Anal. Chem. 1992, 64, 583-589
M, H. 0.; Jwtke, J. B., Jr. phermecd. Bkchem. Behev. 1989, 34, 809-904. (28) Pettlt, H. 0.;Justice, J. B., Jr. &ah Res. 1991, 539, 94-102. (29) pdk9rin0, L.; Memino, A.; Cuahman, A. A Stetuotaxk A L s of the Fbt h; pknum:-New York, 1979. (27)
(30) Ute. *' J. B., In Jr. Eda.; Elsevlwthe Amsterdam. Nwasclences; 1991;Robhmo pp 155-174. T* (31) Kmjevic, K.; M l t C h d , J. F. J . physkl. 1 0 0 , 153, 562-572.
589
(32) Benvenlste, H. J . Ne". 1989, 52, 1667-1679. (33) Amberg. b.; Undefors, N. J . phennecd. Mth. 1989, 22, 157-183. (34) Menacheny, S. D., Ph.D. TheSlS, EITIOIY Unhrerslty, 1991; pp 111-131.
RECEIVED for review August 14,1991. Accepted December 17, 1991.
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-pm cation exchange particles in the hydrogen form with Naphthrkmdkulfonateacetrcaonnrlkas a moblie phase alone w l l h a ~ w l ~ r b o n p o l y n n r d l c c r c o k m n the eluent to convert the tetrabutylammonium form of anionic surfactants into the more conductive free acids has been rehm drown to k an df.ctive combhatkn for the mparatlon ported.6 A phenyl column with a 75% methanol-25% 0.1 M of allphatk anknlc wrfactants. Indlrod conductlvlty and NaN03 mobile phase could separate long-chain (C12-C19) photomtrlc dotectbn mockr are used to modtor those anaalkanesulfonatesurfactants with refractive index detection.6 lytw. Tho rotontkn of these surfactants k found to depend The evaporative light scattering detector has also been onboththe knlcrb.ng#landtheorganksolvont content of evaluated for the HPLC quantitation of surfactants down to tho moblk phase. Tho mochadam of rotontbn k k l k v e d to 20 nm01.7 k a comblnatkn of both reversed phase and Ion exchange Ion-interaction chromatography utilizing a RP column with p"a8. 8rkctlvo wparatbn of both C&, alkanean aromatic ion-pairing agent has been a popular approach Wnonatw and ahyi sulfates can be achleved In less than @ for the separation and detection of aliphatic anionic surfacmln. DotocUon htb are as low as 5 ng for mort analytw, tants. Alkyl surfactants such as C6-C8sulfonates were sepO X t m hwn at kaSt 500 ppm t0 the WRh knr arated on a Bondapak C18 column with a 6040 methanolwb-ppm kvel. water mobile phase containing 0.2 mM cetylpyridinium
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 alkanesulfonatesand 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-C18alkanesulfonates, Smedes et. all fluorometridy detected the extractad acridiniumaurfactant 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 C& 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-C8sulfonates in 16 min on a polystyrendivinylbenzene column using 28% CH&N/water in 0.002 M tetramethylammonium hydroxide using suppressed conductivity detection.' Recently, a paper describing the postcolumn mixing of an aqueous suspension 0003-2700/92/0364-0583$03.00/0
chloride acting as both the ion-pairing reagent and the means for UV detection at 254 mm?s9 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.'O Ion-interaction chromatography on a polymeric column with iron-l,l0-phenanthroline and indirect photometric detection has also been reported for the separation and detection of alkanesulfonates and alkyl sulfates." 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 Ca-Ci4 alkyl sulfates. Both isocratic and methanol gradient elution (from 30 to 90%)were employed for the RP separation of these surfactanb in about 14 min with detection limits in 3-5-pg 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. Lar50n'~used biphthalate, sulfosalicylicacid, and m-sulfobenzoic acid in 6040 acetonitrilewater as mobile phases to separate C2-C8sulfonates on a strong cation exchange column with indirect UV detection at 297, 320, and 298 nm, respectively. Detection limits at the 0.3-pg level and a linear response ranging from 10 to lo00 ppm were reported. Lack of suitable columns and competitive ion exchange eluents 0 1992 American Chemical Society
584
ANALYTICAL CHEMISTRY, VOL. 64, NO. 6,MARCH 15, 1992
