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Anal. Chem. 1988, 60,1670-1673
Determination of Rare-Earth Elements, Thorium, and Chromium with 2-[ (2=Arsenophenyl)azo]-1,8-dihydroxy-7[ (2,4,6-tribromophenyl)azo]naphthalene-3,6-disulfonic Acid by Reversed-Phase Ion-Pair Liquid Chromatography Xin-Xiang Zhang, Ming-Sheng Wang,*J a n d Jie-Ke Cheng Department of Chemistry, Wuhan University, Wuhan 430072, China
The separatlon and determlnatlon of rare-earth elements (RE), thorium, and chromlum by reversed-phase ion-palr liqukl chromatography have been demonstrated. A precolumn chelating-formation method was used. 24( 2Arsenophenyl)azo]-l,8-dlhydroxy-7-[( 2,4,6-tribromophenyl)azo]naphthalene-3,6-dlslfonlc acid was used as the Chelating reagent. The statlonary phase of the column was packed with 9-11 pm YWG-C18 ODs (250 X 4.0 mm 1.d.) and the Ion (tetramobile phase consisted of a CH,CN-H,O-Ion-palr butylammonlum, TBA') solution. A gradient eluent program was used to change the concentratlon of CH,CN, and the detection wavelength was 635 nm. The linear scale for the metal Ions was (0.02-5.0) X lo-' M (RE,O,,), (0.01-3.5) X lo* M (Th), and (0.02-4.0) X loJ M (Cr), respectively. A detection llmlt of 0.1 ng (20 pL Injected) was achieved for each metal with a relative standard devlation of less than 6.0%.
The application of high-performance liquid chromatography (HPLC) to the separation of metal ions has increased rapidly in the last several years. It seems to be a convenient method for the separation and simultaneous determination of metal ions that have similar chemical preporties. Cassidy and coworkers ( 1 ) have reported complete separation of rare-earth (RE) elements in 10 min on a high-performance C18column using sodium n-octanesulfonate as the modifier and Arsenazo I11 as postcolumn reaction reagent to detect them. This method has been developed to determine the RE in thorium and uranium fuels (1-3) and to determine uranium (1) and thorium (3). The separation of cationic metal species generally needs the postcolumn reaction detector with a suitable color-forming reagent. The choice of a postcolumn reaction detector is greatly influenced by the reaction type. Recently, several reports on the application of HPLC to separation and determination of various metal complexes have been published (4-7). The precolumn chelating formation method consists of the neutral metal complexes or anionic metal complexes being formed before injection into the column and then separation and determination by use of a general HPLC system. The separation method of neutral metal complexes is that classical solvent-extraction reagents such as dithizone, substituted acetylacetonates, 8-hydroxyquinoline, and dithiocarbamates were applied to extration and concentration of metal ions, followed by HPLC analysis. But this method requires a complex procedure to prepare the sample solution, and the neutral metal complexes usually must separate from solution. Many of the papers on ionic metal-complex separations using ion-pair chromatography (IPC) have been published. 'Present address: Department of Chemistry, Purdue University, West Lafayette, IN 47907.
