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Analysis of perfluorinated compounds by HPLC-ICP-MS/MS

(21.01.2020)


Background:
There is a plethora of per- or polyfluoroalkyl substances (PFAS) produced and released to the environment and only a small number of these compounds are currently being monitored in environmental and biological sample using molecular mass spectrometry (MS). Also a high number of produced compounds and their metabolites have not even been identified. One of the reason for the analytical gap is the difficulty to analyse for fluorinated compounds. The small mass deficiency of fluorine limits the possibility to screen the mass spectra for fluorine containing molecules and a lack of ionizable groups even limits their signal generation.  In cases where the compound contains elements detectable by elemental ICP-MS, quantification of such compounds is often possible even without having compound-standards. Unfortunately, the ionization potential of fluorine is with 17.42 eV beyond the capability of the argon plasma limited to 15.76 eV.

New method:
Researchers from the TESLA laboratory in Aberdeen propose a new method for the analysis of fluorine containing compounds by HPLC-ICP-MS/MS. By adding a barium solution post-column, the ICP-MS can indirectly detect the fluorine via the BaF+ ion. A reverse phase high performance liquid chromatography (RP-HPLC) method was developed to separate perfluorinated carboxylic acids (PFCA) and perfluorooctanesulfonic acid (PFOS). The HPLC was then on-line coupled to ICP-MS/MS  for the fluorine-specific detection (see figure 1) and simultaneously to electrospray MS (ESI-MS) for molecular structure identification.


Figure 1: Online coupling HPLC-ICP-MS/MS with post column Ba addition

As a proof of principle the setup was used to analyse a mixture of PFCA and their methyl esters as their degradation products. To evaluate the method, real sample analysis on river water was also conducted. The detection of fluorine is based on the detection of BaF+.  In the plasma organofluorines are atomised and fluorine is ionised to form F- (unfortunately modern ICP-MS instruments only allow for the detection of positive ions !). The F- ion reacts subsequently with Ba2+ ions, which have been added post column to the eluate, to form BaF+. The separation was based on a reverse phase column (ACE Excel 3 C18-Amide, 100 mm 114 x 3 mm x 5 μm) with either isocratic 70% methanol or gradient of acetonitrile/water elution. The use of an organic solvent made it necessary to use the organic mode for the ICPMS/MS with addition of O2 to the plasma. Under isocratic 70% methanol elution, C6 until C10 of PFCA compounds were separated with the same fluorine-specific response as [BaF]+.  Using the gradient method, the response of [BaF]+ changes with increasing acetonitrile concentration in the plasma. By knowing the specific retention time of fluorine response factor, the correct response factor for each fluorine compounds could be determined. For further data analysis, each chromatogram was corrected by taking the intensity data and divides it with the relative intensity from the response curve. By using this correction approach, the calibration was linear and was element-specific with detection limits of 0.49 mg F L-1 under gradient elution method. Finally, the analysis of spiked river water showed that this method could help to mine the huge ESI-MS data in identifying the unknown fluorinated compounds.

The authors are convinced, that their method could be capable to quantify individual PFAS or other fluorinated compounds in the low to sub-ppb concentrations in river water, when combined with a preconcentration method such as SPE.




The original publication:

N.L. Azua Jamari, J.F. Dohmann, A. Raab, E.M. Krupp, J. Feldmann, Novel non-targeted analysis of perfluorinated compounds using fluorine-specific detection regardless of their ionisability (HPLC-ICPMS/MS-ESI-MS), Anal. Chim. Acta, 1053 (2019) 22-31. DOI: 10.1016/j.aca.2018.11.037


Used techniques and instrumentation:



Related studies

J. Feldmann, A. Raab, E.M. Krupp, Importance of ICPMS for speciation analysis is changing: future trends for targeted and non-targeted element speciation analysis, Anal. Bioanal. Chem., 410 (2018) 661–667. DOI:10.1007/s00216-017-0502-8.

N.L.A. Jamari, A. Behrens, A. Raab, E.M. Krupp, J. Feldmann, Plasma processes to detect fluorine with ICPMS/MS as [M–F] + : an argument for building a negative mode ICPMS/MS, J. Anal. At. Spectrom., 33/8 (2018) 1304–1309. DOI:10.1039/C8JA00050F.

N.L.A. Jamari, J.F. Dohmann, A. Raab, E.M. Krupp, J. Feldmann, Novel non-target analysis of fluorine compounds using ICPMS/MS and HPLC-ICPMS/MS, J. Anal. At. Spectrom., 32/5 (2017) 942–950. DOI:10.1039/C7JA00051K

N.L.A. Jamari, J.F. Dohmann, A. Raab, E.M. Krupp, J. Feldmann, HPLC-ICP-MS/MS: Fluorine speciation analysis, in: Agilent Technologies (Ed.), Handb. ICP-QQQ Appl. Using Agilent 8800 and 8900, 4th edition, Agilent Technologies [5991-2802EN], 2019: pp. 241-243.
 




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last time modified: January 22, 2020









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