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-, NO;, Br-, NO,, S203%, SO:-, I-, and SCN-.l+l8 A similar comparison of NDS and NTS mobile phases for ion chromatography with indirect photometric detection has been reported re~ent1y.l~ We have also separated organic anions such as lactate, maleate, and tartrate with NDS" 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 chromatographicsystem was composed of a Model 510 HPLC pump, a Model U6K injector equipped with 20-pL 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-naphthalenesulfonicacid (NDS) as well as the sodium salta of 1-butanesulfonate, 1-hexanesulfonate, n-octyl sulfate, and 1-decanesulfonatewere purchased from Aldrich Chemical Co. (Milwaukee, WI). Sodium salts of n-butylsulfate, 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 (Gibbetown,NJ). TEEPOL HB7, a 39% (w/w) aqueous solution of primary alkyl (C9-Ci3) sulfates, was obtained from Sigma Chemical Co. (St. Louis, MO). Astrowet 0-75, sodium dioctyl sulfosuccinate, was a product sent from Alco 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-pm-diameter silica provided by the DuPont Co. was treated as described in ref 20. It had a pore diameter of 300 A 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
+
-CF2-CH2.CFCI-CF2-
H2N(CH2)j-Silica
>-
Aminosilica
Kel-F 800
-
-
-CF2 C H 2 C F - C F 2
I
-
-
N(CH2)3 Si1 i c a
I-
-CF2- C H 2 - C F C F 2 .
yellow product
Flgure 1.
packlng.
Synthesls scheme of fluoropolymerlc weak anlon slllca
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 polymers.21 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-aminopropy1)trimethoxydane in toluene for 18 h at 80 O 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 OC for 8 h before use. The polymeric fluorocarbon-amineCross-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 OC for 48 h. A schematic of the reaction sequence, in agreement with the mechanism proposed,21is 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 confiied 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% 1-methyl-2pyrrolidone solvent at 10O00 psi using a high-pressure pneumatic pump system located at the Du Pont Co. in Wilmington, DE. The ion exchange column (0.46X 25 cm) was conditioned with the desired mobile phase for at least 1h before use. After 2 days of use, the column was flushed with 100% CH&N 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 preasure 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.
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
ANALYTICAL CHEMISTRY, VOL. 64, NO. 6, MARCH 15, 1992 6-
10
-
50-
6-
L
k
8
b
L
3-
-
su* -
0
0
c
\
L
L
u
E
i c12so4
4-
P,,
585
E
2-
4-
b
\'
c1,0s04
21-
04 0.0
I
0.1
0.2
0.3
0.4
0.5
mM N D S 35% CH3CN Flgure 2. Retentlon of some anionic alkyl surfactants as a function
of eluent concentratlon.
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 surfactanta 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." The direct relationship between log k' and log eluent concentration (E) 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 chromat~graphy.~~ However, the validity of this relationship for the separation of surfactants has not been considered. Linearity was apparetlt for log k'versus log NDS for the capacity factor data a t 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-SO4where R = alkyl chain) are as follows: c4,0.68, 0.56; cg, 0.55,0.52; Cg, 0.43,0.40; clo, 0.31,0.32; clz, 0.26,0.24. The cl4so4 slope value was 0.19. For the shortchain C4,C6,C8alkanesulfonatea 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
+
04 20
I
30
40
50
% CH3CN (0.2 mM NDS)
Flgure 3. Retention of some anionic alkyl surfactants as a function
of organic solvent concentration.
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 Y e ~ n g . 2These ~ 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 a t 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 a t 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 %I 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 effeds 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,C1&303,and Cl2SO3 with both conductivity and W detection. Both detectors produced negative analyte signals indicating indirect detection modes were operative. The hexanesulfonatepeak, with conductivity
586
ANALYTICAL CHEMISTRY, VOL. 64, NO. 6, MARCH 15, 1992
Flgure 4. Separation of a standard mlxture of alkanesulfonates: (A) with indirect conductlvlty (0.5 pSFS) and (6)with IPC at 285 nm and 0.1 AUFS. Mobile phase: 0.2 mM NDS/35% CH,CN, flow rate 1.2 mL/mh. Peak identlflcatlon: (1) 5 ppm C8S03,(2) 10 ppm C8S03,(3) 10 ppm CjOSO3, and (4) 20 ppm C12SO3.
