Anal. Chem. 1984, 56,2073-2078
Registry No. Te, 13494-80-9; Au, 7440-57-5; In, 7440-74-6; Cd, 7440-43-9; Bi, 7440-69-9; Sn, 7440-31-5; Se, 7782-49-2; As, 7440-38-2; Pt, 7440-06-4; Pd, 7440-05-3; Rh, 7440-16-6. LITERATURE CITED (1) Aoki, Fumio Bull. Chem. SOC.Jpn. 1953, 26, 480. (2) Smith, Gilbert W.; Reynolds, S. A. Anal. Chin?. Acta 1955, 72, 15 1-153. (3) Veale, C. R . J . Inorg. Nucl. Chem. 1959, 10, 333-334. (4) Strelnikova. N. P.;Liskova, G. G. Zavod. Lab. 1960, 26, 142. (5) Nelson, Frederik; Micheison, Donna C. J . Chromafogr. 1966, 25, 414-441. (6) Busev, A. I.; Bagbanly, I. L.; Bagbanly, S. I.; Guseinov, I . K.; Rusta-
2073
mov, N. Kh. Zh. Anal. Khim. 1970, 25, 1374-1378. (7) Schindewolf, U. Geochim. Cosmochim. Acta 1960, 79, 134-138. (8) Morris, D. F. C.; Killick, R. A. Talanta 1963, 10, 279-285. (9) Strelow. Franz W. E.; Victor, AndrB H.; van Zyl, Christina R.; Eioff, Cynthia Anal. Chem. 1971, 43, 870-876. (10) Kutil, J.; Cuta, F. Collect. Czech. Chem. Commun. 1967, 32, 1390-1397. (11) Pitts, Antony E.; Beamish, Fred E. Anal. Chem. 1969, 4 1 . 1107-1 109. (12) Strelow, Franz W. E.; Victor, AndrB H.; Steyn, Johannes;Lachmann. Hans H. Talanta 1976, 23, 173-177.
RECEIVED for review March 27,1984. Accepted May 23,1984.
Formation Mechanism of Dip Peaks in Nonsuppressed Ion Chromatography Tetsuo Okada a n d Tooru Kuwamoto* Department of Chemistry, Faculty of Science, Kyoto University, Sakyo-ku, Kyoto 606, Japan
The two dip peaks which were observed in nonsuppressed anion chromatography by using low pH eluent were investigated in detail by conductivity and UV spectrophotometric detectors. The eluent anion fixed on the ion-exchange resin was transferred to the sample water zone, and this rate was especially fast at the low pH of the eluent. Consequently, the first dip peak was caused by the ion exclusion effect of the sample cation from the anlon exchange resin and by the elution of the sample water. The second dip peak was caused by the “compensating effect” for the first dip zone and by the “absent effect” of the eluent anion eluted at the position of the second dip peak.
Ion chromatography developed by Small et al. (1)is a very effective method for the analysis of many common anions (1-10) and cations (1, 11-14). The anion chromatographic system, which consists of a carbonate eluent and a suppressor system, has been widely applied for routine work in various fields (2-4). However, two negative peaks, called “water dip” and “carbonate dip”, were observed on a suppressed chromatography, and they interfered with the determination of some anions (15). Concerning these peaks, Stevens et al. (15) explained that the former was related to the injected water zone adjoining the eluent having higher conductance on either side of it, and the latter was the absent peak of the carbonate ion eluted through the unexhausted portion of the suppressor column not in the injected solution but present in the eluent. Besides, Ishibashi et al. (16) also pointed out that two dip peaks, which were observed in the range of the retention time between fluoride and chloride ions, were due to bicarbonate and carbonate ions, respectively. However, there is doubt about this result because the ion-exchange equilibria are not faster than the ion equilibria in solutions. Recently, the suppressor system was improved to an ionexchange membrane suppressor system by Stevens et al. (15, 17) and Hanaoka et al. (18). It became possible to use it continuously without the regeneration of the suppressor system. This system has some advantages, compared with the ion-exchange resin suppressor. These include things such as, the suppression of the adsorption of nitrite ion, less disturbance of the peak bands, and the simultaneous production
