Anal. Chem. 1994,66, 3151-3164
Naphthalenesulfonates as Electrolytes for Capillary Electrophoresis of Inorganic Anions, Organic Acids, and Surfactants with Indirect Photometric Detection Shahab A. Shams1 and Neil D. Danieison' Department of Chemistry, Miami University, Oxford, Ohio 45056
Naphthalene- mono-, di-, and trisulfonates (NMS, NDS, NTS) are introduced as electrolytes for capillary electrophoresiswith indirect photometric detection (IPD) for a wide range of inorganic anions, organic acids, and aliphatic anionic surfactants. Several parameters such as the concentration of the UV-absorbing electrolyte, borate buffer, and organic modifier as well as temperature are found to influence sensitivity, resolution, and efficiency. However, the migration times of anions are affected only to a minor extent. A separation of 22 inorganic anions and organic acids in about 18 min using NDS is straightforward. A similar separation is possible using NTS in about the same time; however, the peak shapes of low mobility anions are slightly poorer. A close mobility match between an electrolyte and analyte can improve detection limits by at least 5-fold for certain inorganic anions such as fluoride and orthophosphate. Similar results are also obtained for organic acids such as formate and tartarate with NTS and NDS, respectively. Detection limits for the inorganic anions (16300pg/L),andorganicacids (40-100pg/L),areobtained at 284 and 288 nm with IPD using NTS or NDS. At 214 nm, these detection limits can be further improved by at least 2-fold. NMS having a significantly lower mobility is particularily well suited for the rapid separation in 6 min of C4-C14SO4- or SO3type surfactants. NDS was the best electrolyte for the separation of all three classes of anions. Although the possibility of conductivity detection of ions after separation by capillary electrophoresis (CE) has been demonstrated,' indirect detection methods have remained important because of their ease of application with available instrumentation. A wide variety of UV-absorbing or fluorescent electrolytes have been used for both cation and anion analysis, and several reviews are available." Chromate with UV detection at 254 nm has provided separation times on the order of minutes for a wide variety of inorganic anions5 and some organic anions.6 Application of chromate for CE separations of anions found in pulped wood samples7 and extracts of alumina from bauxite8 has been made. A concentration comparison of inorganic electrolytes such as ( I ) Dasgupta, P. K.; Bao, L. Anal. Chem. 1993,65, 1003-1011. (2) Jones, W. R. J. Chromatogr. 1993,640, 387-395. (3) Jandik, P.; Bonn, G. Capillary Electrophoresis of Small Molecules and Ions; VCH Publishers, Inc: New York, 1993; pp 134-138, 265, 270. (4) Jandik, P.; Jones, W. R.; Weston. A,; Brown,P. R. LC-GC1990,9,634-645. ( 5 ) Wildman, B. J.; Jackson, P. E.; Jones, W. R.; Alden, P. G . J. Chromatogr. 1991, 546, 459-466. (6) Jones, W. R.; Jandik, P. J. Chromatogr. 1992, 608, 385-393. (7) Salomon, D. R.; Romano, J. J. Chromatogr. 1992, 602, 219-225. (8) Grocott, S. C.; Jeffries, L. P.; Bowser, T.; Carnevale, J.; Jackson, P. E. J. Chromatogr. 1992, 602, 257-264.
0003-2700/94/0366-3757$04.50/0 0 1994 American Chemical Society
chromate and nitrate indicated little effect on the CE separation of inorganic anion^,^ but the presence of organic solvents tended to increase migration times. Chromate provided a good mobility match for highly mobile anions such as bromide, chloride, sulfate, nitrite, and nitrate.1° Some organic anions including benzoate, phthalate, sulfobenzoate, and benzyl benzoate have been compared as CE electrolytes for indirect detection of inorganic anions." Phthalate at pH 6.5 gave the best detection limits. However, aromatic monocarboxylic acids as CE indirect detection electrolytes provided a better mobility match for some less mobile anions such as fluoride and in particular organic acids.1° CE with indirect detection using benzoate was applied to the separation of organic acids such as citrate, tartarate, succinate, and lactate in a variety of foods.12 Indirect CE detection of organic acids using phthalate and of inorganic acids using pyromellitate has been reported.'3 The electrophoretic mobility of pyromellitate having four carboxyl groups could be adjusted to match that of the analytes by changing the pH.I4 At a pH of 7.7, separation of inorganic anions with symmetric peak shapes was possible. At pH 3.5, separations of halocarboxylic acids and short-chain (Cl-C,) sulfonates were possible. Indirect fluorescence detection using salicylate or fluorescein has been used for CE of anions.15J6 Little CE work has been reported using aromatic sulfonates as indirect photometric detection (IPD) electrolytes. Tiron (1,2-dihydroxybenzene-3,5-disulfonate) was used to separate a few inorganic anions, particularly vanadate.17 One chromatogram of C4-C12 alkanesulfonates using naphthalenemonosulfonate (NMS) as the electrolyte was included in an overview of CE for the determination of anions in real samples.I8 A more complete characterization providing separations of other surfactant classes or any estimate of detection limits was not done. Previously, we characterized NMS, naphtblenedisulfonate (NDS), and naphthalenetrisulfonate (NTS) as mobile phases for ion-exchange chromatography with indirect detection of anion^.'