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System peaks in ion chromatography using cetrimide-coated columns and indirect photometric detection
Journal of Chromatography, 402 (1987) 21 I-220 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands CHROM. 19 567
SYSTEM PEAKS IN ION CHROMATOGRAPHY USING CETRIMIDE-COATED COLUMNS AND INDIRECT PHOTOMETRIC DETECTION
E. PAPP Institute for Analytical Chemistry. University of Chemical Engineering, Veszpdm (Hungary) (Received March 18th, 1987)
SUMMARY
The characteristics of the system peaks in an ion-chromatographic system were studied using non-polar stationary phases (RP- 18, PRP- 1) coated with cetrimide, and using aqueous eluents containing 24% methanol, potassium hydrogenphthalate (suitable for indirect UV detection) and other buffer constituents. One or more system peaks were observed depending on the mobile phase composition.
INTRODUCTION
In order to detect non-UV-absorbing solutes, eluents containing I-IV-active constituents are often used in single column ion chromatography. Beside the solute peaks, one or more “system peaks” also appear in the chromatogram obtained by this indirect UV detection method. System peaks eluted near to solute peaks may disturb the evaluation of the chromatograms. On the other hand, they may provide useful information on the thermodynamics and kinetics of the equilibria1p2 and enhance the detection sensitivity under certain conditions. Barber and Carr3-5 and Hammers et ~1.~studied the role of system peaks and their retention mechanism in reversed-phase ion-interaction chromatography, where inorganic sample anions were detected by using UV-active ion-interaction agents. System peaks arising in indirect W detection or conductometric detection on low capacity anion-exchange columns have also been investigated7q8. Successful separations of anions were also reported with cyano-bonded silicapJo or octadecyl silica’ l-r3 columns coated dynamically or permanently with a long-chain alkyltrimethylammonium salt (Cetrimide). For the indirect UV detection of anions in single column ion chromatography, potassium hydrogenphthalate was applied over a broad pH range (with other buffers) because of its high UV absorbance and ability to replace singly and doubly charged anions14-21. However, this eluent system exhibits unexplained system peaks under some eluent conditions. In this study the origin of system peaks in eluents containing potassium hydrogenphthalate and other buffer components has been investigated using two nonpolar stationary phases, hydrocarbon phase chemically bonded to silica gel, RP-18, 0021-9673/87/%03.50
0
1987 Elsevier Science Publishers B.V.
212
and poly(styrene-divinylbenzene) monium ion.
E. PAPP
copolymer, PRP- 1, coated with cetyltrimethylam-
EXPERIMENTAL
The chromatographic system used consisted of two Model 6000A pumps, a Model OE-308 variable-wavelength UV detector (Labor, MIM, Hungary), a differential refractometer detector (Varian, Palo Alto, CA, USA.), two six-port injection valves (Model 7010, Rheodyne, lo-,ul loop; Valco N60) and a dual-channel recorder (M 4220; Knauer, F.R.G.). The analytical columns contained octadecyl silica (LiChrosorb RP- 18, Merck), 120 mm x 4 mm, 5 pm, and poly(styrene-divinylbenzene) copolymer (prepacked, Hamilton, PRP-l), 150 mm x 4.1 mm, 5 pm. To avoid rapid degradation of the RP-18 column, eluents of pH > 7 were directed first to a precolumn (100 mm x 3 mm) situated between the pump and the injector, containing LiChrosorb Si 60. The columns were dynamically coated with cetyltrimethylammonium bromide (Cetrimide). The quantity of cetrimide adsorbed, was determined according to the breakthrough curves recorded by a refractive index (RI) detector**. The column temperature was maintained at 25 f 0.5”C by use of a water-bath (MLW, GDR Type UlO) and a water-jacket. To remove the adsorbed cetrimide, the columns were washed with eluents containing 70% methanol and 0.1 M potassium bromide. The eluents were prepared as described earlier** with a constant, 24%, methanol concentration. The pH of the eluents was measured with an OP-208 precision digital pH meter (Radelkis, Hungary) and a combined glass electrode calibrated with aqueous buffers pH 4.0 and 7.0. Standard sample solutions were prepared from AnalaR grade salts dissolved in de-ionized water to 0.01 M. RESULTS AND DISCUSSION
First, the column of RP-18 was dynamically coated by cetrimide from an eluent containing 24% methanol, 3 mM potassium hydrogenphthalate, 10 n&f sodium acetate and 1 mM eetrimide (CTABr). The amount of cetrimide adsorbed, i.e., the ion exchange cap.acity, was calculated from the breakthrough curve to be 0.31 mM per column. Solute ions were injected on the equilibrated column and were detected by indirect UV detection. Fig. 1 shows that the injection of the buffer constituent acetate results in a negative peak (decreasing absorption) the area of which is proportional to the acetate concentration. When the injected solute ionss, e.g., Cl-, NO;, Br- have a stronger interaction with the adsorbed quaternary ammonium ions than has acetate, a positive system peak (increasing absorption) at the retention of acetate (Scn--) appears in the chromatogram. In the chromatogram of sample ions having stronger binding than bromide ion, beside the solute peaks, two positive system peaks are produced. The retention times of the first and second system peaks are identical to that of acetate and bromide (Sat-), respectively.. Fig. 2 shows a typical chromatogram obtained by the injection of nitrate ion. The area of the solute peak is approximately equal to the sum of the
213
SYSTEM PEAKS IN ION CHROMATOGRAPHY
+
I
1 . 2
5
1
o.oozAU
time (win)
.
