Anal. Cham. 1992, 64, 583-589 (27) Pettit, . 0.; Justice, J. B„ Jr. Pharmacol. Bochem. Behav. 1989, 34 899—904 (28) Pettit, . 0.; justice, J. B„ Jr. Brain Ras. 1991, 539, 94-102. (29) Pellegrino, L; Pellegrino, A.; Cushman, A. A Steraotaxlc Atlas of the Rat Brain·, Plenum: New York, 1979. (30) Stable, L. In Mlcrodlalysls In the Neurosciences Robinson, T. E„ Justice, J. B„ Jr. Eds.; Elsevier: Amsterdam, 1991; pp 155-174. (31) Km|evlc, K.; Mitchell, J. F. J. Physiol. 1980, 153, 562-572.
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(32) Benvenlste, H. J. Neurochem. 1989, 52, 1667-1679. (33) Amberg, Q.; Undefors, N. J. Pharmacol. Moth. 1989, 22, 157-183. (34) Menacherry, S. D., Ph.D. Thesis, Emory University, 1991; pp 111-131.
Received for review August 14,1991. Accepted December 17,1991.
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Anal. Chem. 1992.64:583-589. Downloaded from pubs.acs.org by UNIV OF CAMBRIDGE on 10/11/18. For personal use only.
Separation and Detection of Aliphatic Anionic Surfactants Using a Weak Anion Exchange Column with Indirect Photometric and Indirect Conductivity Detection Shahbaz A. Maki, Julie Wangsa, and Neil D. Danielson*
Department of Chemistry, Miami University, Oxford, Ohio 45056
of 0.2-gm cation exchange particles in the hydrogen form with the eluent to convert the tetrabutylammonium form of anionic surfactants into the more conductive free acids has been reported.6 A phenyl column with a 75% methanol-25 % 0.1 M NaN03 mobile phase could separate long-chain (C12-C19) alkanesulfonate surfactants with refractive index detection.6 The evaporative light scattering detector has also been evaluated for the HPLC quantitation of surfactants down to 20 nmol.7 Ion-interaction chromatography utilizing a RP column with an aromatic ion-pairing agent has been a popular approach for the separation and detection of aliphatic anionic surfactants. Alkyl surfactants such as C6-C8 sulfonates were separated on a Bondapak C18 column with a 60:40 methanolwater mobile phase containing 0.2 mM cetylpyridinium chloride acting as both the ion-pairing reagent and the means for UV detection at 254 mm.8·9 The same conditions were used, but with a different ion-pairing reagent (phenethylammonium ion) for the quantitation of pentanesulfonate and hexanesulfonate from concentrations of 1.6 to 25 mM.10 Ion-interaction chromatography on a polymeric column with iron-1,10-phenanthroline and indirect photometric detection has also been reported for the separation and detection of alkanesulfonates and alkyl sulfates.11 N-Methylpyridinium chloride or copper sulfate were used as the ion-interaction reagents for the separation and detection of positionally isomeric alkanesulfonates.12 Iodide and nitrate which absorb light in the UV have been used as spectator ions to provide the means of indirect photometric detection of even numbered C8-C14 alkyl sulfates. Both isocratic and methanol gradient elution (from 30 to 90%) were employed for the RP separation of these surfactants in about 14 min with detection limits in 3-5-Mg range.13 In all these methods, the mobile-phase concentration must be optimized carefully since it controls both the ion exchange column capacity and detection ability. Anion exchange chromatography with indirect detection is not a common approach for aliphatic sulfonates although it is mechanistically a simpler method than ion-interaction chromatography. Larson14 used biphthalate, sulfosalicylic acid, and m-sulfobenzoic acid in 60:40 acetonitrile-water as mobile phases to separate C2-C8 sulfonates on a strong cation exchange column with indirect UV detection at 297, 320, and 298 nm, respectively. Detection limits at the 0.3-#/g level and a linear response ranging from 10 to 1000 ppm were reported. Lack of suitable columns and competitive ion exchange eluents
Naphthalenedlsulfon ate-acetonitrile as a mobHe phase alone with a crow United amine-fluorocarbon polymer suca column has shown to be an effective combination tor the separation of aliphatic anionic surfactants. Indirect conductivity and photometric detection modes are used to monitor these analytes. The retention of these surfactants Is found to depend on both the Ionic strength and the organic solvent content of the moMe phase. The mechanism of retention Is believed to be a combination of both reversed phase and Ion exchange processes. Selective separation of both C,-C12 alkanesulfonates and alkyl sulfates can be achieved In less than 9 min. Detection limits are as low as 5 ng for most analytes, with linear responses extending from at least 500 ppm to the sub-ppm level.