^-^^ In addition, the separation of alkanesulfonates (9) Buchberger, W.; Haddad, P. R. J. Chromatogr. 1992, 608, 59-64. (10) Jandik, P.; Jones, W. R. J. Chromatogr. 1991, 546, 431-443. (11) Ma, Y.; Zhang, R. J. Chromatogr. 1992,625, 341-348. (12) Kenney, B. F. J. Chromatogr. 1991, 546, 423-430. (13) Kelly, L.; Nelson, R. J. J. Liq. Chromatogr. 1993, 16, 2103-2112. (14) Harrold, M. P.; Wojtusik, M. J.; Riviello, J.; Henson, P. J. Chromatogr. 1993, 640, 463-47 1. (15) Gross, L.; Yeung, E. S. J . Chromatogr. 1989, 480, 169-178. (16) Xue, Q.; Yeung, E. S. J . Chromatogr. 1994, 661, 287-295. (17) Groh, T.; Bachmann, K. J. Chromatogr. 1993,616, 405-410. (18) Romano, J.; Jandik, P.; Jones, W. R.; Jackson, P. E. J. Chromatogr. 1991, 546, 411-421. (19) Maki, S. A.; Danielson, N. D. J . Chromatogr. 1991, 542, 101-113. (20) Maki, S. A.; Danielson, N. D. Anal. Chem. 1991,63, 699-703.
Analytical Chemistry, Vol. 66, No. 21, November 1, 1994 3757
and alkyl sulfates having chain lengths from C6 to C12 has been facilitated by use of a fluorocarbon polymer-based mixedmode reversed-phase ion-exchange column with either indirect photometric or conductivity detection.21+22In this work, we have compared NMS, NDS, and NTS for the separation of inorganic anions, organic acids, and aliphatic anionic surfactants with IPD. One electrolyte, NDS, could effectively separate all three classes of anions.
EXPERIMENTAL SECTION Instrumentation. All electropherograms were generated using an Applied Biosystems Model 270 A capillary electrophoresis instrument (Foster City, CA). The fused silica capillary (75 cm X 50 pm i.d., 320 pm 0.d.) with various effective lengths (Ld) ranging from 44 to 49 cm was also obtained from Applied Biosystems. A Hewlett-Packard Model 3395 integrator with reversed polarity for IPD was used to obtain positive peaks and to record all the data. A HewlettPackard (Wilmington, DE) Model 8452 A diode array UVvisible spectrophotometer was used for taking absorption spectra of the naphthalenesulfonate electrolytes. Reagents and Chemicals. The monosodium salt of NMS with 99.5% purity was purchased from Eastman Kodak (Rochester, NY). The disodium salt of NDS with 95% purity was obtained from Aldrich (Milwaukee, WI). The trisodium salt of NTS with 97% purity was obtained from American Tokyo Kasei (Portland, OR). Diethylenetriamine (DETA, technical grade), sodium tetraborate decahydrate (99.5%), and boric acid (99.5%) were purchased from Fisher Scientific Co. (Fair Lawn, NJ). Salts of the common inorganic anions and organic acids, reagent grade or better quality, were obtained from different manufacturers. Procedure. Preparation of Electrolyte Solutions. A 100 mM stock solution of each UV-absorbing electrolyte (NMS, NDS, NTS) was prepared in triply distilled water and then used after subsequent dilutions. All final operating buffers contain the required concentration of NMS, NDS, or NTS, with various concentration ratios of H3BO3 and N a z B 4 0 ~ lOH2O buffered to a pH of about 8.0. For all buffers except those for the surfactant separations, 2 mM DETA was added to suppress the electroosmotic flow (EOF). All of these final operating buffers were filtered through a 0.2 pm filter and degassed with helium prior to use. Instrumental Operation. The capillary was first flushed with 0.5 M NaOH for 30 min and then equilibrated with the operating buffer for 10 min before any sample injections. Sample was then introduced by using vacuum injection for various time periods (maximum 30 s). The separation was initiated by applying a voltage (f30 kV) between the two capillary ends, which were immersed in vials containing the operating buffer. In between injections using wash and buffer cycle 1, the capillary was flushed with triply deionized water for 2 min and 0.1 M NaOH for 2 min and then rinsed again with triply deionized water for 2 min. The capillary was then filled with the operating buffer for 2 min by selecting buffer cycle 2. This procedure resulted in very reproducible migration times (50.5% RSD) for the analyte anions. (21) Maki, S.A.; Wangsa, J.; Danielson, N. D. Anal. Chem. 1992,64, 583-589. (22) Danielson, N. D.; Shamsi, S. A.; Maki, S.A. J. High Resoluf. Chromarogr. 1992, 15, 343-346.
