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Anal. Chem. 1996, 68, 293-299
Application of Nonaqueous Capillary Electrophoresis to the Separation of Long-Chain Surfactants Hossein Salimi-Moosavi and R. M. Cassidy*
Chemistry Department, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan S7N 5C9, Canada
The potential of nonaqueous capillary electrophoresis (CE) for the separation of hydrophobic solutes has been examined for a series of alkanesulfonates (C2-C16 ), alkyl sulfates (C8-C18), and linear alkyl benzenesulfonates. Sample loss and band broadening observed in aqueous CE separations was not observed in nonaqueous systems. Changes in separation selectivity induced by solvation and ion pair effects were examined. Separation selectivity was affected by changes in solvation and by a change from protic (methanol) to aprotic conditions (addition of acetonitrile). The addition of alkali metal ion caused changes in migration time via ion pairing but had only small effects on separation selectivity. However, the adjustment of the hydrophobicity of the electrolyte counterion was effective for separation optimization. The analytical features of these nonaqueous systems and applications to samples of shampoo, liquid detergent, and laundry detergents were also briefly examined. Alkanesulfonates and alkyl sulfates were separated in 0.01 mol/L sodium p-toluenesulfonate and 0.005 mol/L ptoluenesulfonic acid in methanol and in methanol/acetonitrile mixtures with indirect UV detection at 214 nm. Linear alkyl benzenesulfonates were separated in methanol and methanol/acetonitrile mixtures with tetramethylammonium as a counterion and direct detection at 214 nm. Capillary electrophoresis (CE) has emerged as a powerful technique for the separation of a wide variety of inorganic and organic compounds1-4 since its introduction by Jorgenson and Lukacs in the early 1980s.5-7 The vast majority of CE separations have been accomplished in aqueous media, but a limited number of studies in nonaqueous systems8-11 have shown the usefulness of this approach. Of particular interest in nonaqueous systems is the potential for adjusting relative migration rates via changes in solvation and ion pairing. Some evidence for such analyte/ (1) Jorgenson, J. W. Trends Anal. Chem. 1984, 52-54. (2) Grossman, P. D.; Colburn, J. C.; Lauer, H. H.; Nielsen, R. G.; Riggin, R, R. M.; Sittampalam, G. S.; Rickard, E. C. Anal. Chem. 1989, 61, 1186-1194. (3) Jones, W. R.; Jandick, P. J. Chromatogr. 1992, 608, 385-393. (4) Monning, C. A.; Kennedy, R. T. Anal. Chem. 1994, 66, 280R-314R. (5) Jorgenson, J. W.; Lukacs, K. D. Anal. Chem. 1981, 53, 1298-1302. (6) Jorgenson, J. W.; Lukacs, K. D. J. Chromatogr. 1981, 218, 209-216. (7) Jorgenson, J. W.; Lukacs, K. D. J. High Resolut. Chromatogr. Chromatogr. Commun. 1981, 4, 230-231. (8) Walbrohel, Y.; Jorgenson, J. W. J. Chromatogr. 1984, 315, 135-143. (9) Sahota, R.; Khaledi, M. G. Anal. Chem. 1994, 66, 1141-1146. (10) Salimi-Moosavi, H.; Cassidy, R. M. Anal. Chem. 1995, 67, 1067-1073. (11) Jansson, M.; Roeraade J. Chromatographia 1995, 40 (3/4), 163-169.