Work has been done on complex reagents such as 4-(2pyridy1azo)resorcinol (PAR) (8), 2-(2-thiazolylazo)-4methyl-5-(sulfomethylamino)benzoic acid (TAMSB) (9), lJ0-phenanthroline ( I O ) , 2-nitroso-1-naphthol-4-sulfonic acid ( I I ) , and meso-tetrakis(1-methylpyridinium-4-y1)porphine (TMPyP) (12). There is no report of the use of azo chromatropic acid complex reagent for the separation and simultaneous determination of RE, Th(IV), and Cr(II1) in ion-pair chromatography (IPC). Yu and co-workers (13) have synthesized a new kind of RE color reagent consisting of polyhalogenated bisazo chromatropic acid derivatives. 2-[(2-Arsenophenyl)azo1-7-[(2,4,6tribromophenyl)azo]-1,8-dihydroxy-3,6-disulfonaphthalene (TBDA) is one of these color reagents. The color reaction of TBDA with RE has been studied (14). It is a highly sensitive reagent for RE. The molar absorptivity (e) of cerium reaches up to 1.33 X lo5 L.mo1-l-cm-l. It is 2-3 times as high as Arsenazo I11 and Chlorophosphenazo 111. It was applied to determine trace amounts of the sum of RE in human hair by spectrophotometric methods. In this paper, the precolumn chelating-formation method was used. TBDA was used as the complexing reagent. The separation and determination of mixed rare earths (RE,O,), thorium, and chromium by IPC has been demonstrated. These complexes were injected into the column packed with bonded-silica stationary phase. Tetrabutylammonium bromide was used as an ion-pair reagent. The method is simple and convenient. EXPERIMENTAL SECTION Apparatus and Chemicals. The high-performance liquid chromatography (HPLC) system (Model LC-6A, Shimaduz, Japan) consisted of three pumps (Model LCGA, Shimaduz, Japan), a UV-vis spectrophotomonitor (Model SPDGAV, Shimaduz, Japan), and a Rheodyne Model 7161 inject valve (20-rL sample loop). A column packed with 9-11 r m YWG-C18 ODS (250 X 4.0 mm i.d. Tianjing Chemical Co., Tianjing, China) was used as the main column, and a column of the same particles (50 X 4.0 mm i.d.) was used as a precolumn. A spectrophotometer (Model UV-3000, Shimaduz, Japan) was used. The stock solution of Ce3+,Y, Dy, Er, Gd, Yb, Th, and Cr3+ were prepared from "Spec Pure" oxides (Matthey Spec Pure, Johnson Matthey & Co., Ltd., England); these oxides were ignited to constant weight at 950 "C and dissolved in 0.5 mo1.L-l HC1 (1.000 mgmL-' CeOz, YzO3, Dyz03, ErzO3 Gdz03,Ybz03,Th, or Cr). The stock solution of mixed rare earths (RE,Oy) was prepared by mixing a stock solution of Yz03, GdzO3, DyzO,, Erz03,Ybz03, CeOz (60:10:10:5:5:10, an ore of ionic absorption type, Jiangxi Province, China) with 0.5 mo1.L-l HC1 (1.000 mgmL-' RE,Oy). The chelating reagent 24 (2-arsenophenyl)azo]-1,8-dihydroxy7-[(2,4,6-tribromophenyl)azo]naphthalene-3,6-disulfonic acid (TBDA) was obtained from Laboratory of Organic Analytical Reagents (Wuhan University, China). The TBDA solution (0.2%) was prepared by dissolving TBDA in water. The ion-pair reagent was tetrabutylammonium bromide (TBABr) (A.R. Shanghai Chemical Co., Shanghai, China). All other reagents were analytical grade. Double distilled water was used for all solution preparations.
0003-2700/86/0360-1670$01.50/0 @ 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
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0
2
0
4.0
2.0
6.0
PH Figure 1. Effect of pH on the complex formation. Experiment conditions were as follows: Cr (A)0.4 pg-mL-', Th (0)0.2 ppmL-', RE,Oy (-) 0.4 pg-mL-'; reference, reagent blank; 635 nm; 1-cm cell.
0
0
2.0
0
4.0
0.2% TBDA (mL/251&)
Figure 3. Effect of concentrationof TBDA on the complex formation. Experimental conditions were as follows: mixed metal sample (Cr3+, 1.0 pg-mL-'; Th, 0.4 pg-mL-'; RE,Oy, 0.2 pg-mL-'), other conditions are the same as in Figure 1.
9 0.
f-d
51
'
n
-0
2i4
Flgure 2. Effect of concentration of acetate buffer on the complex formation. Experimental conditions are given in Figure 1.