'I
Figure 6. Separation of a standard mixture of awl sulfates: (A) wlth lndkect conductivity detectbn (0.5 pSFS) and (B) with IPC at 285 nm and 0.1 AUFS. Conditions are the same as in Fbure 4. Peak tdentlflcatlon: (1) 5 ppm C8S04, (2) 10 ppm C8S04, (3) 10 ppm C,,S04, and (4) 20 ppm C,,SO,, and (5) 100 ppm c14so4.
Flgure 5. Separatlon of a standard mixture of alkyl sulfates: (A) with indkect conductlvlty detection (0.5 pSFS) and (B) with IPC at 285 nm and 0.1 AUFS. Conditions are the same as in Figure 4. Peak Mentlflcatlon: (1) 5 ppm C,,S04, (2) 10 ppm C8S04, (3) 10 ppm CloSO4, and (4) 20 ppm C12S04.
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 C,$04, C8S04,Cl$304, and C12S04 is shown in Figure 5. All anal* 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 C1$04. This mobile phase composition gave a background conductivity of about 41 pS f cm. T h e 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 reault 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 (c14so4)is quite clear in Figure 6. A very large positive conductivity injection peak has resulted, burying the C6SOIand C$04 analyte peaks (Figure 6A). However, there
Flgure 7. Separation of a standard mlxture of alkanesulfonates and alkyl sulfates with IPC at 285 nm and 0.1 AUFS. Moblle phase, 0.2 mM NOS/25% CH3cN;Row rate, 1.2 Wmin. Peak IdentlflcaUon: (1) 10 ppm C,sO,, (2) 20 ppm C,S04, (3) 50 ppm Cl0sO3,and (4) 50 ppm CloSO,, (5) 100 ppm C12SO3 and (6) 150 ppm C12SO4.
was no 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 alkaneeulfonates and alkyl sulfates. Figure 7 shows a chromatogram for the separation of a mixture containing C8S03,C8S04, C10SO3, C1,$04, CI2SO3,and C12S04with indirect photometric detection. Except for the first two analytes, all peaks are well separated in a relatively short time of lese than 20 min using this isocratic elution scheme. The first two peaks were part i d y buried in the quite large positive photometric injection peak. There was no problem in analyzing a mixture with lower
ANALYTICAL CHEMISTRY, VOL. 64, NO. 6, MARCH 15, 1992
567
Table I. Detection Limit Comparison for Various Surfactants'
detection limits, ppm (ng)b conductivity IPD
anal*'
1(20) 1 (20) 0.25 (5) 0.25 (5) 0.25 (5) 0.25 (5) 0.25 (5) 0.25 (5)
'Using 0.2 m M NDS in 35% acetonitrile as the mobile phase. Sample volume, 20 &.
* Signal-to-noise ratio 23.
I
A
0
100
200
300
400
100
600
Cone., ppm
8. uneerlty run comparison between indirect photometric (IPD) and conductMty (COND) detection modes. -0
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 analysesof C6-C12sulfonates and sulfates have shown a linear response range extending from at least 500 ppm to the detaction limit (1 ppm or leas) of each analyte with both indirect conductivity and photometric detection modes. The relative standard deviation of the slopes (n = 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 (n = 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 analyt8s.
