Anal. Chem. 1988, 6 0 , 830-832
830
Separation and Detection of Metal Ions Using in Situ Ligand Exchange Chromatography Sir: The separation and detection of metal ions by using chromatography continues to receive the attention of various workers (1-16). Separations have been achieved by using normal-phase (1-3), reversed-phase (4-13), or ion-exchange (14-16) chromatography. UV-vis (4-6,9,11,12) or electrochemical (7-9, 11, 13, 14) detection have proven the most popular means of detecting metal ions after separation. Usually the separation and/or detection of metal ions involves a metal complexation step. In this regard the dithiocarbamate (DTC) ligand which forms stable complexes with a range of metal ions has proven useful. Both precolumn (5, 6) and in situ (7-13) derivatization methods have been employed with this ligand. The in situ complexation technique allows direct injection of metal ions and increases on-column stability for metal complexes. However this technique results in high background detector levels with either UV or electrochemical detection since the free ligand is UV-absorbing and electrochemically active a t relatively low anodic potentials. A further problem encountered in previous work was the instability of some dithiocarbamate ligands in suitable chromatographic eluents. This and the high organic solvent composition necessary to keep the complexes in solution were concerns when developing an automated on-line system (11). In the course of this work a novel technique involving the use of in situ ligand exchange chromatography, wherein a labile metal complex rather than the free ligand is included in the chromatographic eluent, has been investigated. Previous workers (17)have employed this principle in liquidliquid extraction procedures to enhance reagent stability and to induce more selectivity. In this work a water-soluble dithiocarbamate, bis(2hydroxyethy1)dithiocarbamate (BHEDTC, I shown below), has been employed. This allows use of the water-soluble HOCH,CH,
\N-c4s/
HOCH2CH2
+s
Zn(BHEDTC)2complex as the reagent. This ligand has been used previously for postcolumn complex formation (18) and separation of metal complex species following precolumn derivatization (19). In that work (19) metal complexes such as those containing lead and cadmium did not display ideal chromatographic behavior. The use of in situ complexation chromatography overcomes these problems. EXPERIMENTAL SECTION Instrumentation. All experiments were carried out with a Waters Model M-6000A chromatography pump, Model U6K injector, and Model 450 variable-wavelength detector equipped with a Hewlett-Packard 3390A reporting integrator. All spectra were taken on a UV-160 Shimadzu UV-vis recording spectrophotometer. Electrochemical results were obtained with a Model 174A polargraphic analyzer and an LC-3A amperometric detector (BioanalyticalSystems, Inc.). A Waters pBondapak CI8 (3.9 mm X 30 cm) stainless steel column was used to separate metal complexes. Reagents. Sodium bis(2-hydroxyethy1)dithiocarbamate was prepared by reacting carbon disulfide with diethanolamine in a basic media. BHEDTC and AR grade zinc nitrate (BDH) were dissolved in methanol and/or water as required. Nitrates of nickel(II),cobalt(II),and copper(II),as well as acetates of lead(I1) and cadmium(II),were employed. Metal ion samples were diluted in HPLC solvent without adding ligand. All water was distilled
Table I Half-Life ( t , # of BHEDTC in Different Mediab media
no. 1 2 3
4 5 6 7
0.025 M (triethy1amine)acetic acid pH 6.5 0.025 M (diethano1amine)acetic acid pH 6.5 1X M Zn(I1) + 0.025 M (triethy1amine)acetic acid pH 6.5 0.01 M tetrabutylammonium perchlorate (TBAP)
solvent 2X 2X
M Zn(I1) M Zn(I1) + 0.01 M TBAP
t1/2, h
1.8 1.2 15.0 7.1
6.4 37.0 30.0
Ii t l l z was calculated from the recorded spectra. *Note: spectra were obtained at 296 nm for media 1 and 2, at 289 nm for 3, and at 259 nm for 4-7. In each case the solvent was 40/60 MeOH-HpO and the concentration of BHEDTC was 2 X M.
