Environmental Forensics: Why Current GC/MS Methods Produce Inaccurate Concentrations of Alkylated Polycyclic Aromatic Hydrocarbons (PAH) and Sulfur Heterocycles (PASH)
Oral Presentation
Prepared by N. Wilton, A. Robbat, Jr.
Tufts University, 62 Talbot Ave, Chemistry Department, Medford, MA, 02155, United States
Contact Information: nickdmzwilton@gmail.com; 617-584-5041
ABSTRACT
Polycyclic aromatic hydrocarbons (PAH) and sulfur heterocycles (PASH) are ubiquitous organic pollutants, whose unimpeachable quantitation is vital to hazardous waste site investigation and remediation, as well as toxicological and forensic studies. The analysis of parent and alkylated PAH and PASH is an essential component of assessing environmental risks and associated liability from sources of pollution that contain these compounds. We have proven that the standard gas chromatography/mass spectrometry (GC/MS) methods used to quantify the C1 - C4 homologues of PAH and PASH are subject to systematic errors based on lab-specific retention windows. We showed, for instance, that operating the MS in selected ion monitoring (SIM) mode to acquire one ion per homologue (SIM/1-ion) results in overestimation of some homologues by hundreds of percent.
Our findings make evident that analysis of alkylated PAH by either full scan or SIM detection, where multiple fragmentation patterns per homologue (SIM/MFPPH) are used, do not suffer from matrix effect interferences. Many PAH and PASH have the same quantitative ions and, importantly, elute within the same retention windows. For example, C4 naphthalene and C4 phenanthrene have the same molecular ions as 3-ring and 4-ring PASH, respectively. SIM/1-ion, even SIM/2-ion detection methods produce overestimated PAH and PASH concentrations because the same ion signal is counted twice unless three or more ions are used to differentiate target compounds as is the case with parent PAH. Especially problematic are low-level analytes such as C4 phenanthrene, whose concentration in coal tar is far below that of 4-ring PASH. Falsely identifying the latter for the former yields overestimated homologue concentrations, misleading pollution profiles, and poor interlaboratory comparison studies as lab-specific retention windows and assignments of peaks by pattern recognition highly bias the results.
Results of our combinatorial GC-GC/MS library building process will be described, which will elucidate how the MFPPH fragmentation patterns and ions were established. New spectral deconvolution software for analyzing GC/MS data employing SIM/MFPPH detection eliminates the practice of assigning homologue peaks by pattern recognition within lab-defined retention windows. Findings will show that SIM/MFPPH offers the same sensitivity as SIM/1-ion methods but with the selectivity needed to provide accurate, legally defensible data.
Acknowledgement
The Electric Power Research Institute (EPRI) under contract EP-P39203/C17417 provided partial support for this work.
References
A New Spectral Deconvolution – Selected Ion Monitoring Method for the Analysis of Alkylated Polycyclic Aromatic Hydrocarbons in Complex Mixtures, A. Robbat Jr. and N.M. Wilton, Talanta 125, 114–124, 2014, http://dx.doi.org/10.1016/j.talanta.2014.02.068
More Accurate Analysis of Alkylated PAH and PASH and its Implications in Environmental Forensics, P.M. Antle, C.D. Zeigler, N.M. Wilton and A. Robbat, Jr., Intern. J. Environ. Anal. Chem., 94 (4), 332-347, 2014, http://dx.doi.org/10.1080/03067319.2013.840886
New Spectral Deconvolution Algorithms for the Analysis of Polycyclic Aromatic Hydrocarbons and Sulfur Heterocycles by Comprehensive Two-Dimensional Gas Chromatography-Quadrupole Mass Spectrometery, P.M. Antle, C.D. Zeigler, Y. Gankin, and A. Robbat, Jr., Analytical Chemistry, 85, 10369-10376, 2013, http://dx.doi.org/10.1021/ac402336j
Mass Spectra and Retention Indexes for Polycyclic Aromatic Sulfur Heterocycles and Some Alkylated Analogs C. Zeigler, M. Schantz, S. Wise and A. Robbat, Jr., Polycyclic Aromatic Compounds 32(2), 154-176, 2012, http://dx.doi.org/10.1080/10406638.2011.651679
Oral Presentation
Prepared by N. Wilton, A. Robbat, Jr.
