Anal. Chem. 1989, 61, 122-125
122
(3) Feist, R.; Schneider, G. M. Sep. Sci. Techno/. 1082, 17, 261-270. (4) Wilsch, A.; Feist, R.; Schnelder, G. M. F/uidPhase Equilib. 1083, 10, 299-306. (5) Lauer, H. H.: McManlgill. D.: Board, R. D. Anal. Chem. 1083, 55, 1370-1375. (6) Springston, S. R.; Novotny, M. Anal. Chem. 1084, 5 6 , 1762-1766. (7) Sass&, P. R.: Mourier, P.: Caude, M. H.; Rosset, R. H. Anal. Chem. 1087, 5 9 , 1164-1170. (8) Taylor, G. Proc. R. SOC.London, Ser. A 1053, 219, 186-203. (9) Taylor, G. Proc. R. SOC.London, Ser. A 1054, 223, 446-468.
Taylor, G. Proc. R . Soc. London, Ser. A 1054. 2 2 5 , 473-477. Ark, R. Proc. R . SOC.London, Ser. A . 1856, 235, 67-77. Wilke, C. R.; Chang, P: AIChEJ. 1055, 1, 264-270. Reid, R. C.; Prausnitz, J. M.;Sherwocd, T. K. Le Bas, The Properties of Gases and Liquids, 3rd 4.;McGraw-Hill: New York, 1977; p 58. (14) Iwasaki, H.; Takahashl, M. J. Chem. phvs. 1081, 7 4 , 1930-1943. (15) Diller, D. E.; Ball, M. J. Int. J . Thermophys. 1085, 6 , 619-629.
(10) (11) (12) (13)
RECEIVED for review July 28,1988. Accepted October 24,1988.
Background Suppression by Chelation in the Ion-Exchange Chromatographic Separation of Anions Hisakuni Sato* and Akiyoshi Miyanaga
Laboratory of Analytical Chemistry, Faculty of Engineering, Yokohama National University, Tokiwadai 156, Hodogaya-ku, Yokohama-shi, Japan 241, and Tokyo Research Center, Tosoh Corporation, Hayakawa 2743-1, Ayase-shi, Japan 252
By the use of chelate formatlon reactions, three new background suppresslon methods are developed for conductlvlty detection In anion chromatography. (1) By postcolumn homogeneous reactions, including chelate formatlon and acldbase reactlons, background conductlvlty and water dlp can be considerably reduced. (2) By use of an elutlng anlon that forms a neutral chelate with a metal Ion (M"'), background conductlvlty can be reduced to a very low level with a M"+-form cation-exchange column as the suppressor. (3) Protonated amlne, whlch Is the countercatlon of the eluting anlon, is made to dissociate in a Cu2+-form cation-exchange column. The dlscharged proton nerdralizesthe anlonlc charge of the eluant Ion by an acld-base reaction. Background conductlvlty depends on the eluting anion. Although these new methods wlll not completely surpass the usual suppression method utilirlng only acid-base reactbns, they have their own dlfferent features, are easler to use, and are expected to expand the posslbliltles of analytlcal ion-exchange chromatography.
To permit conductivity detection in ion-exchange chromatography, Small et al. (1)found a method to suppress the background conductivity of the eluant by using a second ion-exchange column called a "stripper column". For the separation of anions, this stripper column (frequently called a "suppressor column") is packed with cation exchanger in the H+ form, cations in the eluant are exchanged with the H+ ions, and then eluting anions are protonated. Later, in place of the cation-exchange column, fiber suppressors using cation-exchange membrane tubes were used (2, 3). These background supression methods are based on the combination of an ion-exchange reaction and an acid-base reaction. However, since the alkaline eluant is changed to a weak acid solution in the stripper (or suppressor) portion, problems arise in the detection of NOz- and in the detection of conjugate anions of weak acids (4). We have found three new background suppression methods by using chelate formation reactions in place of the acid-base reactions of Small et al. (1). This paper present$ the principles *Author t o whom correspondence should b e addressed a t Yokohama N a t i o n a l University.
of these methods and some experimental results.
