ASTM D3605 Test Method for Trace Metals in Gas Turbine Fuels
ASTM D3605 Standard Test Method for Trace Metals in Gas Turbine Fuels by Atomic Absorption and Flame Emission Spectroscopy
8. Procedure
8.1 Fill two clean 25-mL volumetric flasks to the line with sample. With the microlitre syringe add 50 µL of mixed standard to one flask and 100 µL to the other. Touch the needle of the syringe to the inner wall of the flask to ensure quantitative transfer of the standard. Invert and mix the contents. (The two flasks are now spiked with 0.5 mg/L and 1.0 mg/L of sodium, lead, calcium, and vanadium). Alternatively, weigh 25.0 g of sample into each of two clean disposable glass vials and add the standard in the same manner. (The two vials are now spiked with 0.5 mg/kg and 1.0 mg/kg of sodium, lead, calcium, and vanadium.)
8.2 Prepare a third spiked sample by adding approximately 1 mL of the mixed standard to approximately 25 mL of sample. This solution serves only to aid in establishing satisfactory operating conditions for the atomic absorption instrument.
8.3 Establish the atomic absorption instrument operating conditions, which are recommended by the manufacturer, and consider the following special points. Select the mode, flame gases, and spectral lines from the information presented in Table 1.
8.4 Analysis:
8.4.1 With the atomic absorption instrument in operation for monitoring lead absorption and with1,2,3,4-tetrahydronaphthalene nebulizing, zero the instrument. Aspirate into the flame the third spiked sample described in 8.2 and note the net lead absorption signal. Optimize experimental conditions by adjusting the burner position (relative to the hollow cathode beam), the flow rates of the fuel and oxidant gases, and the sample aspiration rate until the net lead absorption signal maximizes. Re-zero the instrument with 1,2,3,4-tetrahydronaphthalene nebulizing, and consecutively introduce into the flame the unaltered sample and the two spiked samples, with 1,2,3,4-tetrahydronaphthalene nebulizing between each sample. Record the absorption signal of each sample and of each blank between samples.
8.4.1.1 At optimal experimental conditions, the analyte concentration that accounts for 1 % absorption should be approximately 1 mg/L. The percent absorption to concentration ratio must be near unity in order to achieve the lower limits of detection that are required. This note does not apply to flame emission measurements.
8.4.2 With the atomic absorption instrument in operation for monitoring vanadium absorption and with 1,2,3,4-tetrahydronaphthalene nebulizing, zero the instrument. Introduce into the flame the third spiked sample described in 8.2. Record the net vanadium absorption signal. Optimize the experimental conditions as in 8.4.1. Re-zero the instrument with 1,2,3,4-tetrahydronaphthalene nebulizing and consecutively introduce into the flame the unaltered sample and the two spiked samples. Record the absorption signal of each sample and of each 1,2,3,4-tetrahydronaphthalene blank between samples.
8.4.3 With the atomic absorption instrument in operation for monitoring sodium absorption, zero the instrument with 1,2,3,4-tetrahydronaphthalene nebulizing, and consecutively introduce into the flame the unaltered sample and the two spiked samples. Record the absorption signal of each sample and of each 1,2,3,4-tetrahydronaphthalene blank between samples.
NOTE 4 - For determining sodium and calcium, the maximization of absorption or emission signals is not critical.
8.4.4 With the atomic absorption instrument in operation for monitoring calcium absorption, zero the instrument with 1,2,3,4-tetrahydronaphthalene nebulizing, and consecutively introduce into the flame the unaltered sample and the two spiked samples. Record the absorption signal of each sample and of each 1,2,3,4-tetrahydronaphthalene blank between samples.
9. Calculation
9.1 For each absorption signal, calculate the net absorption signal as follows:
a = A - [(b1 + b2)/2]
where:
a = net absorption signal,
A = observed absorption signal,
b1 = blank signal before the sample, and
b2 = blank signal after the sample.
NOTE 5 - Blank corrections are usually small. If a large drift in blank signals is observed during a series of measurements, some experimental parameter is out of control. The cause of the variation should be corrected, and the measurements repeated.
9.2 For each element in turn, calculate sensitivities as follows:
S0.5 = 2(a1 - a2)
S1.0 = (a3 - a2)
S = (S0.5 + S1.0)/2
where:
S0.5 = sensitivity for the 0.5-mg/L spiked sample,
S1.0 = sensitivity for the 1.0-mg/L spiked sample,
S = average sensitivity,
a1 = net absorption signal for the 0.5-mg/L spiked sample,
a2 = net absorption signal for the unaltered sample, and
a3 = net absorption signal for the 1.0-mg/L spiked sample.
9.3 Calculate the ratio of sensitivities, R, as follows:
R = S1.0/S0.5
If R falls outside the range of 0.90 ≤ R ≤ 1.10, the data are nonlinear. Readjust experimental conditions and repeat the analysis.
9.4 Calculate the concentration of each element as follows:
mg/L = a2/S
mg/kg = a2/(S x d)
where d = density of the sample in g/ml.
10. Quality Control Checks (QA/QC)
10.1 When QA/QC protocols are already established in the testing facility, these may be used to confirm the reliability of the test method.
10.2 Since reference materials for the various matrices are not available and the test method utilizes the method of standard additions, the only practical quality control check is to verify the accuracy of the organometallic standard in 6.3. A source independent of this standard should be acquired and a sample carried through the analysis using the second-source standard as the known addition. Results of this test can be compared to the original sample result using the original standard material to verify accuracy.
