ASTM D5292 Standard Test Method for Aromatic Carbon Contents of Hydrocarbon Oils by High Resolution Nuclear Magnetic Resonance Spectroscopy
9. Procedures
9.1 Three different procedures are described in this section for determining the aromatic hydrogen content, (see 9.6) Procedures A and B (see 9.7), and the aromatic carbon content of hydrocarbon oils, Procedure C (see 9.8).

9.2 The procedure selected by the analyst will depend on the available NMR instrumentation and on whether an aromatic hydrogen or aromatic carbon content is of greater value in evaluating the characteristics of the hydrocarbon oil.

9.3 Appendix X1 and Practice E386 should be used in conjunction with the NMR spectrometer manufacturer's instructions in order to ensure optimum performance ofthe NMR instrument in the application of these procedures.

9.4 If tetramethylsilane is used as an internal chemical shift standard, prepare a 1 % v/v TMS in solvent solution by adding tetramethylsilane to chloroform-d solvent. Since TMS is very volatile, this solution should be refrigerated or replaced if the characteristic absorption due to TMS is no longer evident in the 1H or 13C NMR spectrum.

9.5 If it is inconvenient to prepare the test solution directly in the NMR sample tube as suggested in the following procedures, the test solution can be prepared in a small vial and transferred into the NMR sample tube after solvent addition and sample dissolution. Care should be exercised to ensure that the final solution concentrations are not different from those indicated in the procedures and that no contamination occurs during the transfer process.

9.6 Procedure A - 1H NMR Measurements Using a Continuous Wave (CW) NMR Spectrometer:
9.6.1 Pipette a homogeneous sample of the hydrocarbon oil into an NMR sample tube compatible with the configuration of the CW spectrometer, usually a 5 mm outside diameter capped NMR tube.

9.6.2 Add chloroform-d to the NMR sample tube to generate a final solution consisting ofup to 50 % v/v hydrocarbon oil in solvent. The concentration of hydrocarbon oil in solvent should be optimized for the spectrometer in use but can be as high as the indicated value. Check to ensure that the final solution is homogeneous and free of undissolved particles.

9.6.3 Using the instrumental conditions indicated in Table 1, acquire and plot the CW 1H NMR spectrum. If tetramethylsilane has been used as an internal standard, assign this absorption a chemical shift value of 0.0 ppm.

9.6.4 Integrate the NMR spectrum over two chemical shift regions, from 5.0 to 10.0 ppm (Region A) and from -0.5 to 5.0 ppm (Region B). See Appendix X1 for recommendations on the integration procedure.

9.6.5 Subtract the portion of integral contributed by the NMR absorption line ofresidual chloroform solvent (7.25 ppm in the 1H NMR spectrum) from the total integral value for Region A. If a residual chloroform absorption line is not apparent, make no correction to the Region A integral value.

9.6.6 If tetramethylsilane was used as an internal chemical shift reference, subtract the portion of integral contributed by the NMR absorption line of TMS (0.0 ppm in the 1H NMR spectrum) from the total integral value for Region B.

9.6.7 Calculate the aromatic hydrogen content using the corrected integral values for Regions A and B and the instructions in 10.1 and 10.2.

9.7 Procedure B - 1H NMR Measurements Using a Pulse Fourier Transform (FT) NMR Spectrometer:
9.7.1 Pipette a homogeneous sample of the hydrocarbon oil into an NMR sample tube compatible with the configuration of the pulse FT spectrometer, usually a 5 or 10 mm outside diameter capped NMR tube.

9.7.2 Add chloroform-d to the NMR sample tube to generate a final solution consisting of up to 5 % v/v hydrocarbon oil in solvent. The concentration of hydrocarbon oil in solvent should be optimized for the spectrometer in use but can be as high as the indicated value. Check to ensure that the final solution is homogeneous and free of undissolved particles.

9.7.3 Using the instrumental conditions indicated in Table 2, acquire and plot the pulse FT 1H NMR spectrum. If tetramethylsilane has been used as an internal standard, assign this absorption a chemical shift value of 0.0 ppm.