PRINCIPLES Method of Postcolumn Homogeneous Reactions. Let the cation and the anion in the eluant be ME and LE, respectively. Similarly, let the cation and the anion in the reacting reagent solution to be added to the effluent from the separation column be Ms and Ls, respectively. Here, the charge on the ion is omitted for the purpose of simplicity. The desirable postcolumn reactions for the reduction of the background conductivity can be expressed as follows:
ME + Ls Ms + LE
+
ME-Ls Ms-LE
(1)
(2) where ME-Ls and Ms-LE are complexes or ion associations. As these reactions progress toward the right, positive and negative charges cancel each other and the conductivity is thereby reduced. Ideally, it is preferable that the charges on the all products are zero. If one of the following acid-base reactions progresses in place of (1)or (2),the background conductivity is also reduced:
ME + Ls Ms + L E
+ nH,O + nH,O
-+
+
ME(OH), + H,Ls Ms(OH), + H,LE
(3)
(4) where ME(OH),, Ms(OH),, H,Ls, and H,LE are water-soluble chemical species having smaller charges than ME, Ms, LE, respectively. Thus, it is expected theoretically that the background conductivity of the eluant can be reduced by the combinations of (1)and (2), (1)and (3),or (2) and (4) given above. ME, LE, Ms, and Ls should be suitable for the above reactions, and ME and LE should further be suitable as the eluant components for anion separation. If both ME and MS are limited to metal cations, two kinds of specific chelate formation reactions, as represented in reactions 1 and 2, must be combined. Of course, ME (Ms) must not associate with LE (Ls). If either ME or MS is an amine cation, it is considered that combinations of (1)and (3),or (2) and (4),may be possible. At present, no ideal combinations of ME, LE, Ms, and Ls have been found. A few examples will be given later, where the principles of this method are partially realized. Method of Changing the Eluting Anion to an Uncharged Chelate. Assume that the eluant components are cation ME and anion LE. The effluent from an anion-exchange
0003-2700/89/0361-0122$01.50/00 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 2, JANUARY 15, 1989
separation column contains ME, LE, and sample component anions. This effluent is allowed to pass into a cation-exchange column, in which cation Ms is held, to produce the following ion-exchange reaction: ME
+ Ms-R
---*
Ms + ME-R
I
123
A
(5)
where -R shows the skeleton of the ion exchanger. If Ms produced by (5) forms an uncharged stable chelate with LE, then we have:
Ms + L E
+
Ms-LE
(6)
The ionic species in the solution will now be the sample anion and its opposed cation (ME) only. Normally, most of the anions being analyzed do not form stable complexes with metal ions. Therefore, if it is possible to find any substances such that reactions (5) and (6) take place, the background may be made very small. An approximately ideal system can be constructed by selecting K+ for ME, ethylenediamine-N,N'diacetate ion for LE, and Cu2+ for Ms. Method of Dissociation of Protonated Amine. The protonated cation of a polyacidic amine (B) such as ethylenediammonium ion, H,B"+, is combined with some anion (X-) to make the eluant component. The effluent from a separation column is allowed to pass into a cation-exchange column in which the cation, Ms, is held, and after chelation with B, then we have
H,B"+ If
+ Ms-R
-*
nH+ + B-Ms-R
(7)
X-accepts the discharged H+, then
+
H+ X--. HX (8) The eluant conductivity is thus reduced. The most typical example is the case where OH- is selected for X-.That is, an amine water solution is the eluant, and the background is merely water. However, in this case OH- as the eluting ion is too weak to be suitable for the separation of bivalent ions in samples. If the conjugate anion of a weak acid in place of OH- is selected for X-, the background conductivity will become a little higher, but chromatographic systems having different elution characteristics can be constructed. This method closely resembles the usual method of ion chromatography developed by Small et al. (I). As described below, the effectiveness of the suppressor column can be visually known by using a Cu2+-form column as the suppressor. EXPERIMENTAL SECTION Equipment. The chromatographic equipment consists of a Tosoh pump system (CCPM, dual pump), a conductivity detector (CM-SOOO),and a Reodyne loop injector (1OO-pL loop). For the postcolumn homogeneous reactions, one of two CCPM pumps was used to transfer the eluant, and the other to transfer the reactant solution. The effluents from the separation column and the reactant solution were joined with a Y-shaped connector and mixed through a Tosoh static mixer (Teflon pipe 0.25 mm X 5 m). SeparationColumn. A polymer-based anion-exchangecolumn (Tosoh IC anion PW, 4.6mm X 5 cm) was used. The ion-exchange capacity of the packings is about 30 wequiv/crn3. Suppressor Column. For the background suppression, strong-acid type cation exchanger columns (Tosoh SCX, polypropylene container, 4.6 mm X 5 cm)were used. The ion-exchange capacity of one column was about 2 mequiv. Before use, it was converted to Cu2+form by an adequate flow of a 0.05 M Cu(NO& solution and was rinsed with water. Once it is used as the suppressor, the Cu column can be regenerated in the same way. The lifetime of the suppressor column depends on the composition and the concentration of the eluant. The state of consumption and regeneration of the column is indicated by a color change, as will be explained later. Reagents. Most reagents used were commercially avaiable, guaranteed reagents. Organic reagents such as ethylenediamine-N,"-diacetic acid (EDDA),(2-hydroxyethy1)imiiodiacetic
IO
0
IO
0
20
time (min.) Figure 1. Chromatograms of anions with and without background suppression by chelation, obtained by postcolumn homogeneous reactions. Conditions: eluant, 1 mM (Tri&NTA (1.O mL/min); sample, (1)Cr, (2) B r , (3)NOa-, (4) S O P ; each ion lo4 M, 100 pL; A, without suppression; B, with suppression by using a solution containing 2 mM Ca(OH), and 6 mM H3B03(0.5 mL/min). acid (HIDA), etc. were Tokyo Kasei Kogyo products. KzEDDA solution was prepared by dissolving EDDA reagent in calculated amounts of KOH solution. Water solutions of amine carbonate were prepared by passing a sodium bicarbonate solution (1-5 mM) through an H+-formcation-exchangecolumn (Tosoh,SCX, same one as used for the suppressor) and neutralizing the resulting solution with calculated amounts of the amine. The standard anion solutions were prepared by dissolving sodium salts or potassium salts in water. Distilled, deionized water was used as solvent. Chromatographic Operating Conditions. The flow rate of eluant was 1.0-1.5 mL/min. The sample injection volume was held constant at 100 wL. The operating temperature was room temperature. There were no special conditions to be set.
RESULTS AND DISCUSSION Method of Postcolumn Homogeneous Reactions. At present, no combinations of substances that ideally realize the above-mentioned charge canceling conditions have been found. However, a few systems that reduce the background conductivity to about could be found. Figure 1 shows an example of the chromatogram obtained by using the eluant containing tris(hydroxymethy1)aminomethane (Tris; 3 mM) and nitrilotriacetic acid (H3(NTA); 1 mmol/L). The postcolumn reactant was a water solution of calcium oxide (2 mM) and boric acid (6 mM). Parts A and B of Figure 1 correspond to the cases without and with postcolumn reactions, respectively. The eluant itself had conductivity of 166 pS/cm, which was the background in A. As the reactant was mixed at flow rate of 'I2 that of the eluant, the background conductivity was reduced to 58 pS/cm. This phenomenon is considered to be realized by the following reactions. In the eluant: 3Tris
+ H3(NTA)
In the reactant:
2H(Tris)+
-+
+ Tris + H(NTA)2- (9)
-
Ca2+ + 2H2B03- + H3B03+ H 2 0 (10) Postcolumn reactions: CaO
+ 3H3B03 Ca2+
+
+ H(NTA)2H2B03-
H+
Ca(NTA)+
H3B03
+ H+
(11) (12)
In short, it is considered that most of the divalent ions (Ca2+ and H(NTA)2-)in the eluant disappear, and Ca(NTA)- and
124
ANALYTICAL CHEMISTRY, VOL. 61, NO. 2, JANUARY 15, 1989 I'
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0
I
IO
I
,
20
30
1
40 time (rntn)
Figure 2. Chromatogram of anions with background suppression by chelation, obtained by chelate formation of eluting ion in Cu column. Conditions: eluant, 2 mM dlpotassium ethylenedlaminaN,N'd~~tate (1.0 mL/min, pH 10.08, 380 pS/cm); suppressor, Cu2+-formcation exchanger column (after suppression, pH 5.74, 5 pS/cm); sample, (1) F-, (2) Ci-, (3) NO2-, (4) Br-, (5) NO,-, (6) HPO?-, (7) SO?-,(8)I-, (9) S20,2-; each ion lo-' M, 100 pL.
