ASTM D6591 Standard Test Method for Determination of Aromatic Hydrocarbon Types in Middle Distillates - High Performance Liquid Chromatography Method with Refractive Index Detection
10. Procedure
10.1 Calibration:
10.1.1 Prepare four calibration standards (A, B, C, and D), at the approximate concentrations given in Table 1, by weighing, to the nearest 0.0001 g, the appropriate materials into 100-mL volumetric flasks and making up to the mark with heptane.
NOTE 19 - The recommended concentrations in Table 1 will cover most petroleum materials distilling in the diesel boiling range. Other standard concentrations may be used, provided they meet the requirements of the test method (that is, linearity, detector sensitivity, and column resolution).
NOTE 20 - The calibration standard solutions should be stored in tightly stoppered bottles (for example, 10-mL volumetric flasks) in a dark place between 5 and 25°C. Under these conditions, the solutions are viable for at least six months.
10.1.2 When operating conditions are steady (see 9.4), inject 10 µL of calibration standard A. Record the chromatogram, and measure the peak areas for each aromatic standard (see Fig. 3). Actuate the backflush valve at the predetermined time (see 9.6) to elute the T+AH standard as a single sharp peak. When the analysis is finished, reverse the flow direction of the mobile phase (that is, return to forward flush) and allow the baseline to stabilize before the next injection.
10.1.3 Repeat 10.1.2 using calibration standards B, C, and D.
10.1.4 If the peak area for phenanthrene in calibration standard D is too small to measure accurately, prepare a new calibration standard D with a higher concentration of phenanthrene (for example, 0.02 g/100 mL) and proceed in accordance with 10.1.1.
10.1.5 Plot percent m/v (g/100 mL) concentration against area counts for each aromatic standard, that is, o-xylene, 1-methylnaphthalene, and phenanthrene. Calibration plots shall be linear with a correlation coefficient greater than 0.999 and an intercept of less than +/- 0.01 g/100 mL.
NOTE 21 - A computer or a data system may be used to interpret these calibrations.
NOTE 22 - It should only be necessary to calibrate the refractive index detector on a daily basis.
NOTE 23 - It is recommended that a reference diesel or one of the four calibration standards be run after every five samples to check the stability of the system.
10.2 Analysis of Samples:
10.2.1 Weigh, to the nearest 0.001 g, between 0.9 and 1.1 g of sample into a 10-mL volumetric flask, and make up to the mark with heptane. Shake thoroughly to mix. Allow the solution to stand for 10 min, and filter (see 6.3), if necessary, to remove insoluble material.
10.2.1.1 For samples in which the concentration of one or more aromatic hydrocarbon types falls outside the calibration range, prepare a more concentrated (for example, 2 g/10 mL) or more dilute (0.5 g/10 mL) sample solution as appropriate.
NOTE 24 - If another dilution factor than the one suggested is used, it could modify the retention time and the amount calculated.
10.2.2 When operating conditions are steady (see 9.4) and identical to those used for obtaining the calibration data (see 10.1), inject 10 µL of the sample solution (see 10.2.1) and start data collection. Actuate the backflush valve at the predetermined time (see 9.6) to elute the T+AHs as a single sharp peak (see Fig. 4). When the analysis is finished, reverse the flow direction of the mobile phase (that is, return to forward flush) and allow the baseline to stabilize before injecting the next sample.
10.2.3 With reference to Fig. 5, devise a suitable method to find and identify correctly the MAHs, DAHs, and T+AHs. Fig. 5 shows a typical chromatogram for a sample of diesel fuel.
10.2.4 Draw a baseline from just before the beginning of the non-aromatics peak (A in Fig. 5) to a point on the chromatogram immediately before the backflush point (D in Fig. 5).
10.2.5 Drop a vertical line from the valley (B in Fig. 5) between non-aromatics and MAHs to the baseline.
10.2.6 Drop a vertical line from the valley (C in Fig. 5) between MAHs and DAHs to the baseline.
10.2.7 Draw a baseline from just before the T+AH peak (E in Fig. 5) to a point just after the T+AH compounds elute (F in Fig. 5). As some baseline disturbance is to be expected following actuation of the backflush valve, wait for the baseline to stabilize before drawing the baseline after the backflush point.
10.2.8 Integrate the area due to MAHs from points B to C (see Fig. 5).
10.2.9 Integrate the area due to DAHs from points C to D (see Fig. 5).
10.2.10 Integrate the area due to T+AHs from points E to F (see Fig. 5).
NOTE 25 - If the chromatographic data have been processed automatically, visually check to see that the integration parameters have correctly identified and integrated the peaks.
11. Calculation
11.1 Percent m/m Aromatic Hydrocarbon Type Contents - Calculate the percent m/m contents for MAHs, DAHs, and T+AHs, using the following equation:
% m/m MAHs or DAHs or T + AHs = [(A x S) + I] x V / M
where:
A = MAH or DAH or T+AH peak area for the sample,
S = slope ofthe MAH or DAH or T+AH calibration plot (% m/v versus peak area),
I = intercept of MAH or DAH or T+AH % m/v calibration plot,
M = mass (g) of sample taken (see 10.2.1), and
V = total volume (mL) of sample solution (see 10.2.1).
11.2 Percent m/m Polycyclic Aromatic Hydrocarbon Type Content - Calculate the percent m/m content for POLY-AH using the following equation:
%m/m POLY - AH = %m/m DAHs + %m/m T + AHs
11.3 Total Aromatic Hydrocarbon Content - Calculate the total aromatic hydrocarbon content of the sample (percent m/m) as the sum of the concentrations of the individual hydrocarbon types (that is, MAHs + DAHs + T+AHs).