electrolyte interactions has been reported for simple inorganic anions where complete reversal of separation orders was possible.10 Nonaqueous CE systems should also offer advantages for hydrophobic ionic species, and in this paper the features of nonaqueous systems are examined further with anionic surfactants as test analytes. Anionic surfactants are important additives in many industrial processes and products, and their analysis is a great concern from industrial, environmental, and pharmaceutical viewpoints. Commonly used analytical approaches include gas chromatography (GC)12 and liquid chromatography (LC).13-16 In GC, timeconsuming and often complex derivatization procedures are required. LC approaches with ion-exchangers or reversed phases have been widely used, but there are problems associated with strong sorption or incomplete separation of all components. Only a few CE methods have been reported for some common anionic surfactants,17-23 and since CE methods are potentially applicable to most classes of surfactants, further development in this area would be of widespread interest. Although analytical applications of CE to surfactant analysis has been limited, surfactants have been used extensively to control both electroosmotic flow and undesirable sorption of analytes. These applications have been mainly a result of the ability of surfactants to sorb strongly onto capillary walls. Unfortunately, this property complicates the analytical application of CE to surfactants, and for long-chain surfactants, micelle formation might also present problems.24 Consequently, nonaqueous CE systems may prove useful for the elimination of some of these drawbacks. The classes of anionic surfactants chosen for this study were linear alkyl benzenesulfonates (LASs), alkyl sulfates (CnSO4), and alkanesulfonates (CnSO3), all anionic surfactants, which, are widely used in many different commercial products.13 Alkanesulfonates and alkyl sulfates are used in many shampoos and liquid (12) Yamini, Y.; Ashraf-Khorassani, M. J. High Resolut. Chromatogr. Chromatogr. Commun. 1984, 17, 634-638. (13) Schmitt, T. M. In Analysis of surfactants; Schick, M. J., Fowkes, F. M., Eds; Surfactant Science Series 40; Marcel Dekker Inc.: New York, 1992. (14) Castles, M.; Moore, B. L.; Ward, S. R. Anal. Chem. 1989, 61, 2534-2540. (15) Hoeft, C. E.; Zollars, R. L. J. Liq. Chromatogr. 1994, 17, 2691-2704. (16) Zhou, D.; Pietrzyk, D. J. Anal. Chem. 1992, 64, 1003-1008. (17) Chen, S.; Pietrzyk, D. J. Anal. Chem. 1993, 65, 2770-2775. (18) Romano, J.; Jandik, P.; Jones, W. R.; Jackson, P. E. J. Chromatogr. 1991, 546, 411-421. (19) Shamsi, S. A.; Danielson, N. D. Anal. Chem. 1994, 66, 3757-3764. (20) Zweigenbaum, J. Chromatogram 1990, 8, 9-10. (21) Desbene, P. L.; Rony, C. M. J. Chromatogr. 1995, 689, 107-121. (22) Desbene, P. L.; Rony, C.; Desmazieres, B.; Jacquier, J. C. J. Chromatogr. 1992, 608, 375-383. (23) Nielen, M. W. F. J. Chromatogr. 1991, 588, 321-326. (24) Weiss, C. S.; Hazlett, J. S.; Datta, M. H.; Danzer, M. H.J. Chromatogr. 1992, 608, 325-332.
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detergents, and mixtures containing similar alkyl chain lengths are difficult to separate by LC. An aqueous CE separation of alkyl sulfates and alkanesulfonates has been reported,17 where alkali metal ions were used to increase migration times, and thus resolution. Although this approach offered some improvement in resolution of mixtures of these compounds, unfortunately, it did not offer changes in relative selectivity, nor were chain lengths above C14 studied. LASs are biodegradable surfactants used in many commercial laundry detergents. The LAS components in detergents are normally a mix of parasubstituted alkyl homologues with chain lengths from C10 to C14. In addition, LAS formulations contain isomers due to attachment of the benzyl group at different positions along the alkyl chain. Chromatographic techniques have been reported for separation of LAS,14-16 but complete characterization is difficult. Liquid chromatographic methods have been developed that provide partial resolution of major and minor components, but complete separation takes ∼1 h.25 Alternate CE approaches have been reported,17,22 but these approaches either gave little17 or no22 positional isomer separation or had long separation times.22 In addition, degraded resolution was observed for real samples,22 and some analysts have also found poor reproducibility for aqueous CE separations of LAS samples.26 In this study, the potential of nonaqueous CE has been further examined with LASs, alkyl sulfates, and alkanesulfonates as test solutes. Experimental factors evaluated for adjustment of selectivity included water content, solvent composition (methanol/ acetonitrile), and nature of the counterion. Alkanesulfonates and alkyl sulfates were detected using indirect detection at 214 nm, and alkyl benzenesulfonates were detected by direct absorbance at 214 nm. Analytical performance factors and potential application to real samples were examined briefly.
