Surfactants quantification

A sensitive determination of alkanesulfonates combines RP-HPLC with an on-line derivatization procedure using fluorescent ion pairs followed by an online sandwich-type phase separation with chloroform as the solvent. The ion pairs are detected by fluorescence. With l-cyano-[2-(2-trimethylammonio)-ethyl]benz(/)isoindole as a fluorescent cationic dye a quantification limit for anionic surfactants including alkanesulfonates of less than 1 pg/L per compound for a 2.5-L water sample is established [30,31]. 

CE has been used for the analysis of anionic surfactants [946,947] and can be considered as complementary to HPLC for the analysis of cationic surfactants with advantages of minimal solvent consumption, higher efficiency, easy cleaning and inexpensive replacement of columns and the ability of fast method development by changing the electrolyte composition. Also the separation of polystyrene sulfonates with polymeric additives by CE has been reported [948]. Moreover, CE has also been used for the analysis of polymeric water treatment additives, such as acrylic acid copolymer flocculants, phosphonates, low-MW acids and inorganic anions. The technique provides for analyst time-savings and has lower detection limits and improved quantification for determination of anionic polymers, compared to HPLC. 

Applications Useful 2D NMR experiments for identification of surfactants are homonuclear proton correlation (COSY, TOCSY) and heteronuclear proton-carbon correlation (HETCOR, HMQC) spectroscopy [200,201]. 2D NMR experiments employing proton detection can be performed in 5 to 20 min for surfactant solutions of more than 50 mM. Van Gorkum and Jensen [238] have described several 2D NMR techniques that are often used for identification and quantification of anionic surfactants. The resonance frequencies of spin-coupled nuclei are correlated and hence give detailed information on the structure of organic molecules. 

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... 

Recent studies, including the use of Microtox and ToxAlert test kits [55,56], were carried out for the determination of the toxicity of some non-ionic surfactants and other compounds (aromatic hydrocarbons, endocrine disruptors) before implementation on raw and treated wastewater, followed by the identification and quantification of polar organic cytotoxic substances for samples with more than 20% inhibition. Furthermore, the study of their contribution to the total toxicity was obtained using sequential solid-phase extraction (SSPE) before liquid chromatography-mass spectrometry (LC-MS) detection. This combined procedure allows one to focus only on samples containing toxic substances. 

Commercial mixtures of surfactants comprise several tens to hundreds of homologues, oligomers and isomers of anionic, nonionic, cationic and amphoteric compounds. Therefore, their identification and quantification in the environment is complicated and cumbersome. The requirement of more specific analytical methods has prompted a replacement of many of the separate steps in traditional methods of analysis, usually non-chromatographic, by chromatographic tools. 

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 order to study simultaneously the behaviour of parent priority surfactants and their degradation products, it is essential to have accurate and sensitive analytical methods that enable the determination of the low concentrations generally occurring in the aquatic environment. As a result of an exhaustive review of the analytical methods used for the quantification within the framework of the three-year research project Priority surfactants and their toxic metabolites in wastewater effluents An integrated study (PRISTINE), it is concluded that the most appropriate procedure for this purpose is high-performance (HP) LC in reversed phase (RP), associated with preliminary techniques of concentration and purification by solid phase extraction (SPE). However, the complex mixtures of metabolites and a lack of reference standards currently limit the applicability of HPLC with UV- or fluorescence (FL-) detection methods. 

Obstacles of ionisation interferences in the quantitative determination of the N-containing surfactants from industrial blends dissolved together with AE compounds in methanol applying FIA-MS can be minimised or even eliminated if quantification was performed in the standard addition mode. So the standard deviations (SD) observed now reached a maximum of 7% for N-containing compounds whereas AE could be quantified with a SD of 4%. In parallel, the time investment for FIA-MS quantification in the standard addition mode, however, increased considerably and reached a factor of 3-4. 