Theoretically, better sensitivity of indirect conductivity detection as compared to indirect photometry is expected. The sensitivity for indirect photometry is proportional to C ~ E , where Cs is the concentration of the sample and QE ie the molar absorptivity of the eluent.23 However, the change in conductance is proportional to loooCsmultiplied by the difference in equivalent conductance of the sample and the eluent.23 Assuming NDS has an aEof 10OOO M-' cm-', 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 W 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 W detection using NMS, NDS, and NTS as mobile phases, respectively.18
-vL
WIN
w,2,q1 I
I
I
I
I
I
6
2
I
4
MIN
IKl121ql 1
I
I
I
I
Flgure 9. Chromatograms of (A) 0.25 ppm octyl sulfate (2) and 0.25 ppm dodecyl sulfate (4) wing Indirect conductivity detectlon (0.05 uSFS), and (6) 1 ppm octyi sulfate (2) and 1 ppm dodecyl sulfate (4) using indirect photometric detection (0.02 AUFS). Condltkns are the same as in Flgwe 4.
Table I lists the detection limits (at SIN 1 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 ie 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
588
ANALYTICAL CHEMISTRY, VOL. 64, NO. 6, MARCH 15, 1992
A
;I
B
1
I
Flgure 11. Chromatograms of a 0.01 % TEEPOL HB7 Solution: (A) with indirect conductivity detection (1 pSFS) and (6) with IPC at 285 nm and 0.2 AUFS. Conditions are the same as in Figure 4. Peak identification: (1) C9S04,(2) C,,SO,, (3) CllS04, (4) C,,S04, and (5)
c,3so,.
/I .-
MIN
l
l
i
l
l
lo
8
l N b , 2 , 4 , l
l
l
l
l
I
12 l
14 ~
l
16 l
l
l
l
Flgure 10. Chromatograms of a 0.1% shampoo solution: (A) with indirect conductlvlty detection (1 pSFS) and (B)with IPC at 285 nm and 0.2 AUFS. Conditions are the same as in Figure 4. Peak Mentification: (1) ammonium lauryl sulfate, (2) unknown.
than those previously reported using other systems. Lmon,14 Boiani,I3 and Pietrzyk et al.ll 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 11shows 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-Cl&sodium sulfates. Peaks 2 and 4 were identified to be C10S04 and Cl2SO4,respectively, while peaks 1,3, and 5 are believed to be C9S04,CllSO4, and C13S04,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 Alco 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 chromatagram of 0.01% Ashwet shows only one laxge peak corresponding to 1.5 pg of sodium dioctyl sulfosuccinate injected (Figure 12). The sensitivity of both detection modes using our method is superior compared to previous work using a Partisill0 SAX column and CH,CN-water (6040) with 2.5
WJ
2
MiN 4
6
8
I
1
10
12
14
Flgue 12. chromatograms of a 0.01 % ASTROWET solutbn: (A) with indirect co~uctivitydetection (0.5 NSFS) and (e) with IPC at 285 nm and 0.1 AUFS. Conditions are the same as in Flgure 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 pg 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 effective 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% CH3CNthe 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 ClotCI2,and C14sulfates took 25 min on the mixed-mode column. This high CH3CNpercentage was found to be necessary to shorten the analpis 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. Chem. 1892, 6 4 , 589-594
column. This may be attributed to the high hydrophobic characteristics of the C8 functional group compared to the fluorocarbon moiety. In general, fluorocarbn 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.
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; C8C04Na,142-31-4;Cl$03Na, 13419-61-9;C4S04Na,1000-67-5; C6SO4Na,2207-98-9; C8S03Na,5324-84-5; CloS04Na,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.