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Procedures. Shampoo, detergent, and LAS samples were dissolved in methanol (0.1% w/v), filtered with Whatman filter paper, Cat. No. 1450-125 (Whatman International Ltd., Maidstone, England), diluted to the desired concentration with the electrolyte, and filtered through a 0.2 µm Nylon-66 membrane syringe filter immediately prior to use. Capillaries were conditioned by washing with methanol for 1 h, followed by another 2 h with the separation electrolyte by the use of a 10 cm height differential; overnight, capillaries were stored in methanol. All glassware was rinsed with a chromic/sulfuric acid solution, followed by water and acetone, and then dried in an oven. The number of theoretical plates was calculated from N ) 5.54(tR/w0.5)2, where tR is the migration time of analyte, and w0.5 is the peak width at half-height. Electroosmotic mobilities were calculated from µeo ) Ll/teoV, where µeo is electroosmotic mobility, teo is the migration time for the neutral marker in seconds, V is the separation voltage in volts, L is the capillary length, and l is the injection-to-detection length in centimeters. The electrophoretic mobilities (µep) of ions were calculated from µep ) µeo + Ll/tepV. Detection limits were determined from DL ) peak height/(2(peak-to-peak noise)), where DL is the detection limit. The linearity of calibration curves for ions was evaluated from plots of sensitivity versus concentration.27 The sensitivity, S, was obtained from S ) (I - b)/C, where I and C are the response and concentration, respectively, and b is the y-axis intercept obtained from the analysis of least-squares regression using the original response-concentration data.27
EXPERIMENTAL SECTION Instrumentation. A Quanta 4000 (Waters Chromatography Division of Millipore, Milford, MA) CE unit was used for UV detection (214 nm). Capillaries were 75 µm i.d. and 370 µm o.d. (Polymicro Technologies, Phoenix, AZ), with an end-to-end length of 50 cm and an end-to-detection window length of 43 cm. Samples were injected hydrostatically by elevation of the sample vials to 10 cm for 5 s unless specified otherwise. Applied voltage for both separation and electroosmotic measurements was 20 kV (either positive or negative). The tip of the solvent peak and also benzyl alcohol were used as a neutral marker for the electroosmotic flow measurements. Capillaries were washed with separation electrolyte for 1 min between runs by purging (15 mmHg). Chemicals. Tetramethylammonium hydroxide (TMAOH) and tetraethylammonium hydroxide (TEAOH) (25% w/v in methanol) were purchased from Sigma Chemical Co. (St. Louis, MO). HClO4, HCl, methanol, and acetonitrile were purchased from BDH (BDH, Toronto, Canada). p-Toluenesulfonic acid (PSAH) and the p-toluenesulfonic acid sodium salt (PSA) were purchased from Aldrich Chemical Co. (Milwaukee, WI). Stock solutions (0.01 mol/L) of all sodium alkanesulfonates and sodium alkyl sulfates (Aldrich) were prepared in methanol, and standard solutions were diluted to the desired concentration with the separation electrolyte. All electrolytes and samples were filtered through a 0.2 µm Nylon-66 membrane syringe filter (Cole-Parmer, Chicago, IL) immediately prior to use.