INTRODUCTION Limited work has been cited in the literature on the liquid chromatographic separation and detection of the industrially and environmentally important class of surfactants, namely, the long-chain alkanesulfonates and alkyl sulfates as well as other aliphatic anionic surfactants. Their separation and detection is somewhat troublesome because of the dual hydrophobic and ionic nature of the compounds as well as their lack of UV absorbance and small equivalent conductance. However, some HPLC methods with direct detection of the separated aliphatic anionic surfactants have been published. For the analysis of C8-C18 alkanesulfonates, Smedes et. al1 fluorometrically detected the extracted acridinium-surfactant ion pair in chloroform after reversed phase (RP) separation with 40-60% acetone/water in sodium dihydrogen phosphate. To eliminate the need for on-line extraction, prederivatization of alkyl sulfates with 4-(diazomethyl)-N,N-dimethylbenzenesulfonamide permitted HPLC detection at 240 nm.2 However, the reaction had to occur in 100% methanol with the surfactants in the acidic form. Williams3 reported the separation of C2-Cs and C6-C8 sulfonates, in 20 and 12 min using 0.001 and 0.003 M bicarbonate/carbonate buffers, respectively, by anion exchange with suppressed conductivity detection. Weiss was able to separate C6-C8 sulfonates in 16 min on a polystyrene-divinylbenzene column using 28% CH3CN/water in 0.002 M tetramethylammonium hydroxide using suppressed conductivity detection.4 Recently, a paper describing the postcolumn mixing of an aqueous suspension 0003-2700/92/0364-0583303.00/0
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has likely hindered this method for longer chained surfactants.
We have shown sodium naphthalenesulfonate derivatives to be effective pH-independent eluents for the separation and detection of a wide variety of analytes by anion exchange chromatography with indirect photometric and conductivity detection. Sodium naphthalenemono-, -di-, and -trisulfonate (NMS, NDS, and NTS, respectively) have been used as mobile phases for the separation of diverse ionic analytes. Among those are the singly, doubly charged, and the large common inorganic anions such as F", Cl-, N02-, Br", N03", S2032-, S042-, I-, and SCN-.15-18 A similar comparison of NDS and NTS mobile phases for ion chromatography with indirect photometric detection has been reported recently.19 We have also separated organic anions such as lactate, maléate, and tartrate with NDS17 and the bulky sulfur oxide anions such dithionate, tetrathionate, and other polythionates with NTS.16 We report here simple chromatographic methods for the determination of C6-C12 aliphatic sulfonates and sulfates using sodium naphthalenedisulfonate-acetonitrile as the mobile phase on a novel polymeric fluorocarbon-amine cross-linked weak anion exchange silica column with either indirect conductivity or photometric detection modes. Indirect conductivity has not been explored previously for the detection of surfactants. Quantitation of alkyl surfactants with a detection limit and a sensitivity comparison between the two indirect detection modes is also provided. The analysis of a dioctyl sulfosuccinate industrial sample as well as a commercial shampoo was carried out without sample pretreatment.
EXPERIMENTAL SECTION Instrumentation. The chromatographic system was
composed
of a Model 510 HPLC pump, a Model U6K injector equipped with 20-#iL sample loop, a Model 490 programmable multiwavelength detector, a Model 430 conductivity detector, and two Model 730 Data Module integrators, all from the Waters Chromatography Division of the Millipore Corp. (Milford, MA). The conductivity detector was connected in series following the UV detector. The polarity of both detectors was switched in order to record a positive analyte signal instead of the actual negative signal. Both UV at 285 nm and conductivity detector outputs were simultaneously monitored on the two integrators. A Model LP-21 Lo-Pulse pulse damper from Scientific System, Inc. (State College, PA), was added to the pump, which was found to eliminate the conductivity base-line noise especially at low detector sensitivities.18 All analyte ions were separated on a polymeric fluorocarbon-amine crosslinked silica column synthesized in our laboratory as described in the procedure section. Reagents. The disodium salt of 1,5-naphthalenesulfonic acid (NDS) as well as the sodium salts of 1-butanesulfonate, 1-hexanesulfonate, n-octyl sulfate, and 1-decanesulfonate were purchased from Aldrich Chemical Co. (Milwaukee, WI). Sodium salts of n-butyl sulfate, n-hexyl sulfate, 1-octanesulfonate, n-decyl sulfate, 1-dodecanesulfonate, n-dodecyl sulfate, and n-tetradecyl sulfate were obtained from Lancaster Synthesis (Windham, NH). Acetonitrile, HPLC grade, was purchased from EM Science (Gibbstown, NJ). TEEPOL HB7, a 39% (w/w) aqueous solution of primary alkyl (C9-C13) sulfates, was obtained from Sigma Chemical Co. (St. Louis, MO). Astrowet 0-75, sodium dioctyl sulfosuccinate, was a product sent from Aleo Chemical Co. (Chattanooga, TN). A shampoo distributed by Proctor & Gamble (Cincinnati, OH) was also analyzed. Kel-F 800, a copolymer of 78% chlorotrifluoroethylene and 22% vinylidene fluoride, provided by the 3M Company (Minneapolis, MN), was found to have a molecular weight of about 800000 by size exclusion chromatography. This polymer is soluble in tetrahydrofuran (THF) but not acetonitrile or methanol. The 10-14^m-diameter silica provided by the DuPont Co. was treated as described in ref 20. It had a pore diameter of 300 Á and a surface area of 40 m2/g. Procedure. A stock solution of 0.01 M NDS was prepared in water and used for further dilution with CH3CN/water. A 500 ppm stock solution of each surfactant was prepared and used to prepare the more diluted samples and mixtures of them. The commercial surfactant samples were used as received after dilution
-CFj-CHj-CFCI-CFj-
+
H2N(CH2)3-Sillca
Kel-F 800
Aminosilica
-cf2.ch2-cf-cf2. I
N(CH2)3-Silica
-CF2.CH2-CF-CF2. yellow product 1.
Figure packing.
Synthesis scheme of fluoropolymerlc weak anion silica
with water. All solutions were prepared with distilled water purified with an E-pure water treatment system (Barnstead/ Thermolyne Corp., Dubuque, IA). Previously, it has been shown that Kel-F copolymers of chlorofluoroethylene and vinylidene fluoride can react with aliphatic primary and secondary amines to form amine cross-linked
However, the reaction has not been carried out heterogeneously on a solid matrix such as aminopropyl silica to the best of our knowledge. Aminopropyl silica was synthesized in batches by reacting 0.50 g of silica with 120 mL of 2.5% (7-aminopropyl)trimethoxysilane in toluene for 18 h at 80 °C. The resulting particles were filtered and washed three times with toluene and then with acetone. Several batches of aminosilica were dried in vacuum at 120 °C for 8 h .before use. The polymeric fluorocarbon-amine cross-linked silica packing was synthesized by reacting 3 g of Kel-F 800 with 2 g of amino silica in 150 mL THF at 57 °C for 48 h. A schematic of the reaction sequence, in agreement with the mechanism proposed,21 is shown in Figure 1. The silica product was washed copiously with THF. Electron microscopy showed discrete uniform particles with no agglomeration. Both infrared spectroscopy and elemental analysis (3.27% fluorine) indicated polymer bound to the silica particles. Bonding was also confirmed by the fact that no change in the retention of surfactants was observed even after washing with 100% THF. Considering the high molecular weight of Kel-F 800, it is likely the polymer is attached to the silica through multiple sites. The column was packed from a silica slurry in a 50% THF/50% l-methyl-2pyrrolidone solvent at 10000 psi using a high-pressure pneumatic pump system located at the Du Pont Co. in Wilmington, DE. The ion exchange column (0.46 x 25 cm) was conditioned with the desired mobile phase for at least 1 h before use. After 2 days of use, the column was flushed with 100% CH2CN to remove any residual hydrophobic species that may have bound to the column from sample impurities. The mobile phase flow rate was 1.2 mL/min, generating a column pressure of 2000 psi. All separations were carried out at ambient temperature. Capacity factors, k were calculated in the usual way; the solvent front (injection peak) was taken as the retention of an unretained compound. polymers.21
RESULTS AND DISCUSSION Some commercial columns were initially tested for the separation of surfactants with indirect photometric detection. A column packed with a polymethacrylate gel (quaternary ammonium functional group) was first used with different
concentrations of NDS eluent having as high as 20% CH3CN. No elution was observed for long-chained surfactants, and no attempt was made to use a higher organic solvent content because of the manufacturer’s precautions. A weak anion exchange polystyrene-divinylbenzene column has also been tried without any success. An aminopropyl silica column likewise gave no separations of the surfactants. Characterization of the polymeric fluorocarbon-amine cross-linked silica column followed. The retention of some anionic surfactants such as octanesulfonate, decanesulfonate, dodecanesulfonate, octyl sulfate, decyl sulfate, and dodecyl sulfate as a function of eluent concentration at a constant organic solvent content is shown
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Figure 2. Retention of some anionic alkyl surfactants as a function of eluent concentration.
