2665
Anal. Chem. 1087, 59. 2665-2669
Table 11. G6Li Values of Seawater bsLi f 2a, ( % o ) ~ -32.8 -32.5
f 0.8 f 0.9 -32.1 f 0.9 -31.4 f 0.8
-32.5
* 1.2
-32.3 f 0.5' u,
= standard deviation of the mean. * M e a n value and 95%
confidence interval.
isotopic fractionation is expected to be negligible during low-temperature acid digestion. The measured G6Li value of a fresh basaltic glass was -4.7 f 0.7% (20,). A weathered submarine basalt sample yielded a G6Livalue of -8.4 f 1.0% (13). The chemical and mass spectrometric procedures thus appear suitable for silicate rocks. In summary, it is possible to determine the lithium isotopic abundance ratio in natural waters by the described procedures to a precision better than 2.5% (20). Preliminary study also indicates the technique is applicable to silicate rocks. The metaborate method therefore offers a viable means to study the variation of lithium isotopic composition in natural sources. ACKNOWLEDGMENT The author thanks J. M. Edmond for the opportunity of a visit at MIT and for his support for this work. S. R. Hart kindly provided the mass spectrometer for this study. The hospitality and help of C. I. Measures are gratefully acknowledged. I also thank A. J. Spivack, M. R. Palmer, and
V. Salters for sharing their expertise. A. J. Spivack and M. R. Palmer critically reviewed the manuscript and their comments allowed me to improve this paper. Registry No. 'Li, 14258-72-1;7Li,13982-05-3;lithium tetraborate, 110271-56-2;water, 7732-18-5. LITERATURE CITED (1) Heier, K. S.; Billings, G. K. "Lithium"; In Handbook of Geochemistry; Wedepohl, K. H., Ed.; Springer-Verlag: Berlin, Heidelberg, 1970, Vol. 1112, p 3-8-1. (2) Isakov, Y. A.; Plyusin, G. S.;Brandt, S. B. Geochem. Int. 1969, 6 , 598-600. (3) Morozova, I . M.; Alferovskiy, A. A. Geochem. Int. 1974, 7 1 , 17-25. (4) Plyusnin, G. S.; Lomonosov, I.S.;Posokhov, V. F. Geokhlm. Dokl. Akad Nauk SSSR 1978, 243, 189-191. (5) Levskiy, L. K.; Murln, A. N.; Zasbvskiy, V. G. Geochem. I n t . 1969, 6 , 601-605. (6)Meier, A. L. Anal. Chem. 1982, 5 4 , 2158-2161. (7) Flesch, G. D.; Anderson, A. R., Jr.; Svec, H. J. I n t . J. Mass. Spectrom. Ion Phys. 1873, 72, 265-272. (8) Michleis, E.;De Bievre, P. Int. J. Mass Spectrom. Ion Phys. 1983, 49, 265-274. (9) Spivack, A.; Edmond, J. M. Anal. Chem. 1986, 5 8 , 31-35. (10) Arthur, M. A.; Anderson, T. F.; Kaplan, 1. R.; Veizer, J.; Land, L. S. Stable Isotopes in Sedimentary Geology, SEPM Short Course No. 10, Dallas, 1983; Chapter 1. (11) Taylor, S. R.; Urey, H. C. J. Cbem. Phys. 1938, 6 , 429-438. (12) Catanzaro, E. J.; Champion, C. E.; Garner, E. L.; Marlnenko, G.; Sappenfield, K. M.; Shields, W. R. NBS Speclal Publication (U. s.) No. 260-17; National Bureau of Standards: Washington, DC, 1970. (13) Chan. L. H.; Edmond, J. M. Trans., Am. Geophys. Unlon 1966, 67, 1057. (14) Moeller, T. Inorganic Chemistry; Wiley: New York, 1952; p 181.
RECEIVED for review February 9,1987. Accepted July 20,1987. This study was conducted in the laboratory of John Edmond at Massachusetts Institute of Technology. This work was supported by National Science Foundation under Grant OCE 85 11352. Partial support was also provided by J. Edmond at MIT.