RESULTS AND DISCUSSION Separation of Alkyl Sulfates and Alkanesulfonates in Methanol. The problems associated with the separation of longchain surfactants in aqueous media are illustrated by the results of separations of alkanesulfonates and alkyl sulfates in aqueous systems, shown in Figures 1A and 2A. As the size of the surfactants increased, peak tailing was observed, and for even larger surfactants, peaks were lost completely. From seven alkanesulfonates (C2-C16), only five (C2-C10, see Figure 1A) were observed, and from four alkyl sulfates (C8-C18), only C8 and C12 (see Figure 2A) could be observed. Although previous studies of the separation of alkanesulfonates and alkyl sulfates17 showed peaks for chain lengths as large as C14, these present results show that aqueous systems are not ideally suited for separation of these types of compounds. The observed sample loss may be due to hydrophobic adsorption of these surfactants on the capillary wall and/or ion-ion interactions that cause precipitation of the longer chain surfactants; the latter case may be more important because after standing for 1 day, the sample solutions became cloudy. For initial evaluation of nonaqueous systems for surfactant separations, methanol was chosen as the solvent, primarily because it possesses a favorable dielectric constant and viscosity, and its solvating properties should reduce hydrophobic interactions. In addition, previous studies have shown that it provides unique separation selectivities for inorganic anions,10 and also that it permits the separation of polyethers via interactions with cation additives, which could not take place in water.28 Since alkanesulfonates and alkyl sulfates could not be detected by UV absorbance, detection was performed in methanol using indirect UV absorbance at 214 nm, which has been shown previously to work well for the detection of inorganic ions.10 Thus separations
(25) Chen, S. H.; Pietrzyk, D. J. J. Chromatogr. 1994, 671, 73-82. (26) James M. Jordan, Procter & Gamble Co., private communication.
(27) Cassidy, R. M.; Janoski, M. LC-GC 1992, 10 (9), 692-696. (28) Okada, T. J. Chromatogr. 1995, 695, 309-317.
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Figure 1. Separation of alkanesulfonates (A) in water and (B) in methanol. Experimental conditions: electrolyte, 0.01 mol/L PSA and 0.005 mol/L PSAH; indirect detection at 214 nm; 50 cm capillary (75 µm i.d.) with injection-to-detection length of 43 cm; separation voltage, -20 kV; injection time, 15 s; sample concentration, 2 × 10-4 mol/L diluted with the separation electrolyte. Peak identification: (1) C2SO3, (2) C4SO3, (3) C5SO3, (4) C8SO3, (5) C10SO3, (6) C14SO3, (7) C16SO3.
Figure 2. Separation of alkyl sulfates (A) in water and (B) in methanol. Experimental conditions: electrolyte, 0.01 mol/L PSA and 0.005 mol/L PSAH. Peak identification: (8) C8SO4, (9) C12SO4, (10) C14SO4, (11) C18SO4. Other conditions as in Figure 1.
were performed with an electrolyte containing 0.01 mol/L sodium p-toluenesulfonate and 0.005 mol/L p-toluenesulfonic acid in water, methanol, and water/methanol mixtures; basic conditions were not used because large counter electroosmotic flows (injection polarity was negative) produced long migration times. The
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Figure 3. Effect of the addition of water on the mobilities of alkanesulfonates and alkyl sulfates in methanol. Experimental conditions as in Figure 2.
indirect detection reagent, p-toluenesulfonate, offered good sensitivity and had a mobility close to those of the alkanesulfonates and alkyl sulfates. PSAH was used along with PSA in the electrolyte because earlier results10 showed that without the presence of excess acidic or basic compounds, electroosmotic flows were irreproducible due to the addition or depletion of protons as a result of electrochemical reactions at the anode and cathode. Thus, acidic or basic conditions were used for all the results discussed in this paper. The potential advantages of nonaqueous systems for the separation of hydrophobic ionic species is readily apparent from a comparison of separations of alkanesulfonates and alkyl sulfates in methanol (Figures 1B and 2B) with those obtained in water (Figures 1A and 2A). The results obtained in methanol show good peak shapes for all of the species, unlike the separations in water, and the separations in methanol also take place in a relatively short time. The good peak shapes for methanol separations are reflected by the efficiency data, which showed that the separation efficiency in methanol (336 000-250 000 theoretical plates) was consistently higher than that in water (N ) 170 000-80 000). In the methanol/water mixtures, separation efficiency for short-chain surfactants (C2-C5) had a maximum value at 50% v/v water in methanol (N ) 680 000-520 000), while maximum efficiency for C8- C10 was at 25% v/v water in methanol (N ) 450 000-310 000). The separation efficiencies for longer chain length surfactants were consistently decreased upon the addition of water in the separation electrolyte. Although the mobility patterns were similar in methanol and water, there were some shifts in relative migration times. To examine the effect of solvent on migration in more detail, electrophoretic mobilities were determined as a function of water content. The results in Figure 3 show that, in general, alkyl sulfates have higher mobility than alkanesulfonates (for the same alkyl chain length) and that, while small-chain surfactants show decreased mobility in methanol relative to that in water, the