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Table 1. Relatlve Mlgratlon Time of Electrolyte and Analyte Anlon Measured wlth Negatlve Polaritya
electrolyte anion
re1 migration timea-b
naphthalenemonosulfonate (NMS) naphthalenedisulfonate (NDS) naphthalenetrisulfonate (NTS)
11.37 2.43 1.83
Atd
analyte anion A. inorganic anion bromide chloride nitrite nitrate sulfate fluoride orthophosphate B. organic acids oxalate malonate formate fumarate maleate succinate citrate malate tartarate
re1 migration timeape
NTS
NDS
1.oo
+0.83 +0.73 +0.69 +0.64 +0.50
+1.43 +1.38 +1.29 +1.24 +1.10 +OS9 -0.09
1.os 1.14 1.19 1.33 1.84 2.52 1.39 1.76 1.88 1.94 1.99 2.17 2.24 2.28 2.53
-0.01 -0.69 +0.44 +0.07 -0.05
+1.04 +0.67
-0.11
+0.49 +0.44 +0.26 +0.19 +0.15 -0.10
-0.16 -0.34 -0.41 -0.45 -0.70
+os5
Measured with respect to bromide. Using 2 mM DETA, 100 mM HsBO3,S mM NazB407.10H20 buffered at pH 8. Using 2 mM DETA, 4 mM NTS, 100 mM HoBO3, 5 mM Na2B40~10H20,buffered at pH 8. d Relative migration time of electrolyte (NTS or NDS) minus relative migration time of analyte.
RESULTS AND DISCUSSION The UV absorption spectra of 0.02 mM NMS, NDS, or NTS showed absorption maxima at 274, 288, and 284 nm with molar absorptivities of 6200,16 420, and 7750 L/mol.cm, respectively. On the basis of this work, most of the electropherograms were run at the X maximum values for NMS, NDS, and NTS. The relative migration times of naphthalenesulfonate electrolytes and some of the inorganic anions and organic acids measured with negative polarity (Le., injection at the cathodic end and detection at the anodic end) are shown in Table 1. It can be seen that NTS with a -3 charge and NDS with a -2 charge have relative migration times and hence mobilities which fall within the approximate relative migration time boundaries for the inorganic anions and organic acids. However, the mobility of NMS with a -1 charge is about 5 times smaller, resulting in a longer migration time. The relative migration time of naphthalenesulfonate electrolyte anions therefore increases in order of NTS < NDS < NMS. The difference in the relative migration time of the electrolyte and analyte ( A t )is smallest for fluoride using NTS, while the NDS electrolyte gives a close At match for orthophosphate. Similarly organic acids such as malonate, formate, fumarate, and maleate possess small At values with the NTS electrolyte, whereas the citrate, malate, and tartarate At values match well with NDS. In CE with IPD, a close mobility match (small At) will certainly improve detectability and resolution because sharper peaks are generated.23 Separation of Inorganic Anions and Organic Acids. To optimize the electrophoretic separation, we first investigated the influence of the borate buffer (H3BO3, Na2B407) on (23) Nielen, W. F.J. Chromarogr. 1991, 588, 321-326.
migration time and the signal-to-noise ratio (S/N) for inorganic anions and organic acids using 3 mM NTS/2 mM DETA at pH 8. Increasing the H3B03/Na~B407concentration from 20 to 200 mM has little effect on the migration time of very high mobility anions such as bromide, chloride, nitrite, nitrate, sulfate, and fluoride. The migration time for phosphate did increase from 9 to 11 min when the 100 mM borate concentration was doubled; the reason is unclear at this time. The borate concentration ranging from 20 to 200 mM has little effect on the S / N for bromide, chloride, nitrite, and nitrate. However, increasing the H3B03, Na2B407 concentration from 20 to 100 mM improves the S / N by a factor of 2 for sulfate, fluoride, and phosphate as peak distortions are reduced. At concentrations greater than 100 mM, a general trend of a decreasing S / N was evident. This reduction in S / N can be explained by the dilution of the zones of the UV-absorbing electrolyte such as NTS by the nonabsorbing buffer such as borate. As a result of this, when a nontransparent sample zone passes through the detector cell, the decrease in absorbance signal is smaller. Baseline noise at high ionic strength is another factor that contributes to the decrease in S/N. The effect of the EOF modifier concentration on the migration of anions has been studied.2 For anions such as nitrate, sulfate, and bromide, ion pairing with the EOF amine can take place, reducing their effective charge and increasing migration times. The influence of NDS or NTS electrolyte concentration at a fixed H3BO3/NazB407 concentration buffered at pH 8 on migration time and S / N for inorganic anions (bromide, chloride, nitrite, nitrate, sulfate, fluoride, orthophosphate) and organic acids (oxalate, malonate, formate, fumarate, maleate, succinate, malate, citrate, tartarate) was