and purified with a Milli-Q water system (MILLIPORE). HPLC grade methanol was used. Before each HPLC run, the solvent was filtered, as well as degassed, through Ultipor NX 0.45-pm membrance filters with the aid of a vacuum filter. RESULTS AND DISCUSSION Previous chromatographic separation of metal dithiocarbamates has involved the use of either acetate (7-13) or triethylamine buffers (19) since the presence of buffer enables electrochemical detection. With UV-visible detection, a solvent composition involving use of an aqueous/organic mixture proved suitable. Initial experiments were concerned with the stability of the BHEDTC ligand and the zinc complex of tt 1s ligand in acetate buffer, in triethylamine buffer, and in water since we were interested in applying electrochemical detection. Stability Data. The stability of the free ligand was found to be concentration dependent. For example the half-life (tliz) for 1 X M BHEDTC in methanol was 50 h whereas for 1X M the ligand is stable for a t least a week. Table I summarizes the half-life of the ligand in various media. In either of the buffer solutions the free ligand is relatively unstable. Upon addition of tetrabutylammonium perchlorate (TBAP), which was used as a supporting electrolyte for EC detection, the stability increases. This may result from formation of the R4N+-BHEDTC- ion pair. Similar results were reported previously for the cetyltrimethylammonium bromide and DTC ion pair (6). In the buffer solutions, addition of zinc improved reagent stability. Addition of the TBAP improved stability even further in this medium. In methanol/water mixtures, addition of zinc ions alone was enough to stabilize the reagent. The complexes of all metals investigated, namely, Hg, Pb, Ni, Cu, Co, and Cd are stable for at least 1 h which is adequate for in situ exchange chromatography. I n Situ Formation, Separation, a n d Detection. Both voltammetry and UV-vis spectrophotometry were employed to confirm the formation of metal dithiocarbamate complexes from the zinc dithiocarbamate reagent. In a conventional voltammetric cell it was found that even in the presence of 10-fold excess zinc, well-defined responses were observed upon the addition of metal ions, Figure 1 shows the effect of zinc ion addition on the voltammogram for oxidation of BHEDTC. As expected the free dithiocarbamate oxidation response decreases markedly and a response for oxidation of Zn(BHEDTQ2 appears a t more positive potentials. This decrease in background current (at potentials less than 1.00 V
0003-2700/88/0360-0830$01.50/0 @ 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 8, APRIL 15, 1988
, 1
0
,
~~1
t35 E vs AgIAgCI
'1
ov
Figure 1. Ligand exchange between Cu(I1) and the Zn(BHDTC), complex, differential pulse voltammograms recorded: solvent, 40160 MeOH-H,O with 0.01 M NaNO,; working electrode, glassy carbon; reference electrode, Ag/AgCI; scan rate, 10 mV/s; duration between pulses, 0.5 s; pulse height, 50 mV; (a) 1 X lo-' BHEDTC, (b) adding 1 x lo3 Zn(I1) to solution in a, (c) adding 1 x 1 0 - ~Cu(II) to soiutlon in b; responses due to (1)ligand oxldatlon, (2)Zn(BHEDTC), oxidation, (3)Cu(BHEDTC), oxidation.
1I
0
L
5
10
15Min
Flgure 2. Separation of metal ions using ligand exchange chromatography: solvent, 40160 MeOH-H,O with 1 X M BHEDTC and 1 X lo4 M Zn(1I); flow rate, 1 mL/min; detector, UV, A = 300 nm; sample, 50 A 2.5 X 10" M Cu(II), Cd(II), Pb(II), 2.0 X 10" M NYII), 1.0 X 10" M Co(I1);peaks, (1) Cd(II), (2)Co(II), (3)Pb(II), (4) Ni(II), (5) Cu(I1); (A) dip due to depletion of BHEDTC; (B) dip due to depletion of Zn(BHEDTC),.
vs Ag/AgCl) obtained upon addition of zinc ions could not be exploited in the chromatographic system due to adsorption problems on the electrode surface (see later). Figure 1 also shows the response obtained upon addition of copper ions to the Zn(BHDTC)z-containingsolution. The solution was observed to go brown, verifying formation of the copper complex. While these experiments established the feasibility of in situ ligand exchange, they provided no information on the kinetics of the reactions. The overall exchange reaction between the zinc complex and a metal (M) may be described as Zn(dtc)2
+ M2+
-
831
M(dtc)2 + Zn2+
(1)
For well-defied chromatographic responses the above reaction must be rapid and proceed almost to completion. The kinetics of this reaction are affected by parameters such as solvent composition and ionic strength. However in this work the prime consideration was the concentration of zinc ion employed in the chromatography eluent. It was found that even with 2-fold excess zinc in the flowing system, well-defined responses were observed for all metal ions investigated (Figure 2). Note, especially, the well-defined peaks for Cd2+and Pb2+. The nature of the Pb2+response is more obvious when Ni2+ is not present in the injection. A 2-fold excess of zinc ions was sufficient to stabilize the chromatographic reagent. Higher concentrations of zinc ion distorted the chromatographic
0
02
OL
06
08
1 0 +V
E v s Ag/AgCl
Figwe 3. Deactivation of the surface of electrode caused by oxidation of BHEDTC: solution, 0.01 M NaNO,, 0.01 M Bu4NCI0,, and 5 X lo4 M BHEDTC in 50/50 MeOH-H,O; electrode, working, glassy carbon; reference, Ag/AgCi 3 M NaCI; auxiliary, Pt. Procedure, put at desired potential for 2 min and then scan at 100 mV/s, from 0.00V vs Ag/ AgCi (see text for details).
