Determination of fluorinated surfactants

Schroder, H.E (2003). Determination of fluorinated surfactants and their metabolites in sewage sludge samples by liquid chromatography with mass spectrometry and tandem mass spectrometry after pressurized liquid extraction and separation on fluorine-modified reversed-phase sorbents. J. Chromatogr. A 1020(1), 131-151. 

H.Fr. Schroder, Determination of Fluorinated Surfactants and their Metabolites in Sewage Sludge Samples by LC/MS and MSre after Accelerated Solvent Extraction and Separation on Fluorine Modified RP-Phases, J. Chromatogr. A, in press. 

In general, the concentration of a fluorinated surfactant in solution can be determined by conventional volumetric or spectroscopic methods used for hydrocarbon-type surfactants [1-5], In addition to the functional groups utilized for the analysis of hydrocarbon-type surfactants, the fluorine content is a unique feature useful for the determination of fluorinated surfactants. If the fluorinated surfactant is the only fluorine-containing species in a solution or a substrate, then the fluorine content indicates the concentration of the fluorinated surfactant. 

The methods used for the determination of fluorinated surfactants in biological samples can be divided into two groups (1) the determination of organic fluorine, which represents the concentration of a fluorinated surfactant if other flu-orochemicals are absent, and (2) a specific method for the fluorinated surfactant of interest. 

The direct determination of fluorinated surfactants is possible if the fluorinated surfactant is amenable to chromatography or spectroscopy. Belisle and Hagen [46] determined perfluorooctanoic acid in blood, urine, and liver tissue. Perfluorooctanoic acid was extracted from blood or other biological samples with hexane in the presence of hydrochloric acid and converted to its methyl ester with diazomethane. The recovery of known amounts of perfluorooctanoic acid added to human plasma was essentially quantitative. The precision of the method was inferior to that of the determination of perfluorooctanoic acid by elemental fluorine analysis but could probably be improved by using a capillary chromatographic column instead of the packed column used by the authors. 

The determination of fluorinated surfactants in water and wastewater is essential for (1) the detection of pollution by fluorinated surfactants, (2) study of biodegradation, and (3) determining the effect of fluorinated surfactants on aquatic life. If a specific method is not needed, the oxyhydrogen combustion method is the most effective [10]. By introducing a 10-mL water sample into the oxyhydrogen torch in several portions, as little as 20 0 ppb fluorinated surfactant can be detected without the need to concentrate the sample before combustion. 

Fluorinated surfactants are both hydrophobic and lipophobic. For example, potassium per-fluorooctanesulfonate—an industrially important surfactant —forms a third phase with octanol and water, and it is impossible to determine its octanol-water partition coefficient. " Similar to fluorocarbon-hydrocarbon bulk solvent mixtures, mixed binary systems containing a perfluorocarbon surfactant and a structurally related hydrocarbon surfactant are known to behave nonideally, that is, exhibit phase separation in insoluble monolayers at the air-water interface or form two types of micelles simnltaneously in solution—one type is fluorocarbon-rich and the other is hydrocarbon-rich. This nonideal behavior of fluorocarbon-hydrocarbon surfactant mixtures is used in firefighting foams and powders—an important technical application of fluorinated surfactants. " ... 

Analytical techniques are employed to determine the purity or the concentration of a fluorinated surfactant and to characterize a fluorinated surfactant and its solutions. Because most fluorinated surfactants are mixtures of homologs, the term purity has to be redefined for each particular case. In most cases, the determination of purity begins with the analysis of intermediates used to synthesize the surfactant. Usually, the intermediates can be readily analyzed by chromatography and the homolog distribution determined. Gas chromatography has only a limited value for the analysis of fluorinated surfactants proper because most fluorinated surfactants are not sufficiently volatile for gas chromatography. 

Muto et al. [252] measured pyrene fluorescence lifetime Tq and the ratio IiHt, of the intensities of the first vibronic and the third vibronic band of the monomeric pyrene. The pyrene fluorescence data revealed the existence of a single type of mixed micelle in solutions of LiDS-LiFOS, LiFOS-hexaoxyethylene glycol do-decyl ether, or LiFOS-octaoxyethylene glycol dodecyl ether mixtures. The lifetime and the intensity ratio of vibronic peaks have been used to determine the cmc of fluorinated surfactant micelles [253]. However, the solubility of pyrene in micelles of fluorinated surfactants is not adequate for determining the micelle aggregation number [253,254]. 

Claesson et al. [271 ] studied the adsorbed monolayers of a cationic, double-chained fluorinated surfactant on mica. The XPS spectrometer was equipped with an AlKa x-ray source. The known number of exchangeable potassium and sodium ions on the mica basal plane served as the internal standard for the quantitative determination of adsorbed surfactant. The surfactant oriented preferentially with both nitrogen atoms or only the quaternary ammonium group toward the surface, depending on the deposition method. 

The determination of bismuth activity as an indicator of non-ionic surfactants also suffers from interference in environmental samples. Substance group specific methods also failed to detect different types of fluorine-containing anionic, cationic and non-ionic surfactants. Already marginal modifications in the precursor surfactant due to primary degradation or advanced metabolisation implicated their lack of detection [45]. 

The trend of discovering the analytical field of environmental analysis of surfactants by LC-MS is described in detail in Chapters 2.6-2.13 and also reflected by the method collection in Chapter 3.1 (Table 3.1.1), which gives an overview on analytical determinations of surfactants in aqueous matrices. Most methods have focused on high volume surfactants and their metabolites, such as the alkylphenol ethoxylates (APEO, Chapter 2.6), linear alkylbenzene sulfonates (LAS, Chapter 2.10) and alcohol ethoxylates (AE, Chapter 2.9). Surfactants with lower consumption rates such as the cationics (Chapter 2.12) and esterquats (Chapter 2.13) or the fluorinated surfactants perfluoro alkane sulfonates (PFAS) and perfluoro alkane carboxylates (PFAC) used in fire fighting foams (Chapter 2.11) are also covered in this book, but have received less attention. 

Different authors [15,143] have tried to determine a limiting value for the cohesive energy of the solvent above which a certain solvent should be able to promote surfactant aggregation. However, it is curiously often overlooked that the hydrophobicity of the surfactant must also be taken into account in this context. A surfactant with a long hydrocarbon chain or a fluorinated hydrocarbon chain will be able to aggregate in solvents where a less hydrophobic surfactant will remain in monomeric form, just as in water. 

Leon-Gonzalez et al.[31] proposed an FI spectrophotometric method for the determination of Triton-type non-ionic surfactants based on their reaction with alizarin fluorine blue. An on-line ion-exchange column was incorporated in the system to eliminate interferences from ionic and amphoteric surfactants. In case of interferences from non-ionic surfactants, an on-line Amberlite XAD-4 adsorption column was used to retain selectively the Triton-type surfactant, which was subsequently eluted by ethanol. However, no information was given regarding interferences from refractive index effects at the ethanol/aqueous interface and their elimination. 

Alizarin fluorine blue Spectrophotometry 0.2-12.0 mg r Determination of Triton-type surfactants... 

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