sampling Procedure:
10.1.1 The objective of sampling is to obtain a test specimen that is representative of the entire bulk quantity. Thus, take lab samples in accordance with the instructions in Practice D4057, if applicable. The specified sampling technique can affect the accuracy of ultimate ICP-AES analysis.
10.1.2 Particularly for used oils, crude oils, or residual fuel oils it is important to homogenize the oil samples in the sample container so that a representative test specimen is obtained. Homogenization can be achieved by using an ultrasonic bath or a vortex mixer, preferably with a sample heated to about 60°C or higher as required to liquefy the sample. Heated sample vessels must have the caps loosened to prevent potential overpressuring the vessel. This practice assumes that the duration and temperature of heating will not result in significant loss of light hydrocarbons from the sample (preconcentration of non-volatile analyte) or potentially loss of volatile analyte species.
10.1.3 In case of samples that require chemical decomposition preparation before ICP-AES measurements (for example, for fuel oils by ashing and fusion in Test Method D5184, for coke by ashing and fusion in Test Method D5600, or for crude oils, and residual fuels by wet ashing in Test Method D5708), it is important to:
10.1.3.1 Homogenize the fuel oil or crude oil in the original sample container to obtain a representative specimen; otherwise it can lead to erroneous results;
10.1.3.2 Avoid foaming and frothing during heating, if the specimen contains considerable amount of moisture. The heating temperature should be slowly raised, or a small amount of 2-propanol or toluene-2-propanol should be added to the specimen; and
10.1.3.3 Prevent contamination from muffle furnace by properly covering the specimen container. The heating element or furnace walls should be replaced if they show flaking.
10.1.4 In some procedures, samples are decomposed by acid treatment using a microwave oven (for example, Practice D7876).
10.2 Consult the manufacturer's instructions for operation of the ICP spectrometer. Follow appropriate operating procedures. Design differences among instruments and different selected wavelengths (Table 1) for individual spectrometers make it impractical to list detailed conditions for operation. Sensitivity, instrument detection limit, linear dynamic range, interference effects, and appropriate background correction should be investigated and established for each individual analyte on each instrument. Although beyond the scope of this standard, consider the impact of background signal on scanning source without background correction especially for organic matrices.
10.3 Aspirate the prepared sample solution into the calibrated instrument using the same conditions established for the calibration procedure. A typical integration time is 10 s. Rinse sufficiently with dilution solvent to prevent carryover and avoid memory effects. Memory effects are suspected if a steady decline in signal is observed from taking multiple measurements of a sample that follows one of very high analyte concentration. Though this suggested solvent rinse time will vary with the solution matrix, sample introduction design, and operating parameters for each instrument, a recommended aspiration of an appropriate rinse solvent or the subsequent sample solution for an at least additional 60 s should be applied between sample solution measurements. Sometimes the subsequent sample is used exclusively for the rinse without a brief rinse of sipper tip is risky by virtue of cross contamination potential to subsequent sample if preceding sample is very high in analyte - not to mention wasting prepared sample solution - still it is a viable option to consider. The rinse time should be extended further if analyte levels well above that of the highest calibration standard (empirically used beforehand to determine appropriate rinse times) are present in the previously aspirated sample solution.
10.4 Run a blank and an instrument check standard (a calibration standard or a calibration verification standard) every five samples or as established necessary for the instrument. The concentration measured should be within the tolerance described in the test method (or within +/- 5 %) for the expected value, assuming 100x detection limit concentration is used. If the concentration is out of range, investigate and correct the problem, recalibrate the instrument, and reanalyze the samples in question. Analyze a blank and a check standard at the beginning and end of each run.
10.5 When the concentration of any analyte exceeds the linear range of the calibration, another test specimen solution should be prepared by mixing the sample with base oil before adding diluent, and reanalyzing, if the sample is easily miscible with base oil (versus kerosine or xylenes). However, perhaps dilution protocol suggested above in 8.8.1 may prove more reliable for insuring representative sub-sampling especially when marginally soluble analyte may be involved (that is, suspended emulsions or water bottoms, some siloxanes and polymers).
10.6 Matrix spikes and duplicates may be performed if the sample concentrations are suspect due to contamination or trace levels of elements of interest. However, this may not identify spectral interferences; alternate line monitoring is recommended for this. Spikes typically identify poor matrix matching of the standards to that of the sample matrix (as does the internal standard recovery) assuming no analyte solubility issues are at play. The duplicates typically just confirm repeatability issues such as sample homogeneity concerns assuming no errors in analytical procedure or operations (such as contamination during preparation) are involved.
10.7 Use the background and interference corrected data to calculate the concentration of each element in the sample. The instrument computer performs this calculation including the dilution factor.
11. Quality Control/Quality Assurance
11.1 Confirm the performance of the instrument system and the test procedure by analyzing a quality control (QC) sample. Guidance on QA/QC is given in several of the ICP-AES methods listed in 2.2, and in Practice D6299 and Guide D6792.
11.2 Where possible, the QC sample should not be the same one used for calibrating the instrument. Where possible, the QC sample should be available in large enough quantities for long term usage, be of same or similar type of material as the samples being analyzed, and be stable and homogenous under the anticipated storage conditions.
11.3 The data should be recorded and analyzed by control chart or other statistically equivalent techniques to ascertain statistical control status of the total test process.
11.4 If the QC charts show out of statistical control behavior, investigate the cause and correct the deficiency before proceeding with sample analysis. It may be necessary to reanalyze the earlier samples from the time frame when out of control behavior was apparent.
11.5 Generally a QC sample should be analyzed each testing day with routine samples. The QC frequency should be increased if a large number of samples is routinely analyzed. However, when it is demonstrated that the testing is under statistical control, the QC frequency may be reduced. The QC sample precision should be periodically checked against the ASTM test method precision to ensure data quality. See Guide D6792 for guidance.
11.6 It may be useful to confirm the optimum performance of the instrument system by analyzing certified reference materials such as available from NIST or other sources, if such materials are available.
