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New Method for Surfactant Quantification by HPLC-GPC.

Gel permeation chromatography is effective on nonionic and anionic surfactants.

SURFACE ACTIVE agents (commonly known as surfactants) are used in many commercial products. Surfactants are found in cosmetics, personal care products, paints and coatings, cleaners and pharmaceutical preparations, to name just a few. In general, a surfactant can be described as a molecule or ion that has a hydrophobic (water-hating) end and a hydrophilic (water-loving) end. Thus one part of the molecule is more attracted to water, while the other side of the molecule is more attracted to interfacial surfaces or immiscible liquids. The observable results of this phenomenon are reduction in surface tension (e.g. foam or bubble formation) and formation of emulsion (e.g. lotions, creams). The ability of a surfactant to reduce surface tension or assist in emulsification is concentration-dependent. Too little, and the surface tension is not sufficiently reduced or the emulsion is not stable. Too much, and excessive cost is incurred with reduced performance. Therefore, the amount of surface active agent used in any given product can be very important to performance, shelf life and cost.

Surfactants can be categorized into four types: anionic (negatively charged), cationic (positively charged), nonionic (no charge) and amphoteric (neutrally charged, yet contains both negative and positive charges). This variety in chemistry makes for some difficulty in quantification of the given surfactant. Many methods have been employed to quantify surfactants, including methylene blue active substance (MBAS), cobalt thiocyanate active substance (CTAS), high performance liquid chromatograph (HPLC) and column chromatography.

MBAS is simple and precise, but is specific for anionic surfactants. Similarly, CTAS is specific to nonionic surfactants. Both are good methods for quantification of total active substances, but these methods only provide quantitative information for a given class of surfactant. Both MBAS and CTAS can have interferences from other compounds, so in some cases sample clean-up steps are necessary prior to analysis. HPLC has demonstrated good results when the identity of the compound is known. HPLC is highly sensitive, can be specific to the compound of interest, and can be automated to analyze numerous samples. Unfortunately, the selection of the analytical column is important and specific to each type of surfactant. Therefore the surfactant must be identified prior to analysis by HPLC to ensure that the proper column is selected.

Column chromatography can be utilized to isolate the surfactants from other ingredients in a commercial product. Quantification can be achieved gravimetrically with diligent efforts. Additionally, identification can be achieved using techniques such as fourier transform infrared spectroscopy. However, column chromatography can be labor-intensive and can utilize large quantities of solvents when many analysis are conducted.

All of the methods historically used can work to quantify and identify surfactants. An alternative method of analysis is gel permeation chromatography (GPC). GPC is a technique of separation of compounds by size or molecular weight. It is independent of interaction with the stationary phase, such as is required by other forms of chromatography. In many cases, it is also independent of chemical structure (i.e. charge, functionalities). The new method, detailed in this article, can be performed on any HPLC equipment, is easily automated, has been demonstrated as effective on anionic and nonionic surfactants, can provide information as to the molecular weight of an unknown compound and can provide quantitative information.

A New Method

Several nonionic surfactants, selected from different families of chemical structures, and one commonly used anionic surfactant were investigated for quantification by GPC. The goal of the research was to study the feasibility of GPC as a versatile method of quantification of surface active agents. Criteria of evaluation were selected with respect to family and molecular weight, detection limit, linear range, reproducibility, and ruggedness between analysts.

With this method, calibration curves for each analyte were generated from standard stock solutions prepared by dissolving surfactant in THF, with serial dilution from a stock solution. Each solution was injected into a series of GPC columns (linked in increasing pore size, covering the molecular weight range of 0-100,000 amu), contained in a Hitachi HPLC system. The mobile phase was 100% THF and the column was maintained at 35 [degrees] C. Peak detection was achieved using Refractive Index (RI) and UV (234 nm) detection. Each injection was fully completed in 40 minutes.

Table 1 contains a list of the actual detection ranges used for each surfactant tested. The specific surfactant and the manufacture of each are also listed in Table 1. Each analyte was investigated by different analysts so that the ruggedness of the method could be measured.


