John Raia - Analytical Problem - Detection of Anionic Surfactants in Water
Surfactants are a class of organic compounds that are frequently utilized as emulsifiers and stabilizers in the formulation of consumer based cosmetic products. Examples include the anionic surfactants sodium lauryl sulfate and sodium octanesulfonate. These compounds are often incorporated into cleaning agents, shampoos and toothpastes with the purpose of facilitating and stabilizing oil/water emulsions.(1)
These compounds are used in large scale by consumers in the personal care product market, and therefore it is within the realm to assume that detection and treatment of water sources contaminated by surfactants is a necessary and frequent occurrence.
After the Deepwater Horizon catastrophe that occurred last spring, the EPA forced the BP oil company to expedite their decision on choosing a dispersant agent to clean up the oil slick covering a large portion of the Gulf of Mexico. Even with the possibility of using a wide array of safer alternatives, the Corexit® "sulfate surfactant "product line of dispersants was chosen by BP, in spite of the knowledge of its toxic effect on both human health and marine life. (2)
Even though the chemical composition for Corexit® has been kept a trade secret for many years, the EPA required its disclosure with the purpose of creating a life cycle assessment for the mixture that was introduced in large quantities in and around the Mississippi delta. Large amounts of the anionic surfactant Dioctyl sodium sulfosuccinate revealed to be a part of the Corexit® mixture. This compound has a wide variety of environmental and biological issues due to its known toxicity. These issues include human health concerns along with contamination and biological disruption of freshwater and marine life. Methods are currently being researched that will hopefully allow the detection of concentration of dioctyl sodium sulfosuccinate efficiently in an oceanic matrix. (3) An image of DOSS is shown below. (8)
Dioctyl sodium sulfosuccinate (CAS 577-11-7) can be purchased from Sigma-Aldrich in the form of a salt at 96% purity. Their product number is D201170. A purchase of 100g can be made for 28.00 USD. (9)
I would like to focus my efforts to research the various methods that are currently practiced in the analytical field to detect and characterize anionic surfactants as contaminants in water sources. Thus far, methods for detecting anionic surfactants that I have read about include
-Liquid Chromatography (4)
-Fluorescent absorption of attached dies (5)
-Chromo- and fluorogenic colorimetric detection (6)
-Ion pair chromatography (7)
The analyte(s) frequently used for these established methods have consisted of linear alkyl chains attached to anionic head groups, since these types of surfactants are most prevalent in consumer based products. I will therefore focus my interest on the detection of anionic, straight chained molecules such as sodium lauryl sulfate and/or sodium octanesulfonate in a water matrix. Hopefully after decent exploration into the subject matter, I will be able to make some assumptions and obtain some insight on an analytical method that may be utilized to detect dioctyl sodium sulfosuccinate in an oceanic matrix.
My hypothesis is that a similar method to those mentioned above can be used, but the process will have to be tweaked to account for the branched nature of dioctyl sodium sulfosuccinate and the other dissolved compounds found in an oceanic matrix. My revised hypothesis is that anionic surfactants should be present in higher concentrations in areas that have been exposed to dispersant chemicals compared to other natural water sources with with no known history of intentional contamination.
My hypothesis that the presence of Dioctyl sodium sulfosuccinate (DSS or DOSS) along with other anionic surfactants that are found in the Corexit dispersant, are still be migrating throughout the Gulf of Mexico with the oil that was not accounted for during its recovery process.
Since migrating oceanic species, such as dolphins and sea turtles, have known toxicology issues to petroleum and surfactants, if one were to take water samples from varying radial regions where these migrating sea animals corpses have been found on the coast of the Gulf, DOSS will be detected.
(1) Kosswig, K. In Surfactants; Ullmann's Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA: 2000
(2) Eilperin, J. Post Carbon: EPA demands less-toxic dispersant. The Washington Post [Online] May 20, 2010. http://views.washingtonpost.com/climate-change/post-carbon/2010/05/epa_demands_less_toxic_dispersant.html. (accessed Sept 19, 2011).
(3) Schor, Elana. Ingredients of Controversial Dispersants Used on Gulf Spill Are Secrets No More. The New York Times [Online] June 9, 2010. http://www.nytimes.com/gwire/2010/06/09/09greenwire-ingredients-of-controversial-dispersants-used-42891.html (accessed Sept 19, 2011).
(4) Boiani, J. Spectator ion indirect photometric detection of aliphatic anionic surfactants separated by reverse-phase high-performance liquid chromatography. Anal. Chem. 1987, 59, 2583.
(5) Qian, J.et al. Selective and sensitive chromo- and fluorogenic dual detection of anionic surfactants in water based on a pair of "on-off-on" fluorescent sensors. Chemistry - A European Journal 2009, 15, 319.
(6) Coll, C. et al. A simple approach for the selective and sensitive colorimetric detection of anionic surfactants in water. Angewandte Chemie (International ed.in English) 2007, 46, 1675.