Metal Ion Interaction with Anion Ion Chromatography Archava Siriraks, James E. Girard,* and Phyllis E. Buell
The American University, Washington, D.C. 20016
The behavlor of the metal Ions Zn2+, Pb2+, and Cu2+ on four dlfferent anion-exchange columns Is descrlbed. It Is concluded that metal Ions form complexes with phthalate eluent, whlch are retained for different lengths of time on Vydac slllca-based, Waters polymethacrylate, and Hamilton polystyrene-dlvlnylbenzene (PS-DVB) columns. Wlth the Dionex latex agglomerated PS-DVB column It Is concluded that metals are retalned by an Ion-exchange process. Anlon analysis on the columns Is affected both by Inclusion of metal Ions In the anlon sample and by prevlous exposure of the column to metal Ions.
The problem of interaction between sample metal ions and anion separator columns during anion analysis has received little attention, although many metal cations are retained on anion-exchange columns for varying lengths of time, and so may interfere with anion analysis. For example, the Dionex latex agglomerated polystyrene-divinylbenzene (PS-DVB) anion resin can separate mono- and divalent cations ( I ) , and metal ions have been shown to adsorb strongly on low-capacity
anion-exchange resins prepared by amination of chloromethylated PS-DVB with diethylenetriamine, triethylenetetramine, and tetramethylenepentamine. These aminated resins were selective for metals capable of forming ammine complexes such as Au3+,Hg2+,and Cu2+(2). Interaction between metal ions and normal phase silica columns is well established, and equilibria between silica and mono-, di-, and trivalent metal ions, in aqueous solution, appear to involve simple ion exchange although the exact nature of the acidic silica surface remains uncertain (3-6). Metal interaction with a silica-based size exclusion column has been reported (7). The behavior of a series of cations on a silica-based anion separator column was described by Jenke and Pagenkopf (8). These authors, who used a potassium hydrogen phthalate (KHP) eluent, divided cations into three groups according to their degree of interaction with the column. Cations in the first group did not interact with the column, and so were not retained. This group included Na+, K+, Ca2+,Mg2+,Ni2+,Mn2+,and Cd2+. Cations in the second group interacted sufficiently with the column to give retention times similar to those observed for certain common anions, and included Cu2+,Pb2+,and Zn2+. Cations in the third group,
0003-2700/87/0359-2665$01.50/00 1987 American Chemical Society
2666
*'A
ANALYTICAL CHEMISTRY, VOL. 59, NO. 22, NOVEMBER 15, 1987
zn
Waste
Figure 1. Modified anion chromatographic system for metal ion de-
tection. Fe3+,A13+,and Hg2+,were strongly retained by the column and did not elute. This paper describes the behavior of three metal ions, Zn2+, Pb2+,and Cu2+,on four different commonly employed anion-exchange resins: silica-based (Vydac 302.IC), polymethacrylate (Waters IC PAK), PS-DVB (Hamilton PRPX-loo), and latex agglomerated PS-DVB (Dionex AS-4). Mechanisms to explain the observed behavior are presented, and the effects of the metal ions on anion analysis are discussed. EXPERIMENTAL SECTION Chromatographic Systems. The anion chromatographic system consisted of a Waters M-45 pump, a Rheodyne 7125 injector with a 100-pL loop, a Wescan 213 conductivity detector, and a Hewlett-Packard 3390A recording integrator. For the cation chromatographic system the anion separator column was replaced with a TSK cation no. ICC-830150 column, 50 X 4.6 mm (Toyo Soda ManufacturingCo., Japan), and following a postcolumn reaction 4-(2-pyridylazo)resorcinol(PAR), cations were detected at 546 nm with a Waters 440 UV-vis detector. The PAR solutionwas delivered,by a Lazar pulseless pump (LPP-279), to a low dead volume tee connected to column and detector outlets. The modified chromatographic system for simultaneous detection of anions and cations was similar to the cation system except that an anion separator column was used, and a conductivity detector was added before the mixing tee (Figure 1). The following anion separator columns were used: Vydac 302.IC (13 pm, 250 X 4.1 mm), Waters IC PAK (10 pm, 50 X 4.6 