Surfactant and Additive Analytes

4 INDUSTRIAL AND POLYMER ANALYTES 4.4.1 Surfactant and Additive Analytes 

Surfactants (surface active agents) are very important constituents of many industrial formulations. In these formulations, it is often not just one compound that is of interest. Rather, the overall identity, as determined by the presence and distribution of the individual components, is critical. Kondoh et al. [224] developed a method for determining non-ionic surfactants containing ester groups, such as sorbitan and sucrose fatty acid esters and polyoxyethylene fatty acid esters, as their o-nitro-phenylhydrazine derivatives. To remove the residual free fatty acid fraction, 6 mM triethylamine (TEA) was added to the 85/15 methanol/water starting mobile phase. The free fatty acids then eluted in the void volume and the separation of the analytes of interest was conducted on a C]g column (A = 550 nm). Elution and identification up to the penta-ester resulted when a 50-min 0/85/1575/25/0 ethanol/ methanol/water (6 mM TEA) gradient was used. 

Tsuda et al. [228] studied three alkylphenol polyethoxylates (4-nonylphenol, and nonylphenobnono- and nonyldiethoxylate) and three alkylphenols (4-r-butylpho)ol, 4-octylphenol, bisphenol A). These were isolated from fish and shellfish and separated on a phenyl column (A = 275 nm, ex 300 nm, em). A 25-min 40/60 -y 20/80 water/methanol gradient was used to generate baseline resolution of all peaks. A linear range of 0.05-2.0 pg/mL and a detection limit of 2 ng/mL (S/N = 3) were reported. 

Linear alkyl sulfonates (C1Q-C14) and sulfophenoxycarboxylic acids (C2-C11) were isolated from marine samples and simultaneously separated on a Cg column 

Detergents, linear alkylbenzenesulfonates (C9-C13), were separated on a Cg column (A = 225nm, ex 290 mn, em) using an 80/20 methanol/water (O.IM NaC104) mobile phase [230]. Peak shapes were excellent and elution was complete in 12 min. River water extracts were studied, with a 4ng detection limit claimed. 

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An unknown commercial detergent may contain some combination of anionic, nonionic, cationic, and possibly amphoteric surfactants, inorganic builders and fillers as weU as some minor additives. In general, the analytical scheme iacludes separation of nonsurfactant and inorganic components from the total mixture, classification of the surfactants, separation of iadividual surfactants, and quantitative determination (131). 

The overall set of partial differential equations that can be considered as a mathematical characterization of the processing system of gas-liquid dispersions should include such environmental parameters as composition, temperature, and velocity, in addition to the equations of bubble-size and residence-time distributions that describe the dependence of bubble nucleation and growth on the bubble environmental factors. A simultaneous solution of this set of differential equations with the appropriate initial and boundary conditions is needed to evaluate the behavior of the system. Subject to the Curie principle, this set of equations should include the possibilities of coupling effects among the various fluxes involved. In dispersions, the possibilities of couplings between fluxes that differ from each other by an odd tensorial rank exist. (An example is the coupling effect between diffusion of surfactants and the hydrodynamics of bubble velocity as treated in Section III.) As yet no analytical solution of the complete set of equations has been found because of the mathematical difficulties involved. To simplify matters, the pertinent transfer equation is usually solved independently, with some simplifying assumptions. 

The high water-solubility of surfactants and their, often more polar, metabolites prevents direct application of gas chromatographic separation (GC) with appropriate detection. The necessary volatilisation without thermal decomposition can be achieved by derivatisation of the analytes, but these manipulations are time- and manpower-consuming and can be susceptible to discrimination. Additionally, each derivatisation step in environmental analysis is normally target-directed to produce volatile derivatives of the compounds to be determined. Unknown surfactants that are simultaneously present, but differ in structure and therefore cannot react with the derivatisation reagent, are discriminated under these conditions. 

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. 

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. 

Anions and uncharged analytes tend to spend more time in the buffered solution and as a result their movement relates to this. While these are useful generalizations, various factors contribute to the migration order of the analytes. These include the anionic or cationic nature of the surfactant, the influence of electroendosmosis, the properties of the buffer, the contributions of electrostatic versus hydrophobic interactions and the electrophoretic mobility of the native analyte. In addition, organic modifiers, e.g. methanol, acetonitrile and tetrahydrofuran are used to enhance separations and these increase the affinity of the more hydrophobic analytes for the liquid rather than the micellar phase. The effect of chirality of the analyte on its interaction with the micelles is utilized to separate enantiomers that either are already present in a sample or have been chemically produced. Such pre-capillary derivatization has been used to produce chiral amino acids for capillary electrophoresis. An alternative approach to chiral separations is the incorporation of additives such as cyclodextrins in the buffer solution. 

