Surfactant metabolites identification

Analytical methodologies, which allow the determination of various surfactants and identification of new metabolites are discussed, especially in Chapter 2. The use of multistep sample preparation methods together with advanced techniques like LC-MS (including various types of interfacing systems) for the final identification of surfactants is described in Chapters 3 and 4. Chapter 5 covers the degradation pathway of the major surfactants, which has been studied... 

There are six papers la this section. They deal with biological insecticide formulations (Ward), water dispersible granules (Wright and Ibrahim), analysis of formulations for quality control and identification of toxic contaminants (Plimmer), identification of surfactant metabolites (Stolzenberg et al.), electrostatic spraying (Law) and compatibility and tank-mix testing (Tarwater). 

Detection, identification and quantification of these compounds in aqueous solutions, even in the form of matrix-free standards, present the analyst with considerable challenges. Even today, the standardised analysis of surfactants is not performed by substance-specific methods, but by sum parameter analysis on spectrophotometric and titrimetric bases. These substance-class-specific determination methods are not only very insensitive, but also very unspecific and therefore can be influenced by interference from other compounds of similar structure. Additionally, these determination methods also often fail to provide information regarding primary degradation products, including those with only marginal modifications in the molecule, and strongly modified metabolites. 

As a consequence, for unequivocal identification of the constituents of complex mixtures found in surfactant blends and also in the analyses of surfactants and their metabolites in environmental samples, MS and tandem mass spectrometry (MS-MS) have proved to be more advantageous and are discussed thoroughly in Chapter 2. To optimise the output of reliable results and to save manpower and time certain procedures in sample preconcentration, clean-up and separation prior to MS examinations are inevitable. These are discussed in the present book in more detail in Chapter 3. 

In environmental analytical applications where analyte concentrations, e.g. surfactants or their metabolites, are quite low, extraction and concentration steps become essential. Solid phase extraction (SPE) with cartridges, disks or SPME fibres (solid phase micro extraction) because of its good variety of SP materials available has become the method of choice for the analysis of surfactants in water samples in combination with FIA as well as LC—MS analysis. SPE followed by sequential selective elution provides far-reaching pre-separations if eluents with different polarities and their mixtures are applied. The compounds under these conditions are separated in the MS spectrometer by their m/z ratios providing an overview of the ionisable compounds contained in a sample. Identification in the sense it has been mentioned before, however, requires the generation of fragments. 

While fast atom bombardment (FAB) [66] and TSI [25] built up the basis for a substance-specific analysis of the low-volatile surfactants within the late 1980s and early 1990s, these techniques nowadays have been replaced successfully by the API methods [22], ESI and APCI, and matrix assisted laser desorption ionisation (MALDI). In the analyses of anionic surfactants, the negative ionisation mode can be applied in FIA-MS and LC-MS providing a more selective determination for these types of compounds than other analytical approaches. Application of positive ionisation to anionics of ethoxylate type compounds led to the abstraction of the anionic moiety in the molecule while the alkyl or alkylaryl ethoxylate moiety is ionised in the form of AE or APEO ions. Identification of most anionic surfactants by MS-MS was observed to be more complicated than the identification of non-ionic surfactants. Product ion spectra often suffer from a reduced number of negative product ions and, in addition, product ions that are observed are less characteristic than positively generated product ions of non-ionics. The most important obstacle in the identification and quantification of surfactants and their metabolites, however, is the lack of commercially available standards. The problems with identification will be aggravated by an absence of universally applicable product ion libraries. 

Indications for the formation of analogous species in microbial metabolism of LAS were found by Knepper and Kruse [33] during biotransformation of commercial LAS surfactant on an FBBR. However, the low concentrations of the tentative metabolites in the test liquor, which eluted under the applied reversed-phase (RP)-HPLC conditions somewhat earlier than the normal SPC, did not permit acquisition of full-scan mass spectra as was needed for unequivocal identification. Further evidence for the formation of the intermediate with a double bond in the alkanoate moiety was reported by Bird [103]. During biodegradation of Cn-LAS by a bacterial strain, a new UV adsorption band centred near 260 nm was detected, which was assumed to result from a double bond, although a definite confirmation could not be provided. 

Surfactant elimination happens mainly through biotransformation, denoted by the identification of several metabolites. These have been found in liver and to a greater extent in the gall bladder, where they are transported and subsequently discharged with bile. They have also... 

Schrodei H.Er. Separation, Identification, Quantification of Surfactants, their Metabolites, in Waste Water, Surface Water, Drinking Water hy LC-TSP-MS, FIA-TSP-MS, MS-MS, in Applications of LC-MS in Environmental Chemistry, ed. Barcelo, D., Elsevier, Amsterdam 1996, p. 263. 

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