Comment on “Comprehensive Profiling of Coal Tar and Crude Oil to

Correspondence/Rebuttal pubs.acs.org/est

Comment on “Comprehensive Profiling of Coal Tar and Crude Oil to Obtain Mass Spectra and Retention Indices for Alkylated PAH Shows Why Current Methods Err”

A

isomers resultsa factor of 10 for this sediment. However, considering retention times eliminates overestimation and obtains accurate results for the C4-alkylnaphthalene isomers. In addition, C4-alkylnaphthalenes show a significant ion at m/z 169 (except for the butyl isomers, which are not significant), while none of the thiophene-type species show that ion. Conversely, the thiophene isomers show a significant ion at m/ z 139, whereas none of the C4-alkylnaphthalenes display that ion, allowing simple verification of compound class. Similar solutions apply to the other alkyl PAHs shown as “overestimates” and “false positives” in their Table 1. We also find it surprising that the methylnaphthalene isomers were listed as “false positives” in their Table 1. Since there are only two methyl naphthalene isomers, both are available as pure standards, and both separate easily using GC from themselves and interfering species that show the SIM ion used (m/z 142), we see no valid reason for reporting them as “false positives.” The paper implies that only GC×GC/MS can yield valid results, but the method is impractical since each sample requires 50 injections (ca. 3 days). However, correspondence with the authors clarified their intent was to require GC/MS for PAH-34 analyses rather than GC/MS-SIM.6 Although scan GC/MS is desirable, it cannot meet detection limits required for PAH-34 analyses, that is, the concentration of each PAH corresponding to 1/34th of a toxic unit in the EPA’s narcosis model.4,7 To illustrate, we analyzed three different sediment extracts twice, once using SIM and once in scan mode (m/z 50−300). All analyses were performed on an Agilent Model 5973 GC/MS (a common model used in contract laboratories) which had just undergone full GC and MS maintenance. For all PAH-34 species, the signal-to-noise ratios obtained using SIM were ca. 50−200-fold higher than those obtained using the scan mode. Detection limits in the scan mode were similarly higher, and only GC/MS-SIM could attain the sensitivities required for PAH-34 determinations. Finally, the most important question is, “does a method give data quality sufficient to support its interpretation and use?” The ultimate purpose of EPA-34 analyses is to predict toxicity to benthic organisms, for example, by correlating results with Hyalella azteca mortality.5,7 Pure compound studies with fluoranthene report the median lethal residue value is 26−40 μmol/g lipid (95% CI).8 Our original set of 97 field samples yielded a value of 31−35 μmol/g lipid (95% CI) with an overall prediction efficiency of 90% 5a remarkable agreement between controlled lab studies and a large set of field samples. (Similar agreement has since been found for 167 additional sediments.) Thus, GC/MS-SIM, used properly, provides high quality data sufficient for its ultimate usethat is, predicting the toxicity of sediments based on the EPA’s PAH-34 list.

recent report using two-dimensional gas chromatography coupled with mass spectrometry (GC×GC/MS) concluded, “Currently employed SIM/SIE methods produce inaccurate and imprecise measurements, which lead to faulty toxicological assessments and, ultimately, risk-based decisions.”1 We find that their conclusion that GC/MS methods with selected ion monitoring (GC/MS-SIM) “produce inaccurate and imprecise measurements...” is unjustified. The paper reports that GC/MS-SIM leads to gross overestimations of alkyl PAHs in the EPA-34 list of 18 parent and 16 groups of alkyl PAHs. Although this is possible if the analyst includes interfering species detected at the same m/z as target species, simple retention time information and periodic checking of full-scan data (as previously recommended2−4) can easily be used to eliminate such overestimations. For example, the ASTM method for freely dissolved PAHs includes SIM chromatograms with brackets showing the target species, and nontarget peaks marked with an “X.” 4 Consider the “worst case” presented in ref 1, that is, the 6fold overestimation of C4-alkylnaphthalene isomers in coal tar. Figure 1 shows a GC/MS-SIM chromatogram (molecular ion m/z 184) from a typical coal tar-impacted sediment. Bracketed peaks are the C4-alkylnaphthalene isomers, while those marked with “X” are dibenzothiophene and naphthothiophene isomers. When the thiophenes are mistakenly included in the integration (as done in ref 1), gross overestimation of the C4-naphthalene

