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TOP Analysis? More to Consider When Monitoring Polyfluorinated Alkylated Substances

Sediment_SquareThe sampling and analysis, of per-/polyfluorinated alkylated substances (PFAS) is on the radar of many environmental managers with new regulatory mandates and recent state and US EPA drinking water action levels. PFASs are a set of emerging compounds that are drawing significant public attention and concern due to several high-profile projects involving public water in Colorado Springs, Colorado, and Hoosick Falls, New York, with detectable levels of PFASs being found in many drinking water systems.

Prior articles have provided background on these chemicals and comments regarding the liquid chromatography/tandem mass spectrometry (LC/MS/MS) method, US EPA Method 537, developed for perfluorinated alkylated acids (PFAAs) in drinking water and numerous modifications to the reference method being advanced by the commercial laboratory community. The number of PFAS compounds included in these various LC/MS/MS analyses varies by laboratory, but is generally limited to key perfluorinated carboxylated acids (PFCAs), perfluoroalkyl sulfonic acids (PFSAs), and a small number of polyfluorinated precursor compounds.

Critical to the understanding of PFAS investigations is the extreme environmental stability of the carbon to fluorine bond, making natural degradation of fully fluoroalkylated compounds essentially nonexistent (Vecitis et al.). However, microbial and/or chemical degradation of the non-fluorinated component of a polyfluorinated molecule has been shown (Yoo et al) and likely leads to perfluorinated chemical endpoints in the environment. The starting chemicals in this scenario are commonly referred to as polyfluorinated precursors. Research has also shown an apparent low bias for most chromatographic methods in some matrices as noted by studies using 19F NMR (Moody et al.), combustion ion chromatography (Miyake et al.,Yueng et al.), and particle-induced gamma-ray emission (PIGE) assays.

These studies are evidence that there can be numerous (perhaps hundreds) of additional compounds with significant contribution to the overall PFAS mass in environmental samples that are not currently being captured by the conventional LC/MS/MS methodologies. Some have informally termed these non-identified chemicals as “dark matter.”

Due to the current limitation in analytical capabilities to measure the majority of these precursors, researchers have developed a degradation approach where PFCAs are the final “dead end” oxidation products (Houtz et al. 2013). This procedure is known as total oxidizable precursor (TOP) analysis and involves chemical and thermal treatment of the aqueous extract or water sample. The TOP procedure is designed to convert all polyfluorinated precursors to detectable PFAAs, which are neutralized and then analyzed by conventional LC/MS/MS for PFAAs. Note that this TOP analysis is only applicable to aqueous samples.

A limitation in the TOP analysis is the potential for exhaustion of the oxidant, with incomplete degradation likely as evident in studies with varying dilution ratios. This empirical method may also be prone to different product ratios depending upon the dilution (Houtz el al. 2012). Houtz noted that less than 75% of the 6:2 and shorter fluorotelomer compounds initially present in a sample are converted into measurable perfluorinated carboxylates indicating that the total PFAA precursor concentration determined by TOP slightly underestimates the actual concentration of precursors in samples that contained a high proportion of C6 and shorter fluorotelomer compounds. Some degradation of PFOS has also been noted via addition of 13C-labeled surrogates prior to oxidation

Further research using TOP analysis is needed to define the limitations as this approach should not at this time be used as proof of total PFAS degradation. Though it may provide a reasonable inference of total PFASs within an aqueous sample, and the degradation potential for both remediation and risk evaluation, the potential for data misinterpretation should not be taken lightly. Use of this technique as a quantitative indication for human or ecological risk assessment activities appears to be of questionable utility at this time.

Cited References

  1. Houtz, Erika F.; David L. Sedlak.  Oxidative Conversion as a Means of Detecting Precursors to Perfluoroalkyl Acids in Urban Runoff.  Environ. Sci. Technol.  2012, 46 (17), pp 9342–9349.
  2. Houtz, Erika F.; Christopher P. Higgins; Jennifer A. Field; David L. Sedlak.  Persistence of Perfluoroalkyl Acid Precursors in AFFF-Impacted Groundwater and Soil. Environ. Sci. Technol.  2013, 47, pp 8187−8195.
  3. Miyake, Yuichi; Yamashita, Nobuyoshi; So, Man Ka; Rostkowski, Pawel; Taniyasu, Sachi; Lam, Paul K.S.; Kannan, Kurunthachalam.  Trace Analysis of Total Fluorine in Human Blood Using Combustion Ion Chromatography for Fluorine: A Mass Balance Approach for the Determination of Known and Unknown Organofluorine Compounds. J. Chroma. A 2007, 1154, 1-2, pp 214–221.
  4. Moody C. A; Kwan W. C; Martin J. W; Muir D. C; Mabury S. A.  Determination of Perfluorinated Surfactants in Surface Water Samples by Two Independent analytical Techniques: Liquid Chromatography/Tandem Mass Spectrometry and 19F NMR.  Anal. Chem., 2001, 73 (10), pp 2200–2206.
  5. Vecitis, C. D.; Park, H.; Cheng, J.; Mader, B. T.;,  Hoffmann, M. R.  Kinetics and Mechanism of the Sonolytic Conversion of the Aqueous Perfluorinated Surfactants, Perfluorooctanoate (PFOA), and Perfluorooctane Sulfonate (PFOS) into Inorganic Products. J. Phys. Chem. A 2008, 112, pp 4261–4270.
  6. Washington, John W.; J. Jackson; Thomas M. Jenkins; John J. Evans; Hoon Yoo; Sarah C. Hafner.  Degradability of an Acrylate-Linked, Fluorotelomer Polymer in Soil. Environ. Sci. Technol.  2009, 43, pp 6617–6623.
  7. Yoo, H.; Washington, J. W.; Ellington, J. J.; Jenkins, T. M.; Neill, M. P.  Concentrations, Distribution, and Persistence of Fluorotelomer Alcohols in Sludge-Applied Soils near Decatur, Alabama, USA. Environ. Sci. Technol.  2010, 44, pp 8397–8402.
  8. Yeung, Leo W. Y.; Miyake, Yuichi; Li, Peng; Taniyasu, Sachi; Kannan, Kurunthachalam; Guruge, Keerthi S.; Lam, Paul K. S.; Yamashita, Nobuyoshi.  Comparison of Total Fluorine, Extractable Organic Fluorine and Perfluorinated Compounds in the Blood of Wild and Pefluorooctanoate (PFOA)-Exposed Rats: Evidence for the Presence of Other Organofluorine Compounds. Anal. Chim. Acta. 2009, 635, 1, pp 108-14.