The mobile phases for the three pumps were acetonitrile,water, and ion-pair ion solution (2.5 mmol-L-' TBABr and 4.0 mmo1.L-' acetate buffer, pH 4.0). All HPLC eluents were filtered through a 0.45-pm filter. Procedure. Formation of Complexes. Five milliliters of 1.0 mo1.L-l acetate buffer (pH 4.0) and 2.0 mL of 0.2% TBDA were added to a sample containing RE,O,, Th, and/or Cr, and the mixture was diluted to 25.00 mL. The solution was heated in boiling water for 10 min and cooled to room temperature for HPLC analysis and spectrophotometric analysis. Separation of the Complexes and Reagent. For HPLC analysis, the 9-11-pm ODS reversed phase was equilibrated with an aqueous mobile phase containing 25% CH,CN, 25% ion-pair ion solution (2.5 mmo1.L-' TBABr, 4.0 mmol-L-' acetate buffer, pH 4.0), and 50% water (v/v). Twenty microliters of the 25.00 niL of prepared solution was injected and separated via a gradient of CH3CN. The total flow rate of the mobile phase was 0.8 mLamin-' and the detection wavelength was 635 nm. The sensitivity was set at 0.005 or 0.002 absorbance unit at full scale (AUFS). The mount of each metal was determined by measuring the peak areas.
RESULTS AND DISCUSSION Formation of Complexes. The report (14) shows that RE elements can complex with TBDA at pH 3.2 and at room temperature. Our experiment showed that Cr3+ complexes slowly with TBDA at room temperature but faster in boiling water. Our experiments also showed that RE complexes are stable. The effects of pH, the concentration of acetate buffer,
600
650
700
WAVELENGTHb)
Flgure 4. Absorption spectra of complexes and reagents. Experimental conditlons were as follows: C?+, RE,O,, Th were 0.4 pg-mL-' respectively, pH 4.0, in the presence of excess TBDA (2.00 mL of 0.2% TBDA), reference was reagent blank: absorption spectrum of TBDA, pH 4.0, reference was water.
and the concentration of TBDA on the chelating reactions have been studied (Figures 1-3). Figure 1 shows that the absorbance of complexes had little change between pH 3.0 and 5.0, so pH 4.0 was used. Figure 2 shows that 0.2 M acetate buffer was suitable for those complexes. Figure 3 shows that the amounts of RE,O,,, Th, and Cr were at the middle of the calibration curve and were mixed as the sample, 2.00 mL of 0.2% TBDA solution added was enough. Also, a further experiment showed that at a higher point of the calibration curve the 2.00 mL of 0.2% TBDA was enough. At the chosen conditions (pH 4.0,0.2 M acetate buffer, 0.016% TBDA), the spectra of these complexes and reagents were obtained (Figure 4). The absorption maxima of RE complexes are at 632.6-638.6 nm (Ce, 632.6 nm; Pr, 634.2 nm; Sm, 635.6 nm; Gd, 636.5 nm; Dy, 637.2 nm; Er, 637.6 nm; Yb, 638.6 nm; Y, 637.6 nm). The molar absorptivity (e) of RE complexes was (0.83-1.33) X lo5 L-mol-l-cm-' (Ce, 1.33; Pr, 1.21; Sm, 1.26;
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 17, SEPTEMBER 1, 1988
E
10 20 30 40 Cone. o f 2 . 5 mKTBABr(5)
0
Figure 5. Effect of concentration of TBABr on retention time: 30% CH,CN (vlv), 0.8 mLqmin-'.
BgL I--*-' ot 0
40
20 R E T E N T I O N TIME (ruin.)
Figure 7. Separation of RE, thorium, and chromium TBDA complexes by HPLC: column, 250 X 4.0 mm 9-1 1-pm YWG C18; mobile phase, CH,CN-H,O-TBABr (2.5mmol-L-', pH 4.0); total flow rate, 0.8 mL min-'; sample, 20 pL containing 8 ng of RE,O,, Th, and C?+; detection wavelength, 635 nm.
Table I. Effect of Foreign Ions
ions Ca2+ MgZ+ 25
30 35 40 COAC. OF CR3CN ($)
45
Flgure 6. Effect of concentration of CH,CN on the retention time: 25% 2.5 mmol-L-' TBABr (v/v), pH 4.0, 0.8 mL-min-'.
