ASTM D7691 Standard Test Method for Multielement Analysis of Crude Oils Using Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES)
9. Sampling
9.1 It is critical that a representative sample be obtained for analysis from the bulk material. Maintaining compositional integrity of these samples from the time of collection until their analysis requires care and effort. Sampling procedure also should not introduce any contaminants into the sample or otherwise alter the sample composition so that the subsequent test results are affected.

9.2 See Practices D4057 and D4177 for manual and automatic sampling of petroleum and petroleum products, respectively. In sampling of crude oils, the material may contain a heavy component, such as free water, which tends to separate from the main component. Guide D5854 provides a guide for selecting suitable containers for crude oil samples for various analyses.

10. Preparation of Apparatus
10.1 Instrument - Design differences between instruments, ICP excitation sources, and different selected analytical wavelengths for individual spectrometers make it impractical to detail the operating conditions. Consult the manufacturer's instructions for operating the instrument with organic solvents. Set up the instrument for use with the particular dilution solvent chosen.

10.2 Peristaltic Pump - Before using the peristaltic pump, inspect the pump tubing and replace it, if necessary, before starting each day. Verify the solution uptake rate and adjust it to the desired rate.

10.3 ICP Excitation Source - Initiate the plasma source at least 30 min before performing analysis. During this warm up period, nebulize dilution solvent. Inspect the torch for carbon buildup during the warm up period. If carbon buildup occurs, replace the torch immediately and consult the manufacturer's operating guide to take proper steps to remedy the situation.

NOTE 1 - Some manufacturers recommend even longer warm-up periods to minimize changes in the slopes of calibration curves.

NOTE 2 - Carbon that accumulates on the tip of the torch injector tube can be removed by using nebulizer gas that consists of approximately 1 % oxygen in argon.

NOTE 3 - Generally, the carbon buildup can be minimized by increasing the intermediate argon flow rate or lowering the torch, or both, relative to the load coil.

10.4 Wavelength Profiling - Perform any wavelength profiling that may be called for in the normal operation of the instrument.

10.5 Operating Parameters - Assign the appropriate operating parameters to the instrument task file so that the desired elements can be determined. Parameters to be included are element, wavelength, background correction points (optional), interelement correction factors (optional), integration time, and internal standard correction (optional). Multiple integrations are required for each measurement, and the integration time is typically 10 s.

11. Calibration
11.1 The linear range shall be established once for the particular instrument being used. This is accomplished by running intermediate standards between the blank and the working standard and by running standards containing higher concentrations than the working standard. Analyses of test specimen solutions shall be performed within the linear range of response.
11.1.1 The linear range will also be determined by the instrument parameters (power, gas flow rates, nebulizer type, nebulizer flow rates, etc.). It is likely that this would need to be established more than once for each instrument. Correlation coefficient should be better than 0.995.

11.2 Working Standard - At the beginning of the analysis of each batch of specimens, perform a two-point calibration consisting of the blank and working standard. Use the check standard to determine if each element is in calibration. When the results obtained with the check standard are within 5 % of the expected concentrations for all elements, proceed with test specimen analyses. Otherwise, make any adjustments to the instrument that are necessary and repeat the calibration. Repeat this procedure with the check standard every five samples.

11.3 Working Standard with Internal Standard - Calibrate the instrument as described in 11.2. Calculate an intensity ratio for each element by the following equation:
I(Re) = (I(e) - I(Be))/I(is)
I(Re) = intensity ratio for element, e,
I(e) = intensity for element, e,
I(Be) = intensity of the blank for element, e, and
I(is) = intensity of internal standard element.

11.3.1 Calculate the calibration factors from the intensity ratios. In most modern ICPAES instruments the computer software provided by the instrument's manufacturer will do these calculations.

11.4 Internal Standardization:
11.4.1 The internal standard procedure requires that every test specimen solution have the same concentration (or a known concentration) of an internal standard element that is not present in the original specimen. Specimen to specimen changes in the emission intensity of the internal standard element can be used to correct for variations in the test specimen introduction efficiency, which is dependent on the physical properties of the test specimen.