, time
, 8
.
.
(min)
Fig. 1. Chromatogram showing the solute peaks and the acetate system peak (&a,-). Mobile phase: 24% methanol in aqueous buffer containing 1m&4cetrimide, 3 mM potassium hydrogenphthalate (KHP), 10 mM sodium acetate @H 5.8). Stationary phase: LiChrosorb RP-18 dynamically coated with cetrimide (0.31 mM cetrimide per column). UV absorption at 300 nm. Fig. 2. Chromatogram Fig. 1.
of NO; showing the induced system peaks &s-
and Ssl-. Conditions as in
areas of the two system peaks,. However, no correlation can be sought between the solute and system peak areas when the retentions of the solute ions are larger, e.g., for SOi- the area of the Sal- peak decreased. In Fig. 3, log k’ for the solute and system peaks are plotted against the logarithm of the concentration of potassium hydrogenphthalate (KHP) in the ehtent. The linear relationship observed suggests an ion-exchange retention mechanism involving competition between the anions of the solutes and of the buffer constituents. Both acetate’ 7 and bromidez3 system peaks were observed in similar systems, where the eluents contained one or the other ion. When mobile phase without CTABr vas applied (relying only on the permanently adsorbed cetrimide) the system peak of bromide (SB~-)w&sno longer observed. The retention times of the solute anions and the selectivity of the separation also decreased. In this eluent the positive acetate system peak can still disturb sample peaks eluted nearby. To avoid this, potassium hydrogenphthalate itself or an other non-UV-absorbing buffer can be used to vary the pi-I in the pobile phase. However, a system peak is also observed when the eiuent contains only KHP. Jackson and Haddad* studied the properties of the phthalatq system pea4 (SW) on fixed anion exchangers. They proposed that the origin of SKH~could be accounted for by the adsorption of the phthalate in its neutral form on the stationary phase. This would result in the absence of this system peak when the elwnt pH exceeds 6.5. On the contrary, they found the log k’ vs. log CKHPrelationship to be linear for the
214
E. PAPI’
pH
5.8
-2.5
-2.4
-1.3
-2.2
tOtJ
CKHP (M 1
Fig. 3. Retention of inorganic anions and system peaks as a function of the potassium hydrogenphthalate concentration. k’ = Capacity factor. Conditions as in Fig. 1.
system peak at pH 5.5, which strongly suggests an ion-exchange mechanism. This contradiction led me to make a detailed examination of the phthalate system peak behaviour as a function of the eluent pH. The eluent pH was varied by use of tris(hydroxymethyl)aminomethane (Tris). Because of the unfavourable dissolution of the base silica material above pH 7, the polymeric phase PRP-1 was used in this experiment instead of RP-18. In order to establish a similar ion-exchange capacity for this more hydrophobic packing, the cetrimide concentration in the coating eluent was decreased to 0.2 mM, which resulted in a capacity of 0.21 m&f cetrimide per column. In Fig. 4, log k’ for the phthalate system peak is plotted against the eluent pH. From pH 3.1 (2 m&f phthalic acid) to pH 7 the retention increases and above this value it steeply decreases. For fixed anion exchangers, Jackson and Haddads at pH 3-6, JupilleZ* at pH 2-5 reported similar behaviour of the lower part of this curve. It should be noted that the pH of the present eluent is about 0.5 unit higher compared to the aqueous eluent because of the presence of 24% methanolz4. As can be seen from Fig. 4, the increasing part of the log k’ vs. pH function for the phthalate system peak is in the pH range where the eluent contains singly charged phthalate anions (pK, = 3.1, p& = 5.49. The tendency of HP- to form intramolecular hydrogen bonding may influence its ion-exchange behaviour. This effect decreases the hydration of the hydrogenphthalate ion, which results in its repulsion from the hydrogen-bonded bulk water phase into the ion-exchanger phase, where water is less structuredz6.
