The Application Notebook
Cationic surfactants are used as disinfectants, corrosion inhibitors, phase-transfer catalysts, wetting agents, dispersants, and even mobile phase modifiers in liquid chromatography.
Mark Tracy and Xiaodong Liu, Dionex Corporation
Cationic surfactants are used as disinfectants, corrosion inhibitors, phase-transfer catalysts, wetting agents, dispersants, and even mobile phase modifiers in liquid chromatography. They face two difficulties for successful analysis by LC. First, they tend to have very poor peak shape on conventional reversed-phase columns due to strong ionic interactions with residual silanol groups. Second, many cationic surfactants have no useful UV chromophore. The first problem may be solved by the use of specialized columns. The second problem may be solved by the use of mass spectrometry, charged aerosol detection (CAD), or evaporative light scattering detection (ELSD) with a volatile mobile phase.
The Acclaim® Surfactant is a high-efficiency, silica-based, specialty reversed-phase column designed for the analysis of anionic, cationic, nonionic, and zwitterionic surfactants. Here, the authors compare acetonitrile and four less expensive and less toxic solvents — methanol, ethanol, isopropanol, and acetone for the analysis of cationic surfactants using ELSD.
UltiMate® HPG 3400 RS pump, WPS 3000 TRS sampler, TCC 3000 column oven (Dionex Corporation, Sunnyvale, CA) and SofTA model 1400 evaporative light scattering detector (SofTA Corporation, Westminster, CO). Acetonitrile and methanol UV grade from B&J; ethanol, denatured 3A from BDH; isopropanol and acetone, semiconductor grade from General Chemical.
Figure 1 shows a comparison of the chromatograms of the five solvents under the same gradient program. Compared to acetonitrile (trace d), isopropanol (e) was a stronger solvent, while acetone (c), ethanol (b), and methanol (a) were the weaker solvents. The selectivity, efficiency, and resolution were quite similar for acetonitrile and acetone. Alcohols have higher viscosity than acetonitrile or acetone, which results in high operating pressures (as noted in Figure 1) and lower peak efficiency.
Figure 1: Alternative solvents for the analysis of cationic surfactants. Column: Acclaim Surfactant 3 Âµm, 3 Ã 150 mm; column temperature: 25 Â°C; flow rate: 0.425 mL/min; ELSD spray temperature: 55 Â°C; drift tube: 65 Â°C. Gradient program: initially at 35% organic solvent until 1.0 min; linear gradient to 75% organic until 13.5 min; maintained at 75% organic until 17.0 min. Mobile phases: buffer, 100 mM acetic acid in water, adjusted to pH 5.0 with ammonium hydroxide; organic solvent (a) methanol; (b) ethanol; (c) acetone; (d) acetonitrile; (e) isopropanol. Components approx. 75 Âµg/mL: 1. chloride; 2. bromide; 3. decyltrimethylammonium; 4. dodecyltrimethylammonium; 5. dodecylpyridinium; 6. dodecylbenzyldimethylammonium; 7. tetradecyltrimethylammonium; 8. tetradecylbenzyldimethylammonium; 9. hexadecyltrimethylammonium; 10. hexadecylpyridinium.
The Acclaim Surfactant column provides excellent separation capability for cationic surfactants. Several less-toxic, lower-cost, and more readily available solvents can replace acetonitrile. When using ELSD or CAD detectors, acetone can substitute for acetonitrile. When UV detection is required, methanol and ethanol are useful alternatives.
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