Capillary electrophoresis (ce) is an analytical technique that can achieve rapid high resolution separation of water-soluble components present in small sample volumes. The separations are generally based on the principle of electrically driven 1011s 111 solution. Selectivity can be varied by (lie alteration of pH, ionic strength, electrolyte composition, or by incorporation of additives, Topical examples of additives include organic solvents, surfactants, and complexation agents. 

If the standard operating conditons do not provide the required separation, selectivity can be modified by changing a number of variables, including the nature of the surfactant and the aqueous phases. Altering the hydrophilic end of the surfactant has a dramatic effect since this is the end of the micelle that interacts with the solutes. Alternatively, a second surfactant can be added to form a mixed micelle. The addition of a nonionic surfactant to an ionic one decreases the migration time window so that the migration time of all the analytes decreases nonionic chiral surfactants are often added to MECC buffers for the separation of enantiomers. 

In aqueous solutions the micellar assembly structure allows sparingly soluble or water-insoluble chemical species to be solubilized, because they can associate and bind to the micelles. The interaction between surfactant and analyte can be electrostatic, hydrophobic, or a combination of both [76]. The solubilization site varies with the nature of the solubilized species and surfactant [77]. Micelles of nonionic surfactants demonstrate the greatest ability for solubilization of a wide group of various compounds for example, it is possible to solubilize hydrocarbons or metal complexes in aqueous solutions or polar compounds in nonpolar organic solutions. As the temperature of an aqueous nonionic surfactant solution is increased, the solution turns cloudy and phase separation occurs to give a surfactant-rich phase (SRP) of small volume containing the analyte trapped in micelle structures and a bulk diluted aqueous phase. The temperature at which phase separation occurs is known as the cloud point. Both CMC and cloud point depend on the structure of the surfactant and the presence of additives. Table 6.10 gives the values of CMC and cloud point for the surfactants most frequently applied in the CPE process. 

In both MEKC methods reported, the mobile phase consisted of borate buffer containing surfactant and acetonitrile. It was foimd that mobile phase prepared at pH 9.75 gave better resolution compared to other conditions, and increasing pH also increased the migration time of ezetimibe. An analyte concentration of 25 mM was chosen due to its lower current, and the sharp peaks observed [41]. In addition, for the analysis of ezetimibe in combination with another drug such as simvastatin, increasing the borate concentration increased both resolution and migration times. 

Capillary electrokinetic chromatography (CEKC) with ESI-MS requires either the use of additives that do not significantly impact the ESI process or a method for their removal prior to the electrospray. Although this problem has not yet been completely solved, recent reports have suggested that considered choices of surfactant type and reduction of electro-osmotic flow (EOF) and surfactant in the capillary can decrease problems. Because most analytes that benefit from the CEKC mode of operation can be effectively addressed by the interface of other separations methods with MS, more emphasis has until now been placed upon interfacing with other CE modes. For small-molecule CE analysis, in which micellar and inclusion complex systems are commonly used, atmospheric pressure chemical ionization (APCI) may provide a useful alternative to ESI, as it is not as greatly affected by involatile salts and additives. 

It has been known for a number of years that FD-MS is an effective analytical method for direct analysis of many rubber and plastic additives. Major components and impurities in commercial additives can be assessed quickly, and the FD-MS data can be used to help determine what (if any) additional analytical characterization is needed. Lattimer and Welch showed that FD-MS gives excellent molecular ion spectra for a number of polymer additives, including rubber accelerators (diWocarbamates, guanidines, benzothiazyl, and thiuram derivatives) antioxidants (hindered phenols, aromatic amines) p-phenylenediamine-based antiozonants, processing oils, and phthalate plasticizers. Zhu and Su characterized alkylphenol ethoxylate surfactants by FD-MS. Jackson et al. analyzed some plastic additives (hindered phenol antioxidants and a benzotriazole UV stabilizer) by FD-MS. ... 

Polymeric samples often contain impurities or additives (such as antioxidants, plasticizers, surfactants), and some of these compounds (for example, surfactants) are known to act as a "poison," depressing the efficiency of tire MALDI process. In order to enhance the number of ions produced by MALDI, it may be necessary to purify the analyte from these contaminants prior to analysis. Some MALDI matrices, such as all-trans retinoic acid, are particularly sensitive to impurities, whereas for other matrices (like HABA and dithranol ) the loss of efficency is small. Hence the latter matrices may be preferable when pmification is a problem. 

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