215
SYSTEM PEAKS IN ION CHROMATOGRAPHY
4.5
loqk
1.0
0.5
0
3.4 TRIS
-
4.8 5.3 6
7
1.9
a3
-0.51
2
4
8
PH
(mM)
Fig. 4. Retention of inorganic anions and the system peak (Sap-) as a function of the eluent pH. Mobile phase: 24% methanol in 2 mM aqueous phthalate; pH varied with Tris. Stationary phase: PRP-1 coated with cetrimide (0.21 mM cetrimide per column).
If the pH of the eluent is increased above 7 then highly hydrated doubly charged phthalate anions are present in the eluent and this hydration outweighs the previous retention-enhancing process and the retention quickly decreases with the PH. A similar ion-exchange behaviour was reported for maleic acid*‘, which has a similar arrangement of carboxylic groups. In Fig. 4 certain limits are also given for log k’ of the phthalate system peak. Chromatograms of the system peaks at different eluent pH values are also shown in Fig. 5. At a given eluent pH the system peak can be either positive or negative, depending on the pH of the injected sample solution. When the sample pH is lower than that of the eluent a positive system peak (increased absorbance) is induced, whereas when the sample pH is higher the peaks are reversed. This effect can be accounted for by the higher absorbance of phthalate at lower pH valuesz8. As a result the height of the phthalate system peak depends on the pH difference between the eluent and the injected sample solution (the numbers in Fig. 5 at
216
a
E. PAPP
_ Eluent pti 3.4 t
., I t
5 /
4.8 5
I, ,
4.7
r.9 f . + 4 2\3
5ml ‘5.9
_8.5 4 !
\
t
3 4nll
6.5
Fig. 5. chromatogr?msof the phthalatesystem peaks. Conditions as in Fig. 4.
the system peaks represent these pH differences). Similar characteristics of the phthalate system peak were found in other experimentsB. The different retentions of the positive and negative system peaks at a given eluent pH are clearly demonstrated in Fig. 5. Between eluent pH 3.1 and 6.0 the retention of the negative system peak is higher than that of the positive one. This order is reversed at pH 7, while at eluent pH 8.5 two oppositely oriented system peaks were obtained with an almost identical retention that was very close to the dead volume. Alternatively a relaxation process might be responsible for this behaviour. The injection of a sample solution having a pH different from that of the eluent causes a local pH change in the sample plug compared to the bulk mobile phase, which influences the distribution equilibrium of phthalate. For example, the injection of sample (pH 9.5) into the eluent (pH 3.1) increases the pH at the top of the column and results in an higher concentration of HP-, which has higher retention (cJ, the rising portion of the retention curve shown in Fig. 4). Injection of the same sample (pH 9.5) into an eluent with pH 7 increases the concentration of P2-, which has lower retention. Consequently, the retention of the negative system peak is lower
SYSTEM PEAKS IN ION CHROMATOGRAPHY
I
217
ml
d
Fig. 6. Chromatograms of NO; annd SO:-. Conditions as in Fig. 4.
than that of the positive one in the latter case (cJ, the decreasing part of the retention curve in Fig. 4). In Fig. 4 the log k’ data for some solute anions significantly increases when the eluent pH is decreased below 5. This is connected with the decreasing concentration of HP- (counter ion in the ion-exchange equilibrium) with decreasing pH. The column efficiency also varies with the eluent pH, as demonstrated by the chromatograms of NO; (solidbne) and SOi- (dotted line) shown in Fig. 6. At eluent
218
E. PAPP
0
’ tim~8(min~z
Fig. 7. Chromatogram of inorganic anions. Mobile phase: 24% methanol, 2 m&f aqueous potassium hydrogenphthalate, 2 mM Tris (PH 7). Stationary phase: PRP-I coated with cetrimide. Eluent flow-rate: 0.5 ml/mm: Sample size: 10 pl containing about 0.007 pmol of each solute. UV absorption at 290 nm.