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reverse is observed for the longer chain surfactants. Within each surfactant group, a similar pattern as a function of water content was followed, and for methanol/water mixtures, this general trend is predicted by the Von Smoluchowski equation from changes in viscosity (η) and dielectric constant ().10 However, changes in separation order were seen between 10 and 50% water compositions. Also, while each surfactant group showed an expected minimum in the plots, this is at ∼50% for alkyl sulfates and ∼25% for alkanesulfonates. The reason behind these different patterns was likely related to the different solvation and ion pairing properties of the sulfonate and sulfate functional groups; alkanesulfonates are expected to be more susceptible to ion pairing, and these aspects will be discussed in more detail later in this paper. The changes in relative separation order for alkanesulfonates and alkyl sulfates shown in Figure 3 would suggest that mixed aqueous/methanol systems might be useful for certain analytical problems. Unfortunately, however, the use of water in this composition range caused considerable broadening and, eventually, disappearance of the peaks for longer alkanesulfonates and alkyl sulfates (Cn > 12). Adjustment of Separation Selectivity for Alkanesulfonates and Alkyl Sulfates. CE separations of alkanesulfonates (C4C10) and alkyl sulfates (C6-C14) in aqueous media17 have shown that solute migration is affected when Mg2+ is added to the electrolyte; however, the performance of this system was not examined for the separation of longer chain alkyl sulfates and alkanesulfonates (>C14). Earlier studies of CE in nonaqueous electrolytes showed some evidence for the possible usefulness of ion-ion interactions for adjustment of selectivity,10 and since the interaction between cations and the surfactants of interest should be more extensive in nonaqueous systems, the addition of Ca2+, Mg2+, and Sr2+was examined. The effect of the addition of these three alkaline earth metals (0-5 mmol/L) on the mobilities of both alkanesulfonates and alkyl sulfates was studied in an electrolyte prepared with 0.01 mol/L PSA and 0.005 mol/L PSAH in methanol. The results in Figure 4 for the addition of Ca2+ show that the mobilities of all alkanesulfonates and alkyl sulfates decrease upon addition of Ca2+, and the extent of the decrease for alkanesulfonates is greater as the concentration of the metal ion is increased. The effect of the addition of Mg2+ and Sr2+ on the mobilities of alkanesulfonates and alkyl sulfates was similar to that of Ca2+, but the order of decreasing mobility was Mg2+ < Ca2+ < Sr2+. This order is consistent with the order of the decrease in solvation radii, with smaller ions being able to approach more closely to the anion to form a stronger ion pair. The possible importance of ion pairing effects was also demonstrated by the fact that the relative change in mobility with increasing concentration of the cation was slightly greater for alkanesulfonates, which are expected to show a greater tendency for ion pairing. Thus, although the addition of cations did not appreciably improve separation selectivity between adjacent peaks within one class of compounds, there was a small improvement in relative retention between the two classes of compounds as a function of the concentration of the metal ions. Consequently, while the separation of sulfonates and sulfates with chain lengths of C8-C16 will overlap without Ca2+, almost complete separation into two groups was expected with 3 mmol/L Ca2+ (see Figure 4). However, although the addition of metal ions will improve selectivity, they can cause precipitation of long-chain surfactants due to strong ion-ion interaction and will also increase the
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Figure 4. Effect of the Ca2+ concentration on the mobilities of alkanesulfonates and alkyl sulfates in methanol. Experimental conditions: (~) none, (9) 1 mmol/L Ca2+, ()) 2 mmol/L Ca2+, (() 3 mmol/L Ca2+, (0) 5 mmol/L Ca2+; solid lines are for alkanesulfonates and dashed lines are for alkyl sulfates. Other conditions as in Figure 2.
