Size exclusion chromatography of aromatic compounds in high-boiling petroleum samples

Knowledge of aromatic compounds in petroleum samples is of paramount interest from processing and environmental viewpoints. Considering the complexity of such samples, a selective analytical strategy viz. aromatic specific size exclusion chromatographic methodology using amino-bonded silica phase, was adopted to obtain the average molecular weights of distilled cuts, residue and the corresponding crude oil. The method was found to be suitable from middle distillates up to the residue. It was found that the increase in boiling temperature of distilled cuts was reflected in an increase in high molecular weight aromatic species. The increase in the molecular weight of aromatic species is linked to the increase in alkylation as verified by studying the elution pattern of alkylated aromatic compounds. The accuracy of the molecular weights obtained from the size exclusion chromatographic method depends on the elution profile of aromatic species in petroleum samples. It was found that the size and shape of the parent aromatic compounds negatively influence the size exclusion elution profile. However, such influence can be assumed to be negligible as all the aromatic species of each distilled cut, residue, and crude oil eluted in the permeation zone of the chromatogram. This method has the potential to be used in the second dimension of a 2D-LC method where the aromatic compounds are separated based on the size (or alkylation). The first dimension will separate compounds based on the number of aromatic rings, thereby eliminating the interference of parent aromatic compounds of different sizes in the total elution profile.

Simultaneous determination of the styrene unit content and assessment of molecular weight of triblock copolymers in adhesives by a size exclusion chromatography method

The content of styrene units in nonhydrogenated and hydrogenated styrene-butadiene-styrene and styrene-isoprene-styrene triblock copolymers significantly influences product performance. A size exclusion chromatography method was developed to determine the average styrene content of triblock copolymers blended with tackifier in adhesives. A complete separation of the triblock copolymer from the other additives was realized with size exclusion chromatography. The peak area ratio of the UV and refraction index signals of the copolymers at the same effective elution volume was correlated to the average styrene unit content using nuclear magnetic resonance spectroscopy with commercial copolymers as standards. The obtained calibration curves showed good linearity for both the hydrogenated and nonhydrogenated styrene-butadiene-styrene and styrene-isoprene-styrene triblock copolymers (r = 0.974 for styrene contents of 19.3-46.3% for nonhydrogenated ones and r = 0.970 for the styrene contents of 23-58.2% for hydrogenated ones). For copolymer blends, the developed method provided more accurate average styrene unit contents than nuclear magnetic resonance spectroscopy provided. These results were validated using two known copolymer blends consisting of either styrene-isoprene-styrene or hydrogenated styrene-butadiene-styrene and a hydrocarbon tackifying resin as well as an unknown adhesive with styrene-butadiene-styrene and an aromatic tackifying resin. The methodology can be readily applied to styrene-containing polymers in blends such as poly(acrylonitrile-butadiene styrene).