Figure 3. Retention of some anionic alkyl surfactants as a function of organic solvent concentration.
in Figure 2. The retention of these and other surfactants studied in this work decreases as the ionic strength of the mobile phase increases. This shows that there is an ion exchange competition between these analytes in the mobile phase and the stationary phase. As expected, the retention of the surfactants increases with the length of the alkyl group. This agrees with the Diamond and Whitney theory which stated that large “hydrophobic” ions are more attracted to the functional group of the ion exchanger because they possess weaker hydrating power and are rejected by the external aqueous phase.22 The retention of the alkanesulfonates is relatively shorter than those of the sulfates, especially those having a longer chain. This trend agrees with previous work.11 The direct relationship between log fe'and log eluent concentration (£) as predicted by the equation log k' = -(y/x) log E + log B where y is the charge of the sample ion, x is the charge of the eluent, and B is a constant dependent on the ion exchange equilibrium constant and the capacity of the resin has been well established for ion chromatography.23 However, the validity of this relationship for the separation of surfactants has not been considered. Linearity was apparent for log k'versus log NDS for the capacity factor data at 0.05, 0.1, and 0.2 mM mobile phase concentrations. The respective slope (y/x) values found for various pairs of surfactants (R-S03, R-SO4 where R = alkyl chain) are as follows:
mM. This can be attributed to the finding that first, narrower analyte peaks are obtained with more concentrated eluents because of the early elution, while secondly, better detection limits are obtained with a low eluent concentration for all indirect detection modes as predicted by Yeung.24 These two opposing factors seem to cancel each other with no noticeable effect. The effect of organic solvent, at constant ionic strength, on the retention of alkyl surfactants is shown in Figure 3. For all surfactants, the retention increases with decreasing organic solvent content This suggests that a reverse-phase mechanism plays a role in the retention of these surfactants. At 40% and higher acetonitrile content, all surfactants elute at the same time regardless of alkyl chain length. At a moderate CH3CN content (between 30 and 40%), the difference in the retention times of the sulfonates and sulfates is enough to allow separation of either group in a reasonably short time. A larger analyte signal is produced at high acetonitrile percentages; there is a 60% increase in peak height in going from 25% to 45% CH3CN. This is mainly due to early analyte elution and consequently sharper peaks. Below 30% CH3CN, selective separation of both surfactant types in a mixture is feasible as will be seen later. The optimized mobile phase requires, therefore, a solvent composition that achieves a balance between the hydrophobic and ion exchange retention effects in order to obtain a good chromatographic performance. A mobile phase composition of 0.2 mM NDS in 35% CH3CN has been found to be quite suitable for the separation and detection of either alkanesulfonates or alkyl sulfates with high sensitivity. Figure 4 shows chromatograms of a standard mixture of alkanesulfonates containing C6S03, C8S03, C10SO3, and Ci2S03 with both conductivity and LTV detection. Both detectors produced negative analyte signals indicating indirect detection modes were operative. The hexanesulfonate peak, with conductivity
C4, 0.68, 0.56; C6, 0.55, 0.52; C8, 0.43, 0.40; C10, 0.31, 0.32; C12, 0.26,0.24. The C14S04 slope value was 0.19. For the short-
chain C4, C6, C8 alkanesulfonates and alkyl sulfates, agreement of the slope values with the theoretical value of 0.5 is fairly good, indicating the ion exchange mechanism is important. However, the increasing divergence between the slope values and 0.5 for the long-chain surfactants appears to indicate support for the reverse phase retention mechanism. The NDS concentration has been found to have no net effect on the analyte signal, within the range of 0.05 to 0.4
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Figure 4. Separation of a standard mixture of alkanesulfonates: (A) with Indirect conductivity (0.5 #tSFS) and (B) with IRC at 285 nm and 0.1 AUFS. Mobile phase: 0.2 mM NDS/35% CH3CN, flow rate 1.2 mL/mln. Peak Identification: (1)5 ppm C6S03, (2) 10 ppm C8S03, (3) 10 ppm C10SO3, and (4) 20 ppm C12S03.