Promising Results

The results of the investigation are summarized in Table 1. The data clearly demonstrate the versatility of the method investigated for quantification of several families of surfactants. The linearity and detection limit for several families of surfactants utilizing both the RI and UV-Vis detectors were determined and were within acceptable limits. The detection limit was as low as 0.001% weight to weight for one nonionic surfactant. All the surfactants analyzed had a detection limit of at least 0.05% weight to weight. The detection limits are shown in Table 1.

The ruggedness of this method was demonstrated by utilizing three different analysts for several of the surfactants and two analysts for all the surfactants listed in Table 1. The analysis was carried out over a six-month period, thus demonstrating another indicator of the ruggedness of the assay. All three analysts that have worked with this method have obtained correlation coefficients of 0.99 or better with only one exception.

That exception, a single analysis of a nonionic surfactant, may have been an abommeration because a second analyst obtained an acceptable correlation coefficient when multiple injections of the same sample were used to demonstrate the reproducibility of the method. The multiple injections of the same sample provided a percent RSD of 0.49. This data is shown in Table 2. The recoveries of several surfactants were greater that 95%, as shown in Table 3. For comparison purposes, the range of 80-120% is an acceptable recovery range, according to the Environmental Protection Agency. The results are well within this acceptable range. The columns used allow quantification of numerous surfactants in the molecular range of 100amu to 100,000amu.
Table 2: Repeatability Results

Amount Retention(
Calculated % Time (min)))

0.105 23.19
0.106 23.21
0.106 23.20
0.106 23.17
0.106 23.20
0.105 23.15
Table 3: Percent Recovery

Surfactant % Recovery

Tween 95.2
Tergitol 97.7
Tergitol 15-S-7 100.6

There are some limitations to this method. The major limitation is the need to know the family of surfactant before beginning the quantification. This may be obtained by direct inject mass spectrometry. Mixtures of surfactants must have molecular weight differences of at least 500amu to be distinguishable on these columns. This limitation is shown in Figure 1, which is the chromatogram of Tween 80 and Tergitol 15-S-7. The average molecular weight of Tween, 80 is 1226amu while the average molecular weight of Tergitol 15-S-7 is 515amu. The separation in not baseline resolved, but the method will identify both peaks and a quantitation is possible. This method could be used to simply determine the molecular weight distribution of a surfactant if a complete quantition were not needed. This would be useful in deformulations to determine whether or not a polymeric surfactant was used.


With a little more refining, this method could also be adapted to the LC/MS. This would eliminate the need to identify the surfactant by directly injecting MS before the quantitation was performed. The LC/MS would also help resolve co-eluting peaks. This might help identify peaks from compounds with similar molecular weights.

There is a problem with mixtures of surfactants with similar molecular weights. The current method uses both RI and UV detectors for identifying the peaks. If two surfactants have similar RI and UV characteristics, the current method can not distinguish between the two compounds. This was demonstrated by analyzing a mixture of Sodium Lauryl Sulfate and PEG 600. The chromatogram for this analysis is shown in Figure 2. The method was unable to distinguish between the two compounds due to similar molecular weights, RI and UV characteristics. This method was able to obtain a total amount of surfactant present in the sample. By quantifying the SLS with a different method, the PEG was determined by the difference.



Our quanitification method is useful for a wide range of matrices. The data obtained from this method may be utilized by several industries to determine the amount of surfactants in personal care products, cosmetics and cleaning products. The method is useful for either the deformulation of products or for quality assurance checks of existing products. The number of products that utilize surfactants is broad and diverse, and the possible applications for this method are significant.

Our company plans to continue the development of this method and future plans include testing a wider range of types of surfactants, which would include anionic and cationic surfactants. Both types of surfactants should work well with the method due to the fact that the compound does not have to interact with the column type. The surfactant only needs to be detected by either a RI or UV detector. The solubility of the surfactant is not an important factor due to the availability of several types of GPC columns. These columns are capable of handling several types of solvents.

Lynette Lobmeyer is a staff chemist with Hauser Laboratories, materials product services division, located in Boulder, CO. Hauser Laboratories provides analytical testing, chemical synthesis, process development, custom chemical manufacturing and testing services to the cosmetics, personal care, household chemical specialty and pharmaceutical industries. The materials products services division specializes in problem-solving, failure analysis, deformulations and formulations. Ms. Lobmeyer can be reached via email at
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Author:Lobmeyer, Lynette
Publication:Household & Personal Products Industry
Date:Mar 1, 2001
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