mm), Hamilton PRP-X100 (10 pm, 100 X 4.1 mm), and Dionex AS-4 (20-30 pm, 250 X 4.1 mm). The unfunctionalized column was a Hamilton PRP-1 (10 pm, 150 X 4.1 mm) column. The flow rate was 1.2 mL/min except for the Vydac column used for Figure 2C (2.0 mL/min) and for Figure 3C (1.5 ml/min). Reagents and Standard Solutions. All chemicals were analytical grade, and solutionswere prepared with deionized water from a Milli-Q water system. Stock standard metal ion and anion solutions (lo00 ppm) were prepared from the nitrate and sodium salts, respectively. All solutions were stored in high-density polyethylene bottles. The eluent was 2 mM potassium hydrogen phthalate (KHP), pH 5.0, except where noted. Eluent pH was adjusted with 1M KOH. Concentrations of metals injected were as follows: 4 X low3M (261 ppm Zn2+,254 ppm Cu2+,829 ppm Pb2+)for Figure 2; 20 ppm Zn2+and Cu2+,60 ppm Pb2+for Figure 3, parts A and C, and 2.0 ppm Zn2+and Cu2+,6 ppm Pb2+for Figure 3, part B; 45.4 ppm Zn2+,10.1 ppm Cu2+,21.0 ppm Pb2+ for Figure 4; 10 ppm Zn2+and Cu2+,20 ppm Pb2+for Figure 5. 4(2-Pyridylazo)resorcinolwas obtained from the Aldrich Chemical Co., Milwaukee, WI. The reagent for the postcolumn reaction mol of PAR, 2 mol of ammonium hydroxide, contained 1 X and 1 mol of ammonium acetate per liter of solution. Atomic Absorption. Measurements were made with a Perkin-Elmer 373 atomic absorption spectrophotometer. Procedures. Column capacities were determined by the method described by Girard and Badio (9). Silanol sites were deactivated by the method of Kirkland and Antle (10). RESULTS AND DISCUSSION Behavior of Metal Ions on Anion Separator Columns. When solutions containing the nitrates of zinc, lead, and copper were injected in turn on the four anion columns, the metal ions were retained on the columns for varying lengths of time. The retention times and the degree of separation of
zn
6 10 RETENTK*I TINE ( MI" I
RETE^?^^ nu€ A% i Flgure 2. Conductometric detection of Zn2+, Pb2+, and Cu2+ eluting from (A) Hamilton PRP-X100, (B) Waters IC-PAK, and (C) Vydac 302.IC columns. 0
O
the metal ions depended on the type of separator column used. The results obtained for the Hamilton, Waters, and Vydac columns are shown in Figure 2. Metal ions present a t concentrations too low to permit conductometric detection, and any nonconductive metal species, were monitored by using a modified chromatographic system incorporating a postcolumn reaction with PAR (Figure 3). The order of elution of the metal species was confirmed as follows. First, for metal ion concentrations too low to permit conductometric detection, metal ions were injected separately and their retention times, observed by using the PAR reagent, were noted. Seondly, for mixtures of the metal ions a t concentrations high enough to be detected conductometrically, the eluting peaks were collected separately and analyzed by atomic absorption. It is of interest to note that the order of elution reported by Jenke and Pagenkopf (8)was Pb2+,Zn2+, Cu2+,and not Zn2+,Pb2+,Cu2+as we observed. In the case of the Dionex column, metal ions were completely retained and could not be detected in the eluent leaving the column. To check the possibility that the metal ions had been trapped in the cation-exchange suppressor column, or precipitated by the Na2C03/NaHC03eluent used with this column, the suppressor column was removed and the eluent was changed to KHP. Again, injected metal ions did not elute from the column. To determine what percentage of injected metal ions passed through the different columns, the quantity of each metal ion eluting from the columns was determined by atomic absorption, and by cation chromatography using the postcolumn reaction with PAR. With the exception of the Dionex column from which no metal ions eluted, by the use of either technique, over 94% of the injected metals was recovered in the eluents leaving the columns. Mechanisms of Metal Ion Retention. It seemed likely that for those resins having acidic sites metal ion retention involved an ion-exchange process whereas for resins without acidic sites retention resulted from adsorption of metal-eluent
ANALYTICAL CHEMISTRY, VOL. 59, NO. 22, NOVEMBER 15, 1987
1.II
pb
I
A.
