The analysis of alkyl phosphates in nitrogen-rich crude oils using GC × GC-NPD with a polar/apolar column configuration

Abstract

Alkyl phosphates in crude oils originate from their use as gellants used in some hydraulic fracturing processes or as additives used to improve physical properties of the oil for transportation. Alkyl phosphates are a potential issue for used oil re-refining and recycling operations because of their use as flame-retardants in, for example, aviation hydraulic fluids. The primary concern with alkyl phosphates in the refinery is their association with refinery fouling, including deposit formation and catalyst deactivation. A comprehensive two-dimensional gas chromatography method, with nitrogen phosphorus detection (GC × GC-NPD) and post-column Deans switching, was previously developed by our group for quantitative analysis of alkyl phosphates in petroleum samples. The original method employed an apolar/polar GC × GC column configuration and was shown to provide trace speciation of alkyl phosphates in distillate samples and some crude oils. However, when petroleum samples having a high concentration of heavy nitrogen-containing species were encountered, coelutions with alkyl phosphates ensued. Even in a phosphorus-optimized NPD configuration, the background signal from nitrogen-containing species masked the signals of some alkyl phosphates. Thus, a new column configuration using a polar/apolar GC × GC column set was developed. The results of this study are presented, showing the applicability of this method to the analysis of alkyl phosphates in petroleum samples with a high concentration of nitrogen-containing species. This method effectively separates the alkyl phosphates from nitrogen containing species with which they previously co-eluted. Additionally, it allows for trace speciation and quantification of alkyl phosphates in crude-oil with detection limits ranging from 0.001 to 0.02 μg phosphorus per mL.