7 Sensitivity of Various Analytes
After optimising the method parameters (solvent, pyrolysis temperature and molecule generation temperature) calibration graphs of various fluorinated organic compounds were recorded to examine the effect of different perfluoroalkyl chain lengths and functional groups on sensitivity (calibration graph gradient). A method for the calculation of the total of organically bound fluorine would ideally have the same sensitivity to all fluorinated organic substances.
As shown in Figure 3, the sensitivity to the compounds 2-fluorobenzoic acid (2-FBA, as representative of phenyl-bound fluorine), PFOS, PFOA, perfluoroheptanoic acid (PFHpA) is within an similar range. The sensitivity to sodium fluoride is in the medium range of the fluorinated organic compounds.
To compensate for the differences in sensitivity as much as possible, a mixed standard was used for calibration (mixed calibration standard, MCS). The gradient of the calibration graph is within a range, so that the deviation of the calibration graph gradients of the individual substances is < 30%. This MCS contains equimolar ratios (related to the fluorine content) of PFOA, PFOS, PFHpA, 2-FBA and NaF.
Since the use of PTFE components in the autosampler of the contrAA results in a significant blind value, the MFA (perfluoroalkoxy polymer) autosampler hose was replaced with a polyamide hose. This has a lesser service life when using organic solvents but significantly reduces the blind value.
Figure 4. Calibration graphs in ultrapure water and galvanic process water of various dilution stages
Application Example:
Galvanic Process Water To demonstrate the robustness of the measuring method an example for inorganic fluoride detection should be explained. Even in samples with extreme matrix loads, e.g. in galvanic process water having a highly oxidising environment with very low ph value inorganic fluoride can still be safely quantified in the mg/L range after dilution (see Figure 4).
Summary
The analysis of inorganic and organic fluoride species using MAS is a unique and simple method for the detection of total fluorine in organic and aqueous samples. Existing methods are supplemented with the option to also quantify fluorinated organic compounds that are hard to access for their fluorine content. Due to the simplicity and high sample throughput of the method it has the potential of versatile utilisation in routine analyses.
Figure 3. Calibration graphs for various fluorinated organic compounds and sodium fluoride
After this modification verification limits in the range of 0.5 µg/L and detection limits in the range of 1.5 µg/L were achieved with the calibration of the mixed standard. By enriching the substances by way of solid phase extraction (SPE), verification limits down to the lower ng/L range are possible. This permits realistic concentration ranges in drinking water or food analysis.
Compounds, such as 8:2-fluorotelomer alcohol (8:2-FTOH) cannot yet be detected with the current method due to their volatility. The corresponding calibration graph has a much lower gradient than for carbonic and sulphonic acid. Here, a derivatisation to carbonic acid derivatives could provide a remedy [7].
Problematic is the simultaneous detection of inorganic fluoride in the mg/L range and fluorinated organic compounds of low concentration in the ng/L range. To separate the fluoride the selective solid phase extraction is recommended.
References
1. Buck RC, Franklin J, Berger U, Conder JM, Cousins IT, et al. 2011. Perfluoroalkyl and polyfluoroalkyl substances in the environment: terminology, classification, and origins. Integr Environ Assess Manag 7:513-41
2. Wagner A, Raue B, Brauch HJ, Worch E, Lange FT. 2013. Determination of adsorbable organic fluorine from aqueous environmental samples by adsorption to polystyrene-divinylbenzene based activated carbon and combustion ion chromatography. J Chromatogr A 1295:82-9
3. Pabon M, Corpart JM. 2002. Fluorinated surfactants: synthesis, properties, effluent treatment. J Fluorine Chem 114:149-56
4. Kissa E. 2001. Fluorinated surfactants and repellents. Marcel Dekker
5. Gleisner H, Einax JW, Mores S, Welz B, Carasek E. 2011. A fast and accurate method for the determination of total and soluble fluorine in toothpaste using high-resolution graphite furnace molecular absorption spectrometry and its comparison with established techniques. J Pharmaceut Biomed 54:1040-6
6. Gleisner H, Welz B, Einax JW. 2010. Optimization of fluorine determination via the molecular absorption of gallium mono-fluoride in a graphite furnace using a high-resolution continuum source spectrometer. Spectrochim Acta B 65:864-9
7. Houtz EF, Sedlak DL. 2012. Oxidative Conversion as a Means of Detecting Precursors to Perfluoroalkyl Acids in Urban Runoff. Environ Sci Technol 46:9342-9
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