separation current, which could potentially deteriorate the separation efficiency. As will be discussed below, the separation of alkanesulfonates and alkyl sulfates can be more easily obtained via changes in solvation. Changes in the solvation properties by the use of solvent mixtures should influence solute solvation and ion-ion interactions, and since mixtures of nonaqueous solvents had previously resulted in changes in separation selectivity for inorganic anions, the potential of this approach was examined for alkanesulfonates and alkyl sulfate separations. Acetonitrile was chosen as the second solvent because it possess significantly different properties compared to methanol,29 and its effect on analyte mobility was studied for the range of 0-75% v/v; the maximum acetonitrile composition examined was 75% v/v, since the electrolyte constituents were not soluble at higher acetonitrile content. The results in Figure 5 show that for the two classes of compounds, two distinctly different patterns in mobilities were observed as a function of acetonitrile content. For the alkanesulfonates, the addition of acetonitrile caused an initial increase in mobility, followed by a decrease at higher concentrations of acetonitrile. With alkyl sulfates, addition of acetonitrile resulted in an increase in electrophoretic mobilities, which then leveled off at the higher concentrations of acetonitrile (Figure 5). Changes in ion mobility are expected for mixtures of solvents that have differing dielectric constants and viscosity (acetonitrile has a higher dielectric constant and lower viscosity), as described by the Von Smoluchowski equation.30 The pattern of behavior predicted by the Von Smoluchowski equation was calculated for C2SO3 and C8SO4 from
µep(normalized) ) µep(methanol)[/η](mixture)/[/η](methanol) Use of this equation assumes that the values for dielectric (29) Stathakis, C.; Cassidy, R. M. J. Chromatogr. 1995, 699, 353-361. (30) Shaw, D. J. Introduction to Colloid and Surface Chemistry, 3rd ed.; Butterworths & Co.: London, 1985; p 173.
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Figure 5. Effect of the addition of acetonitrile on the mobilities of alkanesulfonates and alkyl sulfates in methanol. Peak identification: (1) C2SO3, (2) C4SO3, (3) C5SO3, (4) C8SO3, (5) C10SO3, (6) C14SO3, (7) C16SO3, (8) C8SO4, (9) C12SO4, (10) C14SO4, (11) C18SO4, (12) C2SO3 (theoretical), (13) C8SO4 (theoretical). Other conditions as in Figure 2.
constants and viscosity for the mixed solvents are related to the volume percentage of each solvent and that the ζ potential for the ions is constant. The results of this calculation, shown in curves 12 and 13 in Figure 5, suggest that the mobilities of C2SO3 and C8SO4 should increase as the content of acetonitrile in the separation electrolyte is increased. This pattern is more or less observed for sulfates, but for sulfonates, significant negative deviations are observed for acetonitrile contents >50% v/v. Since the functional groups in alkanesulfonates are more hydrophilic than those in alkyl sulfates, increasing solvent hydrophobicity upon addition of acetonitrile may promote ion pair formation between alkanesulfonates and the counterion (sodium ion) and as a result slow the movement of alkanesulfonates. For alkyl sulfates, ion pair formation is likely not as important, and their mobilities continue to increase upon addition of acetonitrile. Thus, the addition of acetonitrile has a rather interesting effect on the relative electrophoretic mobilities of alkyl sulfates and alkanesulfonates, and this approach should be useful for the separation of mixtures of alkyl sulfates and alkanesulfonates. These results also suggest that the use of mixed nonaqueous solvents may be a useful approach for the optimization of the separation of other ions that differ only slightly in electrophoretic mobility. The separation of alkanesulfonates and alkyl sulfates was also studied in 0.02 mol/L PSAH and 0.01 mol/L butylamine in methanol and in mixtures containing either acetonitrile or water; the difference between this electrolyte and the one discussed above was the presence of n-butylammonium as the electrolyte cation in place of sodium. Addition of water to this n-butylammonium electrolyte had the same effect as for the above electrolyte. However, the patterns obtained from the addition of acetonitrile were only slightly different from those shown in Figure 5. The main difference was that both alkanesulfonates and alkyl sulfates had lower mobilities by almost 30-50% in all methanol/