considered. The migration sequence and the separation of these inorganic anions and organic acids remain essentially the same as both NDS and NTS electrolyte concentrations are increased from 1 to 10 mM. However, migration times in general tend to be slightly longer with NTS. A study comparing electrolytes with increasing number of carboxyl groups (benzoate, phthalate, trimesate, pyromellitate) for the separation of inorganic and organic anions also found longer analyte migration as the charge of the electrolyte is increased.3 Somewhat unexpected are the occasional changes in selectivity observed with changes in electrolyte concentrations for the organic acids. At a NTS concentration between 1 and 4 mM, malonate is migrating slower than formate and malate is migrating close to citrate. At 5 mM NTS, malonate merges with formate and malate comigrates with citrate. As theNTS concentration increases to 10 mM, the malonate peak moves away from formate whereas malate migrates faster than citrate. These shifts in the mobility of malate and malonate are probably due to the change in their net charge with increasing ionic strength. However, it is surprising that such reversal is not observed with an increasing NDS electrolyte concentration. The effect of NDS or NTS concentration between 1 and 10 mM on S / N of some inorganic anions and organic acids was studied. In general, S / N tends to double with the increase in NDS or NTS concentration from 1 to 4 mM, providing sharper peaks and better resolution. However, at concentrations greater than 4 mM NDS or NTS, a downward trend in S / N ratio is evident. This decrease in
I
NMS
NTS
1
7
I
I T 1
Figure 1. Comparison of naphthalenesulfonate electrolytes for the separation of seven common Inorganic anions. Electrolyte: 4 mM NMS, NDS, or NTS in 100 mM H3B03/5mM Na2B407/2mM DETA, pH 8 buffer. Peak identification: 10 ppm each of (1) bromide, (2) chloride, (3) nitrite, (4) nitrate, (5) sulfate, (6) fluoride, and (7) orthophosphate. Vacuum injection for 3 s: -30 kV applied for separation: current, 1618 PA; IPD at 274, 288, and 284 nm, respectively.
I I
9
NDS
3
NTS
21aP
7
Figure 2. Comparison of naphthalenesulfonate electrolytes for the separation of nine organlc acids. Peak identification: 10 ppm each of (1) oxalate, (2) malonate, (3) formate, (4) fumarate, (5) maleate, (6) succinate, (7) malate, (8) citrate, and (9) tartarate. Other conditions, same as in Figure 1.
S / N is obviously due to the increase in the magnitude of the noise, which increases proportionally with increasing concentration of UV-absorbing electrolyte. The optimized electrolyte in IPD requires an electrolyte composition that achieves a balance between background noise and electromigrative peak dispersion or efficiency. Accordingly, a 100 mM H$03/5 mM Na2B407 buffer solution with 4 mM NDS or NTS has been found to be quite suitable for the separation and detection of inorganic anions and organic acids. Figure 1 shows electropherograms of a standard mixture of seven common inorganic anions with NMS, NDS, and NTS electrolytes. Although all three electropherograms were run under similar conditions, the analyte peak shapes are very different. With NMS, poor sensitivity and peak fronting are evident whereas NDS and NTS electrolytes provided almost equally good peak shapes for all seven anions. However, for the NDS electrolyte, the orthophosphate peak became very symmetrical while fluoride gave an excellent peak shape with NTS . These differences in peak shapes for orthophosphate Analytical Chemistty, Vol. 66, No. 21, November 1, 1994
3759
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'1'
r!
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I'
4.0 mM NTS
3
4
5
6
7
8
9
10 11 12 13 14 15
3
Tlme (mlnules)
4
5
6
7
8
0-101112
llme (minutes) II
8.3 mM NDS
8.3 mM NTS
l2
23
2 I4
3
4
5
6
7
8
9
10 1 1 12 1 3 1 4 1 5 16 1 7
,*,,.
.
18 19
20
Flgure 3. Effect of electrolyte (NDS, NTS) concentration on the Separation of 22 Inorganic anlons and organic acids. Electrolyte: either 4 or 8.3 mM NDS or NTS In a 100 mM H3B03/5 mM Na2B407/2mM DETA, pH 8 buffer. All anion peak concentratlons are 2 ppm unless otherwise noted. Peak Mentlflcatlon: (1) bromlde, (2) chloride, (3) nitrite, (4) nitrate, (5) sulfate, (6) oxalate, (7) dlthlonate, (8) chlorate, (9) cyclic trimetaphosphate, (10) bromate, (11) fluoride, (12) formate, (13) fumarate, (14) maleate, (15) chlorite, (16) succinate, (17) malate, (18) cltrate, (19) tartarate, (20) orthophosphate,(21) 5 ppm C1S03-,(22) 5 ppm acetate, and (23) 5 ppm iodate. Vacuum lnjectlon for 8 s; -30 kV applied for separation: currrent varied from 18 to 23 PA.