2667
8.
A.
Zn
cu
A
9
0
io
a
0
io
IS
m
Flgure 4. Elution behavior of ZnZ+, Pb2+, and Cu2+ (A) before and (B) after endcapping of SiOH sites on Vydac 302.IC column. zn
A.
p c u
L4.ili A
0
6
12 RLIZMTION TIME (Yln
1
a4
0
a
a
RCTENTH)* W L ( Y k
I
10 min
0
S
IO
mln
I
NOS
Flgure 3. Simultaneous conductometric and colorimetric detection of Zn*+, Pb2+,and Cu2+ eluting from (A) Hamilton PRP-X100, (B) Waters IC-PAK, and (C) Vydac 302.IC columns.
complexes. Evidence is presented to support this proposition. The Dionex anion-exchange resin is an agglomerate resulting from mixing macroparticles of surface-sulfonated styrendvinylbenzenewith microparticles of strong base-type anion-exchange resin, and it can be assumed that it contains cation-exchange sites, which would be available for interaction with metal ions. Although no metal ions could be detected leaving this column following one injection of metal ions, when several successive injections were made, metal ions gradually eluted from the column. Presumably once the column had become saturated with metal ions, any excess ions were eluted. When all metal ions remaining on the column were removed by injecting EDTA, then a subsequent injection of metal ions again resulted in complete retention of the cations on the column. These findings strongly suggest that the metal ions are held on the Dionex column by a cation-exchange mechanism and that neither the Na2C03/NaHC03nor the KHP eluent was strong enough to elute the metals from the column. It might be argued that metals would be retained by an ion-exchange process on the Vydac silica column also, because the packing material in this column is known to contain SiOH sites capable of cation exchange despite the endcapping with trialkylsilane that is part of the manufacturing process. To determine what role SiOH sites play in the retention of metal ions on silica-based anion separator columns, three Vydac 302.IC columns were studied before and after endcappingwith trimethylsilane. Similar results were obtained on all three columns, and are shown for one of the columns in Figure 4. The funding that the metals were still retained after the silanol sites had been deactivated shows that metal ion retention was not due to interaction of the metal ions with the silanol sites and suggests that retention was the result of adsorption of metal-eluent complexes. Figure 4 shows that, following
Pb Cu A
0
IO
20
30 min
Flgure 5. Effect of eluent pH on elution of Zn2+, Pb2+, and Cu2+ from Waters IC.PAK column: (A) pH 4.0, (B) pH 5.0, and (C) pH 6.0.
endcapping, resolution of the metal ions was improved. This is not unexpected since addition of trimethylsilane to the silanol sites would make the surface of the silica more hydrophobic. To confirm that the anion-exchangesites had not been cleaved, or replaced, during the deactivation process, anion analyses were also undertaken. On all three columns resolution of anions, particularly C1- and NO3-, actually improved following deactivation. We can not, at this time, offer an explanation for this finding. When the pH of the KHP eluent was increased, retention times for the three metal ions increased, as demonstrated in Figure 5, obtained by using the Waters column with eluent at pH 4.0,5.0,and 6.0. Slightly greater increases in retention occurred on the Vydac column when pH was changed from 4.0 to 5.0 (this column was not used above pH 5.0 because of the pH limits recommended by the manufacturer). The increase in retention was even more pronounced on the Hamilton column, and in this caw at pH 6.0 some lead was retained beyond 60 min and copper could only be eluted by injecting EDTA. These findings show that as the degree of ionization of phthalate was increased, the metal ions were retained longer on the columns. This can be explained by the formation of
2688
ANALYTICAL CHEMISTRY, VOL. 59, NO. 22, NOVEMBER 15, 1987