An improved specificity was observed when FIA-MS-MS in product or parent ion mode was used to perform quantification of the surfactants in the methanolic mixtures. The ions selected for quantitation in product or parent ion mode were C13-AE m/z 71, 85, 99, 113, and 127 from alkyl chain together with 89, 133, and 177 from PEG chain generated from parent ions m/z 394, 526, 658, 790 and 922 alkylbenzyl dimethyl ammonium quat m/z 91 and 58 generated from parent ion m/z 214 FADA m/z 88, 106 and 227 generated from parent ions m/z 232, 260, 288, 316, 344 and 372 while the alkylamido betaine was quantified generating the parent ion m/z 343 obtained from product ion at m/z 240. 

The quantification of N-containing surfactants under MS-MS conditions but without standard addition resulted in a SD of 14%. In parallel, an increase in time by a factor of 1.5 was found compared with FIA-MS analysis without standard addition. The results obtained with MS-MS using product, parent or neutral loss scan are more... 

Since the lack of commercially available standards will make quantification impossible, the potential of LC-MS, which is often regarded as panacea in the analysis of polar compounds like surfactants, nevertheless remains limited, also. 

API-MS methods have been successfully applied to the quantification of M2D-C3-0-(E0)n-Me, with reliable and reproducible results obtained after online HPLC separation [29,30]. The method was used to quantify recoveries of the surfactant from the surface of plant foliage and from solid substrates under controlled laboratory conditions. Extension of the method to environmental samples has not been investigated. The entire linear dynamic range for HPLC-APCI-MS was not determined, but linearity was observed within the required... 

The prediction that LC-MS will become a powerful tool in the detection, identification and quantification of polar compounds such as surfactants in environmental analysis as well as in industrial blends and household formulations has proven to be true. This technique is increasingly applied in substance-specific determination of surfactants performed as routine methods. From this it becomes obvious that no other analytical approach at that time was able to provide as much information about surfactants in blends and environmental samples as that obtainable with MS and MS-MS coupled with liquid insertion interfaces. 

An industrial blend of ASs presented with its general structural formula in Fig. 2.11.4 was examined to elaborate a method for their quantification in trace amounts. ASs (CraH2n+i-0-S03) only were applied as surfactants in special applications. Moreover, these compounds are the basis of synthesis in the production of AES and therefore the trace analysis of AS in AES is an important task to estimate impurities. Applying APCI-FIA-MS(-), the overview spectrum in Fig. 2.11.5(a) containing [M - H] ions at mlz 265 and 293 was obtained. The mixture then was separated by RP-Cig and recorded by ESI-MS( —). The same [M - H] ions could be recognised that belonged to... 

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. 

Consequently, for an accurate quantification of surfactants in the water column, sampling must be performed by taking surface microlayer samples (at depths between 0 and 3—5 mm), using a surface sampler, and at various greater depths with Ruttner or similar bottles. 

REFERENCE COMPOUNDS IN QUANTIFICATION OF SURFACTANTS, THEIR METABOLITES AND REACTION BY-PRODUCTS... 

For sensitive quantification in LC-MS analysis of non-ionic surfactants, selection of suitable masses for ion monitoring is important. The nonionic surfactants easily form adducts with alkaline and other impurities present in, e.g. solvents. This may result in highly complicated mass spectra, such as shown in Fig. 4.3.1(A) (obtained with an atmospheric pressure chemical ionisation (APCI) interface) and Fig. 4.3.2 (obtained with an ESI interface). 

For several other non-ionic surfactant metabolites, standards are either commercially available, such as for dicarboxylated polyethylene glycols (DCPEGs) [20], or can be synthesised relatively easily, such as for nonylphenoxy acetic acids (NPECs) [21]. For quantification of metabolites for which standards are absent, the most similar available... 

The quantification of surfactants in environmental samples needs further development, particularly in so far as quality assurance of the analysis is concerned. Since the majority of the individual isomers and oligomers involved are not yet available as standards, quantification has to be based in part on external standards of commercially available mixtures. As this holds for both LC-FL and LC-MS analyses, both suffer from this shortcoming. Yet, samples analysed with both the methods have shown good agreement of resulting data. 

For the quantification of ethoxylated non-ionic surfactants in LC-MS, the effect of changing response factors with changes in EO chain length requires particular attention. The urgent need for proper internal standards has recently been addressed with the 13C-labelled NP and NPEO compounds that have become available. These will help improve the quality of LC-MS quantification of ethoxylated non-ionic surfactants significantly. 

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