ASTM D5292 Standard Test Method for Aromatic Carbon Contents of Hydrocarbon Oils by High Resolution Nuclear Magnetic Resonance Spectroscopy
4. Summary of Test Method
4.1 Hydrogen (1H) nuclear magnetic resonance (NMR) spectra are obtained on solutions of the sample in chloroform-d, using a CW or pulse FT high-resolution NMR spectrometer. Carbon (13C) NMR spectra are obtained on solutions of the sample in chloroform-d using a pulse FT high-resolution NMR spectrometer. Tetramethylsilane is preferred as an internal reference in these solvents for assigning the 0.0 parts per million (ppm) chemical shift position in both 1H and 13C NMR spectra.
4.2 The aromatic hydrogen content of the sample is measured by comparing the integral for the aromatic hydrogen band in the 1H NMR spectrum (5.0 to 10.0 ppm chemical shift region) with the sum of the integrals for both the aliphatic hydrogen band (−0.5 to 5.0 ppm region) and the aromatic hydrogen band (5.0 to 10.0 ppm region).
4.3 The aromatic carbon content of the sample is measured by comparing the integral for the aromatic carbon band in the 13C spectrum (100 to 170 ppm chemical shift region) with the sum of the integrals for both the aliphatic carbon band (−10 to 70 ppm region) and the aromatic carbon band (100 to 170 ppm region).
4.4 The integral of the aromatic hydrogen band must be corrected for the NMR absorption line due to residual chloroform (7.25 ppm chemical shift) in the predominantly chloroform-d solvent.
4.5 The integrals of the aliphatic hydrogen band and of the aliphatic carbon band must be corrected for the NMR absorption line due to the internal chemical shift reference tetramethylsilane (0.0 ppm chemical shift in both 1H and 13C spectra).
5. Significance and Use
5.1 Aromatic content is a key characteristic of hydrocarbon oils and can affect a variety ofproperties ofthe oil including its boiling range, viscosity, stability, and compatibility of the oil with polymers.
5.2 Existing methods for estimating aromatic contents use physical measurements, such as refractive index, density, and number average molecular weight (see Test Method D3238) or infrared absorbance and often depend on the availability of suitable standards. These NMR procedures do not require standards of known aromatic hydrogen or aromatic carbon contents and are applicable to a wide range ofhydrocarbon oils that are completely soluble in chloroform at ambient temperature.
5.3 The aromatic hydrogen and aromatic carbon contents determined by this test method can be used to evaluate changes in aromatic contents of hydrocarbon oils due to changes in processing conditions and to develop processing models in which the aromatic content of the hydrocarbon oil is a key processing indicator.