Figure 6. Separation of a standard mixture of alkyl sulfates: (A) with Indirect conductivity detection (0.5 juSFS) and (B) with IRC at 285 nm and 0.1 AUFS. Conditions are the same as In Figure 4. Peak Identification: (1) 5 ppm C8S04, (2) 10 ppm C8S04, (3) 10 ppm C10SO4, and (4) 20 ppm C12S04, and (5) 100 ppm C14S04.
Figure 5. Separation of a standard mixture of alkyl sulfates: (A) with Indirect conductivity detection (0.5 #¿SFS) and (B) with IRC at 285 nm and 0.1 AUFS. Conditions are the same as In Figure 4. Peak Identification: (1) 5 ppm CeS04, (2) 10 ppm C8S04, (3) 10 ppm C10SO4, and (4) 20 ppm C12SC4.
detection (Figure 4A), was not quite resolved from the injection peak. This was not, however, the case with indirect photometric detection (Figure 4B). The separation of an alkyl sulfate mixture containing CgS04, CgS04, C10SO4, and C12S04 is shown in Figure 5. All analyte peaks are well resolved with high sensitivity using both detection modes, and no injection peak interference is observed. The equivalent conductance for NDS was about 7 times greater than C10SO4. This mobile phase composition gave a background conductivity of about 41 #iS/cm.
The injection peak in indirect detection modes results from the elution of the sample solvent as well as the eluent ions that have been displaced from the stationary phase by the analyte ions. The direction and size of the injection peak can be predicted by the sample concentration relative to the eluent concentration and the detector background signal. Negative and relatively large injection peaks are the result of analyzing very dilute samples. Positive and still large injection peaks are generated from concentrated samples. Sometimes both combinations are seen. The effect of the injection peak on the separation of these surfactants in the presence of a highly retained one (C14S04) is quite clear in Figure 6. A very large positive conductivity injection peak has resulted, burying the C6S04 and CgS04 analyte peaks (Figure 6A). However, there
Figure 7. Separation of a standard mixture of alkanesulfonates and alkyl sulfates with IRC at 285 nm and 0.1 AUFS. Mobile phase, 0.2 mM NDS/25% CH3CN; flow rate, 1.2 mL/mln. Peak Identification: (1) 10 ppm C8SOa, (2) 20 ppm C8S04, (3) 50 ppm C10SO3, and (4) 50 ppm C10SO4, (5) 100 ppm C12S03 and (6) 150 ppm C12S04.
major problem with indirect photometric detection (Figure 6B), due to the smaller positive injection peak. The use of 0.2 mM NDS in 25% CH3CN was found to effectively separate a mixture containing both alkanesulfonates and alkyl sulfates. Figure 7 shows a chromatogram for the separation of a mixture containing CgS03, CgS04, C10SO3, CiqS04, C12S03, and Ci2S04 with indirect photometric detection. Except for the first two analytes, all peaks are well separated in a relatively short time of less than 20 min using this isocratic elution scheme. The first two peaks were partially buried in the quite large positive photometric injection peak. There was no problem in analyzing a mixture with lower was no
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Table I. Detection Limit Comparison for Various Surfactants0 detection limits, ppm (ng)6 analyte6 CgSOs
ceso4 CgS03 CgS04 CioS03 CioS04 Ci2S03
c12so4
conductivity
IPD
(20) (20) 0.25 (5) 0.25 (5) 0.25 (5) 0.25 (5) 0.25 (5) 0.25 (5)
0.5 (10)
1 1
1(20) 0.5 (10) 1 (20) 1 (20) 1 (20) 1 (20) 1 (20)
Using 0.2 mM NDS in 35% acetonitrile as the mobile phase. 6 c Signal-to-noise ratio S3. Sample volume, 20 #iL. 0
Figure 8. Linearity run comparison between Indirect photometric (IPO) and conductivity (CONO) detection modes.
analyte concentrations (5-50 ppm) and/or fewer analyte components. The quantitation of alkanesulfonates and alkyl sulfates has also been studied with both detection modes using 0.2 mM NDS in 35% CH3CN as the mobile phase. Least-squares analyses of C6-C12 sulfonates and sulfates have shown a linear response range extending from at least 500 ppm to the detection limit (1 ppm or less) of each analyte with both indirect conductivity and photometric detection modes. The relative standard deviation of the slopes ( = 9) ranged from 0.057% to 0.66% with an average of 0.30% for all analytes with indirect conductivity detection. With indirect photometric detection, the relative standard deviation of the slopes ( = 9) were similar from 0.13% to 0.68% with an average of 0.35% for all analytes. The correlation coefficients were excellent with both detection modes ranging from 0.9998 to 0.9999 for all analytes.