(7) Ding, W. et al. Determination of linear alkylbenzenesulfonates in sediments using pressurized liquid extraction and ion-pair derivatization gas chromatography-mass spectrometry. Anal. Chim. Acta 2000, 408, 291.
(8) Dioctyl sodium sulfosuccinate. http://upload.wikimedia.org/wikipedia/commons/thumb/d/d5/Dioctyl_sodium_sulfosuccinate.png/320px-Dioctyl_sodium_sulfosuccinate.png (accessed Oct 26, 2011).
(9) Dioctyl sulfosuccinate sodium salt 96%. Sigma-Aldrich. http://www.sigmaaldrich.com/catalog/ProductDetail.do?D7=0&N5=SEARCH_CONCAT_PNO%7CBRAND_KEY&N4=D201170%7CALDRICH&N25=0&QS=ON&F=SPEC (accessed Oct 26, 2011).
UV-Vis absorption spectrometry
Anionic surfactants must be coupled with specific dyes in order to be detected by UV/Vis spectrometry. The dyes are the actual molecules that are being detected by UV/Vis, and not the surfactant itself. According to Yuxiu An, et al (10), their group was successful in detecting Sodium Lauryl Sulfate both in the UV and Visible spectrum. The dyes used in the experiment were the compounds:
- 8-hydroxy-1,3,6,-pyrenetrisulfonic acid tridsodium salte [HPTS]
- poly(3-(4-methyl-3'-thienyloxy)propyltrimethylammonium) [PMTPA]
With major absorptions found at 532 and 409 nm, respectively
Their method has an keen advantage because the presence of the specific surfactant could be detected qualitatively by a color change in solution (yellow to pink), along with a quantitative measurement by UV absorbance with a detection limit in the order 10^-9 M.
The only issue that was found was the inability of their chosen dyes to couple with shorter carbon chained surfactants, specifically octyl sulfates. The assumption was that the sterics of the short chained surfactants limited the stoichiometric dissociation of the the dyes, giving varied experimental results for known standard concentrations of surfactants dissolved in aqueous solutions. I feel as if this method still has potential, but specific types of dyes must be found/synthesized that will selectively couple to shorter chained surfactants without any steric interaction.
(10) Yuxiu A. et al. Disassembly-driven colorimetric and fluorescent sensor for anionic surfactants in water based on a conjugated polyelectrolyte/dye complex. Soft Matter. 2011, 7, 6873.
After searching the blog, I found that Joe Zibley's analytical problem, entitled I-131 in Japanese Milk Supply , is quite similar to my problem in question. Please read his posting and my comments thereafter to understand my reasoning.
update 9/29 at 6:15PM
My comment post does not seem to be showing up on Joe's blog post. In summary...
I found our analytical problems similar in nature because we are both require to use some type of coupling reagent that will make our analyte detectable in the UV/Vis spectrum. Also both of our analytes in question are found in an aqueous matrix.
Our problems differ however, in the fact that his analyte is a radioactive element, while mine is a low molecular weight, anionic chemical compound.
Similar Analytical Problem
Erik Sahlin's analytical problem involving Brevetoxin levels in the ocean I found most similar to mine. His hypothesis is that one can detect the amount of Brevetoxin levels in certain oceanic areas based upon the concentration of Brevetoxin found in dead fish that have been washed up on shore. His analyte/matix is obviously brevetoxin in the ocean.
I found this problem similar to mine in that both of our analytes are of a toxicity concern to both marine wildlife and human health. Erik said that according to his research thus far, the best method for detection of his analyte would involve using a multiwave spectrophotometer, which has the ability to detect particle size and structure as well as wavelength absorption. This differs from mine in the fact that my method for detection will most likely include UV-Vis absorption detection through the usage of selective dyes. Also brevetoxin is a cyclic ether, and therefore much larger molecule than SLS and sodium octaneosulfate.
We both decided an issue that we would have to overcome would be interference from other compounds dissolved in the oceanic matrix that may interfere with our analytic technique of choice. Other compounds that are common in specific oceanic areas need to be investigated and categorized by standards or filters to discriminate them from our analyte in question.
1) Suitable methods for separation of DOSS would include GC , reverse phase , HILIC , and ion exchange chromatography . I would not consider size exclusion much use, because DOSS is not a large scale macromolecule, or would be separated from such in the matrix. Also, affinity chromatography would not be that applicable as retention factors that can be used in biological systems with proteins and the like would not find use with DOSS or other anionic surfactants. Finally chiral chromatography for an alkyl sulfate anionic surfactant doesn't serve much purpose, because the molecules chirality would not be nearly as detectable as other forms of separation.
2) After skimming through a couple of papers that use the techniques above for the identification and separation of surfactant compounds from water sources, the most logical and straight forward method for separation involves reverse phase chromatography. This method is simple enough that, after sample cleanup, standards can be used to compare the presence of DOSS in a sample, in the low ppb range.