and fluoride are consistent with their very close mobility match with NDS and NTS, respectively (Table 1). It is quite clear that while NDS and NTS provided good electrophoretic separations, NMS, having a much lower electrophoretic mobility than the inorganic anions, is not a suitable electrolyte for this mixture. The separation of a mixture of nine organic acids by using NMS, NDS, and NTS electrolytes was compared. Again the separation is quite satisfactory either with the NDS or NTS electrolyte (Figure 2). In contrast, NMS provides unsatisfactory peak resolution with fronting similar to that shown in Figure 1. In general, theoxalate, malonate, formate, fumarate, and maleate peak shapes are relatively sharper with NTS, whereas the peak shapes for citrate and tartarate are better with NDS. For succinate, there is not much difference in peak shape in switching from NDS to NTS because the At value (Table 1) is not significantly different with either electrolyte. Fumarate was not detected using benzoate as the IPD electrolyte for CE.12 Figure 3 shows a high efficiency separation of an analyte mixture containing 23 inorganic anions and organic acids at two different electrolyte concentrations of 4.0 and 8.3 mM for both NDS and NTS. First, it should be noted that nitrate and oxalate are well resolved in all these electropherograms. 3760
AnalyticalChemistry, Vol. 66, No. 21, November 1, 1994
This peak pair is difficult to separate with baseline resolution using ~ h r o m a t e .The ~ 4.0 mM NDS electropherogram (top left) shows that all anions are fully resolved except cyclic trimetaphosphate, bromate, and the very low mobility pair of acetate and iodate. The electropherogram with 4 mM NTS (top right) shows baseline separation of cyclic trimetaphosphate and bromate, but with partial resolution of orthophosphate and methanesulfonate and no separation of iodate and acetate. Upon increasing the electrolyte concentrations to 8.3 mM NDS or NTS, the analyte migration times are only slightly increased for most anions (peaks 1-21); for very low mobility anions such as iodate and acetate, this increase is more significant. However, complete baseline resolution of acetate and iodate with NDS (bottom left), and of orthophosphate and methanesulfonate with NTS (bottom right), is possible. This effect of improving resolution with increasing concentration of electrolyte is due to a phenomenon known as stacking, in which increasing the ratio of carrier electrolyte concentration to sample concentration increasespeak efficiency and hence resolution. Previously, this phenomenon was observed with even a lower increase in concentration of lightabsorbing electrolyte (1 s-2.25 mM pyr~mellitate).'~ Figure 4a shows the separation of halogenated anions using NTS; an approximately equally good separation can be
~~
~~
Table 2. Detection LlmH Comparlson Obtalned wHh NDS and NTS as Background Electrolytes for Various Anlons
detection limits. (pg/L) NDSb analyte anion
NTSb
288 nm
214 nm
284 nm
214 nm
350 175 150 160 100 32 20
275 85 -220c -14W 50 16 10
300 150 110 125 80 16 40
250 75 -2off -1off 40 8 20
105 105 80 105 100 120 70 75 65
55 55 40 55 50
80 50 40 60 70 100 75 90
40 25 20 30 35 50 40 45 45
A. inorganic anion
bromide chloride nitrite nitrate sulfate fluoride orthophosphate B. organic acids oxalate malonate formate fumarate maleate succinate malate citrate tartarate 3
4
5
6
Figure 4. (a) Separation of halogenated anlons. Peak identificatlon:
10 ppm each of (1) bromide, (2) chloride. (3) chlorate, (4) bromate, (5) perchlorate, (6) chlorite, and (7) iodate. (b) Effect of temperature on separation of sulfur-containing anions. Peak identiflcation: 10 ppm eachof (l)iodlde, (2)thbsulfate. (3)thiocyanate,(4)sulfate, (5)dittrlonate, and (6) tetrathlonate. Electrolyte: 3 mM NTSIP mM DETAI100 mM H3B03/5 mM Na2B407.Vacuum injection 3 s; -30 kV applled for separation; current 16 fiA.
