Methodology for the derivatization and quantification of dialkyl phosphate esters in petroleum samples

Abstract

Dialkyl phosphate esters are used as gellants in some oil well fracturing processes for conventional oil production. Residual amounts of these compounds that remain in the well represent a source of potential contamination of the crude oil produced from the well. This contamination results in fouling of refinery equipment. Current methodologies for the analysis of alkyl phosphates have a relatively poor detection limit (0.5 AE 1.0 mg mL À1 ) and provide no speciation information: only a total phosphorus concentration. Herein, we present an approach that permits speciation of individual alkyl phosphates and quantification of individual compounds with limits of detection and precision that represent an improvement on the existing methodology. The method that we present relies upon derivatization of dialkyl phosphates to their trimethylsilane (TMS) esters followed by a comprehensive two-dimensional gas chromatographic separation. Two dialkyl phosphate esters, dibutyl and bis(2ethylhexyl) phosphate were used as model compounds. Recovery of spiked samples in pure solvent and a model petroleum mixture ranged from 92% to 120%. This approach represents a significant first step in the development of an analytical solution to the challenge of phosphorus contamination facing the oil industry.

Profiling Alkyl Phosphates in Industrial Petroleum Samples by Comprehensive Two-Dimensional Gas Chromatography with Nitrogen Phosphorus Detection (GC × GC–NPD), Post-column Deans Switching, and Concurrent Backflushing

“…8,15−23 For the speciation of di-and trialkyl phosphates in petroleum samples, we first introduced a method based on trimethylsilylation derivatization followed by GC × GC separation using a flame ionization detector (FID). 24 Although this analytical approach yielded quantification limits and a level of precision that exceeded the capabilities of the currently accepted ICP− OES methodology, our research demonstrated the necessity for selective detection in this application. We achieved selective detection through the use of time-of-flight mass spectrometry (TOFMS), 25 and overall, this technique was capable of both speciation and quantification of alkyl phosphates in petroleum samples with better precision and lower limits of quantification when compared to ICP−OES.…”

“…For this situation, a NPD is a better choice, although it requires a Deans switch for detector selection to divert excess silylation reagents at the start of a chromatogram to a FID . This protects the NPD bead, which would be rapidly (<10 injections) passivated with an inert layer of SiO 2 when exposed to excess silylation reagents. , Using the Deans switch after the secondary column in GC × GC for detector selection extended the lifetime of the NPD bead from approximately 5 injections to over 1000, performed over the course of 15 months . The resulting GC × GC–FID/NPD system offered an approach that was capable of both quantifying and speciating individual alkyl phosphate compounds in industrial petroleum samples.…”

“…Triphenyl phosphate was added to each solution as an internal standard at a concentration of 0.5 μg mL −1 for normalization of the resulting peaks areas. As described in our previously established protocol, 24 50 μL of derivatization reagent mixture was added to 500 μL aliquots of each sample in GC vials. The vials were then sealed, shaken, left to derivatize for 30 min at 30 °C, and then analyzed in triplicate using GC × GC−FID/ NPD.…”

“…All reagents were used as received, unless otherwise stated. Derivatization was performed according to a previously established protocol using a mixture of N , O -bis(trimethylsilyl)trifluoroacetamide (BSTFA) (Sigma-Aldrich, Oakville, Ontario, Canada), pyridine (Caledon, Georgetown, Ontario, Canada), and trimethylchlorosilane (TMCS) (Sigma-Aldrich) in a 10:5:2 ratio by volume. A total of 50 μL of the derivatization reagent mixture was added to 500 μL of sample and left to derivatize at 30 °C for at least 30 min before analysis.…”

“…For the speciation of di- and trialkyl phosphates in petroleum samples, we first introduced a method based on trimethylsilylation derivatization followed by GC × GC separation using a flame ionization detector (FID) . Although this analytical approach yielded quantification limits and a level of precision that exceeded the capabilities of the currently accepted ICP–OES methodology, our research demonstrated the necessity for selective detection in this application.…”