Gd, 1.29; Dy, 1.14; Er, 1.06; Yb, 0.83; Y, 1.03). HPLC Separation of RE, Th, and C$+ Complexes. All these complexes eluted without separation when only a acetonitrile-water mixture was used as mobile phase. When TBABr was added to the mobile phase, these complexes could be separated. The effects of the concentration of TBABr and CH3CN on the retention time were examined (Figures 5 and 6). As expected, the greater the concentration of TBABr, the longer the retention time of each complex and the better the resolution of peaks for these complexes became. On the contrary, the greater the concentration of CH,CN, the shorter the retention time and the worse the resolution of peaks for these peaks became. The concentration of CH3CN had more effect than that of TBABr on the retention and the resolution. So gradient elution, which changes the concentration of CH3CN, was used. With this gradient elution program, Ce, Gd, Yb, and Y complexes were eluted with little difference when they were injected. When the mixed rare-earth complexes were injected, they eluted as one peak. The chromatogram is shown in Figure 7. Calibration Curves. The calibration curves for the determination of RE, Th, and Cr3+ are linear over the range (0.02-5.0) X lo4 M for RE,O,, (0.01-3.5) X lo4 M for Th, and (0.02-4.0) X M for Cr: RE,O,, C(pgmL-') = (3.05 X lo4) X area; Th, C(pgmL-') = (4.50 X lo4) X area; Cr, C(pgmL-') = (6.02 X lo4) X area.
tolerant limit, pg-mL-'
~13+
40 1600 0.8
cu2+
0.8
ratio4
ions
100 4000 2 2
Fez+ Sc3+
tolerant limit, pp.mL-l
ratio"
2 0.4 0.8 2
5 1 2 5
U6+ Zr4+
'Foreign ion/determined ion.
The detection limits (at a signal-to-noise ratio of 2:l) for metals are 0.1 ng (20 pL injected) for each metal. Relative standard deviations calculated from nine replicate analyses of a sample containing 0.2 pgmL each of RE,O,, Th, and Cr are 0.03, 0.06, and 0.02, respectively. Effect of Foreign Ions. The effect of the presence of other ions on the determination of RE, Th, and Cr2+was examined. Table I shows that ions affect the peak areas of these chelates with an error of less than 10% in the determination of 0.4 pg.mL-l RE,O,, Th, and Cr. Fe3+seriously interfered with the determination, it can be changed to Fe2+ by adding ascorbic acid to decrease interference. The determination of RE,O,, Th, and Cr in some samples, such as ore, tap water, and neutral water, using the method described is now under investigation.
ACKNOWLEDGMENT The authors thank Xi-Mao Yu for providing the chelating reagent. LITERATURE CITED (1) Knight, C. H.; Cassidy, R. M.; Recoskie, B. M.; Green, L. W. Anal. Chem. 1984, 56, 474-478. (2) Cassidy. R. M.; Elchuk, S.; Elliot, N. L.; Green, L. W.; Knight, C. H.; Recoskie, 0. M. Anal. Chem. 1906, 5 8 , 1181-1186. (3) Barkley, D.J.; Blanchette. M.; Cassidy, R. M.; Elchuk, S.Anal. Chem. 1986, 58, 2222-2226. (4) O'Laughlln, J. W. J . L i g . Chromatogr. 1984, 7(S1), 27-204. (5) Schwedt, G. Chromafographie 1979, 12(9). 613-619. (6) Smith, R. M. Anal. Proc. 1984, 21, 73-75.
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Anal. Chem. 1988, 60, 1673-1677 (7) Yamada, H.; Hattori, T. J . Chromatogr. 1986, 361, 331-336. (8) Hoshino, H.; Yotsuyanagi, T. Anal. Chem. 1985, 57, 625-628. (9) Wada, H.; Nezu, S.; Nakagawa, G. Bunseki K a g a b 1983, 32, 600-605. (10) O'Laughlin, J. W. Anal. C h m . 1982, 5 4 , 178-181. (11) Sawatanl, I.; Oshima, M.;Motomizu, S. Bunsekl Kagaku 1984, 33, 119-1 21. (12) Igarashi. S.; Nakano, M.;Yotsuyanagi, T. Bunsekl Kagaku 1983, 32, 67-68.