11.4.2 The internal standard is usually combined with the dilution solvent. Internal standard compensation is typically handled in one of two different ways, summarized below
11.4.2.1 Calibration curves are based on the measured intensity of each analyte divided by the measured intensity of the internal standard per unit internal standard element concentration. The concentrations for each analyte in the test specimen solution are read directly from these calibration curves.

11.4.2.2 For each analyte and the internal standard element, calibration curves are based on measured intensities. Uncorrected concentrations for each analyte in the test specimen solution are read from these calibration curves. Corrected analyte concentrations are calculated by multiplying the uncorrected concentrations by a factor equal to the actual internal standard concentration divided by the uncorrected internal standard concentration determined by analysis.

11.4.3 Internal Standard Solution - Weigh 2 g to 4 g of 0.500 mass % cadmium, cobalt, scandium, or yttrium (or any other suitable metal) organometallic concentrate into a 1 L volumetric flask and dilute to 1 L with the dilution solvent. Prepare fresh, at least weekly, and transfer this solution into a dispensing vessel. The stability of this solution shall be monitored. The concentration of the internal standard element is not required to be 1a specific concentration; however, the concentration of the internal standard element in the test specimen solution should be at least 100 times its detection limit. A concentration in the range of 10 mg/kg to 20 mg/kg is typical. This would require one part of oil sample to be diluted tenfold with the internal standard solution.

NOTE 4 - This test method specifies that the internal standard is combined with the dilution solvent because this technique is common and efficient when preparing many samples. However, the internal standard can be added separately from the dilution solvent as long as the internal standard concentration is constant or accurately known.

12. Sample Handling
12.1 Homogenization - It is extremely important to homogenize the crude oil in the sample container in order to obtain a representative test specimen. Failure to follow this homogenization procedure can invalidate the results because non-representative aliquots could be obtained and this could lead to erroneous results.

12.2 Place the sample container in an oven at a temperature of 50 °C to 60 °C. Keep the container in the oven until the sample comes to temperature. These three ways of mixing the sample after heating are permissible.
12.2.1 Insert the shaft of a high speed homogenizer into the sample container so that the head of the shaft is immersed to approximately 5 mm from the bottom of the sample container. Mix the sample for about 5 min.

12.2.2 Ultrasonic Homogenization - Place the crude oil (in the sample container) into the ultrasonic bath. Leave the sample in the bath until immediately before dilution.

12.2.3 Vortex Homogenization - As an alternative to ultrasonic homogenization, vortex mix the crude oil in the sample container, if possible.

13. Preparation of Test Specimen and Standards
13.1 Internal Standard - Add internal standard solution to the working standard, check standard, and test specimen before diluting with the dilution solvent. Ensure that the standard or test specimen concentration is 10 mass %. Alternatively, the internal standard can be present in the dilution solvent.

13.2 Blank - Prepare a blank by diluting the base oil or white oil tenfold by mass with the dilution solvent.

13.3 Working Standard, 10 µg/g - Weigh a quantity of the 0.0500 mass % multielement standard to the precision of the balance used (see 7.1) into an appropriately sized container, add four times the weight of the multi-element standard of base oil and dilute with 45 times the weight of the multi-element standard of dilution solvent. Working standards containing higher or lower concentrations can be prepared depending on the concentrations of elements in the crude oils. In addition, solutions containing single elements can also be prepared. However, ensure that the tenfold dilution is maintained for all solutions.

13.4 Check Standards - Prepare instrument check standards in the same manner as the working standards such that the concentrations of elements in the check standards are similar to the concentrations of elements in the test specimen solutions. It is advisable to prepare the check standard from alternative sources of certified organometallic standards.

NOTE 5 - Organometallic standards are not as stable as aqueous standards when diluted to working standard strength. They should be prepared fresh before use.

13.5 Test Specimen - Weigh a portion of the well-homogenized sample into a suitable container. Record the mass to three significant figures. Add dilution solvent until the test specimen concentration is 10 mass %.