588
REFERENCES (1) S d e s , S.; Kraak, J. C.; Werkhoven-Goewie, C. F.; Brhkman. U. A. Th.; Frei. R. W. J . chromatogr. 1982, 247, 123-132. (2) Kudoh, M.; Tsuji, K. J . Chromatog. 1984, 294. 458. (3) Wllllams, I?.J. J . C h f o m m . Sci. 1982, 20, 580-585. (4) Weiss, J. J . Chfomtogr. 1988, 353, 303-307. (5) Li. J. B.; Jandlk, P. J . chrometogr. 1991, 546, 395. (8) Xkokn. H.; Yao, R. F m i Huexue 1988, 18 (5). 408-409. (7) Bear, 0. R. J . Chfmetogr. 1988, 91. 459. (8) Bidiingmeyer, B. A. J . C h t o g r . Scl. 1980, 18, 525-539. (9) Bldiingmeyer, B. A.; Warren, F. V., Jr. Anal. Cbem. 1882, 54,
2351-2356. (10) Sachok. B.; Deming, S. N.; Bidiingmeyer, B. A. J . Llq. chnxnatogr. 1982, 5 (3). 389-402. (11) Pietrzyk. D. J.; Rigas, P. 0.; Yuan, D. J . Chfmetogr. Scl. 1989, 27, 485. (12) Eppert, 0.; Liebscher, 0. J . Chrometogr. S d . 1991, 29, 21-25. (13) Bolanl, J. A. Anel. Chem. 1987, 59, 2583-2588. (14) krson, J. R. J . chromatogr. 1988, 356, 379-381. (15) Maki. S. A.; Danielson, N. D. J . Chfmetogr. Sci. 1990, 28, 537. (18) Maki, S. A.; Danlelson, N. D. Anel. chsm. 1991, 63, 899-703. (17) Maki, S. A.; Danielson, N. D. J . Chromatogr. 1991, 542(1), 101. (18) Maki, S. A.; Danieison. N. D. Ctwomatographle, in press. (19) Motomku. S.; Oshima, M.; Hironaka, T. AneEyst 1991, 116. 895. (20) Kohier, J.; Kirkland, J. J. J . chromatogr. 1987, 385, 125. (21) Paciorek, K. L.; Mitchell, L. C.; Lenk, C. T. J . pdym. Scl. 1980, 45, 405-413. (22) Diemond, R. M.; Whitney, D. C. I n Ion Exchenge; Marinsky, J. A., Ed.; Marcel Dekker: New York, 1986 Voi. 1. (23) Small, H. I n Ion Chfomtognrphv; Plenum Press: New Yo&, 1989. (24) Yeung, E. S. Acc. Chem. Res. 1989, 22, 125-130. (25) Larson, J. R.; Pfeiffer. C. D. J . Chfomatogr. 1983. 259, 519-521. (28) Danielson, N. D.; Wangsa. J.; Beaver, L. G. J . chrometcgr. 1991, 544, 187-199.
RECEIVED for review August 19,1991. Accepted December 13, 1991.
Interpretation of Retention Behaviors of Transition-Metal Cations in Micellar Chromatography Using an Ion-Exchange Model Tetsuo Okada Faculty of Liberal Arts, Shizuoka University, Shizuoka 422, Japan
Uses of moblle phases contalnlng an anlonlc surfactant and tartark add 818 mld compbxhg agent permttthe separatbn d transltkmnaal knr wlth revemd-phasechromatography. Effects of the surfactant and tartaric acld are quantltatlvely evaluated on the bad8 of an lon-exchange model. Comphxatlon d tartarlc acld wtth metal Ions does not Influence the partltlon to the anlonlc mlcellar phase because of the negathrervface potentlal dthemlcdle, whkh d e p ” 8 n o t only the dssoclatbn af tartrrrk add but ako the complexatbn wtth metal Ions at the mkelldsolutlonInterface. Thk result suggests that the partltlon of metal catlons to the anlonlc mlcdk can be treated as a simple lon-exchange equlllbrlum even In the presence d tartaric acM. On the other hand, the partltlon to the statknary phase Is affected by the complexatlon. Quantltatlve lnterpretatlon of these results allows us to prodkt the retentkn and the optlmwn moblle-phare compolnlon. Three prlnclpal factors affecting the retentlon and the wparatbn are dmunaneously upthnked wlth the rknpkx method on the bask of the retentlon model developed.
INTRODUCTION Micellar mobile phases have been extensively used in the reversed-phase chromatographic separation of various organic 0003-2700/92/0384-0589$03.00/0
compound^.^-^^ Interaction between analytes and micelles results in unique separation selectivity and permits the determination 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.16J7 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 macroscopicallyhomogeneous 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 ~ctanol.’~J~ Thus, we can regard the interaction involved in micellar chromatographyof neutral organic compounds as lipophilic or hydrophobic. On the contrary, the use of micellar mobile phases in inorganic chromatography has been limited.lgZ4 The author indicated the efficiency of micellar mobile phases in the separation of inorganic anionsmaZ1and cations;n 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 0 1992 American Chemical Society
