ASTM D5708 Method for Nickel, Vanadium, and Iron in Crude Oils and Residual Fuels
ASTM D5708 Standard Test Methods for Determination of Nickel, Vanadium, and Iron in Crude Oils and Residual Fuels by Inductively Coupled Plasma (ICP) Atomic Emission Spectrometry
TEST METHOD A - ICP WITH AN ORGANIC SOLVENT SPECIMEN SOLUTION
7.1 Inductively Coupled Plasma Atomic Emission
Spectrometer - Either a sequential or simultaneous spectrometer, equipped with a quartz torch and radio-frequency generator to form and sustain the plasma, is suitable.
7.2 Nebulizer - The use of a high-solids nebulizer is optional but strongly recommended. This type of nebulizer minimizes the probability of clogging. A concentric glass nebulizer can also be used.
7.3 Peristaltic Pump - This pump is required for non-aspirating nebulizers and optional for aspirating nebulizers. The pump must achieve a flow rate in the range of 0.5 to 3 mL/min. The pump tubing must be able to withstand at least a 6 h exposure to the solvent. Fluoroelastomer copolymer tubing is recommended.
7.4 Specimen Solution Containers, glass or plastic vials or bottles with screw caps having a capacity of appropriate size. One hundred millilitre glass bottles are satisfactory.
8.1 Dilution Solvent - Mixed xylenes, o-xylene, tetralin and mixed paraffin-aromatic solvents are satisfactory. Solvent purity can affect analytical accuracy when the sample contains low concentrations (typically, a few mg/kg) of the analytes.
8.2 Mineral Oil - A high-purity oil such as U.S.P. white oil.
8.3 Organometallic Standards - Pre-prepared multielement concentrates containing 100 mg/kg concentrations of each element are satisfactory.
8.4 Quality Control (QC) Samples, preferably are portions of one or more liquid petroleum materials that are stable and representative of the samples of interest. These QC samples can be used to check the validity of the testing process as described in Section 19.
9. Preparation of Standards and Specimens
9.1 Blank - Prepare a blank by diluting mineral oil with dilution solvent. The concentration of mineral oil must be 10 % (m/m). Mix well.
9.2 Check Standard - Using organometallic standards, mineral oil, and dilution solvent, prepare a check standard to contain analyte concentrations approximately the same as expected in the specimens. The concentration of oil in the check standard must be 10 % (m/m).
9.3 Test Specimen - Weigh a portion of well-mixed sample into a container and add sufficient solvent to achieve a sample concentration of 10 % (m/m). Mix well.
9.4 Working Standard - Prepare an instrument calibration standard that contains 10 mg/kg each of vanadium, nickel, and iron. Combine the organometallic standard, dilution solvent and, if necessary, mineral oil so that the oil content of the calibration standard is 10 % (m/m).
9.5 Quality Control (QC) Samples - Weigh a portion of the well-mixed QC sample into a container and add sufficient solvent to achieve a sample concentration of 10 % (m/m).
10. Preparation of Apparatus
10.1 Consult the manufacturer's instructions for the operation of the ICP instrument. This test method assumes that good operating procedures are followed. Design differences between instruments make it impractial to specify required parameters.
10.2 Assign the appropriate operating parameters to the instrument taskfile so that the desired analytes can be determined. Parameters include: (1) element, (2) analytical wavelength, (3) background correction wavelengths (optional), (4) interelement correction factors (refer to 10.3), (5) integration time of 1 to 10 s, (6) two to five consecutive repeat integrations. Suggested wavelengths are listed in Table 1.
10.3 Spectral Interferences - Check all spectral interferences expected for the analytes. If interference corrections are necessary, follow the manufacturer's operating guide to develop and apply correction factors.
10.3.1 Spectral interferences can usually be avoided by judicious choice of analytical wavelengths. If spectral interferences cannot be avoided, the necessary corrections should be made using the computer software supplied by the instrument manufacturer or by using the empirical method described in Test Method D5185.
10.4 Consult the instrument manufacturer's instructions for operating the instrument with organic solvents.
10.5 During instrument warm-up, nebulize the blank solution. Inspect the torch for carbon build-up. When carbon build-up occurs, replace the torch and adjust the operating conditions to correct the problem.
10.5.1 Carbon build-up within the torch can be caused by improperly adjusted argon flow rates, improper solution flow rates, and positioning the torch injector tube too close to the load coil. Carbon deposits can invalidate a calibration and extinguish the plasma.
11. Calibration and Analysis
11.1 Using the blank and working standard, perform a two-point calibration at the beginning of the analysis of each batch of specimens. Additional working standards can be used, if desired.
11.2 Use the check standard to determine if the calibration for each analyte is accurate. When the result obtained on the check standard is not within +/-5 % of the expected concentration for each analyte, take corrective action and repeat the calibration.
11.3 Analyze the specimens in the same manner as the calibration standards (that is, same integration time, plasma conditions, and so forth). Calculate concentrations by multiplying the concentration determined for the test specimen solution by the dilution factor. Calculation of concentrations can be performed manually or by computer when such a feature is available.
11.4 When the measured intensities for the test specimen solution exceed the corresponding intensities for the working standard, either ensure that the calibration curve is linear to the concentration of the element in the test specimen solution or dilute the test specimen solution with the blank solution and reanalyze.
11.5 Analyze the check standard after every fifth specimen. If any result is not within 5 % of the expected concentration, take corrective action, repeat the calibration, and reanalyze the specimen solutions back to the previous acceptable check standard analysis.
11.6 The use of spectral background correction is highly recommended, particularly when the test specimen solutions contain low concentrations of the analytes (typically less than 1 mg/kg). When concentrations are low, background changes, which can result from variability in the compositions of test specimen solutions, can affect the accuracy of the analysis. Background correction minimizes errors due to variable background intensities.