pH 3.1 the injection of NO; results in two negative and one positive peak, while the injection of SOi- results in one peak (beside the system peak of phthalate). At higher eluent pH, the anions give negative peaks, with narrower widths. An increase in the peak area of SO:- relative to that of NO; (identical concentration injected) is also observed. When the eluent pH is increased to 8.5 the solute peaks become broader again. The peak area of SOi- is about twice that of NO;, demon-
I
-3.0
-2.i
-2.5 lo+.+
Fig. 8. Retention of inorganic anions and the system peak (SW-) as a function of the potassium hydrogenphthalate concentration. Mobile phase: 24% methanol, 14 mA4 potassium hydrogen phthalate, 14 mM Tris (pH 7). Stationary phase: PRP-1 coated with cetrimide.
SYSTEM PEAKS IN ION CHROMATOGRAPHY
219
strating that one doubly charged SOi- is equivalent with one doubly charged P2in the ion-exchange equilibrium and one moleNOi to a half mole P2- ion. Accordingly, mobile phases with pH 5-7 are best suited to the separation of common anions (F-, Cl-, Br-, NO;, NO;, SO:-) in this chromatographic system (Fig. 7). Alternatively, the retention of these anions can be influenced by varying the concentration of potassium hydrogenphthalate, at a constant eluent pH. The retentions of both the solute and the system peaks (phthalate) decrease with increasing phthalate concentration as demonstrated in Fig. 8. This effect should be taken into account for the efficient optimization of this eluent system. CONCLUSIONS
A systematic study of the induced peak patterns can reveal interesting details of column processes in ion chromatography. When the aqueous-organic eluent three additives (cetyltrimethylammonium bromide, sodium acetate and potassium hydrogenphthalate), more system peaks (Sacetate,%romide, f$Ualate ) may appear in the chromatogram. This behaviour can be accounted for by the competition between the injected solute anions and the anions of the mobile phase additives (buffer, surfactant), which results in positive induced peaks (with given retention) of the respective eluent anions. In mobile phases prepared with tris(hydroxymethyl)aminomethane and potassium hydrogenphthalate, only one system peak (Sphthalate)is observed. The dependence of the capacity factor of the phthalate system peak as a function of the eluent pH exhibits a maximum. This behaviour can be rationalized in terms of the stronger bonding of the hydrogenphthalate anion relative to the phthalate anion to the positively charged groups of the adsorbed cetrimide. The phthalate system peak can be either positive or negative depending on the relative pH difference between the eluent and the injected sample solution. This effect can be explained by the difference in the absorption coefficients of P2- and HP-, and the dependence of the concentration ratio of these two forms of phthalate on the local pH in the sample zone. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
S. Levin and E. Grushka, Anal. Chem., 58 (1986) 1602. A. Sokolowski, Chrumurogruphia, 22 (1986) 177. W. E. Barber and P. W. Cat-r,/. Chrumalugr., 260 (1983) 89. W. E. Barber and P. W. Carr, J. Chruma~ugr.,301 (1984) 25. W. E. Barber and P. W. Carr, i. Chromutogr., 316 (1984) 211. W. E. I-I-, C. N. M. Anssemsand M. Janssen, J. Chromurugr.,360 (1986) 1. T. Okada and T. Kuwamoto, Anal. Chem., 56 (1984) 2073. P. E. Jackson and P. R. Haddad, J_ C/wm/~., 346 (1985) 125. R. N. Reeve, J. Chrumutogr., I77 (1979) 393. J. P. de Kleijn, Andysf (Luzdat), lo7 (1982) 223. B. B. Wheals, J. Chromufqr., 262 (1983) 61. F. G. P. Mullins and G. F. Kirkbright, Analyst (London), 109 (1984) 1217. I. W. Burns, Anal. Pruc., 21 (1984) 200. D. R. Jenke, Anal. Chum., 56 (1984) 2468. D. R. Jenke, Anal. Chem., 56 (1984) 2674.
