Analysis of surfactants

Surfactants are often classified according to the nature of the polar (hydrophilic) part of the molecule, which is explained in the table below  

A surfactant molecuie that can dissociate to yield a surfactant ion whose polar group is negatively charged. Example sodium dodecyl sulfate (SDS), CH3(CH2)iiSO, -Na+ 

A surfactant molecule for which the ionic character of the polar group depends on solution pH. For example, lauramidopropyl betaine 

A surfactant molecule whose polar group is not electrically charged. Example poly(oxyethylene) alcohol, 

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The system of anionic surfactants is another example of organic compounds mixtures. The procedure of their determination is proposed using coordinate pH in two-dimensional spectra of ionic associates anionic surfactants with rhodamine 6G. This procedure was tested on the analysis of surfactant waters and domestic detergents. 

T. M. Schmitt, Analysis of Surfactants, Surfactant Science Series Vol. 40, Marcel Dekker, New York (1992). 

In Table 16 particular problems and analytical processes are summarized. The literature quoted in the table makes quick access to this subject possible for the reader. A general and detailed overview of the analysis of surfactants can, for example, be found in books of Longman [192], Cross [193], and Schmitt [194], as well as in the reviews of Raid [195,196] and Kunkel [197-199]. In the area of analysis of surfactants present in trace quantities and for metabolites the book by Swisher can be put to good use [200]. 

Today the analysis of surfactants includes three aspects of practical interest Process control in the manufacturing of surfactants... 

Remarkably, the first priority in process control of manufacturing alkanesulfonates is not the analysis of surfactants. Important criteria, such as for the sulfoxidation process, are the reactor temperature (30-38°C), the composition of... 

M. R. Porter, Recent Developments in the Analysis of Surfactants, Elsevier, London, 1991. 

Interfacial Phenomena in Biological Systems, edited by Max Bender Analysis of Surfactants, Thomas M. Schmitt (see Volume 96)... 

D.O. Hummel, Analysis of Surfactants, Atlas of FTIR-Spectra with Interpretations, Hanser/Gardner, Cincinnati, OH (1996). 

Some typical areas for SFC comprise waxes, surfactants and dyes. Marked advantages of SFC over GC in the analysis of surfactants (e.g. ethoxylates) have been reported [246]. It is arguably the best chromatographic method for the separation of nonionic surfactants, as reviewed by Cserhati and Forgacs [306]. pSFC-UV-ELSD has been used for separation of oligomers of the nonionic surfactant Triton X-100 [249]. 

The application range of cSFC-DFI-MS (Table 7.41) appears to be restricted either to the analysis of low-MW substances or to problems related to high-MW samples where low detection limits are not needed [124,444,445], The analysis of surfactants [446] by SFC-MS is frequently performed to demonstrate the feasibility of newly developed interface technology for practical applications. A rugged cSFC-MS method has been developed for the analysis of ethoxylated alcohols (AEs), which are non-ionic surfactants incorporated into a wide variety of industrial and consumer products [447]. cSFC-DFT-DFS was used for the analysis of low-MW, thermally unstable peroxides, and the higher-MW surfactants Triton X-100 and... 

For analysis of surfactants, i.e. detection, identification and quantification, LC-TSP-MS and MS/MS are also qualified methods for substance-specific information [600-602]. A mixture of non-ionic surfactants, comprising nonylphenol ethoxylates [C9Hi9-(CeH4)-0-(CH2-CH2-0)m-H], anionic surfactants and PEG, was... 

Capillary gel electrophoresis is becoming very widely used in the biotechnology field for the high resolution separation of DNA and peptides according to molecular weight, but it has limited application for the analysis of surfactants (Wallingford, 1996). CGE does result in an increase in the resolution per unit time over SEC for charged polymers (Poli and Schure,1992). 

Schmitt [17] in his book on the analysis of surfactants includes details of a number of HPLC-based procedures. LC-MS can be used for positive identification. Figure 29 shows the molecular ion mass spectrum for the surfactant lauryl hydrogen sulfate detectable as its (M—H) ion by positive ESI. 

Flow injection analysis—mass and tandem mass spectrometry in the analysis of surfactants—advantages and disadvantages 123 Horst Fr. Schroder... 

In addition to a general introduction to surfactants, the book comprises a comprehensive variety of analytical techniques, including sample handling, for the analysis of surfactants in the aquatic environment. Sample preparation includes automated solid phase... 

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. 

In off-line coupling of LC and MS for the analysis of surfactants in water samples, the suitability of desorption techniques such as Fast Atom Bombardment (FAB) and Desorption Chemical Ionisation was well established early on. In rapid succession, new interfaces like Atmospheric Pressure Chemical Ionisation (APCI) and Electrospray Ionisation (ESI) were applied successfully to solve a large number of analytical problems with these substance classes. In order to perform structure analysis on the metabolites and to improve sensitivity for the detection of the various surfactants and their metabolites in the environment, the use of various MS-MS techniques has also proven very useful, if not necessary, and in some cases even high-resolution MS is required. 

In this book, methods and data covering the state-of-the-art of modern analysis and environmental fate of the entire synthetic surfactant spectrum is provided. The first part deals with the analysis of surfactants... 

Gas chromatography (GC) has developed into the most powerful and versatile analytical separation method for organic compounds nowadays. A large number of applications for the analysis of surfactants have emerged since the early 1960s when the first GC papers on separation of non-ionics were published. The only major drawback for application of GC to surfactants is their lack of volatility. This can be easily overcome by chemical modification (derivatisation), examples of which will be discussed extensively in the following paragraphs. This chapter focuses on surfactant types, and in addition discusses some structural aspects of alkylphenol ethoxylates (APEOs) that are important for, as well as illustrative of, aspects of separation and identification that are linked to the complexity of the mixtures of surfactants that are involved. 

Numerous applications have been shown to exist that overcome the general problems of lack of volatility and instability at higher temperatures that principally hamper direct analysis of surfactants by GC methods. Thus, a whole suite of derivatisation techniques are available for the gas chromatographist to successfully determine anionic, non-ionic and cationic surfactants in the environment. This enables the analyst to combine the high-resolution chromatography that capillary GC offers with sophisticated detection methods such as mass spectrometry. In particular, for the further elucidation of the complex mixtures, which is typical for the composition of many of the commercial surfactant formulations, the high resolving power of GC will be necessary. 

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