ASTM D7111 Standard Test Method for Determination of Trace Elements in Middle Distillate Fuels by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES)
15. Analysis
15.1 Determine the ICP detection limits for all elements of interest as follows: Prepare a kerosine blank with an internal standard by pipetting 1000 µL of the internal standard stock solution into a 50 mL volumetric flask, and fill to the volume marker with kerosine. Seal the flask, and mix well. Perform ten consecutive analyses of this solution for all elements of interest under the same conditions/parameters that the two-point calibration standards were run. With the ICP instrument software, determine the standard deviation of the ten results for each element of interest. The detection limit of each element is its standard deviation multiplied by three. Detection limits should be determined daily after calibration.
15.2 Conduct fuel sample analyses under the same conditions/parameters that the two-point calibration standards were run. Aspirate kerosine between fuel samples to clear the system of elements from the prior sample. The ICP instrument software generates concentration values for each element.
15.3 In a manner similar to that described in 14.4 for the calibration, the analysis of the fuel sample generates an intensity ratio as follows:
Rf = If/Iintf
where:
Rf = intensity ratio for an element e in the fuel,
If = emission intensity of element e in the fuel, and
Iintf = emission intensity of internal standard added to the fuel.
Thus, by comparison with the calibration curve, the ICP instrument software determines the element concentrations as follows:
Cf = (Rf x Ccs)/Rws
where:
Cf = concentration (mg/kg) of element e in the fuel,
Ccs = concentration (mg/kg) of element e in the calibration standard,
Rf = intensity ratio of element e in the fuel, and
Rws = intensity ratio of element e in the working standard.
15.3.1 Eq 3 applies if the density of the working standards and the test specimen are similar. If the density of the test specimen is dissimilar (greater than 6 %) to that of the working standards, the accuracy of the result calculated by Eq 3 will be impacted. The effect of the density difference can be corrected for by multiplying the result of Eq 3 (Cf) by the density of the working standard divided by the density of the test specimen. Alternatively, the working standards can be prepared using a diluent of similar density to that of the test specimen.
NOTE 2 - The procedure described in 15.3 only works if the internal standard fully compensates for sample transport and nebulization differences between the sample and the standard. This may not be the case for larger density differences. However, the middle distillate fuels covered in this test method have all distillation fractions contained within the boiling range of 150 °C to 390 °C and the differences in density typically should not be significant enough to cause sample transport and nebulization differences.
15.4 Analyze the check standard after every fifth fuel test specimen. If any result is not within 5 % of the prepared concentration, recalibrate the instrument and reanalyze the previously analyzed fuel test specimens back to the previously acceptable check standard analysis.
16. Calculation
16.1 Calculate the concentrations of the elements in the calibration standard solution as follows:
Ccs = (M100/Mcs)C100
where:
Ccs = the concentration (mg/kg) of element e in the calibration standard solution,
C100 = the concentration (mg/kg) of element e in the nominal 100 mg/kg organometallic standard,
M100 = the mass (g) of the nominal 100 mg/kg organometallic standard, and
Mcs = the mass (g) of the prepared solution of the nominal 100 mg/kg organometallic standard and kerosine.