3) A commercial column that would be suitable for this application is the LiChrospher 100 RP-18 according to Petrovic M. and Barcelo D. This can be purchased from Merck (cat # 1.50983.0001), has a particle size of 5um, length of 250mm and i.d. of 4mm. It has a pH range of 2-7.5 and is packed with particles of silica and octadecyl derivatives. ,
4) The mobile phase that would be utilized for this column would be varying mixtures of acetonitrile/water (80:20) or methanol/acetonitrile (50:50)
5) Although they in the literature for this specific study by Petrovic M. they stated that they utilized an atmospheric pressure chemical ionization (ACPI) mass spec with scanning ranges of (100-800 m/z) and injection mode (ESI).This is because their study focused on detecting a range of surfactants, being both amphoteric, nonionic, and ionic in nature.
Further research into chemical ionization methods found that a Quadrupole Mass Spec set to a negative chemical ionization (NCI) would be the best bet... since DOSS is a stable negative ion by itself. It is also been seen through multiple papers in the literature that those choosing to use a NCI Quad were often focusing on environmental detection issues.
(1) Antonio, D. C. Characterization of surfactants and their biointermediates by liquid chromatography-mass spectrometry. Journal of Chromatography A 1998, 794, 165-185.
(2) Boiani, J. Spectator ion indirect photometric detection of aliphatic anionic surfactants separated by reverse-phase high-performance liquid chromatography. Anal. Chem. 1987, 59, 2583.
(3) Bruins, A. P.; Drenth, B. F. H. Experiments With The Combination Of A Micro Liquid Chromatograph And A Chemical Ionization Quadrupole Mass Spectrometer, Using A Capillary Interface For Direct Liquid Introduction : Some theoretical considerations concerning the evaporation of liquids from capillaries into vacuum. Journal of Chromatography A 1983, 271, 71-82.
(4) de Hoffmann, E. In Mass spectrometry : principles and applications /; pp 14.
(5) Ding, W. -.; Ding, W. Determination of linear alkylbenzenesulfonates in sediments using pressurized liquid extraction and ion-pair derivatization gas chromatography-mass spectrometry. Anal. Chim. Acta 2000, 408, 291.
(6) Furton, K. G. Determining the critical micelle concentration of aqueous surfactant solutions: Using a novel colorimetric method. Journal of chemical education 1993, 70, 254.
(7) Guo, P.; Guo, P. Determination of linear alkylbenzene sulfonates by ion-pair solid-phase extraction and high-performance liquid chromatography. Talanta 2011, 84, 587.
(8) Petrovic, M. Determination of anionic and nonionic surfactants, their degradation products, and endocrine-disrupting compounds in sewage sludge by liquid chromatography/mass spectrometry. Anal. Chem. 2000, 72, 4560.
(9) Poppe, A. Negative-ion mass spectrometry. X. A spurious [CH5]- ion: problems with negative chemical-ionization quadrupole instrument. Org. Mass Spectrom. 1986, 21, 59.
(10) Rivera-Rodríguez, L. B.; Rodríguez-Estrella, R.; Ellington, J. J.; Evans, J. J. Quantification of low levels of organochlorine pesticides using small volumes (≤100 μl) of plasma of wild birds through gas chromatography negative chemical ionization mass spectrometry. Environmental Pollution 2007, 148, 654-662.
(11) Wu, S. H.; Pendleton, P. Adsorption of Anionic Surfactant by Activated Carbon: Effect of Surface Chemistry, Ionic Strength, and Hydrophobicity. J. Colloid Interface Sci. 2001, 243, 306-315.
(12) Yokoyama, Y. Determination of alkylbenzenesulphonates in environmental water by anion-exchange chromatography. J. Chromatogr. 1993, 643, 169.
(13) LiChrospher® 100 RP-18 and RP-18 Endcapped | Merck Chemicals International http://www.merck-chemicals.com/lichrospher-100-rp-18-and-rp-18-endcapped/c_DMOb.s1LSAoAAAEWsOAfVhTl (accessed 11/10/2011)
1) The CZE method along with the MEKC method would work for the detection of anionic surfactants.
2) The best method would most likely be the CZE method. This should help separate out the other compounds that are found it he matrix, and most importantly even separate other surfactants found in the dispersant by size and charge.
3) Conditions should be reversed to obtain positive peaks. Detections dime set at a constant of 0.5 s with a collection of 20 pts per second. A fused silica capillary with an i.d of 50um and lenthg of 75 cm (50 cm from point of injection). pH of 6-7 with methanol as the solution. To separate out different types of anionic surfactants, buffer of napthalenemonosulfonate or p-toluenesulfoate can be mixed 50:50 with methanol. 
4) While the paper states using an IDP detection with inversed detection of other ionic compounds in solution. I think that using an ESI to a mass spec would work just fine, and therefore would not have to deal with UV detection, since surfactants do not exhibit any fluorescence without dye conjugation.
 Shamsi S., Danielson N. Individual and Simultaneous Class Separations of Cationic and Anionic Surfactants Using Capillary Electrophoresis with Indirect Photometric Detection. Analytical Chemistry 1995 67 (22), 4210-4216