(13) Yu, X.-M.; Cai, R.-X.; Tian, S.-2.; Zeng, Y.-E. C b m . J . Chinese Univ. 1986, 2(2), 23-31. (14) Cai, R.-X.; Wang. L.-S.; Yu, X.-M. Gaodeng Xuexiao Huaxue Xuebao 1985, 6, 120-122.
for review December 1 4 9 1987* Accepted
25,
1988.
Separation of Oxygen Isotopic Benzoic Acids by Capillary Zone Electrophoresis Based on Isotope Effects on the Dissociation of the Carboxyl Group Shigeru Terabe* and Toshiyuki Yashima
Department of Industrial Chemistry, Faculty of Engineering, Kyoto Uniuersity, Sakyo-ku, Kyoto 606, Japan Nobuo Tanaka and Mikio Araki
Faculty of Textile Science, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan
Oxygen isotopic benzoic acids are chosen as examples of closely related compounds for the study of ultrahigh-resolutlon separation by capillary zone electrophoresis (CZE). Optimum condltlons for the Separation of acids having sllghtly dlfferent dissoclatlon constants are theoretically discussed on the basis of the resolution equation In CZE. The effects of the following factors on the resolution have been experlmentally Investlgated: applied voltages, capillary tube lengths, pH of the buffers, and electroosmotlc velocltles. Three isotopic aclds, C,,H,C1e02H, C,H,C'eO1eOH, and C8H5C1802H,have been successfully separated In 100 mln under the optlmum conditions.
Separations of various kinds of isotopic compounds by ionization control in reversed-phase liquid chromatography (RPLC) have been very successful: oxygen isotopic benzoic acid (1-3) and phenol ( 2 , 3 )derivatives and nitrogen isotopic aniline derivatives ( 3 , 4 )have been separated on the basis of the very slight differences in the dissociation constants of carboxyl, phenolic hydroxyl, and amino groups. In these separations, the use of highly efficient chromatographic systems has been essential, because the isotopic separation factors are generally less than ca. 1.012. Therefore, either recycle chromatography with conventional columns (1-4) or a 6-mlong microbore column system ( 3 ) has been employed. The isotopic separation requires long separation times, e.g., from 5 to 20 h, for adequate resolutions. Recently developed capillary zone electrophoresis (CZE) (5-7) is a highly efficient separation technique and has proved to be useful in a wide area of analysis (8-16). In electrophoresis, separation is brought about on the basis of the difference in the electrophoretic mobility, which is controlled by the difference in an electrical charge, shape, or size of the solute. CZE includes no stabilizing media to prevent unfavorable convection; therefore, CZE constitutes one of the simplest systems for theoretical considerations. From the viewpoint of general interest, we have decided to explore the capability of CZE for a primary separation by employing
isotopic compounds as examples of closely related compounds to be resolved. In this paper, separation of solutes having slightly different dissociation constants is theoretically studied and the separation of oxygen isotopic benzoic acids in short times is reported to reveal the potential capability of CZE in ultrahigh resolution.
THEORY According to Giddings (17) resolution R, in electrophoresis is expressed as
R, = ( W I 2 / 4 ) ( A u / D )
(1)
where N is the theoretical plate number and Au/ij is the relative velocity difference of two solutes. Since electrophoresis is usually accompanied by electroosmosis in CZE (6),the average velocity of the two solutes is given as the s u m of the average electrophoretic velocity ueP and the electroosmotic velocity ueo;thus
u = oep+ u,,
(2)
where the sign of the velocity is taken as positive when the migration is toward the negative electrode. The average velocity ij is rewritten as
u = (Pep + peJ(V/L)
(3)
where pepis the average electrophoretic mobility, pe0 may be called electroosmotic mobility (6),and V is the applied voltage between both ends of the capillary tube with a total length L. If band broadening in CZE can be attributed only to the molecular diffusion of the solute along the tube axis, the plate number is given as (6)
N = 12/2Dt
(4)
where D is the diffusion coefficient of the solute, 1 is the effective length of the tube used for the separation or the length from the injection end to the detection portion (see Experimental Section), and t is the time required for the solute to migrate through the tube of length 1. Equation 3 is combined with eq 4 by using the relation t = 110, yielding
0003-2700/S8/0360-1673$01.50/00 1988 American Chemical Society
