Cationic surfactants environmental samples

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]. 

Equidistant or clustered signals, characteristic of some anionic, nonionic or cationic surfactants (cf. Fig. 2.5.1(a) and (b). So the presence of non-ionic surfactants of alkylpolyglycolether (alcohol ethoxylate) type (AE) (structural formula C H2 i i-0-(CH2-CH2-0)x-H) could be confirmed in the formulation (Fig. 2.5.1(a)) applying APCI-FIA-MS in positive mode. AE compounds with high probability could also be assumed in the heavily loaded environmental sample because of the patterns of A m/z 44 equally spaced ammonium adduct ions ([M + NH4]+) shown in its FIA-MS spectrum in Fig. 2.5.1(b). 

Detections of cationic surfactants in commercial products and environmental samples... 

Several ELISA methods for determining cationic surfactants have been reported although we have not found any examples of their application to real environmental samples. The detectability obtainable by these methods is very good an LOD of 0.04 pg L 1 has been reported for benzyldo-methyldodecylammonium chloride (BDD12AC), a component of benzalkonium chloride (BAK).94... 

Conventional ionization techniques like El or Cl are less well suited for the characterization of quaternary amines, which are the most common cationic surfactants. Because of their thermal instability and low volatility their corresponding mass spectra only show decomposition products and fragment ions which make it impossible to analyze environmental samples of unknown composition. By the use of EAB-MS and FD-MS, however, ionization of quaternary amines can be achieved without decomposition. FAB spectra are characterized by strong quasimolecular ions as well as structure specific ions. ° FAB in combination with collisionally activated decomposition (CAD) in a tandem mass spectrometer enables a clear differentiation between quasimolecular and fragment ions, which is often difficult using FAB alone. FD spectra of quaternary amines are dominated by quasimolecular ions as already described for other surfactant... 

IR spectroscopy is used for the qualitative identihcation of surfactants and for differentiating between them and nonsurfactant compounds. Prior to IR spectroscopy, however, separation of the organic compound complex into different fractions, performed by, e.g., the use of thin-layer chromatography, is required to obtain meaningful spectra. °" ° By comparing the IR spectra of the isolated fractions with IR spectra of standard compounds with regard to characteristic bands, the qualitative determination of surfactants in environmental samples is possible. The method is equally applicable to anionic, ° nonionic, °" and cationic surfactants.The prerequisite for a clear identification of surfactants, however, is the availability of suitable standards. Moreover, considerable experience and knowledge are needed to interpret IR spectra of environmental samples. 

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]. 

De Ruiter, C., J. C. H. F. Hefkens, U. A. T. Brinkman, R. W. Frei, M. Evers, E. Matthijs, J. A. Meijer, LC determination of cationic surfactants in environmental samples using a continuous post-column ion-pair extraction detector with a sandwich phase separator. Intern. J. Environ. Anal. Chem., 1987, J/, 325-339. 

In general, the same two-phase titration methods based on formation of water-insoluble ion pairs can be used for determination of cationic surfactants as are used in the determination of anionic surfactants, described earlier in this chapter. This means that most methods have the disadvantage that anionic surfactants interfere with cationic determination. This is not a serious problem in formulations, since cationics are not blended with anionics. However, in environmental samples anionics are usually present at much higher levels than cationics. All of the following methods suffer from anionic interference. 

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. 

The general test for cationic surfactants in environmental samples is a spectrophoto-metric test based on formation and extraction of a colored ion pair with an anionic dye, disulfme blue. The result of this test is designated as disulfme blue active substance, ... 

Waters, J., Cationic surfactants in laboratory test liquors and environmental samples, in J. Cross and E. J. Singer, eds.. Cationic Surfactants Analytical and Biological Evaluation, Marcel Dekker, New York, 1994. 

For this reason, ion-exchanger based ISEs have historically been used for the detection of perchlorate and nitrate, as well as a range of lipophilic organic ions including many dmgs and cationic/anionic surfactants. Anion exchanger-based membrane electrodes are routinely used to assess nitrate in environmental and laboratory samples. While there is some interference from bicarbonate, calibration in samples with very similar background electrolytes can minimize such effects. 

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