Anal. Chem. 1997, 69, 3333-3338
Cyanuric Acid-Based Eluent for Suppressed Anion Chromatography V. Maurino and C. Minero*
Dipartimento di Chimica Analitica, Universita` di Torino via Pietro Giuria 5, 10125 Torino, Italy
The performance of cyanuric acid (CA, in the 1,3,5triazine-2,4,6-trihydroxy form) as eluent for suppressed anion chromatography has been investigated. The distinctive features of this eluent are reported with respect to the others that are now routinely utilized. CA-based eluents for SAC have several of advantages over carbonate and tetraborate buffers: (i) wider range of eluent strength, (ii) lower background conductivities, (iii) good calibration linearities, (iv) improved sensitivity for carboxylic acids, (v) the possibility to perform gradient analysis with limited baseline drifts, and (vi) ease of eluent purification. This makes the CA-based eluent well-suited for the analysis of strongly retained anions like chromate, thiosulfate, iodide, thiocyanate, and perchlorate with good limits of detection and reasonable capacity factors. Suppressed anion chromatography (SAC)1 is generally recognized as a prominent technique for anion analysis at trace and ultratrace levels. Isocratic elution, especially with carbonate/ hydrogen carbonate buffers, is widely used, as documented by numerous papers already published.2 Since its discovery, major improvements have been made in column technology,3 membrane4 and electrodialytical5,6 suppressors, and system hardware. The research carried out during the last few years has focused on electrodialytical eluent generation6,7 and quantification of anions of very weak acids through two-dimensional conductivity detection.8 Eluents commonly used in SAC are few. Besides carbonate/ hydrogen carbonate, only hydroxide and tetraborate eluents have found widespread use, while zwitterionic buffers, like aminoalkanesulfonates9 and amino acids,10 ethylenediaminediacetic acid (EDDA),11 phenol,1 and cyanophenol,12 have not gained popularity. Despite its common use, the carbonate/hydrogen carbonate buffer suffers from various drawbacks. (i) The suppressed (1) Small, H.; Stevens, T. S.; Baumann, W. C. Anal. Chem. 1975, 47, 18011809. (2) Dasgupta, P. K. In Ion Chromatography; Tarter, J. G., Ed.; Marcel Dekker: New York, 1987; pp 191-367. (3) Stillian, J. R.; Pohl, C. A. J. Chromatogr. 1990, 499, 249-266. (4) Stevens, T. S.; Davis, J. C.; Small, H. Anal. Chem. 1981, 53, 1488-1492. (5) Strong, D. L.; Dasgupta, P. K. Anal. Chem. 1989, 63, 939-945. (6) Strong, D. L.; Joumg, C. U.; Dasgupta, P. K. J. Chromatogr. 1991, 546, 159-173. (7) Strong, D. L.; Dasgupta, P. K.; Friedman, K.; Stillian, J. R. Anal. Chem. 1991, 63, 480-486. (8) Sjo ¨rgen, A.; Dasgupta, P. K. Anal. Chem. 1995, 67, 2110-2118. (9) Irgum, K. Anal. Chem. 1987, 59, 358-362 (10) Zolotov, Y. A.; Shpigun, O. A.; Pazukhina, Y. E.; Voloshik, I. N. Int. J. Environ. Anal. Chem. 1987, 31, 99-105. (11) Sato, H.; Miyanaga, A. Anal. Chem. 1989, 61, 122-125. (12) 1994-95 Dionex Product Selection Guide; Dionex Corp.: Sunnyvale, CA, 1994; p 55. S0003-2700(97)00205-9 CCC: $14.00
© 1997 American Chemical Society
conductivity is relatively high (12-15 µS cm-1 for a total concentration of 3 mM), largely due to the dissociation of carbonic acid formed in the suppressor. This precludes the use of carbonate/hydrogen carbonate-based eluents in gradient elution. (ii) The analytical response is non-linear, in particular when considering weak acid anions.13,14 (iii) High background conductivities cause high levels of baseline noise, which negatively affect the limit of detection. The hydroxide ion could give, at least theoretically, the lowest background conductance.6,7 It is unquestionably better for gradient SAC. However, OH- is a very weak eluent, and relatively high concentrations of OH- (100-150 mM) are required in order to elute multiply charged anions, like sulfate, phosphate, and oxalate, or hydrophobic anions, like iodide, thiocyanate, and perchlorate. Moreover, commercial NaOH/KOH solutions contain varying amounts of dissolved carbonate and/or other anionic impurities that can affect the retention behavior and the background conductivity. Anionic impurities have very adverse effects in gradient elutions. They concentrate on the analytical column in the initial part of the analysis and then elute as interfering peaks at larger eluent concentrations. This problem could be partially solved by using an anion trap column before the injector. Ultralow background conductivities were also achieved through electrodialytic eluent (OH-) production (EEP),6,7 coupled with an electrodialytic membrane