10. Procedure
10.1 Calibration:
10.1.1 Prepare four calibration standards (A, B, C, and D), at the approximate concentrations given in Table 1, by weighing, to the nearest 0.0001 g, the appropriate materials into 100-mL volumetric flasks and making up to the mark with heptane.
NOTE 19 - The recommended concentrations in Table 1 will cover most petroleum materials distilling in the diesel boiling range. Other standard concentrations may be used, provided they meet the requirements of the test method (that is, linearity, detector sensitivity, and column resolution).
NOTE 20 - The calibration standard solutions should be stored in tightly stoppered bottles (for example, 10-mL volumetric flasks) in a dark place between 5 and 25°C. Under these conditions, the solutions are viable for at least six months.
10.1.2 When operating conditions are steady (see 9.4), inject 10 µL of calibration standard A. Record the chromatogram, and measure the peak areas for each aromatic standard (see Fig. 3). Actuate the backflush valve at the predetermined time (see 9.6) to elute the T+AH standard as a single sharp peak. When the analysis is finished, reverse the flow direction of the mobile phase (that is, return to forward flush) and allow the baseline to stabilize before the next injection.
10.1.3 Repeat 10.1.2 using calibration standards B, C, and D.
10.1.4 If the peak area for phenanthrene in calibration standard D is too small to measure accurately, prepare a new calibration standard D with a higher concentration of phenanthrene (for example, 0.02 g/100 mL) and proceed in accordance with 10.1.1.
10.1.5 Plot percent m/v (g/100 mL) concentration against area counts for each aromatic standard, that is, o-xylene, 1-methylnaphthalene, and phenanthrene. Calibration plots shall be linear with a correlation coefficient greater than 0.999 and an intercept of less than +/- 0.01 g/100 mL.
NOTE 21 - A computer or a data system may be used to interpret these calibrations.
NOTE 22 - It should only be necessary to calibrate the refractive index detector on a daily basis.
NOTE 23 - It is recommended that a reference diesel or one of the four calibration standards be run after every five samples to check the stability of the system.
10.2 Analysis of Samples:
10.2.1 Weigh, to the nearest 0.001 g, between 0.9 and 1.1 g of sample into a 10-mL volumetric flask, and make up to the mark with heptane. Shake thoroughly to mix. Allow the solution to stand for 10 min, and filter (see 6.3), if necessary, to remove insoluble material.
10.2.1.1 For samples in which the concentration of one or more aromatic hydrocarbon types falls outside the calibration range, prepare a more concentrated (for example, 2 g/10 mL) or more dilute (0.5 g/10 mL) sample solution as appropriate.
NOTE 24 - If another dilution factor than the one suggested is used, it could modify the retention time and the amount calculated.
10.2.2 When operating conditions are steady (see 9.4) and identical to those used for obtaining the calibration data (see 10.1), inject 10 µL of the sample solution (see 10.2.1) and start data collection. Actuate the backflush valve at the predetermined time (see 9.6) to elute the T+AHs as a single sharp peak (see Fig. 4). When the analysis is finished, reverse the flow direction of the mobile phase (that is, return to forward flush) and allow the baseline to stabilize before injecting the next sample.
10.2.3 With reference to Fig. 5, devise a suitable method to find and identify correctly the MAHs, DAHs, and T+AHs. Fig. 5 shows a typical chromatogram for a sample of diesel fuel.
10.2.4 Draw a baseline from just before the beginning of the non-aromatics peak (A in Fig. 5) to a point on the chromatogram immediately before the backflush point (D in Fig. 5).
10.2.5 Drop a vertical line from the valley (B in Fig. 5) between non-aromatics and MAHs to the baseline.
10.2.6 Drop a vertical line from the valley (C in Fig. 5) between MAHs and DAHs to the baseline.
10.2.7 Draw a baseline from just before the T+AH peak (E in Fig. 5) to a point just after the T+AH compounds elute (F in Fig. 5). As some baseline disturbance is to be expected following actuation of the backflush valve, wait for the baseline to stabilize before drawing the baseline after the backflush point.
10.2.8 Integrate the area due to MAHs from points B to C (see Fig. 5).
10.2.9 Integrate the area due to DAHs from points C to D (see Fig. 5).
10.2.10 Integrate the area due to T+AHs from points E to F (see Fig. 5).
NOTE 25 - If the chromatographic data have been processed automatically, visually check to see that the integration parameters have correctly identified and integrated the peaks.
11. Calculation
11.1 Percent m/m Aromatic Hydrocarbon Type Contents - Calculate the percent m/m contents for MAHs, DAHs, and T+AHs, using the following equation:
% m/m MAHs or DAHs or T + AHs = [(A x S) + I] x V / M
where:
A = MAH or DAH or T+AH peak area for the sample,
S = slope ofthe MAH or DAH or T+AH calibration plot (% m/v versus peak area),
I = intercept of MAH or DAH or T+AH % m/v calibration plot,
M = mass (g) of sample taken (see 10.2.1), and
V = total volume (mL) of sample solution (see 10.2.1).
11.2 Percent m/m Polycyclic Aromatic Hydrocarbon Type Content - Calculate the percent m/m content for POLY-AH using the following equation:
%m/m POLY - AH = %m/m DAHs + %m/m T + AHs
11.3 Total Aromatic Hydrocarbon Content - Calculate the total aromatic hydrocarbon content of the sample (percent m/m) as the sum of the concentrations of the individual hydrocarbon types (that is, MAHs + DAHs + T+AHs).