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Perfluoralkylated substances is a general term used to describe substances that are largely comprised of or contain a perfluorinated or polyfluorinated carbon chain moiety such as F(CF2)n- or F(CF2)n-(C2H4)n. Perfluorooctane sulfonate (PFOS) and other perfluorinated compounds (PFCs) are widely used in industrial and consumer applications, including stain-resistant coatings for textiles, leather, and carpets; grease-proof coatings for paper products approved for food contact; firefighting foams; mining and oil-well surfactants; floor polishes; and insecticide formulations. In recent years, there has been increasing concern over the levels of perfluorinated and polyfluorinated chemicals such as PFOS and PFOA (perfluorooctanoic acid) in the global environment and their fate and possible adverse effects.
In animal studies, some PFCs disrupt normal endocrine activity; reduce immune function; cause adverse effects on multiple organs, including the liver and pancreas; and cause developmental problems in rodent offspring exposed in the womb.1 Data from some human studies suggest that PFCs may also impact human health, while other studies have failed to find conclusive links.2 Additional research in animals and humans is needed to better understand the potential adverse effects of PFCs for human health.
Two compounds in particular, PFOS and PFOA, represent the final environmental degradation products of (and contaminants in) a wide range of other perfluorinated products and have been most extensively studied. PFOS is now subject to varying but increasing levels of control in a number of countries. PFOA, also a widespread contaminant but with a far lower bioaccumulation potential, is still under evaluation.
In the United States, U.S. EPA Method 537 is generally followed for the processing of water samples for PFCs. This article describes the first automated solid-phase extraction system made specifically for PFC extraction and concentration. Primarily effective at reducing background contamination, extraction and concentration of aqueous samples take less than two hours. These applications are capable of handling drinking water and wastewater.
Material and methods
The solid-phase extraction (SPE) system was loaded with PFC 500-mg cartridges (Fluid Management Systems, Watertown, MA) that were each conditioned with 15 mL methanol and 40 mL water. Five 250-mL water samples were spiked with 25 µL of 1 µg/mL PFC standard solution. Samples were loaded onto the SPE system and passed across the cartridge under –12 psi vacuum. After loading, bottles were rinsed with 25 mL of water and loaded onto the cartridge under negative pressure. The cartridges were dried using nitrogen until no residual water was present (typically 20 min). The cartridges were then eluted with 25 mL methanol to collect the PFCs.
Concentration
The collection tubes with sample were preheated in an FMS SuperVap concentrator to 50 oC for 20 min, followed by heating in the sensor mode under 9 psi of nitrogen, which assured automatic shut-off at 0.5 mL. The extracts were concentrated to 500 µL, after which internal standard was added. The samples were diluted to a final volume of 1 mL of water for LC/MS analysis.
Analysis
An Acquity UPLC coupled to a Q-TOF (Xevo G2-XS) and HR-MS (Waters, Milford, MA) was used for the analysis of PFCs. Separation was performed with an Acquity HSS T3 column (2.1 mm × 100 mm, 1.8 µm) and samples were analyzed by electrospray in negative ionization mode. Samples were reconstituted in 1 mL methanol (1:3 ratio) and then sonicated prior to injection (20 µL). A detailed description of chromatography and spectrometry methods is provided below.
A gradient solvent program was used with 0.1% formic acid in LC/MS-grade methanol (solution A) and 0.1% formic acid in LC/MS-grade water (solution B). Details of the gradient method are given in Table 1.
Table 1 – Gradient solvent program for chromatography method
Samples were analyzed in negative electrospray ionization mode. Desolvation gas (nitrogen) flow rate was kept at 750 L/hr, capillary voltage was 0.5 kV, and the source and desolvation temperatures were 120 ºC and 250 ºC, respectively. Mass range was set at 50–950 Da with a scan rate of 0.055/sec.
The concentration and recovery of each PFC were determined by comparing the spiked sample response to that of a reference standard at the same concentration (0.5 ng/mL). A labeled PFC (d3-N-MeFOSA-M) was spiked to each sample and reference standard in order to obtain a normalized response for better accuracy.