Theoretically, better sensitivity of indirect conductivity detection as compared to indirect photometry is expected. The sensitivity for indirect photometry is proportional to CgoE, where Cs is the concentration of the sample and aE is the molar absorptivity of the eluent.23 However, the change in conductance is proportional to 1000CS multiplied by the difference in equivalent conductance of the sample and the eluent.23 Assuming NDS has an oE of 10000 M'1 cm"1, the difference in equivalent conductance has only to be >10 to give a larger slope for indirect conductivity as compared to photometry. This is certainly the case for most mobile phase-analyte pairs. Figure 8 shows a comparison between typical linearity runs of two surfactants with indirect conductivity and photometric detection modes. The sensitivity values for indirect conductivity detection have ranged from 6 to 7 times higher than those for indirect UV detection with all the analytes studied in this work. This is consistent with our previous study which showed that the conductivity detection values of inorganic anions averaged 13,8, and 5 times more sensitive than those of the indirect UV detection using NMS, NDS, and NTS as mobile phases, respectively.18
Figure 9. Chromatograms of (A) 0.25 ppm octyl sulfate (2) and 0.25 ppm dodecyl sulfate (4) using Indirect conductivity detection (0.05 uSFS), and (B) 1 ppm octyl sulfate (2) and 1 ppm dodecyl sulfate (4) using Indirect photometric detection (0.02 AUFS). Conditions are the same as In Figure 4.
Table I lists the detection limits (at S/N> 3) of various anionic surfactants using 0.2 mM NDS in 35% CH3CN. Except for hexanesulfonate and hexyl sulfate, the indirect conductivity detection limits are 2-4 times lower than those obtained with indirect photometric detection. This trend is in good agreement with a previous ion chromatography comparison study of inorganic anions between conductivity and indirect photometric detection.18 Figure 9 shows chromatograms of octyl sulfate and dodecyl sulfate at their detection limits of 1 and 0.25 ppm using indirect photometric and conductivity detection, respectively. The large base-line noise of the UV detector presumably due to shot noise is evident. The relatively high detection limits of hexanesulfonate and hexyl sulfate can be attributed to the large negative conductivity injection peaks which have resulted in a partial overlap with the analyte peaks at low concentrations. However, all the listed detection limits (5-20 ng) are generally lower
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Figure 11. Chromatograms of a 0.01% TEEPOL HB7 solution: (A) with indirect conductivity detection (1 mSFS) and (B) with IPC at 285 nm and 0.2 AUFS. Conditions are the same as In Figure 4. Peak Identification: (1) CeS04, (2) C10SO4, (3) C11S04, (4) C12S04, and (5) c13so4.
Figure 10. Chromatograms of a 0.1% shampoo solution: (A) with Indirect conductivity detection (1 mSFS) and (B) with IPC at 285 nm and 0.2 AUFS. Conditions are the same as In Figure 4. Peak Identification: (1) ammonium lauryl sulfate, (2) unknown.
c2hs MIN INJ
than those previously reported using other systems. Larson,14 Boiani,13 and Pietrzyk et al.11 have reported detection limits of 300,3000-5000, and 20-500 ng, respectively, for some anionic aliphatic surfactants. The simplicity of our method to reproducibly detect low levels of these surfactants is another important factor. These low detection limits of both indirect detection modes may be attributed to the bulky structure of the NDS eluent and its stable pH-independent effective charge. This allows the use of a relatively low mobile phase concentration having a low absorbance or conductivity background levels, while still maintaining sufficient elution strength. The analysis of commercial samples containing different kinds of aliphatic surfactants using 0.2 mM NDS in 35% CH3CN are shown in Figures 10-12. Figure 10 shows chromatograms of a 0.01% shampoo solution with both detection modes. Peak 1 was identified to be lauryl sulfate; however, no attempt was made to identify the earlier peaks and peak 2. Other ingredients such as xylenesulfonate or laureth sulfate present in the sample could be causing the other peaks. Figure 11 shows chromatograms of a 0.01% TEEPOL HB7 solution. This sample, which has been formulated by Sigma, contains 39% (w/w) aqueous solution of primary alkyl (C9-C13) sodium sulfates. Peaks 2 and 4 were identified to be C10SO4 and C12S04, respectively, while peaks 1,3, and 5 are believed to be C9S04, CnS04, and Ci3S04, respectively. This conclusion was verified from the linearity of a plot of log k' versus the number of solute carbon atoms. Astrowet 0-75 is the Aleo Chemical commercial name for a 74% sodium dioctyl sulfosuccinate solution. This sulfonated aliphatic polyester surfactant, besides having industrial uses, is also a laxative drug. A chromatogram of 0.01% Astrowet shows only one large peak corresponding to 1.5 Mg of sodium dioctyl sulfosuccinate injected (Figure 12). The sensitivity of both detection modes using our method is superior compared to previous work using a Partisil 10 SAX column and CH3CN-water (60:40) with 2.5
2
4
6
-4—I ............—CH ,
!