220 16 17 18 19 20 21 22 23 24 25 26 27 28
E. PAPP
H. J. Cartes and T. S. Stevens, J. Chromarogr., 295 (1984) 269. M. Cooke, J. High Resoluf. Chromatogr. Chromatogr. Commun., 7 (1984) 515. G. Domazetis, Chromatographia, 18 (1984) 383. P. J. Naish, Analyst (London), 109 (1984) 809. D. R. Jenke and N. Raghavan, J. Chromatogr. Sci., 23 (1985) 75. T. Jupille, Am. Lab. (Fairfield Corm.), 18 (1986) 114. A. Bartha and Gy. Vigh, J. Chromatogr., 260 (1983) 337. Z. Iskandarani and D. J. Pietrzyk, Anal. Chem., 54 (1982) 2427. P. L. T. Mussini and S. Rondinini, Anal. Chem., 58 (1986) 2290. G. K. Pagenkopf, Introduction to Water Chemistry, Marcel Dekker, New York, 1978. B. Chu, D. C. Whitney and R. M. Diamond, J. Znorg. Nucl. Chem., 24 (1962) 1405. A. Marton and J. In&dy, J. Chromatogr., 102 (1979) 165. C. A. Hordijk, C. P. C. M. Hagenaars and Th. E. Cappenberg, J. Microbial. methods, 2 (1984) 49.
SYSTEM PEAKS IN ION CHROMATOGRAPHY USING CETRIMIDE-COATED COLUMNS AND INDIRECT PHOTOMETRIC DETECTION
E. PAPP Institute for Analytical Chemistry. University of Chemical Engineering, Veszpdm (Hungary) (Received March 18th, 1987)
SUMMARY
The characteristics of the system peaks in an ion-chromatographic system were studied using non-polar stationary phases (RP- 18, PRP- 1) coated with cetrimide, and using aqueous eluents containing 24% methanol, potassium hydrogenphthalate (suitable for indirect UV detection) and other buffer constituents. One or more system peaks were observed depending on the mobile phase composition.
INTRODUCTION
In order to detect non-UV-absorbing solutes, eluents containing I-IV-active constituents are often used in single column ion chromatography. Beside the solute peaks, one or more “system peaks” also appear in the chromatogram obtained by this indirect UV detection method. System peaks eluted near to solute peaks may disturb the evaluation of the chromatograms. On the other hand, they may provide useful information on the thermodynamics and kinetics of the equilibria1p2 and enhance the detection sensitivity under certain conditions. Barber and Carr3-5 and Hammers et ~1.~studied the role of system peaks and their retention mechanism in reversed-phase ion-interaction chromatography, where inorganic sample anions were detected by using UV-active ion-interaction agents. System peaks arising in indirect W detection or conductometric detection on low capacity anion-exchange columns have also been investigated7q8. Successful separations of anions were also reported with cyano-bonded silicapJo or octadecyl silica’ l-r3 columns coated dynamically or permanently with a long-chain alkyltrimethylammonium salt (Cetrimide). For the indirect UV detection of anions in single column ion chromatography, potassium hydrogenphthalate was applied over a broad pH range (with other buffers) because of its high UV absorbance and ability to replace singly and doubly charged anions14-21. However, this eluent system exhibits unexplained system peaks under some eluent conditions. In this study the origin of system peaks in eluents containing potassium hydrogenphthalate and other buffer components has been investigated using two nonpolar stationary phases, hydrocarbon phase chemically bonded to silica gel, RP-18, 0021-9673/87/%03.50
0
1987 Elsevier Science Publishers B.V.
212
and poly(styrene-divinylbenzene) monium ion.