metal-phthalate complexes. With increasing pH, the phthalate ions formed would become available for complexation with the metal ions. The formation of complexes of the type MP and MP?-, where M2+and P2-represent the metal and the doubly deprotonated phthalate ions, respectively, was discussed by Jenke and Pagenkopf (8)in their study with a silica-based column. They calculated that for copper, under similar eluent conditions at pH 4, Cu2+was the dominant species (81.7%),with CUP contributing 17.5%, and CUP2-less than 1%. A t pH 6.0 the distribution was very different, with Cu2+accounting for only 16%, CUP80%, and CuPZ2-4%. If the mechanism of retention involves an affinity between the column matrix and a neutral metal-eluent complex, the Hamilton PS-DVB column would be expected to retain the three metals more strongly than the less hydrophobic Waters polymethacrylate column, as was found to be the case experimentally. Jenke and Pagenkopf (8) suggested that the hydrogen ion concentration of the phthalate eluent they used was the determining factor in the elution of metal ions from a silica anion column. We found that when the KHP eluent was replaced with nitric acid a t the same pH, the three metals were no longer retained on the Vydac silica-based column and eluted as a group. This indicates that phthalate rather than H+ is the main factor in controlling elution behavior. The fact that the three metals were separated from each other on the column when the KHP eluent was used strongly suggests that zinc, lead, and copper complexes, with differing affinities for the column, were formed. Ion exclusion probably accounts for the rapid elution, and lack of separation, of the metals when the nitric acid eluent was used. To demonstrate that the hydrophobic backbone of the polymer resin columns, rather than the ionic functional groups, was the main factor in metal ion retention, the three metal ions were injected in turn on to a Hamilton PRP-1 PS-DVB column. As was observed with the functionalized Hamilton resin (Figures 2 and 3), the three metal ions were retained on the unfunctionalized column. The finding of two unresolved peaks for zinc prompted further investigation using an eluent at lower pH. With phthalic acid at pH 2.9, elution of metal ions was rapid and multiple peaks were observed: three for zinc, two for copper, and one for lead. In the case of zinc it is probable that the first metal peak, which eluted rapidly, is due to Zn2+,and subsequent peaks represent metal-eluent complexes. A metal ion would not be retained on a PS-DVB column, but metal-eluent complexes would be expected to be adsorbed to some extent, and so retained. For lead and copper, similar species may have been present but not distinguishable under the conditions chosen. At pH 2.9 less than 0.6% of the total phthalate would exist as the doubly deprotonated P2-ion, but approximately 50% would be present in the singly protonated form, P- (pK, = 2.89, pK2 = 5.51). It therefore seems likely that metal ions can form complexes with both P2-and P-, the dominant complex depending on PH. Behavior of Fe3+on Anion Separator Columns. It was reported that Fe3+and A13+were very strongly retained by a silica anion separator column (8). In general, trivalent metal ions interact more strongly with cation exchangers than do mono- and divalent metal ions. It would, therefore, be expected that if Zn2+,Pb2+,and Cu2+are retained via a cation-exchange process, Fe3+would be even more strongly retained than the divalent cations. A solution containing Fe3+ was, therefore, injected in turn on to Vydac, Waters, and Hamilton coluinns. As expected, Fe3+was strongly retained on the two Vydac columns that were tested. However, after deactivation of the SiOH sites, Fe3+was no longer retained and eluted rapidly from the column, strongly suggesting that
the Fe3+ions, unlike the divalent metal ions, had initially been bonded to the acidic sites on the Vydac column. This difference in behavior between Fe3+and the divalent metal ions is further evidence for the formation of neutral metal complexes as a result of interaction between the divalent metal ions and the phthalate eluent. Fe3+too may form a complex with phthalate, but such a complex would be positively charged and so, unlike a neutral complex, would be attracted to the silanol sites. Once the silanol sites had been capped, a charged complex would no longer be retained. With the polymer-based Hamilton column, Fe3+was not retained when KHP eluent at pH 4.0 or 5.0 was used, but at pH 6.0 there was strong retention. Changing the eluent pH from 6.0 back to 4.0 resulted in elution of the retained