8 ¡
I...
X)
CHzCOOCHzCHICHdsCHs 121
COOCH^HtCHtiiCHg C2H5
Figure 12. Chromatograms of a 0.01 % ASTROWET solution: (A) with Indirect conductivity detection (0.5 mSFS) and (B) with IPC at 285 nm and 0.1 AUFS. Conditions are the same as In Figure 4. See text for peak Identification.
mM potassium biphthalate and 1% acetic acid (pH 5.0) as a mobile phase with indirect photometric detection at 293 nm. A chromatogram of a standard sample containing 2.5 Mg of sodium sulfosuccinate generated two peaks in both the positive and negative directions in that method.25 The work done in this study has been a result of a fortuitous choice of a specially formulated stationary phase and an ef-
fective mobile phase. A mixed mode RP C8/weak anion silica column was also tried to separate some surfactants. This column could separate with 0.2 mM NDS-50% CH3CN the small alkyl surfactants such as butanesulfonate, butyl sulfate, hexanesulfonate, and hexyl sulfate. Using a mobile phase of 0.2 mM NDS in 80% CH3CN, a separation of C10, C12, and C14 sulfates took 25 min on the mixed-mode column. This high CH3CN percentage was found to be necessary to shorten the analysis time, which is still considered quite long compared to that obtained with the polymeric fluorocarbon-amine cross-linked silica column (see Figure 6). Using 0.2 mM NDS in 35% CH3CN, the octyl sulfate retention time was 4 min on our column compared to 22 min with the mixed mode
Anal. Cham. 1992, 64, 589-594
column. This may be attributed to the high hydrophobic characteristics of the C8 functional group compared to the fluorocarbon moiety. In general, fluorocarbon based columns are less retentive in the reversed-phase mode compared to hydrocarbon types.26 This work has described an ion exchange chromatographic method for the analysis of different classes of aliphatic surfactants using conventional HPLC equipment with UV or conductivity detectors. It has shown the effectiveness of utilizing a powerful eluent with a moderately hydrophobic weak ion exchange column to permit a simple isocratic elution method for the separation of alkanesulfonates and alkyl sulfates with high sensitivity and low detection limits to be developed. It is expected that commercial surfactants having different anionic functional groups such as sarcosine could also be easily analyzed using this method without sample pretreatment.
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REFERENCES (1) Smedes, S.; Kraak, J. C.; Werkhoven-Goewle, C. F.; Brinkman, U. A. Th.; Freí, R. W. J. Chromatogr. 1982, 247, 123-132. (2) Kudoh, M.; Tsuji, K. J. Chromatogr. 1984, 294, 456. (3) Williams, R. J. J. Chromatogr. Scl. 1982, 20, 560-565. (4) Weiss, J. J. Chromatogr. 1986, 353, 303-307. (5) LI. J. B.; jandlk, P. J. Chromatogr. 1991, 546, 395. (6) Xlaolan, H.; Yao, R. Fenxl Huaxue 1988, 16 (5), 406-409. (7) Bear, G. R. J. Chromatogr. 1988, 91, 459. (8) BkJlIngmeyer, B. A. J. Chromatogr. Scl. 1980, 18, 525-539. (9) BkJlIngmeyer, B. A.; Warren, F. V., Jr. Anal. Chem. 1982, 54,
2351-2356.