obtained with NDS (not shown). A separation of a similar mixture by ion chromatography (IC) requires a complex gradient24 as iodate is a weakly retained anion whereas the perchlorate anion has a strong affinity for the ion exchanger. With isocratic conditions using a stronger IC eluent such as trimesate or phthalate, anions such as iodate, bromate, chlorate, and chlorite are not easily resolved in a mixture containing chloride and bromide, while the use of a very weak eluent such as sodium hydroxide or benzoate can result in a very long separation time.25 The effect of temperature on the separation of some anions of similar electrophoretic mobility was also briefly investigated. Figure 4b shows the separation of iodide along with a variety of sulfur-containing anions at three different temperatures. At 25 "C, the thiocyanate/ sulfate pair comigrate, but as the temperature is increased, the peaks sharpen and this anion pair can be nearly baseline separated at 35 OC. A further increase in temperature above 35 OC (not shown) improves the resolution further for the thiocyanate/sulfate pair, but degrades the separation of the dithionate/tetrathionate pair. Hence, the control of temperature can result in an additional selectivity for this separation mixture. The separation time for a similar mixture of sulfur-containing anions ever. with a very powerful IC eluent such as NTS is at least 18 min,20 which is about 3 times greater than the CE separation time. Table 2 compares the detection limits (at S / N 2 3) of seven inorganic anions and nine organic acids using NDS and (24) Saini, C.; Pohl. C. A. Presented at the Pittsburgh Conference, Atlanta, GA, March 8-12, 1993; Abstract P285. (25) Haddad, P. R.; Jackson, P. E. Ion Chromatography. Principles and Applications: J . Chromatogr. Lib. 1990, 46.
60
35 40 30
90
a S / N 2 3 based on peak height; injection, vacuum for 30 s. b Using the optimized background electrolyte, 4 mM NDS,4 mM NTS, all in 2 mM DETA, 100 mM H,B03,5 mM NazB407.10H20 buffered at pH 8. Negative numbers indicate the analyte anion is more absorbing than the background electrolyte.
NTS electrolytes at two different wavelengths. The detection limits of higher mobility inorganic anions (bromide, chloride, nitrite, nitrate, sulfate) range from 100 to 350 pg/L and 80 to 300 pg/L using NDS and NTS, at their X maxima (288 and 284 nm), respectively. Slightly lower detection limits for NTS are observed compared to those of NDS. This is mainly due to two reasons. First, even though the molar absorptivity of NDS is about 2 times larger than NTS, the background noise of NDS was greater than that of NTS. Second, since NTS is triply charged, its mobility being relatively higher than NDS is a more suitable match (smaller A f )for these fast migrating anions. This factor of 2 improvement in detection limit for fluoride with NTS and orthophosphate with NDS is mainly due to a very close mobility match of these analytes with the light-absorbing electrolytes. For organic acids, detection limits range from 65 to 105 pg/L and 40 to 100 pg/L using NDS and NTS, respectively. Again, relatively fast migrating organic acids such as oxalate, malonate, formate, fumarate, and maleate have smaller At values with NTS, and therefore, better detection limits are achieved with NTS rather than NDS. Relatively slower migrating anions such as malate, citrate, and tartarate have smaller At values with NDS and improved detection limits with NDS. These trends in detection limits for inorganic anions and organic acids are in good agreement with the Kohlrausch theory and studies by Foret et a1.26and Nie1e1-1.~~Basically, detection limits are improved when the transfer ratio between the chromophore electrolyte and analyte is high due to a close mobility match.27 Table 2 also shows a detection limit comparison at another convenient wavelength, 214 nm. Detection limits for most of the inorganic anions and organic acids were improved by nearly a factor of 2 at 214 nm. The ~
~
~~
(26) Foret, F.; Fandi, S.; Zhu, M. D. J . Chromatogr. 1989, 470, 299-308. (27) Camilleri, P. Capillary Electrophoresis Theory and Practice; CRC Press: Boca Raton, FL, 1993.
Analytical Chemistry, Vol. 66, No. 21, November 1. 1994
3701
NUS
NDS
I
Figure 5. Comparlson of naphthalenesulfonate electrolytes for the Separation of alkyl sulfates. Peak Identification: 20 ppm each of (1) c14so4-, (2) C12SO4-, (3) c1oso4, (4) CaS04-, (5) CaS04-, and (6) C4S0,-. Electrolyte: 5 mM NMS, NDS, or NTS, In 100 mM H3B03/5 mM NaBO buffer, pH 8.0. Vacuum injection 4 s for NMS and 5 s for NDS and NTS; 30 kV applied for separation; current 10 MA.
