Correspondence/Rebuttal pubs.acs.org/est
Response to 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”
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small data set the responders cite might be consistent (due to training of the participants),6 but it does not mean the data are accurate. We believe a method should be standalone and not require coaching to achieve acceptable quality data. For example, even within the same release, different microenvironments can arise that produce varying hydrocarbon background profiles7 and peak patterns due to differences in organisms and other environmental factors from one location to the next.8 Since most commercial laboratories rely on data analysis software to quantify target compounds, our work provides the analyst with the retention windows, ions and relative abundances, and the criteria to deconvolve MFPPH data by SIM or full scan MS to obtain defensible data. The finding of “false positive” for methylnaphthalenes in a highly weathered sample is not surprising, since these compounds are volatile. Nonetheless, the disagreement over whether C1 naphthalenes are present in the sample speaks directly to the issue of data quality. We agree the methylnaphthalenes separate well from one another. A trained analyst is easily able to identify these target compounds using their spectral pattern. However, multiple peaks were found within the 1 min retention window based solely on the molecular ion. In this sample none of the peaks exhibited the correct fragmentation pattern, which, along with other homologue data, supports the conclusion that the composition of the sample had significantly changed from that of fresh oil. Confirmation of single ion detection to determine which (if any) peaks are C1 naphthalenes is not possible. Finally, with respect to single versus MFPPH ion detection limits,9 forthcoming research will show MFPPH by SIM produces ∼5 ppb detection limits for parent PAH and a 0.99 linear response from 10 ppb upward of 100 ppm. These detection limits are well below the 100−200 ppb threshold of 1 /34th of the final chronic value for sediment/soil containing 1% total organic carbon as discussed by Hawthorne et al.10 Moreover, single ion detection per homologue only lowers the detection limit and improves sensitivity compared to MFPPHSIM by ∼2.4-fold. We leave it to the reader to decide if detection by a single ion per homologue is sufficient to produce defensible data as compared to full scan MS or SIM analysis using MFPPH and spectral deconvolution.
he comment posited by Hawthorne and Miller in response to our paper asks: is it good enough for “a method [to] give data quality sufficient to support its interpretation and use” or should the method produce the correct information needed to answer the scientific and engineering question under study? The responders believe single ion detection meets the EPA’s criterion of a performance based method and, thus, provides sufficient data quality to assess whether a given sample is toxic or not.1 The responders further state: the “ultimate purpose of EPA-34 analyses is to predict toxicity to benthic organisms, for example, by correlating results with Hyalella azteca mortality.” Other investigators use SIM data to answer the question of whether oil and/or coal tar contaminated soils and sediments have weathered sufficiently to require no further action (cleanup), to apportion liability, and to defend a position in a court of law. For the latter, single ion detection is indefensible since insufficient data are produced to determine if peak identity and peak area assignments are made correctly, especially when the matrix complicates such assignments. In situations where SIM data is on the cusp of exceeding ∑ESBTUFCV, that is, on the edge of “toxic,” overestimation due to false positives or additive matrix/target compound signals causes actions to occur that diverts funds from areas of concern to areas that pose little to no environmental damage, with law suits arising that could be avoided. With this in mind, we understood analysts use different retention windows, and some a confirming ion to produce alkylated PAH concentrations. We also knew that isomers within the same homologue produced different fragmentation patterns and feared some PAH might be missed due to relative abundance differences in their spectral patterns. Prior to our work, little research was available documenting how polycyclic aromatic sulfur heterocycles (PASH), a major matrix interferent, eluted or how they fragmented under EI conditions. Since standards for the hundreds of alkylated PAH and PASH are not available, we used GC-GC/MS (not GC×GC/MS as responders continue to misstate) to obtain this information from actual fresh and weathered oil and coal tar samples. Our goal was to develop a legally defensible method that would provide correct assessments of PAH and PASH toxicity, fate, and transport. We showed single ion compared to MFPPH detection produces substantial differences in homologue concentrations due to matrix interferences.2−5 Given the vast number of scientists that rely on PAH and PASH data worldwide, training analysts to recognize peak patterns and eliminate interferences is a difficult process to standardize, since patterns and interferences may change dramatically from one sample to the next. The responders suggest that their round-robin study, based on a NIST SRM sample, shows single ion detection provides data quality sufficient to answer site-specific questions. We argue that the © 2012 American Chemical Society
Christian D. Zeigler Albert Robbat*
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Department of Chemistry, Tufts University, 62 Talbot Avenue Medford, Massachusetts, United States 02155
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Published: October 3, 2012 11477
dx.doi.org/10.1021/es3035472 | Environ. Sci. Technol. 2012, 46, 11477−11478
Environmental Science & Technology
Correspondence/Rebuttal
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Nicholas Wilton produced the MFPPH-SIM data. REFERENCES
(1) Procedures for the Derivation of ESBs for the Protection of Benthic Organisms: PAH Mixtures, ; Office of Research and Development: Washington D.C, 2003. (2) Zeigler, C. D.; 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, 46 (7), 3935−3942, DOI: 10.1021/es2030824. (3) Zeigler, C.; MacNamara, K.; Wang, Z.; Robbat, A., Jr Total alkylated polycyclic aromatic hydrocarbon characterization and quantitative comparison of selected ion monitoring versus full scan gas chromatography/mass spectrometry based on spectral deconvolution. J. Chromatogr., A 2008, 1205 (1−2), 109−116, DOI: 10.1016/ j.chroma.2008.07.086. (4) Zeigler, C.; Schantz, M.; Wise, S.; Robbat, A. Mass spectra and retention indexes for polycyclic aromatic sulfur heterocycles and some alkylated analogs. Polycyclic Aromat. Compd. 2012, 32 (2), 154−176, DOI: 10.1080/10406638.2011.651679. (5) Zeigler, C. D.; Wilton, N.; Robbat, A. J. Toward the accurate analysis of C1 to C4 polycyclic aromatic sulfur heterocycles. Anal. Chem. 2012, 84, (5), 2245-2252; DOI http://dx.doi.org/10.1021/ ac202845x. (6) Hawthorne, S. personal communication, June 2012. (7) Bence, A. E.; Kvenvolden, K. A.; Kennicutt, M. C. Organic geochemistry applied to environmental assessments of Prince William Sound, Alaska, after the Exxon Valdez oil spill–a review. Org. Geochem. 1996, 24 (1), 7−42, DOI: 10.1016/0146-6380(96)00010-1. (8) Wardlaw, G. D.; Nelson, R. K.; Reddy, C. M.; Valentine, D. L. Biodegradation preference for isomers of alkylated naphthalenes and benzothiophenes in marine sediment contaminated with crude oil. Org. Geochem. 2011, 42 (6), 630−639. (9) Definition and Procedure for the Determination of the Method Detection Limit, EPA, CFR Title 40, Part 136, Appendix B. Revision 1.11; Washington DC, 2011. (10) Hawthorne, S. B.; Miller, D. J.; Kreitinger, J. P. Measurement of total polycyclic aromatic hydrocarbon concentrations in sediments and toxic units used for estimating risk to benthic invertebrates at manufactured gas plant sites. Environ. Toxicol. Chem. 2006, 25 (1), 287−296, DOI: 10.1897/05-111r.1.
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