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acetonitrile mixtures (compared to results in Figure 5). The primary cause of these differences relative to Figure 5 is likely from the butylamine cation forming more extensive ion pairs than sodium with both alkanesulfonates and alkyl sulfates. Reproducibility of migration times was good, and %RSD was 1-4% for both electrolytes (for three runs) over a period of 3 h. Alkanesulfonates and alkyl sulfates were also separated in basic solution (0.01 mol/L PSA and 0.005 mol/L butylamine in methanol), but their migration times were much longer than those in acidic conditions (by almost 5 times) because of larger electroosmotic flow. Consequently, this system was not investigated further. Separation of Linear Alkyl Benzenesulfonates. With direct UV absorbance detection at 214 nm and separation in a basic salt electrolyte (0.01 mol/L HClO4 and 0.02 mol/L NaOH) in methanol, long migration times were observed (about 50 min) because of a relatively fast counter electroosmotic flow. Faster separations were observed with an electrolyte containing 0.01 mol/L HClO4 and 0.02 mol/L butylamine in methanol with just as good resolution, possibly due to the fact that butylammonium cations are more effective than sodium for ion pairing interactions (see above discussion on alkanesulfonates and alkyl sulfates). The results showed that all major LAS components were well resolved in about 15 min, but there was no evidence of any positional isomers. To provide both more extensive ion pairing and longer retention times, LAS samples were separated with tetramethylammonuim hydroxide (TMAOH) instead of NaOH; previous studies with inorganic anions had indicated that TMAOH provided more effective ion pairing interactions. For these separations of LAS, 0.01 mol/L acetic acid was used instead of HClO4 because tetramethylammonuim perchlorate was not soluble in methanol. Separations were obtained with this electrolyte system for four different samples containing LASs (C15LAS was added to all samples as an internal standard), and two representative electropherograms are shown in Figure 6. Each of these separations shows a number of secondary peaks, which are most likely positional isomers, and it can be seen that the relative amounts of these components vary with the sample type. Further work would be required to fully characterize the components present in these secondary peaks. Additional enhancement of ion pairing interactions via use of tetraethylammonium cations caused very long migration times (>180 min), without any appreciable improvement in resolution of the minor positional isomers. The above results clearly show that separation resolution in nonaqueous systems can be manipulated via the selection of the electrolyte counterion (relative to analyte charge); further manipulation of resolution might be possible via selection of counterions with different steric and hydrophobic properties, but this was not examined further in this study. Since the addition of acetonitrile had provided interesting changes in the relative migration of alkanesulfonates and alkyl sulfates, this approach was also examined for LAS separations. Additions of 20, 30, and 50% v/v acetonitrile to a TMAOH/HClO4 methanol electrolyte increased migration times (>1 h in 50% acetonitrile), but provided only small improvements for the separation of either the major components or the positional isomers. Thus, not unexpectedly, it would appear that changes in solvent properties are of primary use where there are differences in chain lengths or in the properties of the analyte functional groups (i.e., sulfonates versus sulfates) and of less use for
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Figure 6. Separation of LASs in two different commercial LAS samples; (A) a detergent (1000 mg/L) and (B) a commercial LAS (100 mg/L). Separation electrolyte, 0.01 mol/L CH3COOH and 0.02 mol/L TMAOH in methanol; 50 cm capillary (75 µm i.d.) with injection-todetection length of 43 cm; injection time, 15 s; separation voltage, -20 kV; direct detection at 214 nm. Peak identification: (1) C10, (2) C11, (3) C12, (4) C13, (5) C14, (6) C15, (I) impurity associated with C15.
