Electrospray ionization surfactants

Zhu, J. (2004). Liquid chromatography-mass spectrometry of nonionic surfactants using electrospray ionization. J. Surfactants Detergents 7(4), 421 -23. 

Til. Ozaki, H., Itou, N., Terabe, S., Takada, Y., Sakairi, M., and Koizumi, H. (1995). Micellar electrokinetic chromatography mass-spectrometry using a high-molecular-mass surfactant - online coupling with an electrospray-ionization interface.. Chromatogr. A 716, 69—79. 

Rundlett, K. L., and Armstrong, D. W. (1996). Mechanism of signal suppression by anionic surfactants in capillary electrophoresls-electrospray ionization mass spectrometry. Anal. Chem. 68, 3493-3497. 

Zhu, X., Kamande, M. W, Thiam, S., Kapnissi, C. P., Mwongela, S. M., and Warner, I. M. (2004). Open-tubular capillary electrochromatography/electrospray ionization-mass spectrometry using polymeric surfactant as a stationary phase coating. Electrophoresis 25, 562-568. 

Further information regarding the composition of mixed micelles can be obtained by coupling to mass spectrometry. However, the use of surfactants in electrospray ionization will always be fraught with difficulties because contamination of the interface with nonvolatile tensides causes undesirable effects. Stable mixed micelles can be measured only by employing nonequilibrium conditions (only buffers without micellar components). Two further variants of MEKC-MS have been developed and successfully used in recent years, but these are not readily employed for the electrophoretic characterization of micelles, since either a partial filling technique or surfactants of high molecular mass have to be used (37). 

The characterization of water-soluble components in slurries is one use of SPME with mixed solid-liquid samples. In one application, dried homogenized solid samples (10 mg of sewage sludge or sediment) were slurried in 4 ml of H,0 saturated with NaCl and adjusted to pH 2 with HCl for extraction for 1-15 h, which was followed by desorption into 4 1 methanol/ethanol over 2 min. The extracted compounds were either injected into a liquid chromatograph or fed directly via an electrospray ionization interface to a mass spectrometer with 1 s miz scans from 50-700 or selected-ion monitoring. The major components extracted included phthalates, fatty acids, non-ionic surfactants, chlorinated phenols and carbohydrate derivatives [235]. 

W Lu, GK Poon, PL Carmichael, RB Cole. Analysis of tamoxifen and its metabolites by on-line capillary electrophoresis-electrospray ionization mass spectrometry employing nonaqueous media containing surfactants. Anal Chem 68 668, 1996. 

Loo, R.R., Dales, N., Andrews, RC. (1994) Surfactant effects on protein structure examined by electrospray ionization mass spectrometry. Protein Science, 3(11), 1975-1983. 

However, an alternative to using surfactant systems is to use nanopartide-based PSPs directly. They are more compatible with mass spectrometric (MS) detection and do not hamper electrospray ionization (ESI) [49]. In that respect, nanopartides from silica [50], gold [51] and polymers [52] and even molecularly imprinted nanoparticles [53] have been used. Imprinting is based on a technique for tailor-making network polymers, where templates for a specific solute give a high separation affinity. 

The following protocol describes a mass spectromet-ric method for characterization of a block copolymer consisting of methoxy poly(ethylene oxide) (mPEO), an -e-caprolactone (CL) segment, and linoleic acid (LA), used as surfactant in water-based latex paints by liquid chromatography electrospray ionization (LC-ESI) or API-MS [10]. 

Several methods have been developed specifically for naphthenic acids, a class which includes the surface active carboxylate surfactants. Naphthenic acids are present as a complex mixture of a number of homologues with only a small range in molar mass (166-450 mol/g), little change in solubility character, and have been difficult to assay using conventional analytical methods. Methods such as negative ion-mode mass spectrometry using fast atom bombardment (FABMS), have been successfully applied to the analysis of naphthenic acid mixtures [93, 94], Other promising techniques include fluoride ion chemical ionization mass spectrometry (FI-MS) [95], and electrospray ionization mass spectrometry (ESIMS), which may allow for the quantification of the various naphthenic acid fractions [96]. 

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. 

Eichhom P, Knepper TP. Electrospray ionization mass spectrometric studies on the ampho-terie surfactant cocamidopropylbetaine. J Mass Spectrom. 2001 26 677-84. 

Eielihom, R Knepper, T.P. Electrospray Ionization Mass Spectrometric Studies on the Amphoteric Surfactant Cocami-dopropylbetaine. J. Mass Spectrom. 2001, 36, 677-684. 

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. 

It is a challenge to analyze surfactants by the most common configuration of MS GC-MS. As discussed in Chapter 8, most surfactants are not sufficiently volatile to pass through the GC unless they are first derivatized or degraded to form volatile products. Liquid chromatography-MS presents problems of its own, since there is no one LC-MS interface suitable for all compounds. The use of LC-MS in characterizing surfactants in the environment has been reviewed (1). Electrospray ionization gives better results for surfactants than thermospray, with not much published on particle beam or atmospheric pressure ionization of surfactants. 

Electrospray ionization. ESI-MS in the negative-ion mode is effective for MS of alkyl sulfates in the surfactant range of C12-C15 chain length (16). The parent ion by itself is not sufficient for unequivocal identification, but this is provided by coupling with LC (31) or by CAD analysis of the parent ion (15). 

Atmospheric pressure chemical ionization. LC-MS with acetonitrile/water/ammo-nium acetate or even columnless injection of acetonitrile solutions of APE gives molecular positive ion peaks or pseudomolecular adducts for all oligomers in the sample, limited only by the mass range of the spectrometer. Identification and quantification of individual peaks is difficult and requires experience to differentiate PEG from the surfactant (61,62). Positive molecular ions characteristic of NPE are of the form [44x + 220]", while those for PEG are [44x + 18] (18). Plomley et al. report that APCI is superior to electrospray ionization for determination of APE in the environment, because fewer interfering compounds are ionized (79). APCI is also more suitable for use with normal phase LC. 

We have previously reported on the coupling of an SEC to a mass spectrometer operated in the electrospray mode of ionization and its application to the molecular weight characterization of octylphenoxy-poly(ethoxy)ethanol oligomers (20). The analysis of nonionic surfactants... 

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. 

Mass spectrometric detection allows analysis of most nonionic surfactants without deriva-tization. Thermospray, electrospray, or atmospheric pressure chemical ionization interfaces permit direct introduction of the effluent of the LC into the MS and make the MS a very selective detector for nonionics. Quasimolecular ions are produced for each discrete compound, so that the HPLC system is not required to separate both by degree of ethoxylation and by alkyl character. A relatively simple HPLC separation, coupled with MS anal-... 

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