Figure 1. C4-alkyl naphthalene isomers (bracket) and nontarget dibenzothiophene/naphthothiophene isomers (“X”) in the GC/MSSIM chromatogram at m/z 184 from a coal tar-contaminated sediment extract. © 2012 American Chemical Society

Published: September 13, 2012 11475

dx.doi.org/10.1021/es302826v | Environ. Sci. Technol. 2012, 46, 11475−11476

Environmental Science & Technology

Correspondence/Rebuttal

(9) Schantz, M.; Wise, S. , National Institute of Standards and Technology, personal communication, May 2012.

A solution to such analytical disagreements will soon be available.9 The U.S. National Institute of Standards and Technology (NIST) is certifying PAH-34 concentrations on a mixed coal tar/petroleum nonaqueous phase liquid (NAPL). Two years before NIST obtained the sample we used the same diluted NAPL as a test sample for the round-robin lab study required for the ASTM porewater PAH method.4 Seven independent operators in three contract laboratories and one Civil Engineering department determined PAH-34 concentrations using GC/MS-SIM and the principals described above. Reported concentrations agree well with the values NIST determined nearly three years later.9 For example, the mean concentrations determined by the seven operators for the “worst case” species (the C4-alkylnaphthalenes and C4alkylphenanthrene/anthracenes) were 31.5 and 9.1 μg/mL, respectively, which is good agreement with the preliminary NIST values of 32.6 and 10.2 μg/mL, respectively. This good agreement demonstrates that GC/MS-SIM can accurately determine the parent and alkyl PAHs on the EPA-34 list, and suggest that any future related studies utilize the new SRM 1991 to evaluate other analytical approaches.

Steven B. Hawthorne* David J. Miller

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Energy and Environmental Research Center, University of North Dakota, 15 North 23rd Street, Grand Forks, North Dakota, United States, 58202

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Zeigler, C.; Robbat, A. Comprehensive profiling of coal tar and crude oil to obtain mass spectra and retention indices for alkylated PAH shows why current methods err. Environ. Sci. Technol. 2012, 39, 2795−2803. (2) Hawthorne, S. B.; Miller, D. J.; Kreitinger, J. P. Measurement of “total” PAH concentrations and toxic units used for estimating risk to benthic invertebrates at manufactured gas plant sites. Environ. Toxicol. Chem. 2006, 46, 3935−3942. (3) Hawthorne, S. B.; Grabanski, C. B.; Miller, D. J.; Kreitinger, J. P. Solid-phase microextraction measurement of parent and alkyl polycyclic aromatic hydrocarbons in milliliter sediment pore water samples and determination of KDOC values. Environ. Sci. Technol. 2005, 39, 2795−2803. (4) ASTM Standard Test Method D 7363-11. Standard Test Method for Determination of Parent and Alkyl Polycyclic Aromatics in Sediment Pore Water Using Solid-Phase Microextraction and Gas Chromatography/ Mass Spectrometry in Selected Ion Monitoring Mode; ASTM International: West Conshocken, PA, 2011. (5) Hawthorne, S. B.; Azzolina, N. A.; Neuhauser, E. F.; Kreitinger, J. P. Predicting bioavailability of sediment polycyclic aromatic hydrocarbons to Hyalella azteca using equilibrium partitioning, supercritical fluid extraction, and pore water concentrations. Environ. Sci. Technol. 2007, 41, 6297−6304. (6) Robbat, A. Jr.; personal communication, June, 2012. (7) Procedures for the Derivation of ESBs for the Protection of Benthic Organisms: PAH Mixtures, EPA/600/R-02/013; Office of Research and Development: Washington D.C., 2003. (8) Schuler, L. J.; Landrum, P. F.; Lydy, M. Comparative toxicity of fluoranthene and penatachlorobenzene to three freshwater invertebrates. Environ. Toxicol. Chem. 2006, 25, 985−994. 11476

dx.doi.org/10.1021/es302826v | Environ. Sci. Technol. 2012, 46, 11475−11476