E. PAPP
copolymer, PRP- 1, coated with cetyltrimethylam-
EXPERIMENTAL
The chromatographic system used consisted of two Model 6000A pumps, a Model OE-308 variable-wavelength UV detector (Labor, MIM, Hungary), a differential refractometer detector (Varian, Palo Alto, CA, USA.), two six-port injection valves (Model 7010, Rheodyne, lo-,ul loop; Valco N60) and a dual-channel recorder (M 4220; Knauer, F.R.G.). The analytical columns contained octadecyl silica (LiChrosorb RP- 18, Merck), 120 mm x 4 mm, 5 pm, and poly(styrene-divinylbenzene) copolymer (prepacked, Hamilton, PRP-l), 150 mm x 4.1 mm, 5 pm. To avoid rapid degradation of the RP-18 column, eluents of pH > 7 were directed first to a precolumn (100 mm x 3 mm) situated between the pump and the injector, containing LiChrosorb Si 60. The columns were dynamically coated with cetyltrimethylammonium bromide (Cetrimide). The quantity of cetrimide adsorbed, was determined according to the breakthrough curves recorded by a refractive index (RI) detector**. The column temperature was maintained at 25 f 0.5”C by use of a water-bath (MLW, GDR Type UlO) and a water-jacket. To remove the adsorbed cetrimide, the columns were washed with eluents containing 70% methanol and 0.1 M potassium bromide. The eluents were prepared as described earlier** with a constant, 24%, methanol concentration. The pH of the eluents was measured with an OP-208 precision digital pH meter (Radelkis, Hungary) and a combined glass electrode calibrated with aqueous buffers pH 4.0 and 7.0. Standard sample solutions were prepared from AnalaR grade salts dissolved in de-ionized water to 0.01 M. RESULTS AND DISCUSSION
First, the column of RP-18 was dynamically coated by cetrimide from an eluent containing 24% methanol, 3 mM potassium hydrogenphthalate, 10 n&f sodium acetate and 1 mM eetrimide (CTABr). The amount of cetrimide adsorbed, i.e., the ion exchange cap.acity, was calculated from the breakthrough curve to be 0.31 mM per column. Solute ions were injected on the equilibrated column and were detected by indirect UV detection. Fig. 1 shows that the injection of the buffer constituent acetate results in a negative peak (decreasing absorption) the area of which is proportional to the acetate concentration. When the injected solute ionss, e.g., Cl-, NO;, Br- have a stronger interaction with the adsorbed quaternary ammonium ions than has acetate, a positive system peak (increasing absorption) at the retention of acetate (Scn--) appears in the chromatogram. In the chromatogram of sample ions having stronger binding than bromide ion, beside the solute peaks, two positive system peaks are produced. The retention times of the first and second system peaks are identical to that of acetate and bromide (Sat-), respectively.. Fig. 2 shows a typical chromatogram obtained by the injection of nitrate ion. The area of the solute peak is approximately equal to the sum of the
213
SYSTEM PEAKS IN ION CHROMATOGRAPHY
+
I
1 . 2
5
1
o.oozAU
time (win)
.
, time
, 8
.
.
(min)
Fig. 1. Chromatogram showing the solute peaks and the acetate system peak (&a,-). Mobile phase: 24% methanol in aqueous buffer containing 1m&4cetrimide, 3 mM potassium hydrogenphthalate (KHP), 10 mM sodium acetate @H 5.8). Stationary phase: LiChrosorb RP-18 dynamically coated with cetrimide (0.31 mM cetrimide per column). UV absorption at 300 nm. Fig. 2. Chromatogram Fig. 1.
of NO; showing the induced system peaks &s-
and Ssl-. Conditions as in
areas of the two system peaks,. However, no correlation can be sought between the solute and system peak areas when the retentions of the solute ions are larger, e.g., for SOi- the area of the Sal- peak decreased. In Fig. 3, log k’ for the solute and system peaks are plotted against the logarithm of the concentration of potassium hydrogenphthalate (KHP) in the ehtent. The linear relationship observed suggests an ion-exchange retention mechanism involving competition between the anions of the solutes and of the buffer constituents. Both acetate’ 7 and bromidez3 system peaks were observed in similar systems, where the eluents contained one or the other ion. When mobile phase without CTABr vas applied (relying only on the permanently adsorbed cetrimide) the system peak of bromide (SB~-)w&sno longer observed. The retention times of the solute anions and the selectivity of the separation also decreased. In this eluent the positive acetate system peak can still disturb sample peaks eluted nearby. To avoid this, potassium hydrogenphthalate itself or an other non-UV-absorbing buffer can be used to vary the pi-I in the pobile phase. However, a system peak is also observed when the eiuent contains only KHP. Jackson and Haddad* studied the properties of the phthalatq system pea4 (SW) on fixed anion exchangers. They proposed that the origin of SKH~could be accounted for by the adsorption of the phthalate in its neutral form on the stationary phase. This would result in the absence of this system peak when the elwnt pH exceeds 6.5. On the contrary, they found the log k’ vs. log CKHPrelationship to be linear for the
214
E. PAPI’
pH
5.8
-2.5
-2.4
-1.3
-2.2
tOtJ
CKHP (M 1
Fig. 3. Retention of inorganic anions and system peaks as a function of the potassium hydrogenphthalate concentration. k’ = Capacity factor. Conditions as in Fig. 1.