H(Tris)+ are produced. To what extent these reactions progress depends on the acid dissociation constants of H(Tris)+, boric acid, and Hi(NTA) (i = 1-3) and on the chelate formation constant of Ca-NTA. In this case, about 90% of the total Ca2+is estimated to form a 1:l chelate with NTA by equilibrium calculations. A comparison between parts A and B of Figure 1indicates that each peak in B has a height of about twice as much as in A. The composition of the sample zone is different from the background components, not only in the anion, but also in the cation, because of the postcolumn chemical reaction. This is reflected in the peak height. The change of conductivity just after sample injection is very different in parts A and B of Figure 1. In A, the so-called water dip is large and the peak of C1- is thereby affected, making its quantitative analysis difficult. In contrast, the water dip is hardly seen in B. The peak of C1- is judged as an isolated peak. This effect is caused by the addition of the postcolumn reactant and is the third advantage of this method. In place of Tris used in the above example, triethanolamine or imidazole having a comparable pK value can be used, but they are not specifically superior to Tris. At present, any combination of substances that is superior to the above example has not been found, but it is obvious that the possibility exists in principle. Method of Changing the Eluting Anion to an Uncharged Chelate. When a water solution of dipotassium ethylenediamine-N,N'-diacetate (K,EDDA) is used as the eluant and a cation-exchange resin column of Cu2+form is used as the suppressor, a nearly ideal background suppression is realized. The suppression reaction may be expressed as EDDA2-
+ CU-R + 2K'
-
CU-EDDA
+ K2-R
(13)
Figure 2 shows a typical chromatogram of anions obtained by this method. The conductivity of the 2mM KzEDDA used as the eluant was 380 pS/cm (pH 10.08). After the eluant was passed through a Cu column, the conductivity was reduced to 5 pS/cm (pH 5.74). As a result, in the ordinary sensitivity region (full scale 10 pS/cm or so), the base line is very stable. As compared with the usual suppressor type ion chromatographic method, the cation to be replaced is not H+, but Cu2+. Therefore, the detection sensitivity (conductivity change per mole of analyte) is relatively small. Since almost all of the eluting anions (EDDA2-) are changed to uncharged chelates, the background is very low, about 113 of the background obtained by the method using carbonate as eluant and a H+-form cation exchanger column. Moreover, the pH buff-
ering ability of the solution after suppression is small. This does not promote the protonation of the conjugate anion of a weak acid. In the usual method, HC03- and C032- are changed to HzC03in the suppressor, and the detection sensitivity for the conjugate anion of the weak acid is lowered because of forced acidification. As for the working curve for common anions, the linearity of peak height was confirmed in a range of 5 x lo* to lo4 M. The practical convenience of this method is that the degree of suppressor consumption and regeneration can easily be judged by the color change. Sulfonated St-DVB resin is light yellow. When it is packed into a polypropylene or a glass column and a C U ( N O ~solution )~ is allowed to flow through it, it is changed to green (Cu2+form). When this column is used as the suppressor, Cu2+is