Tufts University, 62 Talbot Ave, Chemistry Department, Medford, MA, 02155, United States
Contact Information: nickdmzwilton@gmail.com; 617-584-5041
ABSTRACT
Polycyclic aromatic hydrocarbons (PAH) and sulfur heterocycles (PASH) are ubiquitous organic pollutants, whose unimpeachable quantitation is vital to hazardous waste site investigation and remediation, as well as toxicological and forensic studies. The analysis of parent and alkylated PAH and PASH is an essential component of assessing environmental risks and associated liability from sources of pollution that contain these compounds. We have proven that the standard gas chromatography/mass spectrometry (GC/MS) methods used to quantify the C1 - C4 homologues of PAH and PASH are subject to systematic errors based on lab-specific retention windows. We showed, for instance, that operating the MS in selected ion monitoring (SIM) mode to acquire one ion per homologue (SIM/1-ion) results in overestimation of some homologues by hundreds of percent.
Our findings make evident that analysis of alkylated PAH by either full scan or SIM detection, where multiple fragmentation patterns per homologue (SIM/MFPPH) are used, do not suffer from matrix effect interferences. Many PAH and PASH have the same quantitative ions and, importantly, elute within the same retention windows. For example, C4 naphthalene and C4 phenanthrene have the same molecular ions as 3-ring and 4-ring PASH, respectively. SIM/1-ion, even SIM/2-ion detection methods produce overestimated PAH and PASH concentrations because the same ion signal is counted twice unless three or more ions are used to differentiate target compounds as is the case with parent PAH. Especially problematic are low-level analytes such as C4 phenanthrene, whose concentration in coal tar is far below that of 4-ring PASH. Falsely identifying the latter for the former yields overestimated homologue concentrations, misleading pollution profiles, and poor interlaboratory comparison studies as lab-specific retention windows and assignments of peaks by pattern recognition highly bias the results.
Results of our combinatorial GC-GC/MS library building process will be described, which will elucidate how the MFPPH fragmentation patterns and ions were established. New spectral deconvolution software for analyzing GC/MS data employing SIM/MFPPH detection eliminates the practice of assigning homologue peaks by pattern recognition within lab-defined retention windows. Findings will show that SIM/MFPPH offers the same sensitivity as SIM/1-ion methods but with the selectivity needed to provide accurate, legally defensible data.
Acknowledgement
The Electric Power Research Institute (EPRI) under contract EP-P39203/C17417 provided partial support for this work.
References
A New Spectral Deconvolution – Selected Ion Monitoring Method for the Analysis of Alkylated Polycyclic Aromatic Hydrocarbons in Complex Mixtures, A. Robbat Jr. and N.M. Wilton, Talanta 125, 114–124, 2014, http://dx.doi.org/10.1016/j.talanta.2014.02.068
More Accurate Analysis of Alkylated PAH and PASH and its Implications in Environmental Forensics, P.M. Antle, C.D. Zeigler, N.M. Wilton and A. Robbat, Jr., Intern. J. Environ. Anal. Chem., 94 (4), 332-347, 2014, http://dx.doi.org/10.1080/03067319.2013.840886
New Spectral Deconvolution Algorithms for the Analysis of Polycyclic Aromatic Hydrocarbons and Sulfur Heterocycles by Comprehensive Two-Dimensional Gas Chromatography-Quadrupole Mass Spectrometery, P.M. Antle, C.D. Zeigler, Y. Gankin, and A. Robbat, Jr., Analytical Chemistry, 85, 10369-10376, 2013, http://dx.doi.org/10.1021/ac402336j
Mass Spectra and Retention Indexes for Polycyclic Aromatic Sulfur Heterocycles and Some Alkylated Analogs C. Zeigler, M. Schantz, S. Wise and A. Robbat, Jr., Polycyclic Aromatic Compounds 32(2), 154-176, 2012, http://dx.doi.org/10.1080/10406638.2011.651679