Fe3+, presumably because the un-ionized phthalate formed at the lower pH could compete with the retained Fe3+for sites on the column. Retention on the Waters column was not as strong, and even at pH 6.0 Fe3+was not retained. This difference in behavior can be explained by the less hydrophobic nature of the polymethacrylatecolumn. These results suggest that an iron-phthalate complex with a positive charge, which would not have an affinity for the polymer matrix, is formed. The retention of Fe3+at pH 6.0 on the Hamilton column can be explained by assuming that at this pH the concentration of dissociated phthalate ions would be sufficient to neutralize the positively charged complex, which could then be strongly adsorbed on the resin. Effects of Metal Ions on Anion Analysis. Jenke and Pagenkopf (8)demonstrated that Pb2+,Zn2+,and Cu2+eluted from a silica column with retention times similar to those of C1-, Br-, and NO3-, but other than noting the overlapping peaks, they did not investigate this further. We, also, observed the possible coelution of anions and metal cations, and it is clear that the retention of metal ions on anion separator columns may interfere with anion analysis. Metal ions when present in anion samples may interfere in several ways. If present in high concentration they may, by eluting as charged species together with anions, increase the response of the conductivity detector, or they may interact with anions during passage down the column and, by removing a conducting species, cause a decreased detector response. For example, if a sample containing both lead and chloride ions is injected on the column the C1- will be concentrated as it travels down the column, and this could result in precipitation of PbC12 on the column, with a consequent decrease in the C1- leaving the column. Metal ions might also form soluble, but nonionized, complexes with sample anions. To determine the degree to which inclusion of metal ions interferes with anion analysis (specifically chloride analysis), standard chloride calibration curves, in the range 10 to 40 ppm, were prepared by using the Waters, Hamilton, and Vydac columns before and after metal nitrates, at several concentrations, had been added to the standard chloride solutions. The results obtained varied with the column that was used. Zn2+,Pb2+,and Cup+,in C1- solution, were injected in turn onto the Hamilton column. Zinc and lead eluted early, overlapped very little with C1-, and, at concentrations as high as 20 ppm, had little effect on C1- peak height. Copper, on the other hand, gave a peak that overlapped the peaks of both C1- and NO3-, and, when present a t a concentration of only 10 ppm, caused a significant reduction in chloride peak height. With the Waters and Vydac columns, the metal ions were not injected separately with the chloride standards, but as a group. Zn2+,Pb2+,and Cu2+at concentrations up to 25 ppm of each metal ion could be injected onto the Vydac column without any effect on C1- peak height being observed, but above this concentration peak height decreased with increasing metal concentration. With the Waters column the effect”
ANALYTICAL CHEMISTRY, VOL. 59, NO. 22, NOVEMBER 15, 1987
2668
Table I. Percent Decrease in Anion Peak Height Following Exposure of Anion Separator Columns to Metal Ions"
Cl-
sot-
NO;
column
10 PPm
100 ppm
10 PPm
100 ppm
10 PPm
100 ppm
Hamilton (1) Hamilton (2) Vydac Waters Dionex
33.82 36.85 3.42 2.28 1.78
4.76 8.19 0 0 3.78
40.63 47.28 7.36 7.32 5.88
9.09 14.28 0 0 3.84
43.54 45.70 14.77 6.11 b
16.80 13.79 0 0 5.43
"Eluent: 10 ppm, 1 mM KHP (pH 5.5): 100 ppm, 2 mM KHP (pH 5.0). Flow rate: 1.2 mL/min for Hamilton and Waters columns; 2.0 mL/min for Dionex and Vvdac columns. bInterferencefrom svstem ueak. ~~
~~
Table 11. Percent Decrease in Retention Time Following Exposure of Anion Separator Columns to Metal Ionso
c1-
sot-
NO,
column
10 PPm
100 ppm
10 PPm
100 ppm
10 PPm
100 ppm
Hamilton (1) Hamilton (2) Vydac Waters
4.02 8.95 12.50 5.17 3.94
6.39 8.21 14.61 5.79 4.26
2.63 9.76 29.73 4.29 8.10
7.16 8.97 30.00 5.26 7.37
9.84 12.17 13.57 7.38 15.15
6.43 11.29 16.49 7.99 14.33
Dionex