+
one exception noted was nitrite, which does not absorb as strongly as nitrate at this wavelength. However, the sensitivity of detection of nitrite, nitrate, and bromide can be improved further by direct UV detection at a wavelength of 206 nm. In addition, malonate can also be detected directly at 206 nm. Separation of Surfactants. Few reports of the separation of aliphatic anionic surfactants by CE have been made. The separation of C2-Clz alkyl sulfates using a barbiturate (barbital) IPD electrolyte could be done with detection limits at 1 X 10-5 M.23 Chen and PietrzykZ8separated both alkyl sulfates and alkanesulfonates using salicylate as the IPD electrolyte and provided C E separations of benzenesulfonates with direct detection. We investigated naphthalenesulfonates as IPD electrolytes for the CE separation of this important class of compounds. As explained previously, with negative polarity, NMS has the lowest mobility. This situation is reversed when naphthalenesulfonate electrolytes migrate with positive polarity (i.e., conventional CE in which injection is at the anodic side and detection is at the cathodic end). The relative migration times for these naphthalenesulfonate electrolytes now follow the opposite order of migration, NMS < NDS < NTS. Under these conditions, the least mobile electrolyte such as N M S (1.54 min) is eluted first as it is carried more rapidly by the bulk EOF flow than the more mobile NDS (2.44 min) and NTS (4.61 min) electrolytes toward the cathode. This situation is especially favorable for the separation of aliphatic anionic surfactants, such as alkanesulfonates (RS03-)and alkyl sulfates (RS04-). NMS has a relative migration time that falls within the approximate range of 1.39 and 1.71 for C4-C14SO3- or C4-C14SO4surfactants. In general (with positive polarity), C1+214SO3or C1+214SO4- have slightly shorter migration times (+At of 0.01-0.1 5) than NMS, whereas C4-CgSO3- or C4-CgS04have slightly longer migration times (-At of 0.01-0.18) as compared toNMS. TheS/N (datanot shown) at an optimized electrolyte concentration of 5 mM NMS in a 100 mM H3B03/5 mM Na2B407 buffer was found to be 2-4 times (28) Chen, S.; Pictrzyk, D.J. Anal. Chem. 1993, 65, 2170-2715.
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8
Tlms (minutes)
Figure 6. Comparlson of reversed-phasehon chromatography (RPIC) and capillary electrophoresis (CE) for the separation of alkane sulfonates. For RP-IC (left),mobile phase: 10 pM NTS, 82% ACN, 20 pL Injection, 2.2 mL/mln, IPD at 284 nm, 0.05 AUFS. Peak Identification: 40 ppm each of (1) CBSOS-,(2) C8S03-, (3)CIOSOS-,(4) C12SO3-, and (5) c14so3- and 80 ppm of (6) CI8SO3-. For CE (rlght), electrolyte composition: 5 mM NMS/ 100 mM H3B03/5 mM Na2B407. Vacuum lnjectlon 4 s; +30 kV applled for separation; current 10 pA: IPD at 274 nm; 0.01 AUFS. Peak Identlflcatlon: 20 ppm each of (1) c&03-, (2) c12so3-, (3) closo3-. (4) CaSOa-, (5) c7so3-, (6) c&03-, (7) c4so3-, (8) c3so3-, and (6) C1SO3-.
higher for C4-CgSO3- or C4-CgSO4- as compared to (210C14SO3- or C1&14S04-. Figure 5 compares the separation of C4-C&04- achieved with NMS, NDS, and NTS electrolytes. NTS because of its very high mobility gives an inadequate separation for this particular mixture. A separation of the same mixture with NDS, although quite adequate, resulted in loss of resolution for Cl2- and C14S04-, and some degree of sensitivity loss, in particular of C4- and C&O4-. Although NMS has a lower molar absorptivity than either NDS or NTS, the electropherograms show quite clearly that NMS is the best electrolyte for the separation of this surfactant mixture. This indicates that selection of a UV-absorbing electrolyte in IPD must include the consideration of achieving higher sensitivity by employing not only an electrolyte with
Alkyl Ether Sulfates ES1
Alkyl Ether Sulfates
L(-
Laurvl Ether Sulfonates
ES2
I o
I
I
I
a
n m irmnmmj
6
6
0
I
I
a
nm irmnmr)
a
I
I
I
Figure 7. Electropherograms of 0.1 % of (a-c) Standapol and (d) Avanel solutions using a 5 mM NMS/100 mM H3B03/5 mM Na2B4O7/30% acetonltrile electrolyte. Vacuum injection 3 s for Standapol and 6 s for Avanel solutions; +30 kV applied for separation; current: 6-10 PA; IPD at 274 nm.
high molar absorptivity but one with a close mobility match to the sample components. Capillary electrophoresis was found to be complementary to reversed-phase ion chromatography (RP-IC) for surfactant separations. A separation for a RS03- mixture shown in Figure 6 (right) by CE is compared to one by RP-IC (left). The elution order was reversed for the two techniques. With RP-IC, the more polar sulfonated surfactants eluted first but with CE, the more nonpolar and less mobile long-chain analytes are rapidly swept toward the negative electrode by the electroosmotic flow. A higher peak capacity and speed are compelling advantages of CE as compared to an isocratic RP-IC separation. However, long-chain RS03- or RSO4such as C14 or greater are difficult to separate because of their very short migration times with normal (positive polarity) CE or very long migration times with reversed (negative polarity) CE. To the best of our knowledge, there are no reports for the separation of >C14SO3- or C14S03- surfactants by CE. The detection limits of RSO3- and RS04- type surfactants with NMS are listed in Table 3. With positive polarity, analytes such as c4-cSs03- or C4-CsS04- that have higher mobilities than NMS have lower detection limits of about 1 mg/L. On the other hand, analytes having lower mobilities than NMS have slightly higher detection limits (2-4 mg/L). These results obtained for aliphatic anionic surfactants (with positive polarity) are opposite to the results obtained by Haddad and c o - ~ o r k e r for s ~ ~the study of inorganic anions (using negative polarity) in which the carrier anion having a shorter migration time and hence higher mobility than the analyte anions gave the best transfer ratio (Tr) and IPD response. These detection limits were further improved by a factor of 2 by working at 214 nm, at which NMS has a higher molar absorptivity. Even though CsS04- provided a very close mobility match with NMS, the detection limit for this particular analyte was not improved over the others. Upon inspection, it was revealed that this sodium salt was rather (29) Cousins,S. M.; Haddad, P. R.; Buchberger, 391.
