Surfactants are widely used for industrial, agricultural, and
pharmaceutical applications, in products as diverse as pesticides,
detergent powders, petroleum products, cosmetics,
and pharmaceuticals. Their separation and identification
can be challenging due both to the diversity of surfactants
and the complexity of the sample matrix.
Many methods are available for surfactant determination,
such as titration, spectroscopy (nuclear magnetic resonance [NMR], infrared [IR], ultraviolet-visible [UV-VIS],
and mass spectrometry [MS]), and chromatography
(high-performance liquid chromatography [HPLC], gas chromatography [GC], thin-layer chromatography
[TLC], supercritical fluid chromatography [SFC], and capillary electrophoresis [CE]). HPLC is typically the
preferred technique for many surfactant analyses.
Reversed-phase and ion-exchange chromatography are
the most popular approaches, while normal-phase and size-exclusion chromatography are also used, depending
on the application. Although many HPLC stationary
phases are available and have been used for the analysis
of surfactants, none of these phases is capable of separating
anionic, nonionic, and cationic surfactants in a single
chromatographic run.
The Acclaim® Surfactant column (Dionex Corp., Sunnyvale,
CA) is designed for the separation of a variety of
different surfactants. Its proprietary surface chemistry
offers very high selectivity and capacity for separating
cationic, nonionic, and anionic surfactants in a single
chromatographic separation. The simple, volatile
mobile phases are compatible with mass spectrometry
detection, facilitating application of the column to
trace-level analyses of surfactants in various matrices,
including pharmaceutical formulations and environmental
samples.
The column offers the following advantages:
- High selectivity for the separation of anionic, nonionic,
and cationic surfactants
- Good peak shapes, especially for cationic surfactants
- Improved resolution for ethoxylated surfactants
- Compatibility with highly aqueous mobile phases
- Methods that are compatible with UV, evaporative light scattering detection (ELSD), MS, and conductivity detectors.
Features
High selectivity for the separation of anionic,
nonionic, and cationic surfactants
Figure 1 - Separation of cationic, nonionic, and anionic surfactants.
Reversed-phase C18 columns often provide satisfactory
peak shapes for anionic and nonionic surfactants. However,
when the analys i s of a mixture of anionic,
cationic, and nonionic surfactants is required, C18
columns fail to provide effective separation, due to the
inherent selectivity of such stationary phases as well as
the presence of residual silanols on the silica surface.
On the other hand, as shown in Figure 1, the Acclaim
Surfactant column offers high selectivity and capacity
for separating cationic, nonionic, and anionic surfactants
in a single run.
Good peak shapes for cationic surfactants
When using silica-based reversed-phase columns to analyze
cationic surfactants, it is often difficult to obtain
sharp, symmetrical peaks due primarily to the presence of
free silanols. The bonding chemistry of the Acclaim Surfactant
phase effectively deactivates free silanols toward
positively charged cationic surfactants, resulting in the
peak shapes shown in Figure 2. Conventional C18
columns yield severely tailing peaks with excessively long
retention times.
Figure 2 - Peak shape for cationic surfactants.
Figure 3 - Resolution for ethoxylated surfactants.
Improved resolution for ethoxylated
surfactants
The polarity of the Acclaim Surfactant column provides significantly
improved resolution for individual oligomers of
ethoxylated surfactants compared with conventional C18
columns. Figure 3 shows a comparison between the Acclaim
Surfactant column and a conventional C18 column for the
characterization of Triton X-100 (Rohm and Haas,
Philadelphia, PA). The Acclaim exhibits significantly
improved resolution between the oligomers.
Compatibility with highly aqueous mobile
phase conditions
Figure 4 - Analysis of highly hydrophilic hydrotropes.
Conventional C18 columns are often unsuitable for analyzing
strongly hydrophilic hydrotropes such as sodium naphthalene
sulfonate and xylene sulfonate. The problem arises because
these analyses require a highly aqueous mobile phase to
enhance hydrophobic retention, which often leads to undesirable
column dewetting. The surface chemistry of the Acclaim
Surfactant column makes it compatible with highly aqueous
mobile phases and suitable for analyzing highly hydrophilic
compounds. As illustrated in Figure 4, the column provides
high resolution between isomers of xylene sulfonate with good retention, while under the same conditions little or no retention
is observed on the conventional C18 column.