peaks, presumably due to slower exchange reactions in situ. In all cases injection of metal ions caused two "dips" to appear in the chromatogram (Figure 2). It was verified that the early eluting dip was due to depletion of BHEDTC ion concentration, while the later eluting dip was due to depletion of Zn(BHEDTC)2 when metal ions were injected. When 0.01 M TBAP was added to the mobile phase, the ligand dip appeared between peaks 3 and 4. If the concentration of TBAP is reduced or TBAP is substituted by 0.01 M TEAP, which has smaller aIkyl groups, the ligand dip appeared before peak 1. This is presumably due to an ion-pairing phenomenon involving the BHEDTC ion. With EC detection, the later eluting dip only appeared at more anodic (>0.8 V) potentials as would be expected. By use of the buffer-containing eluents, excellent results were obtained with UV-vis detection. However, under none of the circumstances investigated could well-defined amperometric responses be obtained except for injection of copper ions. Optimum copper responses were obtained at +1.00 V vs Ag/AgCl. Detection levels of 42 ppb (for a 50-wL injection) were obtained. This was not as sensitive as UV detection (see later) or other previously reported EC detection methods (7). Voltammetry in a stationary cell indicated that oxidation of the ligand in the chromatographic eluent results in electrode fouling, presumably due to absorption of the oxidation product, a thiuram disulfide dimer, at more positive potentials. Figure 3 demonstrates the loss of electrode activity when more positive potentials are applied to glassy carbon in the chromatographic eluent. In each case the potential is held at a predetermined potential prior to recording the voltammogram in a stirred solution. Voltammograms 1-6 correspond to application of +O.O, +0.1, +0.2, +0.3, +0.4, and + L O V, respectively. Obviously the oxidation response due t~ free ligand decreases with more anodic potentials applied. The original response could be regenerated by electrochemically cleaning the electrode at 0.00 V. However, the use of pulse waveforms in the flowing system in an attempt to minimize this problem was unsuccessful due to the magnitude of pulse required to clean the electrode. Detection is achieved at +OB0 V and cleaning at 0.OOV vs Ag/AgCl and therefore the large pulse required resulted in very high background charging currents. Detection limits attainable with UV-vis are summarized in Table 11. These limits were obtained by using h = 300 nm which is suitable for multielement analysis. Increased sensitivity for selected elements can be achieved by tuning the wavelength. For example the determination of nickel and
832 ANALYTICAL CHEMISTRY, VOL. 60, NO. 8, APRIL 15, 1988 Lajunen, Lauri H. J.; Erjarvi, Erkki; Niemi, Plrjo Finn. Chem. Lett. 1984, 6 . 146. Irth, Hubertus; de Jong, G. J.; Brinkman, U. A. Th.; Frei. R. W, Anal. Ch8m. 1987, 59,98-101. Bond, A. M.; Wallace, G. G. Anal. Chem. 1981, 5 3 , 1209. Bond, A. M.; Wallace, G. G. Anal. Chem. 1982, 5 4 , 1706. Bond, A. M.; Wallace, G. G. Anal. Chem. 1983, 55, 718. Bond, A. M.; Wallace, G. G. J . Ll9. Chromatogr. 1983, 6 , 1799. Bond, A. M.; Wallace, G. G. Anal. Chem. 1984, 56, 2085. Rlordan, K. P.; Heneghan, G.; Wallace, G. G. Anal. Chem. 1985,5 7 , 1354. Heneghan, G.; Wallace, G. G. Anal. R o c . (London) 1988, 2 3 , 29. Hojabrl, H.; Lavin, A. C.; Wallace, G. G. Anal. Chem. 1987, 59, 54. Cassidy, R. M.; Eichuck, S.Anal. Chem. 1982, 5 4 , 1558. O'Laughlln, J. W. Anal. Chem. 1982, 5 4 , 178. Wytianbach. A.; Bajo, S.Anal. Chem. 1975, 4 7 , 1813. Fu, Chengguang; Zuo, Bencheng Fenxi Huaxue 1981, 9 , 635. Klng, J. N.; Frltz, J. S.Anal. Chem. 1987, 59, 703.