W. J . Chromatogr. 1994, 671,
Table 3. Detection LlmHs Obtalned with NMS as Background Electrolyte for Aikanesutfonate and Alkyl Sulfate Surfactants
detection limits' (mg/L) NMSb analyte anion
214 nm
214 nm
4.0 2.0 1.5 1 .o 1.o 1.0
2.0 1 .o 0.7
3.0 2.0 2.0 2.0 1 .o 1 .o
1.5 1 .o 1 .o 1 .o 0.5 0.5
0.5 0.5 0.5
S/N 1 3 based on peak height; injeciton, vacuum for 12 s; stock solution of analyte anions diluted in 50% ACN/H*O. Using the optimized background electrolyte 5 mM NMS/100 mM HaHBOo/S mM NazB407.10H20 buffered at pH 8.0. (I
wet, which might be the reason for higher than expected detection limits for this analyte. Ether sulfates and sulfonates widely used in heavy-duty laundry formulations, toilet soaps, and shampoos have considerable importance in the detergent industry.30 To the best of our knowledge, this class of surfactants has not been shown in the literature to be separated by CE. Figure 7a-c shows electropherograms for commercial mixtures of ether sulfate surfactants of structure RCH2(0CH2CH2),OS03-. This sample, which has been formulated by Henkel under the name Standapol, has a concentration of about 25% active material. The three materials, ES1, ES2, and ES3, are all mixtures of surfactants with alkyl chain length varying from R = C12 to R = C18 and differ only by thedegreeof ethoxylation (n = 1, 2, 3). These mixtures were also run by RP-IC with a NTS eluent, and the elution order was found to be reversed (30) Ainsworth, S.J. Chem. Eng. News 1994, 72 (4), 34.
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with similar relative peak height ratios. An electropherogram of lauryl ether sulfonate (trade name Avanel) having a structureof CH3(CH2)11(0CH2CH~),S03-is shown in Figure 7d. It is suspected that this sample could be a mixture with various degrees of ethoxylation for lauryl (R = C12) chainlength ether sulfonates. No further attempts were made to confirm this assumption because of the nonavailability of pure standard solutions. Addition of 30%acetonitrile in the mobile phase increases analysis times from 4 to 5 min, but resolution and peak height are somewhat improved as compared to electropherograms obtained at 0% acetonitrile. A slight improvement in resolution and peak height is also observed for standard solutions of RS03- or RS04- surfactants upon addition of 30% acetonitrile.
CONCLUSIONS A comparison of electropherograms achieved with NMS (low mobility), NDS (intermediate mobility), and NTS (high mobility) revealed that NMS is the best electrolyte for aliphatic anionic surfactants, whileNDS and NTS caused more efficient separations for inorganic anions and organic acids. These electrolytes are stable for at least 10days at room temperature and can be used over a wide pH range (Le., their intrinsic mobility as well as stability is independent of pH). It is wellknown that chromate (Cr042-) should be prepared the day of use and is not a stable electrolyte at acidic pH. At pH 2.0-
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6.0, Cr0d2- can dimerize to C r ~ 0 7 ~which -, can be reduced to Cr3+ in acidic solution. This can cause problems in the analysis of some real samples like oxalate in urine, which requires an acidic condition not only toprevent the precipitation of calcium oxalate but also to inhibit the conversion of ascorbate to ~ x a l a t e . ~Phthalate and benzoate are suitable IPD electrolytes only for organic acids while chromate and pyromellitate are useful for high-mobility inorganic anions. None of these electrolytes can separate long-chain aliphatic anionic surfactants (Le., surfactants with chain length of >8).3J4 Using the same borate buffer, NDS was found to be applicable for the separation and detection of three different classes of anions, namely, inorganic anions, organic acids, and aliphatic anionic surfactants. In addition, detection limits found are competitive with other indirect photometric reagents used previously for CEa3
ACKNOWLEDGMENT The CE instrument was purchased using funds provided by the Committee for Faculty Research and the Research Challenge Program of Miami University. We thank H. T. Kalinoski (Unilever Research U.S., Inc.) for providing the ethoxylated surfactant samples. Received for review March 25, 1994. Accepted July 13, 1994." Abstract published in Aduonce ACS Abstructs, September 1 , 1994