To accommodate various detection methods, such as UV, ELSD,
MS, and suppressed conductivity detection, several mobile phase
systems have been developed, depending on the specific application
requirements. For example, the ammonium acetate/acetonitrile
system is not only compatible with UV, but also the ELSD
and MS detectors. Another useful mobile phase contains acetic
acid and acetonitrile. In addition to its compatibility with UV,
ELSD, and MS, this mobile phase system provides satisfactory
results when analyzing cationic surfactants using suppressed conductivity
detection. To separate anionic surfactants using suppressed
conductivity detection, borate buffers are found to provide
low background and good results using the Acclaim
Surfactant column. As shown in Figures 1–8, the column demonstrates
its compatibility with all of the above-mentioned mobile
phases, providing quality results using various detection methods.
Figure 5 - Analysis of sodium dodecylbenzene sulfonate.
Figure 6 - Separation of cationic surfactants (ELSD).
Figure 7 - Analysis of Zonyl FSO fluorosurfactant.
Figure 8 - Analysis of polyethylene glycols.
Applications
Analysis of anionic surfactants
Anionic surfactants account for most of the surfactant use in the
United States, where they are popular ingredients in detergent
powders. This popularity results from their effectiveness compared
to other surfactants in particulate soil removal, especially
from natural fabrics, and because they are easily spray-dried.
Linear alkylbenzenesulfonates (LAS) are the most widely used
surfactants due to their low cost and rapid degradation under aerobic
conditions. The synthesis of LAS typically leads to a mixture
of positional isomers that results in a very complex sample matrix.
To simplify quantitative analysis, isocratic conditions are often
used to produce only single peaks for the same size homologue
species. As shown in Figure 5, LAS can be separated on the
Acclaim Surfactant column into simple, single peaks corresponding
to a homologous series, which is usually desirable for routine
quantitative analysis, whereas the Acclaim Polar Advantage (PA)
(Dionex) or conventional C18 columns provide higher resolution
between positional isomers, but rather complex chromatograms.
The Acclaim Surfactant column is suitable for analyzing other
anionic surfactants, such as alkyl sulfates, ether sulfates, alkanesulfonates,
petroleum sulfonates, and phosphate esters. Due to
space limitations, the relevant results will not be reported here.
Analysis of cationic surfactants
Cationic surfactants are used as fabric softeners, corrosion
inhibitors, and antimicrobial agents. The most popular
cationic surfactants include alkyl quaternary ammonium
salts, benzylalkylammonium salts, pyridinium salts, ester
quats, ethoxylated quats, and quaternary imidazolium compounds.
Figure 6 presents an example of separating a mixture
of common cationic surfactant on the Acclaim Surfactant column using ELSD and a volatile acetic acid buffer. Note
that this application can also be conducted by MS or suppressed
conductivity detection without modification.
Analysis of individual nonionic surfactants
Nonionic surfactants are another class of widely used surfactants.
Most nonionic surfactants are considered low-foaming
products, have good cold water solubility, and exhibit low critical
micelle concentrations. Figure 7 shows the chromatographic
analysis of Zonyl™ FSO perfluorosurfactant (DuPont, Wilmington,
DE) on an Acclaim Surfactant column using ELSD.
Analysis of polyethylene glycols
Polyethylene glycols (PEGs) are often nonsurfactant impurities
found in ethoxylated surfactants, typically in the range of 1–10%.
The oligomer distribution is similar to but broader than that of
the surfactant. Figure 8 illustrates the resolution of the Acclaim
Surfactant column for individual oligomers in various PEGs.
Analysis of surfactants in consumer products
The column is a convenient tool for analyzing surfactants in
various consumer products, such as laundry detergent, fabric
softener, shampoo, and mouthwash. Due to space limitations,
the relevant results will not be reported here.
Reproducible manufacturing
To meet the exacting needs of users, each Acclaim Surfactant
column is manufactured to stringent specifications to ensure
column-to-column reproducibility. Each column is shipped
with a lot validation sheet showing the test results and specifications
for the lot of bonded silica packed into the column. In
addition, each column is individually tested and shipped with
an individual test chromatogram validating the column performance,
with respect to selectivity, capacity, and efficiency.
Conclusion
The Acclaim Surfactant column is a high-efficiency specialty
column that is well suited for analyzing different kinds
of surfactants. By using volatile mobile phases, anionic,
cationic, and nonionic surfactants can be separated within a
single chromatographic run on this column using ELSD.
The column provides good peak shape for cationic surfactants,
improved resolution for oligomers in ethoxylated surfactants,
and compatibility with highly aqueous mobile
phases. It is not only suitable for the quality assurance of
individual surfactants, but also for the analysis of a variety of
surfactant-containing samples such as consumer products,
pharmaceuticals, plating baths for semiconductor manufacturing,
and environmental waters.
The authors are with Dionex Corp., 1228 Titan Way, Sunnyvale, CA
94088, U.S.A.; tel.: 408-737-0700; fax: 408-730-9403; e-mail:
[email protected].