One-Step Method for Separation and Identification of <i>n</i>-Alkanes/Oligomers in HDPE Using High-Temperature High-Performance Liquid Chromatography

Abstract

n-Alkanes are often an important ingredient of waxes, oils, gasoline, and a byproduct in polyolefins. Separation and identification of alkanes in commercial high-density polyethylene (HDPE) are of vital importance from various points of view. Currently used chromatographic techniques require extraction and laborious preconcentration. Porous graphite Hypercarb as a stationary phase in high-performance liquid chromatography (HPLC) in combination with a mobile phase gradient n-decane → 1,2-dichlorobenzene (ODCB) enables a selective separation of C40–C160 n-alkanes at temperatures between 100 and 160 °C. Thus, it is possible to resolve the individual alkanes in HDPE of 1 kg/mol. An unambiguous assignment of the individual homologues in the chromatogram was possible by doping the HDPE with a C60 standard, and the results were also confirmed by MALDI TOF. Using this method, the presence of alkanes in a HDPE sample with a weight-average molar mass of 70 kg/mol could be shown. n-Hexane as isocratic mobile phase enables to separate and identify n-alkanes in the range C18–C36 in a mixture at a temperature of 50 °C while alkanes with lower carbon numbers are not detected with the evaporative light scattering detector used in this study. The newly developed HPLC system allows separation and identification of alkanes in model blends as well as in high molecular weight HDPE without any prior extraction and work-up. The upper limit of detection with regard to carbon number is superior to the traditionally used gas chromatography.