of the water dip and carbonate dip (3, 15). In nonsuppressed ion chromatography (6-11) introduced by Gjerde et al., greater dip peaks were found, and some of their production mechanisms were discussed. Gjerde et al. (6) mentioned that this pseudopeak was produced by the difference in the conductance between the background eluent and the sample water band containing the sample cations and the eluent anion replaced by the sample anions from the anion-exchange resin a t the top of the column. They stated that the base line disturbance, not the pseudopeak, was reduced or eliminated by adding eluent to the sample and occurred because the eluent anion initially was adsorbed by the resin (8). Hershcovitz et al. (19) investigated the dip peaks in this method using benzoic acid eluent. They found that a linear relationship existed between the injected ion concentrations and dip peak heights regardless of the injected ion species and that the dip peak was able to be utilized for the determination of some ions retained strongly in the ion-exchange resin. The authors previously investigated the sensitivity of nonsuppressed ion chromatography using dibasic acid eluents, such as tartaric or malic acid. We observed the production of two dip peaks which interfered with the determination of the fluoride ion (20). In this paper, we undertook to elucidate the mechanism of these two dip peaks by using a conductivity detector and a UV spectrophotometric detector which monitored the absorbance at the isosbestic point of the tartaric acid. EXPERIMENTAL SECTION Apparatus. A Toyo Soda Model HLC-601 nonsuppressed ion chromatograph equipped with a conductivity detector and a UV spectrophotometricdetector (Japan Spectroscopic Co., Ltd.) was used. The separation column was a porous polymer anion exchange resin column TSKgel IC-Anion-PW (particle size 10 i 2 pm, capacity 0.03 f 0.005 mequiv/g, 4.6 mm i.d. X 50 mm). The volume of the sample loop was 100 hL. The flow rate was maintained at 1.2 mL/min under a pressure of 20-25 kg/cm2. The column and the conductivity detector were set in the oven regulated at 30 O C . Reagents. Stock solutions of anions were prepared by dissolving their potassium salts, dried under vacuum at 110 O C overnight, in distilled deionized water. Working standard solutions were obtained by diluting the stock solutions with distilled water. The eluents were prepared by dissolving guaranteed reagent
0003-2700/84/0356-2073$01.50/0 0 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56,
NO. 12, OCTOBER
1984
PH 2.98
150ys pH 3.34
a
PH 349
b
pH 5.77
0.064A
Flgure 3. Dip peaks by the conductivity and UV spectrophotometric detectors: solid line, conductivity detector: broken line, UV spectrophotometric detector; eluent, 2 mM of tartaric acid (pH 2.98); concentration of the injected chlorlde ion, (a) 0 ppm, (b) 10 ppm, (c) 40 ppm, (d) 100 ppm.
Time
Figure 1. Lip peaks upon injecting 40 ppm of chloride ion. IOOV
I
I I
0-
W
>
-
I
m W
E
200 - + 0
i,
5
6
-
7
PH
Flgure 2. Variation of the retention time of the first and second dip peaks with the eluent pH.
L-tartaric acid in pure water and by adjusting the pH with a 0.1 M potassium hydroxide solution.
RESULTS AND DISCUSSION Dip Peaks by Conductivity Detector. Two negative peaks were observed in the front of the phosphate peak in the analysis of inorganic anions using 2 mM tartaric acid (pH 2.98) as eluent (20). Similar peaks were also observed by using 0.8 mM of benzoic acid (pH 3.5) or a 2 mM citric acid (pH 3.1) solution. From these facts, it was estimated that two negative peaks were not due to the use of dibasic acid eluents. Accordingly, the following experiments were carried out in order to elucidate the production mechanism of these unknown negative peaks called "dip peaks". Figure 1shows the dip peaks obtained by the injection of 40 ppm of chloride ion at various pHs of the eluents, and Figure 2 shows the relationship between the retention times of the first and second dip peaks and the eluent pH. From these two figures, the retention time of the first dip peak was constant regardless of the sample concentration, and its relative peak conductance increased with the increase in the eluent pH at pH C3.4. Then it decreased to a negative with the increase in the eluent pH at pH >3.4. On the other hand, the retention time of the second dip peak increased with the eluent pH, its relative peak conductancegradually approached
0
50 C o n c e n t r a l i o n of
CI-
1W , ppm
Flgure 4. Variation of the relative peak conductance with the concentration of the Injected chloride ion: solid line, first dip: broken line, second dip; eluent pH, (a) 2.98, (b) 3.37, (c) 3.56, (d) 4.08.
