Separation of Branched PFOS Isomers by UPLC with MS/MS Detection

Waters ACQUITY UltraPerformance LC (UPLC) is shown to achieve rapid separation of perfluoroalkane sulphonate homologs and isomers. Tandem quadrupole MS/MS detection provides rich spectral information to facilitate compositional analysis and structural elucidation of perflourinated alkane sulphonates. Sensitive quantitation of perfluorinated octyl sulphonate (PFOS) isomers is demonstrated using the mass spectrometer in the highly selective multiple reaction monitoring (MRM) mode. 

Perfluorooctane sulfonate (PFOS) is a fully fluorinated anion that can be formed by degradation from a large group of related substances, close to 100 in total, referred to as PFOS-related substances. These substances are used in a wide variety of applications including the paper and packaging industries, the textiles/upholstery industries and as a surfactant. Extensive biological monitoring of PFOS and other related perfluorinated compounds in recent years has revealed PFOS as a global pollutant¹ and perfluorinated compounds have even been found in remote areas of the Arctic.² 

PFOS has been detected at parts per billion levels in human serum and the livers of fish, birds and marine mammals.^2,3,4 Toxicity tests in rodents have raised concerns about potential developmental, reproductive and systemic effects of PFOS. It is for this reason that sensitive detection of all perfluorinated compounds including PFOS and its branched isomers is of great importance. 

An LC-MS/MS method of analysis using negative ion electrospray is described. Separations were performed using a Waters ACQUITY UltraPerformance LC System. Sample components were separated using a Waters ACQUITY UPLC Column and gradient elution. All mass spectra were acquired using a Waters Quattro Premier XE Tandem Quadrupole mass spectrometer with ZSpray interface. When operated in negative ion electrospray mode, the mass spectra acquired provided useful structural information for analysis of isomer composition and proved suitable for sensitive quantitative analysis. 

In the first instance, UPLC-MS/MS was shown to provide fast and efficient separation of perfluorinated alkyl sulphonates in the homologous series containing C₄ to C₉. Rapid separation of the PFOS isomer group was achieved and structural information for isomers was obtained from analysis of product ion spectra. Subsequent analysis targeted the PFOS isomer group demonstrating the quantitative potential of the UPLC-MS/MS approach. 

Software

Data was acquired and processed with Waters MassLynx Software. 

Compositional Analysis

Analytical methods which provide good chromatographic separations of perfluorinated alkane sulphonates, when coupled with mass spectroscopic detection, allow for the characterization and/or quantitation of complex mixtures. 

The chromatographic separation of a Technical Grade sample of PFOS in under seven minutes using Waters ACQUITY UPLC Technology in shown in Figure 1 below.

MS data were collected in full scan mode. Extracted ion chromatograms (XICs) corresponding to the homologous series [C_nF_n+1SO₃]^-  from n=4 to n=9 are shown in Figure 2 below. This shows that the Technical Grade product contains material of different chain lengths. The number of isomers for each homolog is seen to increase with n as expected. 

Further experiments focused upon the group of isomers associated with PFOS (n=8 homolog from Figure 2). A product ion scan experiment monitoring product ions of m/z 499 was conducted. Below, in Figure 3, are the XICs for product ions m/z 80 and m/z 130 (red trace). 

The chromatograms show different relative intensity profiles depending upon which product ions are monitored. This indicates that the relative intensities of product ions in the spectra of PFOS isomers are different. These differences are the result of structural variations between isomers. Mass spectrometry can therefore provide useful information for structural elucidation. To illustrate this further the spectra of several isomer groups labelled in the chromatograms are shown in Figure 4 overleaf. Further investigations revealed a minor eighth peak with common fragments at Rt 3.71 min and there are clearly further unresolved components within the labelled isomer groups. 

Spectral information was obtained over the range m/z 65–510 with the use of tandem quadrupole MS/MS enabling structurally significant low mass fragments to be observed, which would not be seen on a conventional ion trap mass spectrometer due to low mass cut-off.

The peaks observed in the product ion spectra from the precursor m/z 499 generally fit the series [C_nF2_n+1]^- for peaks from m/z 119 to m/z 419; or [C_nF_2nSO₃]^-, which is characterized by peaks from m/z 80 to m/z 330. Previous work5 has postulated that the relative abundance of such series may be an indicator of degree of branching in the isomers, which is an important structural characteristic. In particular, the [C_nF2_n+1]^- ions are indicative of cleavage in the aposition of a CF₃ branched chain and the [C_nF_2nSO₃]^- series of the linear alkyl sulphonate portion of the molecule (Figure 5). 

The mixture analyzed is a complex one and the spectra recorded show the characteristic fragments associated with PFOS. Further work with branched model compounds would facilitate the assignment of exact structures. Nevertheless, it is clear that MS/MS can produce useful spectral information containing structurally significant information. 

Further optimization of chromatographic separation and MS/MS fragmentation is proposed in order to enable better characterization of complex PFOS mixtures. Investigation of simpler systems containing specific isomers will enable better characterisation of the perfluorinated alkyl compounds. 

The obvious differences in response factor also have an implication for accurate quantitation of PFOS mixtures. 

Quantitative Analysis

In addition to determining the structural composition of PFOS it also important to offer sensitive quantitative methods for the analysis of the material in a variety of environmental matrices. To improve quantitation, a BEH C₈ Column chemistry was used to produce a rapid separation of PFOS as a single chromatographic peak. The mass spectrometer was operated in MRM mode using the conditions described in the method section. MRM use offers the ultimate combination of selectivity and sensitivity as demonstrated by the chromatogram shown in Figure 6. This shows the trace for the MRM tranisition m/z 499 > 80. The calibration curve for PFOS over the range 0.01–100 ng/mL is also shown (Figure 7). 

The combination of UPLC with tandem quadrupole mass spectrometry has considerable potential for the compositional analysis and quantitative determination of perfluorinated alkyl compounds. 

Rapid separation of perfluorinated alkane sulphonate homologs can be achieved in under seven minutes. 

Tandem quadrupole mass spectral data provides useful information for determining the structural composition of PFOS isomers. 

The use of MRM enables highly selective detection of PFOS at low levels. 

1. Giesy J.P., Kannan K., Environ. Sci. Technol. 35 (2001) 1339. 
2. Bossi R, Riget FF, Dietz, Sonne C, Fauser P, Dam M, Vorkamp K. Environ Pollut. 2005 Jul; 136 (2):323–9. 
3. Olsen G.W., Church, T.R., Miller J.P., Burris J.M., Hansen K.J., Lundberg J.K., Armitage J.B., Herron R.M., Medhdizadehkashi Z., Nobiletti J.B., O’Neill M.E., Mandel J.H., Zobel L.H., Environmental Health Perspectives. Vol 111, No.16, Dec 2003. 
4. Koichi I, Okado F, Ito R, Kawaguchi M, Okanouchi N, Nakazawa H., Journal of Chromatography B, 810 (2004) 49–56. 
5. Langlois I, Oehme M; Organohalogen Compounds, Volume 66 (2004) 3973–3978. 

Acknowledgement

The authors would like to thank Wellington Laboratories (Guelph, Ontario, Canada) for the provision of perfluorinated standards.