CORRESPONDENCE/REBUTTAL pubs.acs.org/est
Response to Comment on “HILIC-NMR: Toward the Identification of Individual Molecular Components in Dissolved Organic Matter”
W
e thank Dr. Blough and Dr. Del Vecchio for their comments.1 The authors very much agree that these signals are not due to the fluorescence of the model quinones used but that instead that the components indicated are somehow affected by quinones as the fluorescence intensities are altered by quinone addition without changing loadings (i.e., excitation and emission spectra). We further apologize for not expressing this more clearly as well as not clarifying a particular sentence in the text. “Correlation data indicate that all quinones...”2 should more specifically read “Correlation data indicate that all quinoneinfluenced signals.” Taken out of context this sentence is certainly misleading but the remainder of the text utilizes the phrase “quinone-influenced” and the authors have clearly stated that “quinones are not attempted to be identified” with this parallel factor analysis (PARAFAC).2 We would further like to point out that excitation emission matrices (EEMs) and PARAFAC were not the primary focus of this paper and that our objectives were first and foremost to explore hydrophilic interaction chromatography (HILIC) with nuclear magnetic resonance (NMR) spectroscopy for application to organic matter. Fluorescence-based approaches are now widely used in environmental research and contain a range of useful information.3 6 Extracting distinct fluorescence signals, however, and relating components to chemical structure is at present challenging. Therefore we hoped that additional structural information afforded by NMR and HILIC separation may now, or in the future, be useful in explaining the congested fluorescence data sets. In simple terms we wanted to ask “can HILIC-NMR help explain fluorescence characteristics commonly seen in DOM”? We did not aim to promote or discredit these fluorescence-based methods in any way. The limitations of PARAFAC have been addressed previously,3,4 but the technique is growing in application to humic substances and similarities have been drawn between various researches such that source and diagenetic processes appear to be linked to broadly similar PARAFAC components.4 6 Aside from method standardization, the limitations also are likely to include (a) PARAFAC components represent a number of different fluorophores, (b) complex energy transfers (e.g., charge transfer, excited-state electron transfer) are occurring, complicating both the location and intensity of excitation and emission bands, and (c) ultimately mathematical modeling is applied to look for patterns within the excitation emission matrices. The resulting “spectra” should certainly be regarded as the extrapolation of simpler phenomena from a much more complex fluorescence emission and very likely do not resemble standards run alone (which were run but not reported in conjunction with spiked samples during analysis of this study). The PARAFAC model is very sensitive to minor additions of a standard and the authors hoped to spark further interest in applying standards to PARAFAC modeling. Among the most interesting finds in this preliminary analysis is that some standards would affect multiple components and can perhaps lend insight into energy relationships between varying components. r 2011 American Chemical Society
The complexity of humic substances is such that a variety of approaches are necessary to understand the molecular characteristics, sources and fate of this material in the environment. New techniques as well as attempts to link data from varying approaches are of use in understanding the overlapping and often nondistinct signals provided by many analytical methods and arguably critical to the future of the field. Gwen Woods and Andre Simpson* Department of Chemistry, Univerisity of Toronto, Toronto, Ontario, Canada M1C 1A4
’ REFERENCES (1) Blough, N. V.; Del Vecchio, R. Comment on “HILIC-NMR: Toward the identification of individual molecular components in dissolved organic matter. Environ. Sci. Technol. 2011, DOI: 10.1021/ es201531v (2) Woods, G. C.; Simpson, M. J.; Koerner, P. J.; Napoli, A.; Simpson, A. J. HILIC-NMR: Toward the identification of individual molecular components in dissolved organic matter. Environ. Sci. Technol. 2011, 45, 3880–3886. (3) Stedmon, C. A.; Bro, R. Characterizing dissolved organic matter fluorescence with parallel factor analysis: a tutorial. Limnol. Oceanogr. Methods 2008, 6, 572–579. (4) Murphy, K. R.; Hambly, A.; Singh, S.; Henderson, R. K.; Baker, A.; Stuetz, R.; Khan, S. J. Organic matter fluorescence in municipal water recycling schemes: Toward a unified PARAFAC model. Environ. Sci. Technol. 2011, 45, 2909–2916. (5) Walker, S. A.; Amon, R. M. W.; Stedmon, C.; Duan, S.; Louchouarn, P. The use of PARAFAC modeling to trace terrestrial dissolved organic matter and fingerprint water masses in coastal Canadian Arctic surface waters. J. Geophys. Res. 2009, 114, G00F06, DOI: 10.1029/2009JG000990. (6) Murphy, K. R.; Stedmon, C. A.; Waite, T. D.; Ruiz, G. M. Distinguishing between terrestrial and autochthonous organic matter sources in marine environments using fluorescence spectroscopy. Mar. Chem. 2008, 108, 40–58.
Published: May 27, 2011 5910
dx.doi.org/10.1021/es201716u | Environ. Sci. Technol. 2011, 45, 5910–5910