the background conductance, and this peak disappeared at a pH of 4.08. Generally, no attention was paid to these two peaks because it was necessary to simultaneously analyze the anions eluted at the wide range of the retention time. Therefore, the eluent, the pH of which was adjusted over 4, was used on the nonsuppressed ion chromatography. For this reason, only one dip peak was studied by Gjerde et al. (6) or Hershcovitz et al. (19). Dip Peak by a UV Spectrophotometric Detector. The absorbance at 220 nm, which was the isosbestic point of the tartaric acid, was observed by the UV spectrophotometric detector in order to obtain information concerning the concentration of the eluent in the dip peaks zone. Figure 3 shows the dip peaks observed by the conductivity and UV spectrophotometric detectors upon the injection of (3-100 pprn of the chloride ion at a pH of 2.98 of the eluent. As the first dip peak was varied considerably as shown in Figure 3, the relationships between the concentration of the injected chloride ion and the relative peak conductance and absorbance were investigated at various pH levels of the eluents. The results are shown in Figures 4 and 5.
ANALYTICAL CHEMISTRY, VOL. 56, NO. 12, OCTOBER 1984
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Table 11. Relative Conductance ( b S ) of Second Dip Peak 2.2
2.7
sample pH" 3.0 3.1
2.98
44
-43
-61
3.35
-25
eluent pH
-36
5.gb
11.6
-82 -42
-60
The sample pHs were adjusted by adding hydrochloric acid or potassium hydroxide solution to the distilled water. The distilled water absorbed the carbon dioxide in the air and became an acidic solution.
50
0
C o n c e n t r a t i o n of
100 CI- , pprn
Flgure 5. Variation of the relative peak absorbance with the concentration of the injected chloride ion: solid iine, first dip; broken iine, second dip; eluent pH, (a) 2.98, (b) 3.37, (c) 3.56 (d) 4.08.
Table I. Relative Peak Conductance and Absorbance" cOncn of cl-,
PPm 0 20 40 100
re1 peak conductance, $3 1st dip 2nd dip
re1 peak abs, A 1st dip 2nd dip
-129
0
0.031
-63
-128 -132
-b
-135
0 -31
0.067 0.148
-0.174 -0.173 -0.178
-0.186
"The eluent was 4 mM tartaric acid. bThe first dip peak was the W-shaped peak. Concerning the first dip peak from these figures, a linear relationship existed between the relative peak absorbance and the injected chloride ion concentration,regardless of the eluent pH. However, the relative peak conductance decreased on injecting the chloride ion a t 3.5, the relative peak conductance increased with the injected chloride concentration. On the other hand, the relative conductanceand absorbance of the second dip peak decreased slightly with the increase of the sample concentrationand its peak height decreased with the pH of the eluent. The effect of the concentration of the eluent was investigated next. Table I shows the relative conductance and absorbance of the first and second dip peaks using 4 mM tartaric acid as eluent. The relative first dip peak conductance and absorbance using 2 mM tartaric acid were larger than those obtained by using 4 mM tartaric acid, though their differences were small. On the other hand, the second dip peak obtained by using 2 mM of tartaric acid was considerably smaller than that obtained by using 4 mM of tartaric acid. Effect of t h e Sample Matrix on the Dip Peak. The retention times and the dip peak heights were measured by injecting 0-10 mM of tartaric acid solution adjusted to the pH of the eluent. However, no definite tendency was found
3
4
PH
Flgure 6. Variations of the relative peak conductance and absorbance with the sample pH eluent, 2 mM tartaric acid (pH 2.98); (0)relative first dip conductance, (A)relative second dip conductance, (0)relative first dip absorbance, (A)relative second dip absorbance.