system peak at pH 5.5, which strongly suggests an ion-exchange mechanism. This contradiction led me to make a detailed examination of the phthalate system peak behaviour as a function of the eluent pH. The eluent pH was varied by use of tris(hydroxymethyl)aminomethane (Tris). Because of the unfavourable dissolution of the base silica material above pH 7, the polymeric phase PRP-1 was used in this experiment instead of RP-18. In order to establish a similar ion-exchange capacity for this more hydrophobic packing, the cetrimide concentration in the coating eluent was decreased to 0.2 mM, which resulted in a capacity of 0.21 m&f cetrimide per column. In Fig. 4, log k’ for the phthalate system peak is plotted against the eluent pH. From pH 3.1 (2 m&f phthalic acid) to pH 7 the retention increases and above this value it steeply decreases. For fixed anion exchangers, Jackson and Haddads at pH 3-6, JupilleZ* at pH 2-5 reported similar behaviour of the lower part of this curve. It should be noted that the pH of the present eluent is about 0.5 unit higher compared to the aqueous eluent because of the presence of 24% methanolz4. As can be seen from Fig. 4, the increasing part of the log k’ vs. pH function for the phthalate system peak is in the pH range where the eluent contains singly charged phthalate anions (pK, = 3.1, p& = 5.49. The tendency of HP- to form intramolecular hydrogen bonding may influence its ion-exchange behaviour. This effect decreases the hydration of the hydrogenphthalate ion, which results in its repulsion from the hydrogen-bonded bulk water phase into the ion-exchanger phase, where water is less structuredz6.
215
SYSTEM PEAKS IN ION CHROMATOGRAPHY
4.5
loqk
1.0
0.5
0
3.4 TRIS
-
4.8 5.3 6
7
1.9
a3
-0.51
2
4
8
PH
(mM)
Fig. 4. Retention of inorganic anions and the system peak (Sap-) as a function of the eluent pH. Mobile phase: 24% methanol in 2 mM aqueous phthalate; pH varied with Tris. Stationary phase: PRP-1 coated with cetrimide (0.21 mM cetrimide per column).
If the pH of the eluent is increased above 7 then highly hydrated doubly charged phthalate anions are present in the eluent and this hydration outweighs the previous retention-enhancing process and the retention quickly decreases with the PH. A similar ion-exchange behaviour was reported for maleic acid*‘, which has a similar arrangement of carboxylic groups. In Fig. 4 certain limits are also given for log k’ of the phthalate system peak. Chromatograms of the system peaks at different eluent pH values are also shown in Fig. 5. At a given eluent pH the system peak can be either positive or negative, depending on the pH of the injected sample solution. When the sample pH is lower than that of the eluent a positive system peak (increased absorbance) is induced, whereas when the sample pH is higher the peaks are reversed. This effect can be accounted for by the higher absorbance of phthalate at lower pH valuesz8. As a result the height of the phthalate system peak depends on the pH difference between the eluent and the injected sample solution (the numbers in Fig. 5 at
216
a
E. PAPP
_ Eluent pti 3.4 t
., I t
5 /
4.8 5
I, ,
4.7
r.9 f . + 4 2\3
5ml ‘5.9
_8.5 4 !
\
t
3 4nll
6.5
Fig. 5. chromatogr?msof the phthalatesystem peaks. Conditions as in Fig. 4.