situations where there are differences only in the orientation of hydrophobic functional groups. Electroosmotic Flow. While the direction of electroosmotic flow in all electrolytes containing acetic acid and TMAOH in methanol was from the positive electrode toward the negative electrode, significant changes in electroosmotic flow were seen when acetonitrile was used to adjust the selectivity or when different electrolytes were used. Since electroosmotic flow is an important consideration in the optimization of CE separations, this aspect was examined briefly. Figure 7 shows a typical pattern observed for electroosmotic flow in methanol/acetonitrile mixtures. Changes in viscosity and dielectric constant predict a steadily increasing electroosmotic flow with acetonitrile concentration, as was the case for electrophoretic mobilities (see theoretical curves 12 and 13 in Figure 5). The decrease in electroosmotic flow for >50% acetonitrile (Figure 7) coincides with the changes in electrophoretic mobility for alkanesulfonates (see Figure 5). While increased ion pairing with the analytes would decrease analyte mobility, ion pairing of the separation electrolyte would normally be expected to increase electroosmotic flow due to a decrease in ionic strength. The expected connection between ion pairing and ionic strength was observed when tetraethylammonium was replaced with tetramethylammonium cations; with TEAOH, the electroosmotic mobility was 2.622 × 10-4 cm2 V-1 s-1, while that of TMAOH was 1.630 × 10-4 cm2 V-1 s-1. A possible reason for the decrease in electroosmotic flow beyond 50% acetonitrile (Figure 7) may be a decrease in the effective charge at the capillary surface due to an increase of the pKa of silanol groups upon the addition of acetonitrile, which would decrease the ζ potential10 at the silica surface. Applications. The separation of LASs was accomplished in three LAS products and two laundry detergents. The separations
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Figure 7. Electroosmotic mobility as a function of acetonitrile in the separation electrolyte. Separation electrolyte, 0.01 mol/L CH3COOH and 0.02 mol/L TMAOH in methanol; 50 cm capillary (75 µm i.d.) with injection-to-detection length of 43 cm; separation voltage, 20 kV; detection at 214 nm.
Figure 8. Separation of alkyl sulfates in shampoo. Sample concentration, 800 mg/L. Peak identification: (1) C12SO4, (2) C14SO4. Curve B is an expansion of a portion of curve A. Other conditions as in Figure 2.
showed clear evidence of different distributions of LAS homologues in the different products. Representative electropherograms for two samples are shown in Figure 6. These electropherograms show the presence of different average alkyl chain lengths for the alkyl benzenesulfonates and also reveal differences in the composition of some of the positional isomers (small side peaks). To evaluate the behavior of the nonaqueous system used
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for the separation of alkyl sulfates and alkanesulfonates, commercial liquid shampoo and liquid detergent samples were separated. The results in Figure 8 (liquid shampoo) show that two alkyl sulfates with chain lengths C12 and C14 were identified in these liquid soap products; higher chain length sulfates may also be present in these samples, but we did not have standards above C14. The same types of alkyl sulfates were seen in the electropherograms of a liquid detergent. The separation reproducibility was good for all LAS and alkyl sulfate samples, but some showed migration times 10-20% higher than those for standards. Recent results suggest that large concentrations of other ionic additives are responsible for these shifts in migration times. Calibration curves were determined for the indirect detection of alkyl sulfates and alkanesulfonates in the concentration range 5 × 10-5-5 × 10-4 mol/L in both nonaqueous and aqueous media. Detection limits (peak height of twice peak-to-peak baseline noise) in nonaqueous media for alkyl sulfates and alkanesulfonates were in the ranges 9.6 × 10-6-1.3 × 10-5 and 1.2 × 10-5-7.5 × 10-5 mol/L, respectively, in water systems. Samples were injected by elevation of sample vials at a height of 10 cm for 15 s. Sensitivity
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plots showed that the maximum deviations in sensitivity over the total calibration range varied from 8% for C4SO3 to 25% for C16SO3. The above results show that nonaqueous media should offer advantages for CE analysis of some types of samples containing hydrophobic and surface active compounds. For some samples, there are some problems related to shifts in migration times, but further studies on this phenomenon may provide solvent and electrolyte systems that can eliminate these effects. ACKNOWLEDGMENT We acknowledge the Natural Science and Engineering Research Council of Canada and the Waters Corp. for financial assistance on certain portions of this research. Received for review August 10, 1995. Accepted October 25, 1995.X AC950813J X
Abstract published in Advance ACS Abstracts, December 1, 1995.