Results and discussion
Figure 1 shows the FMS TurboTrace PFC SPE system (Fluid Management Systems), a fully automated sample preparation system for analyzing PFCs in wastewater. It incorporates a vacuum or positive pressure pump to load samples for compliance with U.S. EPA Method 537. Sample loading rates are programmable. A liquid sensor detects when the sample has been loaded, triggering the system to initiate next steps. The system is expandable from 1 to 6 modules with parallel extraction. The control module with touchscreen technology allows for easy programming of the various steps. All surfaces are made of inert PEEK material and stainless steel.
Figure 1 – TurboTrace PFC SPE system.Recoveries for a number of PFCs run on the TurboTrace system are shown in Figure 2. These vary between 80 and 120%. The compounds shown are from two major classes of PFCs: perfluorocarboxylic acids, which have a linear chain of carbon atoms all substituted with fluor and a terminal carboxylic acid group, and perfluorosulfonic acids, which are similar in structure but have a terminal sulfonic acid functional group. The PFCs shown in Figure 1 contain between 4 and 9 carbon atoms.
Figure 2 – Recoveries for a number of perfluorinated compounds (4–9 carbon atoms).Figure 3 shows recoveries of linear PFCs with 10–14 carbon atoms and for three other compounds (4:2 FTS, 6:2 FTS, and 8:2 FTS with between 6 and 10 carbon atoms), sulfonic acids in which the two carbon atoms nearest to the sulfonic functionality have no fluor atoms substituted. Recoveries here varied between 65 and 120%.
Figure 3 – Recoveries for a number of perfluorinated compounds (6–14 carbon atoms).Background concentrations for method blank runs done on the TurboTrace SPE system are shown in Figure 4. Contributions found from the system are low: the highest blank concentration is that of PFOA at ~0.2 ng/L. The system is clearly suitable for trace-level determination of PFCs in water. The other PFCs shown are present at lower concentrations, mostly <0.05 ng/L. The presence of PEEK in the SPE system is critical, as opposed to PTFE, which contains PFCs.
Figure 4 – Background concentrations for a number of perfluorinated compounds.The sequential PFC SPE system has much of the same characteristics of the TurboTrace, the difference being that the sequential system runs each sample sequentially within the same module. Since five samples can be run per module, and six modules can be added in one system, the sequential system can run up to 30 samples unattended over a roughly five-hour period. When composed of six modules, the system will run position #1 of each module at the same time, then all positions #2, etc. As seen in Figure 5, recoveries and background contributions are comparable to the TurboTrace system. All recoveries were between 80 and 100%, and blank concentrations were <0.15 ng/L.
Figure 5 – Background concentrations for a number of perfluorinated compounds.Conclusion
The automated extraction and analysis of perfluorinated compounds hold promise for environmental laboratories in the U.S. Both drinking and wastewater can be analyzed quickly and reproducibly with the systems described. The recoveries for PFCs easily meet the U.S. EPA Method 537 requirements of the 70–130% window.
The background contribution from the automated systems is low, as shown in Figure 4, and is <0.2 ng/L for all PFCs. The design of the systems, with all PEEK and stainless-steel surfaces, ensures sample extraction with very low background contamination.
The TurboTrace system runs samples in parallel and can process up to six wastewater samples in 2 hours. With the sequential FMS SPE PFC system, because of its modular nature, 30 water samples can be reliably processed within a 5-hour time frame.
References
- Cui, L., Zhou, Q.F. et al. Studies on the toxicological effects of PFOA and PFOS on rats using histological observation and chemical analysis. Arch. Environ. Contam. Toxicol. 2009, 56(2), 338–49.
- Grandjean, P. and Clapp, R. Changing interpretation of human health risks from perfluorinated compounds. Public Health Rep. 2014, 129(6), 482–5.
The authors are with Fluid Management Systems, 580 Pleasant St., Watertown, MA 02472, U.S.A.; tel.: 617-393-2396; e-mail: [email protected]; www.fms-inc.com. The authors would like to thank Dr. Sujan Fernando at the Center for Air Resources Engineering and Science, Clarkson University, Potsdam, NY, for sample analysis.