"Eluent: 10 ppm, 1 mM (pH 5.5); 100 ppm, 2 m M KHP (pH 5.0). Flow rate: 1.2 mL/min for Hamilton and Waters columns; 2.0 mL/min for Dionex and Vvdac columns. level was 50 ppm, but in contrast to the Vydac column, C1peak height increased rather than decreased when metal ion concentration was further increased. In the case of the Dionex column, which had 'already been shown to retain metals so strongly that they could not be removed by either Na2C03/NaHC03or KHP eluents, a different experiment was performed. Standard solutions containing C1-, NO3-, and SO-: were analyzed when the column was partially, and completely, saturated with copper ions. Peak heights and peak resolution, obtained before and after metal injection, were compared. Copper, rather than lead or zinc, was chosen because its interaction with the three anions was the smallest. KHP was used as eluent. It was found that there were no significant changes in either peak height or peak resolution when the column was partially saturated with copper. However, a marked decrease in peak height was observed once the column became completely saturated, although peak resolution was unaffected, which implies that metal adsorption and anion exchange proceed via different mechanisms. Prior exposure of anion separator columns to metal ions may also degrade chromatographic performance. This possibility was investigated by determining C1-, NO,-, and Sod2-, at two concentrations, on the four different anion columns and comparing peak heights, retention times, and resolution, before and after exposure of the columns to metal ions. To simulate long-term use of the columns with samples containing metal ions, 50 consecutive injections of a solution containing lo00 ppm each of Zn2+,Pb2+,and Cu2+were made in turn on the Hamilton, Waters, Vydac, and Dionex columns. Metal ions were eluted from the columns with KHP eluent between injections (except in the case of the Dionex column where, as mentioned earlier, this was not possible). The results of this study are shown in Tables I and 11. In general, the peak heights of anions at low concentration (10 ppm) were affected more by metal exposure than were peak heights for anions at high concentration (100 ppm). The
greatest effect on peak height occurred with the Hamilton columns: decreases between 34 and 47% of the initial values for all three anions at 10 ppm were observed. On all columns retention times were shorter following exposure to metal ions, but the extent of the decrease did not depend on anion concentration. The Vydac column was most affected, retention times for nitrate being decreased by as much as 30%, which adversely affected resolution between C1- and NO3-. On the other columns resolution was not degraded. Overall the Waters column was least affected by exposure to metal ions and on this column peak height and retention decreased by less than 8% at the anion concentrations that were tested. The effect of exposure to Zn2+,Pb2+, and Cu2+ on the ion-exchange capacity of the columns was also investigated. For this study the capacities of a set of new columns were first determined, and then, following 15, 30, and 50 injections of the three metal ions, capacities were again determined. The results obtained indicated that, with the exception of an initial 13% loss of capacity for the Vydac column following 15 injections, very little change in capacity occurred. Registry No. Zn, 7440-66-6; Pb, 7439-92-1;Cu, 7440-50-8; C1-, 16887-00-6; NO3-, 14797-55-8; SO4-, 14808-79-8; Vydac 302.IC, 100552-31-6;Waters IC PAK, 110026-28-3;Hamilton PRPX-100, 92229-59-9; Dionex AS-4, 110026-27-2. LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)
Wimberley, J. W. Anal. Chem. 1881, 53, 1709. Egawa, H.; Saeki, H. Kogyo Kagaku Zasshi 1971, 7 4 , 772-775. Iler, R. K. The Chemistfy of S//;ca;Wiley: New York, 1979. Dugger, D. L.; Stanton, J. J.; Irby, B. N.; McConnel, B. L.; Cummings, W. W.; Maatman, R. W. J . Phys. Chem. 1984, 68, 757-760. Smith, R. L.; Pletrzyk, D. J. Anal. Chem. 1984, 56, 610-614. Pletrzyk, D. J.; Brown, D. M. Anal. Chem. 1988, 58, 2554-2557. Pituck, M. R.; Collard, 8. P.; Haworth, D. T. LC Mag. 1988, 4 , 115. Jenke, D. R.; Pagenkopf, G. K. Anal. Chem. 1983, 55, 1168. Girard. J. E.; Badio, D. Y. Anal. Chem. 1984, 56, 2992. Kirkland, J. J.; Antle, P. E. J . Chromafogr. Sci. 147 , 75, 137.
RECEIVED for review December 23,1986. Accepted June 25, 1987.