the system peaks represent these pH differences). Similar characteristics of the phthalate system peak were found in other experimentsB. The different retentions of the positive and negative system peaks at a given eluent pH are clearly demonstrated in Fig. 5. Between eluent pH 3.1 and 6.0 the retention of the negative system peak is higher than that of the positive one. This order is reversed at pH 7, while at eluent pH 8.5 two oppositely oriented system peaks were obtained with an almost identical retention that was very close to the dead volume. Alternatively a relaxation process might be responsible for this behaviour. The injection of a sample solution having a pH different from that of the eluent causes a local pH change in the sample plug compared to the bulk mobile phase, which influences the distribution equilibrium of phthalate. For example, the injection of sample (pH 9.5) into the eluent (pH 3.1) increases the pH at the top of the column and results in an higher concentration of HP-, which has higher retention (cJ, the rising portion of the retention curve shown in Fig. 4). Injection of the same sample (pH 9.5) into an eluent with pH 7 increases the concentration of P2-, which has lower retention. Consequently, the retention of the negative system peak is lower
SYSTEM PEAKS IN ION CHROMATOGRAPHY
I
217
ml
d
Fig. 6. Chromatograms of NO; annd SO:-. Conditions as in Fig. 4.
than that of the positive one in the latter case (cJ, the decreasing part of the retention curve in Fig. 4). In Fig. 4 the log k’ data for some solute anions significantly increases when the eluent pH is decreased below 5. This is connected with the decreasing concentration of HP- (counter ion in the ion-exchange equilibrium) with decreasing pH. The column efficiency also varies with the eluent pH, as demonstrated by the chromatograms of NO; (solidbne) and SOi- (dotted line) shown in Fig. 6. At eluent
218
E. PAPP
0
’ tim~8(min~z
Fig. 7. Chromatogram of inorganic anions. Mobile phase: 24% methanol, 2 m&f aqueous potassium hydrogenphthalate, 2 mM Tris (PH 7). Stationary phase: PRP-I coated with cetrimide. Eluent flow-rate: 0.5 ml/mm: Sample size: 10 pl containing about 0.007 pmol of each solute. UV absorption at 290 nm.
pH 3.1 the injection of NO; results in two negative and one positive peak, while the injection of SOi- results in one peak (beside the system peak of phthalate). At higher eluent pH, the anions give negative peaks, with narrower widths. An increase in the peak area of SO:- relative to that of NO; (identical concentration injected) is also observed. When the eluent pH is increased to 8.5 the solute peaks become broader again. The peak area of SOi- is about twice that of NO;, demon-
I
-3.0
-2.i
-2.5 lo+.+
Fig. 8. Retention of inorganic anions and the system peak (SW-) as a function of the potassium hydrogenphthalate concentration. Mobile phase: 24% methanol, 14 mA4 potassium hydrogen phthalate, 14 mM Tris (pH 7). Stationary phase: PRP-1 coated with cetrimide.
SYSTEM PEAKS IN ION CHROMATOGRAPHY
219
strating that one doubly charged SOi- is equivalent with one doubly charged P2in the ion-exchange equilibrium and one moleNOi to a half mole P2- ion. Accordingly, mobile phases with pH 5-7 are best suited to the separation of common anions (F-, Cl-, Br-, NO;, NO;, SO:-) in this chromatographic system (Fig. 7). Alternatively, the retention of these anions can be influenced by varying the concentration of potassium hydrogenphthalate, at a constant eluent pH. The retentions of both the solute and the system peaks (phthalate) decrease with increasing phthalate concentration as demonstrated in Fig. 8. This effect should be taken into account for the efficient optimization of this eluent system. CONCLUSIONS
A systematic study of the induced peak patterns can reveal interesting details of column processes in ion chromatography. When the aqueous-organic eluent three additives (cetyltrimethylammonium bromide, sodium acetate and potassium hydrogenphthalate), more system peaks (Sacetate,%romide, f$Ualate ) may appear in the chromatogram. This behaviour can be accounted for by the competition between the injected solute anions and the anions of the mobile phase additives (buffer, surfactant), which results in positive induced peaks (with given retention) of the respective eluent anions. In mobile phases prepared with tris(hydroxymethyl)aminomethane and potassium hydrogenphthalate, only one system peak (Sphthalate)is observed. The dependence of the capacity factor of the phthalate system peak as a function of the eluent pH exhibits a maximum. This behaviour can be rationalized in terms of the stronger bonding of the hydrogenphthalate anion relative to the phthalate anion to the positively charged groups of the adsorbed cetrimide. The phthalate system peak can be either positive or negative depending on the relative pH difference between the eluent and the injected sample solution. This effect can be explained by the difference in the absorption coefficients of P2- and HP-, and the dependence of the concentration ratio of these two forms of phthalate on the local pH in the sample zone. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
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220 16 17 18 19 20 21 22 23 24 25 26 27 28
E. PAPP
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