9.7.4 Fig. 1 shows an acceptable pulse FT 1H NMR spectrum of a gas oil test sample dissolved in chloroform-d.

9.7.5 Integrate the NMR spectrum over two chemical shift regions, from 5.0 to 10.0 ppm (Region A) and from -0.5 to 5.0 ppm (Region B). See Appendix X1 for recommendations on the integration procedure.

9.7.6 Subtract the portion of integral contributed by the NMR absorption line ofresidual chloroform solvent (7.25 ppm in the 1H NMR spectrum) from the total integral value for Region A. If a residual chloroform absorption line is not apparent or if carbon tetrachloride was used as solvent, make no correction to the Region A integral value.

9.7.7 If tetramethylsilane was used as an internal chemical shift reference, subtract the portion of integral contributed by the NMR absorption line of TMS (0.0 ppm in the 1H NMR spectrum) from the total integral value for Region B.

9.7.8 Calculate the aromatic hydrogen content using the corrected integral values for Regions A and B and the instructions in 10.1 and 10.2.

9.8 Procedure C - 13C NMR Measurements Using a Pulse Fourier Transform (FT) NMR Spectrometer:
9.8.1 Pipette a homogeneous sample of the hydrocarbon oil into an NMR sample tube compatible with the configuration of the pulse FT spectrometer, usually a 5 or 10 mm outside diameter capped NMR tube.

9.8.2 If a relaxation reagent is used, weigh 10 mg of chromium 2,4-pentanedionate per 1 mL of final solution volume directly into the tube or vial containing the hydrocarbon oil.

NOTE 1 - A relaxation reagent is recommended but is not required for this procedure (see X1.4.3). If relaxation reagent is not used, however, the "sequence delay time" (see Practice E386) instrumental setting must be increased to a significantly longer time than that used when relaxation reagent is present. Failure to use the longer "sequence delay time" as indicated in Table 2 will generate erroneous results.

9.8.3 Add chloroform-d to the NMR sample tube to generate a final solution consisting of up to 50 % v/v for petroleum distillates in solvent and up to 30 % v/v for coal liquids in solvent. The concentrations of sample oil in solvent should be optimized for the spectrometer in use but can be as high as the indicated values. Check to ensure that the final solution is homogeneous and free of undissolved particles.

9.8.4 Using the instrumental conditions indicated in Table 2, acquire and plot the pulse FT 13C NMR spectrum. If tetramethylsilane has been used as an internal standard, assign this absorption a chemical shift value of 0.0 ppm.

9.8.5 Fig. 2 shows an acceptable pulse FT 13C NMR spectrum of a gas oil test sample dissolved in chloroform-d containing relaxation reagent.

9.8.6 Integrate the NMR spectrum over two chemical shift regions, from 100 to 170 ppm (Region A) and from −10 to 70 ppm (Region B). See Appendix X1 for recommendations on the integration procedure.

9.8.7 If tetramethylsilane has been used as an internal chemical shift reference, subtract the portion of integral contributed by the NMR absorption line of TMS (0.0 ppm in the 13C NMR spectrum) from the total integral value for Region B.

9.8.8 Calculate the aromatic carbon content using the corrected integral values for Regions A and B and the instructions in 10.1 and 10.3.

10. Calculation
10.1 Calculate the aromatic hydrogen or aromatic carbon content as follows:
aromatic hydrogen or aromatic carbon content = [A/(A + B)] x 100%
where:
A = integral value of the aromatic portion of the spectrum, and
B = integral value of the aliphatic portion of the spectrum.

10.2 For the aromatic hydrogen content: A is the corrected integral value for Region A (from 5.0 to 10.0 ppm) and B is the corrected integral value for Region B (from -0.5 to 5.0 ppm). The result is expressed as mole percent aromatic hydrogen atoms or % H (Ar).

10.3 For the aromatic carbon content: A is the integral value for Region A (from 100 to 170 ppm) and B is the corrected integral value for Region B (from -10 to 70 ppm). The result is expressed as mole percent aromatic carbon atoms or % C (Ar).