Table 11. Detection Limits' of Metal Ionsb metal ion
detection limits, ppb
Cd(I1) Co(I1) Pb(I1) Ni(I1) Cu(I1)
53.0 49.0 46.0 7.7 13.0
'Detection limit taken as twice the signal to noise ratio. bSolvent 40/60 MeOH-H,O with 1 X lo4 BHEDTC and 1 X low4 Zn(I1): detector. UV. X = 300 nm: iniection, 50-ULsample.
cobalt at X = 321 nm gives detection levels 4.9 and 3.5 ppb, respectively.
Hailin Ge G. G. Wallace*
Registry No. Cd, 7440-43-9; Co, 7440-48-4; Pb, 7439-92-1; Ni, 7440-02-0; Cu, 7440-50-8; Zn(B&IEDTC),, 19163-92-9.
Chemistry Department University of Wollongong P.O. Box 1144 Wollongong, New South Wales 2500 Australia
LITERATURE CITED (1) Liska, 0.;Gulochon, G.; Colin, H. J . Chromatogr. 1979, 171,
145-151. (2) Liska, 0.;Lehotay, J.; Brandsteterova, E.; Guiochon, G.; Colin, J. J . Chromatogr. 1979, 172, 384-387. (3) Edward-Inatimi, E. B. J . Chromatogr. 1983, 256, 253. (4) Smlth, R. M.; Butt, A. M.; Thakur, A. Ana/y.st (London) 1985, 710, 35.
RECEIVED for review July 7, 1987. Accepted December 14, 1987. This work was supported by the Dionex Corp.
ADDENDUM The revised Table I1 appears below. The data set proper and the conclusions of the paper are unchanged. With the last two GISP values dropped, the mean ad with standard deviation becomes 1.041 474 f 0.000 11. The mean a with standard deviation becomes 1.041463 f 0.000193. After combining all the errors in precision for each process, we would now assign a a value of 1.0414 f 0.0003.
Carbon Dioxide-Water Oxygen Isotope Fractioilation Factor Using Chlorine Trifluoride and Guanidine Hydrochloride Techniques Joseph P. Dugan, Jr., and James Borthwick (Anal. Chem. 1986,58, 3052-3054).
Correspondence subsequent to publication has revealed several mix ups concerning the basis for calculation of the values in Table 11. An extensive review of laboratory records showed the p values to be incorrect. Also, the last two GISP results were found to be poorly documented in laboratory records.
There were also wrong mathematical signs in eq 5 p(6180,
aCOrH~=
(6180,
-
PO,)
- EO,)
and a =
1000 + 6C02 1000 + 6H20
Table 11. Fractionation Factors for C 0 2 - H 2 0 Obtained from Equilibration, Chlorine Trifluoride Analysis, a n d Guanidine Hydrochloride Technique P
6180c02ion'
6180co;
6180H20 ione
a
V-SMOW
4.06 4.06 469.50 469.50 469.50
13.591 13.591 19.308 19.308 19.308
35.78 35.78 41.56 41.56 41.56
-26.372 -26.292 -21.415 -21.521 -21.955
-5.54' -5.50' 0.22d 0.1Id -0.33d
1.041655 1.041 561 1.041330 1.041 444 1.041908
SLAP
3.68 3.68 3.86 3.86 461.40 461.40 461.40
-31.497 -31.497 -30.102 -30.102 -37.910 -37.910 -37.910
-9.90 -9.90 -8.51 -8.51 -16.34 -16.34 -16.34
-66.932 -67.024 -68.472 -68.330 -75.556 -75.877 -75.905
-49.17' -49.27' -48.03d -47.8ad -55.2ad -55.61d -55.64d
1.041 290 1.041 399 1.041 514 1.041 350 1.041 186 1.041 582 1.041 616
GISP
3.65 3.65 3.65
-6.909 -6.909 -6.909
15.01 15.01 15.01
-44.792 -25.41' 1.041 514 -44.752 -25.37' 1.041 472 -44.715 -25.33' 1.041 429 mean of aCand standard deviation 1.041 474 i 0.000 11 mean of ad and standard deviation 1.041491 i 0.000 21 mean of CY and standard deviation 1.041463 i 0.000 193
"Measured 6l80 of COz in equilibrium vessel after equilibration (6l80,). *Data normalized to V-SMOW/SLAP using the result from analyses of the standards before equilibration. Data obtained from chlorine trifluoride analysis. Data obtained by using guanidine hydrochloride technique. 'Raw data corrected for machine effects and effects of and 170.