chelated with EDDA2- and is separated from the resin. The resin converted to the K+ form appears light yellow. The Cu-EDDA chelate is blue and diffuses into the pores of the Cu2+-formresin, so that the effective part of the suppressor column can be distinguished distinctly. Other divalent cations may be utilized in place of Cu2+,but it is difficult to find any that are superior to Cu2+ in the color change and the stability of the chelate with EDDA. On the other hand, the cation in the eluant is required not to form any chelate with the eluting anion. Alkali-metal and alkaline-earth-metal ions seem to be candidates. However, Ca2+ is apt to form hardly soluble precipitates with F- or HP04%as the sample ions and is not favorable. Mg2+is easily hydrolyzed in alkaline solution and is difficult to use. Then, the remaining cations are Na+ and K+. K+ is caught by the cation-exchange resin more strongly than Na', and therefore, K+ ion was selected. As the eluting anion, ethylenediamine-N,"-dipropionate (EDDP) ion can be used in place of EDDA. However, this reagent is commercially available in the hydrochloride form and it is troublesome to prepare in the free acid form. EDDA and EDDP have almost the same characteristics as the eluting anion. As for other eluting anions, iminodiacetate (IDA) and (2hydroxyethy1)iminodiacetate (HIDA) were tested in the form of K+ salts. With these anions, the suppression effect appears for a while to be similar to that with EDDA. However, when about one-tenth and two-thirds of a Cu column are discolored with IDA and HIDA, respectively, the background conductivity begins to rise, and it becomes necessary to regenerate the Cu column. Thus, the Cu column utilization efficiency is poor. In the case of EDDA, the suppression effect lasts until the whole Cu column is discolored. With a single Cu column of 4.6 mm X 5 cm size, about 0.5 L of 2 mM KzEDDA solution can be allowed to flow. The probable reason why IDA and HIDA are inferior to EDDA is the difference in complexation capability with Cu2+. The logarithmic formation constants of the copper chelates with IDA, HIDA, and EDDA are reported as 10.63 @), 13.38 (6), and 16.2 (7), respectively. Moreover, in the case of IDA, there is probably a possibility of 1:2 chelation in addition to 1:l. It is desirable that the ligand, which serves also as the eluting ion, should satisfy the coordination number of the central metal ion and form an uncharged water-soluble chelate. At present, those that satisfy this requirement are EDDA and EDDP. The anion separation pattern varies with the ratio of EDDA or EDDP and KOH. Especially, the phosphate ion peak is easy to shift with the eluant pH. However, even if the eluant pH varies somewhat, the suppression effect of the Cu column is not affected. If the HC0,- ion coexists in the sample, its peak will appear between those of Br- and NO3-. This may be a drawback of this method in its practical application. The detection sensitivity for HCO; is as low as about lll0 of that for NO3-, and
ANALYTICAL CHEMISTRY, VOL. 61, NO. 2, JANUARY 15, 1989
125
I
2
0
0
IO
fime (min.) Figure 3. Chromatogram of fluoride and chloride ions with background suppression by chelation, obtained by dissociation of protonated amine in Cu column. Conditions: eluant, 2 mM ethylenediamine (1.2 mL/min); suppressor, Cu2+-formcation exchanger column; sample, (1) F- (5.3 x 10-5 MI, (2) C I (2.8x 10-5 M).