because of the shift of the equilibrium of the tartaric acid between the eluent and the sample solution. Subsequently, the effect of the sample pH on the dip peaks was investigated. When distilled water was used as a sample, the pH of which was adjusted by adding hydrochloric acid or potassium hydroxide solution, the relative conductance of the second dip peak decreased with the increase in the sample pH regardless of the eluent pH as shown in Table 11, and its retention time was constant. On the other hand, the relative conductance of the first dip peak always increased with the sample pH. Figure 6 shows the change in the dip peak heights upon injecting 2 mM of tartaric acid adjusted to various pH levels by adding hydrochloric acid or potassium hydroxide solution using 2 mM tartaric acid (pH 2.98) as eluent. The first dip peak depended on whether the sample pH was lower than eluent pH (2.98) or not. The relative first dip conductance and absorbance were nearly equal to the backgrounds with a sample of pH x2.98. However, the relative first dip peak absorbance increased with the increase in the sample pH for the sample of pH X.98. The W-shaped peak was observed by the conductivity detector for the sample of pH >3.4, as seen in Figure 3. The relative second dip peak conductance and absorbance decreased with the increase in the sample pH, and continuous curves were obtained. From these results, it was considered that the second dip peak depended upon the sample pH and the first dip peak depended upon the other parameter. A s u m of the positive and negative peak areas recorded with the UV spectrophotometric detector, which contained the first dip, the second dip, and the indirect photometric peak accompanied with the elution of the chloride ion (21),was equal to zero at every pH of the eluent. Tartaric acid was added to the sample solution of the chloride ion so as to be equal to the eluent concentration (2 mM) in order to exclude the effect of the sample water and
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ANALYTICAL CHEMISTRY, VOL. 56,
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Table 111. Relative Peak Conductance and Absorbancen re1 peak cOncn of cl-, PPm 0 20 40 80
conductance, rS 1st dip 2nd dip 0
-25 -35 -b
0 -2
-2 -8
re1 peak abs, A 1st dip 2nd dip 0 0.036 0.068
0.113
0
-0.003 -0.002 -0.009
“The eluent was 2 mM tartaric acid. *The first dip peak was the W-shaped peak. the eluent and the sample pHs were not adjusted, because it was difficult to accurately reproduce the eluent components in the sample solution if the pHs were varied by the hydrochloric acid or potassium hydroxide solution. Table I11 shows the relative dip peak conductance and absorbance when injecting the chloride ion solution containing the eluent components. The relative first dip peak conductance and the absorbance of the sample containing the eluent components were equal to that of the sample in the absence of the eluent components. However, the second dip peak became much smaller than the other case, and the relative second dip peak conductance and the absorbance decreased slightly with the increase in the injected chloride ion concentration in the sample. The differences between the second dip peak conductance and absorbance upon injecting the distilled water and those obtained when injecting 100 ppm of chloride ion were 10 pS and 0.01 A, respectively, as shown in Figures 4 and 5. These values were equivalent to the results in Table 111. The second dip peak was influenced slightly by sample concentration in every case. These phenomena were observed not only for the chloride ion but for the other anions (Br03, NO3-,etc.), and the dip peak heights of the latter were equal to those of the former. The dip peaks for the divalent ions were not investigated because of the long retention time at low pH of the eluent. However, Hershcovitz et al. (19) mentioned that the dip peak heights of the equivalent concentration of the divalent ions were equal to those of the monovalent ions. Thus, the dip peaks were independent of the species of the injected ion. Formation Mechanism of the First Dip Peak. From the results mentioned above, the following phenomena were summarized concerning the