Direct Analysis of Surfactants using HPLC with Charged Aerosol Detection

Importance of the Topic

Surfactants are widely used chemicals in consumer, industrial, and environmental applications. Their amphiphilic nature and structural diversity pose analytical challenges, as most lack chromophores and exist as mixtures of homologues. Reliable quantitation and characterization are essential for quality control, regulatory compliance, and advanced research.

Aims and Overview

This study evaluates high-performance liquid chromatography (HPLC) methods coupled with charged aerosol detection (CAD) using a dedicated Acclaim Surfactant Plus column. The goal is to demonstrate direct separation and detection of anionic, cationic, non-ionic surfactants, block copolymers, and complex mixtures under gradient and inverse-gradient conditions.

Methodology and Instrumentation

Samples were prepared by dissolving surfactants in isopropanol, isopropanol/water (1:1), or acetonitrile/water (1:1) at 10–20 mg/mL. The HPLC system comprised a Thermo Scientific Dionex UltiMate 3000 RSLC, employing an Acclaim Surfactant Plus column (4.6×250 mm) maintained at 30–40 °C. Mobile phases included 100 mM ammonium acetate (pH 5.4), 50 mM ammonium acetate in water/acetonitrile (9:1), n-propyl alcohol, and acetonitrile with 50 mM acetate. Gradient and inverse gradient programs optimized resolution and response stability. Detection used a Corona Ultra RS charged aerosol detector, with data acquisition and inverse gradient calculations performed in Chromeleon 7.1 SR1.

Main Results and Discussion

• A mixture of eight surfactants (five anionic, two non-ionic, one cationic) was baseline separated under dual-pump inverse gradient, yielding flattened baselines and consistent response factors.
• TWEEN 80 and TWEEN 85 polymers showed distinct elution profiles correlating with oleate content, confirming subcomponent resolution.
• Span 80, 83, and 85 homologues were differentiated by hydrophobic moieties, reflecting the increasing trioleate fraction.
• Pluronic F68 block copolymer quantitation achieved high precision (0.6–6.5 % RSD), linearity (r2 0.9995), and a limit of quantitation of 115 ng on-column.
• Analysis of a commercial laundry detergent revealed alcohol ethoxylates, sulfonate varieties, and other hydrophilic compounds in a single run.

Benefits and Practical Applications

• Universal, mass-based detection suitable for nonvolatile and semi-volatile surfactants lacking UV activity.
• High sensitivity (low ng on-column), broad dynamic range (>10^4), and reproducible quantitation (RSD <2%).
• Flexible method development via gradient/inverse gradient to optimize baseline and response stability.
• Applicable to quality control of formulations, environmental monitoring, and process analytics.

Future Trends and Potential Applications

• Integration with high-throughput systems and multidimensional chromatography for complex matrices.
• Extension to novel biosurfactants, fluorosurfactants, and microemulsion components.
• Coupling CAD with mass spectrometry or spectral detectors for structural elucidation.
• Advances in data processing algorithms for automated homolog distribution analysis.

Conclusion

The combination of an Acclaim Surfactant Plus column with charged aerosol detection offers a streamlined, sensitive, and reproducible platform for comprehensive surfactant analysis. Gradient strategies, including inverse gradients, enhance quantitation accuracy and facilitate the separation of structurally diverse and polymeric surfactants within 21 minutes.

References

• Nalco North America. News & Events. http://www.nalco.com/news-and-events/4297.htm (accessed February 2012).
• Chung H.H., Zhou C., Khor H.K., Qiu J. J. Chromatogr. A 2011, 1218(15), 2106–2113.