(10) Sachok, B.; Doming, S. N.; BkJlIngmeyer, B. A. J. Uq. Chromatogr. 1982, 5 (3), 389-402. (11) Pietrzyk, D. J.; Rlgas, P. G.; Yuan, D. J. Chromatogr. Scl. 1889, 27, 485. (12) Eppert, G.; Llebscher, G. J. Chromatogr. Scl. 1991, 29, 21-25. (13) Bolán!, J. A. Anal. Chem. 1987, 59, 2583-2586. (14) Larson, J. R. J. Chromatogr. 1988, 356, 379-381. (15) Makl, S. A.; Danielson, N. D. J. Chromatogr. Scl. 1990, 28, 537. (16) Makl, S. A.; Danielson, N. D. Anal. Chem. 1991, 63, 699-703. (17) Makl, S. A.; Danielson, N. D. J. Chromatogr. 1991, 542(1), 101. (18) Makl, S. A.; Danielson, N. D. Chromatographla, in press. (19) Motomizu, S.; Oshlma, M.; Hlronaka, T. Analyst 1991, 116, 695. (20) Kohler, J.; Kirkland, J. J. J. Chromatogr. 1987, 385, 125. (21) Paclorek, K. L.; Mitchell, L. C.; Lenk, C. T. J, Pofym. Scl. 1980, 45,
ACKNOWLEDGMENT We thank J. J. Kirkland and G. R. Wooler (DuPont Co.) for the gift of silica and for packing the HPLC column as well as L.G. Beaver (Dap, Inc.) for the molecular weight characterization of Kel-F 800. Support by the Du Pont Co. is gratefully appreciated. Registry No. C4C03Na, 2386-54-1; C6C03Na, 2832-45-3;
405-413.
(22) Diamond, R. M.; Whitney, D. C. In Ion Exchange·, Marlnsky, J. A., Ed.; Marcel Dekker: New York, 1966; Vol. 1. (23) Small, H. In Ion Chromatography·, Plenum Press: New York, 1989. (24) Yeung, E. S. Acc. Chem. Fies. 1989, 22, 125-130. (25) Larson, J. R.; Pfeiffer, C. D. J. Chromatogr. 1983, 259 , 519-521. (26) Danielson, N. D.; Wangsa, J.; Beaver, L. G. J. Chromatogr. 1991,
544, 187-199.
C8C04Na, 142-31-4; CioS03Na, 13419-61-9; C4S04Na, 1000-67-5; C6S04Na, 2207-98-9; C8S03Na, 5324-84-5; C10SO4Na, 142-87-0; C12S03Na, 2386-53-0; C12S04Na, 151-21-3; C14S04Na, 1191-50-0; Teepol HB7, 134092-79-8; Astrowet 0-75, 577-11-7.
Received for review August 19,1991. Accepted December 13, 1991.
Interpretation of Retention Behaviors of Transition-Metai Cations in Micellar Chromatography Using an Ion-Exchange Model Tetsuo Okada Faculty of Liberal Arts, Shizuoka University, Shizuoka 422, Japan
compounds.1"15 Interaction between analytes and micelles results in unique separation selectivity and permits the de-
Uses of mobile phases containing an anionic surfactant and tartaric add as a mid complexlng agent permit the separation of transitlon-metal Ions with reversed-phase chromatography. Effects of the surfactant and tartaric acid are quantitatively evaluated on the basis of an Ion-exchange model. Compiexation of tartaric acid with metal Ions does not Influence the partition to the anionic micellar phase because of the negative surface potential of the micelle, which depresses not only the dseodatlon of tartaric add but also the complexation with metal Ions at the mlcelle/solutlon Interface. This result suggests that the partition of metal cations to the anionic micelle can be treated as a simple Ion-exchange equilibrium even In the presence of tartaric acid. On the other hand, the partition to the stationary phase Is affected by the complexation. Quantitative Interpretation of these results allows us to predict the retention and the optimum mobile-phase composition. Three principal factors affecting the retention and the separation are simultaneously optimized with the simplex method on the basis of the retention model developed.
termination of partition coefficients of analytes into the micellar phase. Usually, the partition of analytes is thought to occur in the micellar lipophilic core and/or in the palisade region of micelles.16,17 The solute location in a micelle is determined by the nature of the micelle and the balance of hydrophilicity and hydrophobicity of the solute. We do not have to note where a solute of interest is located in a micelle, because micelles are regarded as macroscopically homogeneous
media in micelle chromatography. It has therefore been indicated that there usually exists an apparent correlation between the partition coefficients of a number of neutral organic compounds into various micelles and those into usual organic solvents such as octanol.13,14 Thus, we can regard the interaction involved in micellar chromatography of neutral organic compounds as lipophilic or hydrophobic. On the contrary, the use of micellar mobile phases in inorganic chromatography has been limited.18"24 The author indicated the efficiency of micellar mobile phases in the separation of inorganic anions20,21 and cations;22 i.e. micellar mobile phases permit the alternation of selectivity and the evaluation of the partition to micellar phases. An important distinction between organic and inorganic micellar chromatography is the interaction involved in the
INTRODUCTION Micellar mobile phases have been extensively used in the reversed-phase chromatographic separation of various organic 0003-2700/92/0364-0589S03.00/0
©
1992 American Chemical Society