first dip peak: (a) The retention time was constant regardless of the eluent pH or the injected ion concentration (Figure 2). (b) The relative absorbance increased in proportion to the injected chloride ion concentration (Figure 5 ) . (c) The relative conductance decreased with the concentration of the chloride ion upon injecting 0-40 ppm of chloride ion. It increased by injecting the chloride ion >40 ppm when using low pH (4.08. (Figures 1, 4, and 5). (c) The second dip peak was independent of the injected ion concentration (Figures 4 and 5). (d) The second dip peak became large with the increase in the sample pH (Figure 6). (e) The variation of the relative peak conductance corresponded to that of the relative peak absorbance (Figures3-6). From these phenomena, it WEB considered that the residence time of the eluent anion flowing into the column was equal to the retention time of the second dip peak and this peak was caused by the “absent effect” of the eluent anion, which should be eluted at the position of the second dip peak, in the sample solution. It is difficult to observe the retention time of the eluent anion because of the equilibria between the anion in the eluent and that in the sample solution. However, when distilled water was injected upon using the eluent containing the chloride ion, the negative peak was observed at the eluting position of the chloride ion, as seen in Figure 9. This fact supports the formation mechanism of the second dip peak named ”eluent anion absent effect”. However, this is not the only effect to cause the second dip peak because of the observation of the small second dip peak upon injecting the chloride ion solution containing the eluent components (Table 111). The other mechanism of the second dip peak is considered as follows: The ion exchange site is saturated with the eluent anion, and the fixed eluent anion is replaced by the hydroxide ion accompanied with the movement of the first dip zone. After
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 12, OCTOBER 1984
-==--=-
E.l
s- s-
€.I
_I
-
E-
E*/ E'
- -
sample band
E
. .
IOH IOH E
j
0
sample waler zone
5- E- E-I E - E- E - € 7 ' ~ - E - H E ll-ll-ll~,,-il-,l-ll~il-~l-ll-~
E' E - ! E-
E-
I
S- S-IOH. E- OH-OH.! E- € - E., €- E-! I
1
2nd dip zone
I
ion excluded zone
1st dip zone
Flgure 10. The formation mechanism of the dip peaks: (a) mobile phase; (b) cation excluded layer: (c) anion exchange resin. H+, K+, OH-, E-, S-,and HE represent the hydrogen Ion, potassium ion, hydroxide ion, eluent anion, solute anion, and assoclated eluent anion, respectively.
that, the hydroxide ion is again replaced by the eluent anion. Consequently, the second dip peak is caused by this zone in which the eluent anion is insufficient. The decrease in the second dip peak height with high eluent pH was caused by the decrease in the transfer of the eluent anion to the first dip zone. The decrease in the transfer of the eluent anion is caused by the decrease in the amount of the fixed eluent anion in the ion exchange column and by the slow transfer rate of the divalent eluent anion. The difference between the transfer rate of the monovalent ion and that of the divalent ion was proved by Figure 9B. Though the absent peaks of the chloride and nitrite ions were observed,that of the sulfate ion was not. This fact explains why the transfer rate of the monovalent anion is faster than that of the divalent anion. The second dip peak became large with the increase in the sample pH, because the hydroxide ion in the sample directly replaced the fixed eluent anions. These two formation mechanisms of the second dip peak are essentially the same effect, that is, the hydroxide ion replaced the eluent anion fixed on the ion-exchange resin accompanied by the movement of the first dip peak zone. In other words, the second dip peak was caused by the compensating effect for the first dip peak. Although Gjerde and Fritz stated that the base line disturbance occurred because the eluent anion (benzoate) was adsorbed by the resin (8),this effect was not considered because of its appearance upon using the eluent containing organic solvent since the aqueous acid such as tartaric acid was hardly adsorbed by the used resin matrix.
