Cationic surfactants mass spectrometry

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. 

Applications of FAB have been succesfully performed in the characterization of a wide range of compounds (dyes, surfactants, polymers...) but little attention has been devoted to the capabilities of this technique to solve environmental concerns, such as organic pollutants identification in water. The widespread use of surfactants in the environment has required the emplo yment of both sensitive and specific methods for their determination at trace levels. GC/MS and HPLC procedures has been used for the determination of anionic (LAB s) and non ionic surfactants (NPEO) in water (1-4). Levsen et al (5) identified cationic and anionic sirrfactants in surface water by combined field desorption/ collisionally activated decomposition mass spectrometry (FD/CAD), whereas FAB mass spectrometry has been used for the characterization of pine industrial surfactants (6-8). 

A detailed analytical scheme is described elsewhere (9). Briefly, organic compounds from raw and drinking water were collected with GAC or XAD-2. A soxhlet extraction with dichloromethane was nsed to disadsorb the organics, evaporated and redissolved in ether. The ether solnble componnds were fractionated into acids and base+neutrals and analyzed by GC/MS and FAB. The ether-insolnble fraction was fractionated by HPLC-UV diode array detection usually in four fractions, and analyzed by FAB mass spectrometry. FAB spectra of surfactants were obtained using triethanolamine or thioglycerol+NaCl for anionic and cationic compounds, whereas the last was the most suitable for non ionic snrfactants and real samples. 

The same relates to the analytical methods, where for anionic, nonionic and cationic surfactants, various colorimetric procedures (MBAS, bismuth active substance(s) (BiAS), and disulfine blue active substance(s) (DSBAS)) were introduced. Since these methods analyse only the loss of surface-activity or primary degradation, other analytical approaches have been employed in the last 10 years in order to characterize the total or ultimate degradation. These include methods such as high performance liquid chromatography (HPLC), gas chromatography (GC) GC/mass spectrometry (MS), and the measurement of total organic carbon (TOC) and chemical oxygen demand (COD). 

Hyphenation between GC or LC and mass spectrometry (MS) has improved these techniques, however the high cost of the equipment limits its use in routine quality control. GC-MS has been used to determine different analytes in cosmetics (e.g., essential oils). LC-MS has also been used (e.g., ceramides, cationic and nonionic surfactants, preservatives). 

Chen, Y.-C., Tsai, M.-F. (2000) Sensitivity Enhancement for Nitrophenols Using Cationic Surfactant-modified Activated Carbon for Solid Phase Extraction (SPE)/Surface-assisted Laser Desorption Ionization (SALDI) Mass Spectrometry. Rapid Commun. Mass Spectrom. 14 2300-2304. 

Saraiva, S.S., Abdelnur, P.V., Catharine, R.R., Nunes, G., and Eberlin, M.N. 2009. Fabric softeners Nearly instantaneous characterization and quality control of cationic surfactants by easy ambient sonic-spray ionization mass spectrometry, Rapid Commun. Mass Spectrom., 23 357-362. 

Harvey, G. J. and Dunphy, J. C. Characterisation of cationic, anionic, and nonionic surfactants by positive and negative ion electrospray ionization mass spectrometry. Proceedings of the 40th ASMS Conference on Mass Spectrometry and Allied Topics, Washington, DC, May 31-June 5, 1992. 

Barco, M. Planas, C. Palacios, O. Ventura, F. Rivera, J. Caixach, J. Simultaneous Quantitative Analysis of Anionic, Cationic, and Nonionic Surfactants in Water by Electrospray Ionization Mass Spectrometry with Flow Injection Analysis. Anal. Chem. 2003, 75,5179-5136. 

The most recent advances in MS, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) were applied in positive ion mode to the analysis of three oral rinse and one commercial disinfectant formulation. In the latter, a complex mixture containing three dialkyldimethylammonium and alkylbenzyl-dimethylammonium (Ciq, C14, Cjs) salts was detected and in three oral rinse, cetylpyri-dinium chloride was the cationic surfactant present. Besides, the dissociation was verified as the mode of ionization of quaternary ammonium under MALDI conditions (Morrow et al, 2001). 

Batts and Paul [101b] used time-of-flight secondary-ion mass spectrometry (ToF-SIMS) to investigate the competitive adsorption of a cationic fluorinated surfactant (FC-134) at the gelatin-air interface. ToF-SIMS is a very sensitive surface analysis technique. In the static mode, the sampling depth of ToF-SIMS is only one to two monolayers. However, the ToF-SIMS data are difficult to interpret in quantitative terms and experimental conditions must be carefully controlled. Batts and Paul used positive secondary-ion spectra only, although negative-ion spectra may have been used as well. 

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