usually it will cause no problems. However, it is advisable to confirm the elution position of the HC03- peak previously by injecting distilled water that has been exposed to air. Method of the Dissociation of a Protonated Amine. Figure 3 shows a chromatogram of F- and Cl- when 2 mM ethylenediamine is used as the eluant. The conductivity of the eluant was 180 wS/cm, but by suppression with a Cu column, it dropped to 2 pS/cm. Therefore, without raising the amplification degree of the detector, high-sensitivity detections are feasible. In the 2 m M solution of ethylenediamine, the concentration of OH- ion is estimated as 3.72 X M from pK1 (10.11) and pK2 (7.30) (8). Thus, most of ethylenediamine exists in the form of neutral molecules. OH- as the eluting ion is rather weak, and therefore, this elution system is suitable for the separation of ions that are not held firmly in the anion exchanger. If a water solution of an amine is used for the elution of those ions that have a large affinity with the anion exchanger, such as 50:- ion, it takes too much time until elution. Moreover, if anion-exchange resin is used too long in the OH- form, the ion-exchange groups are liable to be broken. In such a case, salts of an amine and a weak acid can be used. An example is the mixture of ethylenediamine and carbonic acid. The more basic the amine is, the more the weak acid dissociates. Therefore, if the Cu column efficiency is taken into account, it is favorable to use a strongly basic, polyacidic amine. In any case, it is necessary to find proper concentration conditions by calculating the equilibilium concentration with use of the acid-base equilibilium constants of the amine and the weak acid. Figure 4 shows the equilibrium concentrations of HC03- and CO2- in some aminecarbonic acid systems. Ethylenediamine, which is a weak base, gives a small concentration of C032- with the same total concentrations, as compared with propylenediamine, etc. Table I shows the values of the background conductivity of some elution systems before and after the suppression. When a weak acid is added, the conductivity after suppression increases, depending on the dissociation degree of the weak acid. In this respect, this method may be considered quite the same as the ion chromatographic method developed by Small et al. (1).
2
I
0
3
total concentration of. carbonate ion (mM) Figure 4. Equilibrium concentrationsof bicarbonate and carbonate ions in amine-carbonate solutions. The total concentration of each amine is 3 mM. Key: curve 1, ethylenediamine; 2, diethylenetriamine; 3, propylenediamine.
Table I. Changes of Background Conductivity by Chelation for Some Amine-Weak Acid Systems
eluant 3mM EDAa/3mM H3B03 2mM PDAb/4mM H3B03 3mM DENc/3mM H3B03 4mM PDA/2mM H,COS
background conductivity, rS/cm after suppresbefore suppression sion 157 148 151 221
3 6 3 13
'EDA is ethylenediamine (pK1 10.11, pK2 7.30) (8). bPDA is propylenediamine (pKI 10.72, pK, 8.96). DEN is diethylenetriamine (pK, 10.02, pK2 9.21, pK3 4.42).
The Cu column consumption and regeneration can clearly be indicated by the color. change, as in the second method of utilizing uncharged chelate formation. However, this color change is different from the former case. Protonated amine in the eluant dissociates in the Cu column, and the resulting amine coordinates to Cu2+ion. The copper-amine complex remains in the resin phase, and that part changes to dark blue or to violet from yellowish green. When C U ( N O ~solution )~ is allowed to pass into the used suppressor column, the combined amine is eluted and the column is regenerated. Any carbonate salt of the polyacidic amine is not available as a commercial reagent, and its preparation is slightly troublesome.
LITERATURE CITED (1)
Small, H.; Stevens, T. S.;Bauman, W. C. Anal. Chem. 1975, 47,
(2) (3)
Stevens, T. S.; Davis, J. C.; Small, H. Anal. Chem. 1981, 53, 1488. Hanaoka, Y.; Murayama, T.; Muramoto, S.; Matsuura, T.; Nanba, A. J .
1801.
Chromatogr. 1978, 239, 537. Okada, T.; Kuwamoto, T. Anal. Chem. 1985. 57, 829. Chaberek, S.; Martell, A. E. J . Am. Chem. Soc. 1952, 7 4 , 5052. Anderegg, G.; Schwarzenwach,,G.He&. Chim. Acb 1955, 38, 1940. Countney, R. C.; Chaberek, S.;Martell, A. E. J . Am. Chem. SOC. 1953, 75, 4814. (8) Rlngbom. A. Complexation in Analytical Chemistry; W h y : New York, 1963.
(4) (5) (6) (7)
RECEIVED for review June 14, 1988. Accepted October 18, 1988.
