Micellar-enhanced detection

Micellar-Enhanced Detection in Separation Science. In several instances, the use of appropriate surfactant media allows for enhanced and/or new modes for chromatographic detection. Most of the work has been in the area of spectroscopy. For example, it is well known that the presence of suitable surfactant micelles can significantly increase the absorbance of metal complexes (4,8,345). 

Table 12.1 Experimental Charaeteristics of Some MLC Procedures for the Determination of ComjKJunds using Micellar Enhanc l Detection... 

Matson, M. T., Ramstad, T., and Dunn, M. J. (2005). Purity determination of alprostadil by micellar electrokinetic chromatography with signal enhancement involving field-amplified sample stacking and extended path length detection. /. Liq. Chromatogr. Relat. Technol. 28, 3181—3203. 

A simple, rapid, sensitive, and selective spectrofluorimetric method (2ex/ lem = 345/455nm) has been developed for the determination of zaleplon. Tang et al. have studied the influence of micellar medium on the absorption, fluorescent excitation, and emission spectra character of zaleplon The nonionic surfactant of Triton X-100 showed a strong sensitizing effect for the fluorescence of zaleplon in a pH 5.0 buffer. The possible enhancement mechanism was discussed. Based on the optimum conditions, the linear range was 1.32 x 10 8-1.00 x 10 mol/1. The detection limit was 4.0 x 10 mol/1 with a relative standard deviation (RSD) of 0.06%. The proposed method was successfully applied to the determination of zaleplon in tablets, serum, and urine. 

Detection limits for 13 resolved PTH-amino acids ranged from 1.6 to 4.8.10 M, which was by a factor of ten superior to the detection limits reported for micellar electrokinetic chromatography [77]. Most of this improvement in detection limit is due to the higher thermo-optical enhancement produced by the high acetonitrile content in the buffer compared to the aqueous mobile phase used in the micellar electrokinetic separation. 

Likewise, the luminescence properties of many analytes can be altered in the presenoe of surfactant aggregates (4,7.,8.). Consequently, addition of micelle-forming surfactants (present either in the LC mobile phase or added post-column) can improve the sensitivity of fluorimetric LC detectors (49,482). Micellar spray reagents have been utilized to enhance the fluorescence densitometric detection of dansylamino acids or polycyclic aromatic hydrocarbons (483). The effect was observed for TLC performed on cellulose or polyamide stationary phases with the micellar spray reagent being either CTAC, SB-12, or NaC (483). More recently, use of nonionic Triton X-100 has been found to improve the HPLC detection of morphine by fluorescence determination after post-column derivatization (486) as well as improve the N-chlorination procedure for the detection of amines, amides, and related compounds on thin-layer chromatograms (488). 

Lewis acid catalysis in micellar systems [33] was first found in the model reaction in Table 14-6 [34]. While the reaction proceeded sluggishly in the presence of 0.2 eq. Yb(OTf)3 in water, remarkable enhancement of the reactivity was observed when the reaction was carried out in the presence of 0.2 eq. Yb(OTf)3 in an aqueous solution of sodium dodecyl sulfate (SDS, 0.2 eq., 35 mM), and the corresponding aldol adduct was obtained in 50% yield. In the absence of the Lewis acid and the surfactant (water-promoted conditions), only 20% yield of the aldol adduct was isolated after 48 h, while 33% yield of the aldol adduct was obtained after 48 h in the absence of the Lewis acid in an aqueous solution of SDS. The amount of the surfactant also influenced the reactivity, and the yield was improved when Sc(OTf)3 was used as a Lewis acid catalyst. Judging from the critical micelle concentration, micelles would be formed in these reactions, and it is noteworthy that the Lewis acid-catalyzed reactions proceeded smoothly in micellar systems [35]. It was also found that the surfactants influenced the yield, and that Triton X-100 was effective in the aldol reaction (but required long reaction time), while only a trace amount of the adduct was detected when using cetyltri-methylammonium bromide (CTAB) as a surfactant. 

Classically, the partition coefficients have been obtained from total solubility measurements, which are based on the ability of micellar solutions to enhance the solubility of compounds that are otherwise insoluble or sparingly soluble in water. The solubility limit for the solute at different smfactant contents is detected experimentally by, for example, turbidity or density measurements. 

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