Evaluation of integration procedures for PNA analysis by C-13 NMR

Carbon-13 NMR spectroscopy has become a useful tool in the analysis of crude oils and petroleum products. The determination of the relative percentages of paraffinic, naphthenic and aromatic carbon in crude oils and oil fractions by carbon-13 nuclear magnetic resonance spectroscopy has been shown to compare well with results from ASTM D2140 procedures. In the ASTM method, the PNA analysis is determined by the refractive index, the density and the molecular weight of the material. In carbon-13 NMR spectroscopy, the carbon types are observed directly and, under appropriate conditions, can be readily quantified. Aromatic carbon resonances are observed in the 110-170 ppm region of the 25-60 ppm region of the spectrum. The paraffinic carbon signals are seen as a band of overlapping resonances in the 0-25 ppm region and as sharp resonances in the 25-60 ppm region. The relative percentages of these three carbon types is calculated by the ratio of the area of the resonances for each carbon type over the total area, excluding the solvent and reference peak areas. To study the effects of the baseline subtraction and the baseline correction subroutines on the PNA results, a typical crude oil and its asphaltene fraction were selected as modelmore » compounds. Generally, asphaltene samples display poor sensitivity when analyzed by carbon-13 NMR, so this fraction was chosen to examine the abilities of the two subroutines under non-ideal conditions. As well, the effect of line broadening factors on the PNA results was studied. Line broadening will increase the signal to noise ratio, but also leads to a decrease in resolution.« less