CONCLUSION The formation mechanism of the two dip peaks is shown in Figure 10. It was assumed that the eluent was a monobasic acid and did not contain cations except for hydrogen ion. The cation is excluded from the (b) and (c) layers and the diffusion of the ions in the (a) layer exists between the adjacent zones. In Figure 10A, the eluent anion (E-) and cation (H+) is in the ion-exchangeequilibrium between the bulk solution and the ion exchange resin. When a sample solution (K+ and S-) is injected into the column, the ion exchange equilibrium is newly established between the eluent anion (E-) adsorbed by the ion-exchange resin and the sample anion (S-) (Figure 10B). After that, the sample anion (S-)is adsorbed by the ion-exchange resin (Figure lOC). The sample cation (K+) and the replaced eluent anion (E-), sample water zone or first dip zone, flow through in the front of the sample anion (S-) retarded by the resin, and this zone forms the dip peaks (see the left side of Figure 1OC). The ion-exchange equilibrium between the bulk solution and ion-exchange layer is changed by the movement of this sample water zone. Therefore, the eluent anion (E-) transfers from the ion-exchange resin into the sample water zone in which the eluent anion is insufficient and hydroxide ion is caught by the resin instead of the eluent anion in order to keep a charge equilibrium (see the centers of Figure lOC,D). The hydroxide ion adsorbed by the resin is replaced again by the eluent anion and this zone forms the second dip peak. The first dip zone is separated into the sample water zone and the ion-excluded zone because of the difference in the utilizable volume of the column (see the right side of Figure 10E). The latter contains more ionic species than the former. Consequently, as seen in Figure 10E, four zones are formed upon injecting the sample solution in the following order; "ion-excluded zone", "sample water zone", "second dip zone", "sample band". LITERATURE CITED (1) Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975, 47, 1801-1809. (2) Nadkarni, R. A.; Pond, D. M. Anal. Chlm. Acta 1983, 146, 261-266. (3) Pohi, C. A,; Johnson, E. L. J. Chromatogr. Scl. 1980, 18, 442-452. (4) Green, L. W.; Woods, J. R . Anal. Chem. 1981, 5 3 , 2187-2189. (5) Dolzine, T. W.; Esposito, G. G.; Rinehart, D. S. Anal. Chem. 1982, 5 4 , 470-473. (6) GJerde,D. T.; Fritz, J. S.; Schmuckler, G. J . Chromatogr. 1979, 186, 509-5 19. (7) Gjerde, D. T.; Schmuckler, G.; Fritz, J. S. J. Chromatogr. 1980, 187, 35-45. (8) Gjerde, D. T.; Fritz, J. S. Anal. Chem. 1981, 5 3 , 2324-2327. (9) Okada, T.; Kuwamoto, T. Anal. Chem. 1983, 55, 1001-1004. (IO) Okada, T.; Kuwamoto, T. Bunsekl Kagaku 1983, 32, 595-599. (11) Fritz, J. S.; Gjerde, D. T.; Becker, R. M. Anal. Chem. 1980, 52, 1519-1522. (12) Cassidy, R. M.; Elchuk, S. Anal. Chem. 1982, 5 4 , 1558-1563. (13) Nordmeyer, F. R.; Hansen, L. D.; Eatough, D. J.; Rolllns, D. K.; Lamb, J. D. Anal. Chem. 1980, 52, 852-856. (14) Bouyoucos, S. A. Anal. Chem. 1977, 49, 401-403. (15) Stevens, T. S.; Davis, J. C.: Small, H. Anal. Chem. 1981, 53, 1488-1 492. (16) Ishibashi, W.; Kikuchi, R.; Yamamoto, K. Bunseki Kagaku 1982, 3 1 , 207-21 1. ( I 7) Stevens, T. S.; Jewett, G. L.; Bredeweg, R. A. Anal. Chem , 1982, 54, 1206-1 208. (18) Hanaoka, Y.; Murayama, T.; Muramoto, S.; Matsuura, T.; Namba, A. J. Chromatogr. 1982, 239, 537-548. (19) Hershcovitz, H.; Yarnitzky, Ch.; Schmuckler, G. J. Chromatogr. 1982, 244, 217-224. (20) Okada, T.; Kuwamoto, T. J. Chromatogr. 1984, 294, 149-156. (21) Small, H.; Miller, T. E., Jr. Anal. Chem. 1982, 5 4 , 462-469. (22) Turkelson, V. T.; Rlchards, M. Anal. Chem. 1978, 50, 1420-1423.
RECEIVED for review March 19,1984. Accepted May 31,1984. This work has been supported by Grant-in-Aid for Scientific Research (No. 58470029) from the Ministry of Education, Science and Culture, Japan.
