Cationic surfactants environmental analysis

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. 

Depending on the nature of the hydrophilic groups of surfactants, they can be divided into anionic, nonionic, cationic, and amphoteric surfactants. The last-mentioned class only plays a minor role with respect to domestic and industrial applications and practically no methods for the environmental analysis of amphoteric surfactants have been published so far. 

GC analysis is not of practical relevance for the determination of cationic surfactants in environmental matrices. 

Some reviews were published dealing with this type of interface and its application in environmental analysis [24, 42, 123). Qualitative and quantitative analysis of polar pollutants by FAB or CF-FAB was performed with extracts of aqueous matrices, such as wastewater, surface water, seawater, raw and drinking water [124-129], for all types of surfactants (non-ionics, anionics, cationics and amphoterics) in urban wastewaters, receiving waters (rivers and costal receiving areas), and groundwater [124-148], for metabolites of surfactants [130, 149-153], and bromi-nated surfactants [137, 154). 

FAB or LSIMS using a probe inlet does not readily lend itself to quantitative work. Firstly, it is not possible to know how much of the sample has been consumed in the analysis. Secondly, discrimination effects (see section 12.3.3) prevent the comparison of intensities between species of differing surface activity. Semiquantitative results may be readily obtained if discrimination effects are assumed to be constant for the species of interest, for example the determination of homologue distributions in a mixture. For accurate quantitation an internal standard of an isotopically enriched analogue of the analyte should be used. For example, in the determination of cationic surfactants in environmental samples [10], quantitation was achieved by using an internal standard of a trideuterated form of the analyte. In this way the standard will be subject to the same level of discrimination as the analyte. Discrimination effects between different cationic species may also be reduced by adding to the sample an excess of a highly surface-active anionic surfactant. The anionic species will dominate the matrix surface and attract cations into the surface monolayer [10]. 

It is important to note that several works, in both environmental and food analysis, compare the extraction performance of the IL-based surfactant with the conventional cationic surfactant CTAB [20, 36, 56, 60, 61]. For example, Pino et al. have reported much better extraction efficiencies of PAHs from marine sediments when using Cj C Im-Br (being quantitative) than when using CTAB under the same extraction conditions [36], Moreover, this application was validated using a certified reference material (CRM). 

Like the two-phase titration methods (Chapter 16), almost all of the colorimetric methods rely on the formation of an ion pair by the anionic surfactant and a cation, in this case a dye. The ion pair is extractible into an organic solvent, while the dye alone is not, so the color of the organic phase is directly proportional to the surfactant concentration. This approach is suitable for determination of low concentrations and has been used most often for environmental analysis. 

Spectrophotometric determination of cationics without extraction can be performed by forming a ternary complex of iron (HI), chrome azurol S, and the surfactant. The method is susceptible to interference and has not yet been proven in environmental analysis (144). 

A typical analysis of an environmental sample includes preliminary separation by liquid-liquid extraction or SEE cleanup by ion exchange, additional extractions, or alumina adsorption and final determination of cationic surfactant by HPLC or MS. 

Waters has written a well-balanced review article on environmental analysis of cat-ionics giving a historical perspective on the development and application of the various procedures, including official methods (153). Published methods for determination of cationic surfactants in the environment are summarized in Tables 7 and 8. 

Mass spectrometric analysis of cationic surfactants is difficult, so MS has not been applied as much to environmental analysis of cationics as it has to anionics and nonionics. 

Ion-selective electrodes (ISEs) are used in clinical, pharmaceutical or environmental analysis for the detection not only of inorganic ions but also of some organic species such as anionic or cationic surfactants. However, the need for an internal filHng solution causes many problems, such as fragihty, and it is obstructive for miniaturization. The need for microstructures or even nanostructures leads to the concept of an all-solid-state ISE. 

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