Application of individual response factors for accurate quantitation of alkylated PAH homologs in complex environmental samples using gas chromatography/triple quadrupole mass spectrometry (GC-MS/MS)

Alkylated polycyclic aromatic hydrocarbons (alkyl PAHs) are abundant in the environment. They can be more toxic and persistent than their non-alkylated parent PAH compounds, and thus alkyl PAHs can pose an increased threat to environmental quality. However, the US EPA has not listed alkyl PAHs as priority pollutants, which is primarily due to a lack of reliable and accurate analytical method for routine identification and quantification of alkyl PAHs in complex environmental samples. A multiple reaction monitoring (MRM) method in GC-MS/MS has been proposed for more accurate quantification of target alkyl PAHs in complex sample matrices. However, the lack of certified analytical standards for alkyl PAH isomers and the historic use of relative response factors (RFs) of parent PAH to quantify its alkylated PAH isomers can compromise the quantitative results obtained from MRM analyses. In this study, we used commercially available parent and alkyl PAH standards to develop and optimize both SIM and MRM analytical methods. The optimized methods were used to determine and compare RFs of alkyl PAHs to their respective parent PAHs in both SIM and MRM modes. The results showed that the conventional approach of quantifying alkyl PAHs by using the RFs of corresponding parent PAHs underestimates alkyl PAHs by 1.2-6.4 (2.2 +/- 1.2) and 1.2-6.7 (3.4 +/- 2.0) times, respectively, for SIM and MRM analyses. The results further demonstrated that the use of RFs of parent PAHs to quantify their alkylated homologs affects MRM-based analysis more than SIM-based analysis, underestimating the alkyl PAHs in MRM mode to as low as only 15% of the actual concentrations obtained by using individual RFs for each alkylated PAH homologs. The results showed that a GC-MS/MS in MRM mode, with baseline separation and higher selectivity, is a powerful analytical tool for the analysis of alkyl PAHs in complex matrices, however, the use of at least one of the individual RFs for target PAH analyte clusters is critical for accurate quantitative MRM analysis. An accurate quantification of alkyl PAHs is important for determining their transport, fate, and environmental health impacts for next generation of oil spill studies as well as for updating existing EPA priority pollutant list by including alkyl PAHs in the list.

Automated summary

Alkylated polycyclic aromatic hydrocarbons (alkyl PAHs) are more toxic and persistent than non-alkylated parent PAH compounds, posing a greater threat to environmental quality. The lack of reliable analytical methods for quantifying alkyl PAHs has hindered their inclusion as priority pollutants by the US EPA. This study demonstrates that using relative response factors of parent PAHs to quantify alkylated PAH isomers can lead to underestimation of alkyl PAHs, particularly in MRM-based analyses.