suppressor. In addition, EEP generators have the possibility for electrochemical gradient generation. However, they are not yet commercially available, and the very high eluent concentrations required to elute hydrophobic anions such as CNS-, I-, and ClO4- on conventional SAC columns are rather difficult to generate and suppress, even with these systems. The tetraborate buffer has gained acceptance as an eluent for separation of weakly retained, small aliphatic acids and inorganic anions. The background conductance is lower than that with the carbonate/hydrogen carbonate eluent, typically 2-4 µS cm-1 for 5 mM concentration, in agreement with the pKa ) 9.21 of boric acid. Gradient elution leads to a less pronounced rise in the background conductivity (about 8 µS cm-1 under the typical conditions of 5-30 mM tetraborate). The major drawback is the low eluent strength: typically, a concentration of 30 mM (as tetraborate) is required in order to elute multiply charged anions on general purpose SAC columns. Phenol-based eluents have been proposed in the pioneering work of Small and co-workers.1 These show good eluent strength but have not gained widespread use due to their instability and toxicity. Zwitterionic eluents, proposed for their low suppressed (13) Doury-Berthod, M.; Giampaoli, P.; Pitsch, H.; Sella, C.; Poitrenaud, C. Anal. Chem. 1985, 57, 2257-2263. (14) Midgely, D.; Parker, R. L. Talanta 1989, 36, 1277-1283.
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conductivities, are weaker than hydroxide.9 Thus, strongly retained anions, like I-, CNS-, S2O32-, and ClO4-, require specially designed columns, in association with eluent containing organic solvents and cyanophenate.12,15 Perchlorate anions have been determined through nonsuppressed ion chromatography with large retention volumes and low sensivity16 or through suppressed ion chromatography17 using nonstandard SAC columns. In this work, we report on the use of cyanurate (CA) as eluent in SAC. CA is an interesting and versatile alternative to tetraborate and carbonate/hydrogen carbonate eluents, because (i) the eluent strength can be varied over a wide range, enabling the gradient elution of ions from F- to ClO4- on general purpose SAC columns, (ii) the background conductivities are lower than those with the carbonate/hydrogen carbonate eluent, (iii) CA features an excellent chemical stability and low cost, and (iv) it is very easy to obtain CA free from ionic impurities. EXPERIMENTAL SECTION Materials and Reagents. Cyanuric acid (Aldrich, 98%) was recrystallized as monoclinic crystals from hot water and dried at 110 °C to constant weight. Water was treated with a reverse osmosis system (Milli-RO, Millipore) and purified by a Milli-Q apparatus (Millipore) equipped with a 0.45 µm Millistack filter at the outlet. Anion stock solutions (1000-2000 ppm) were prepared by dissolution of the corresponding sodium salts (Aldrich or Merck, at least analytical grade). NaOH (50%) was prepared from analytical grade NaOH pellets (Merck) and filtered (0.45 µm Durapore membrane, Millipore) under N2. Instrumentation. The chromatographic system was a Dionex apparatus composed of a pressurized (He) eluent organizer, a lowpressure gradient mixer, a GP40 gradient pump, a high-pressure gradient mixer, a 25 µL Rheodyne injector, a 25 cm length × 4 mm i.d. Dionex AS4-SC anion separator column, a Dionex AMMS-1 electrodialytic suppressor, and a Dionex ED40 detector equipped with a temperature-compensated conductivity cell. The entire system and the data acquisition were computer controlled through Peaknet software ver. 4.11 (Dionex). The column void volume was determined as the retention volume of the water dip and was corrected for connections and suppressor dead volumes. The void volume and capacity were 1.15 mL and 20 µequiv, respectively. All elutions have been carried out at 1 mL min-1 flow rate. The suppressor was used in the autosuppression recycle mode. We did not test if background conductivities were lower when using the autosuppression external water mode, where some eluent components could permeate through the suppressor membrane. The measurement of pH was carried out at 25 °C using a Metrohm pH-meter (Model 713) equipped with a combined glass electrode, calibrated against two standard buffers (pH 4.00 and 7.00). The background conductivities of the eluents after suppression were measured using the temperature-compensated conductivity cell of the ED40 ionic chromatographic detector, after cell calibration with 1 × 10-3 M KCl, performed following the instructions of Dionex. (15) Brown, P. D.; Morra, M. S. J. Agric. Food Chem. 1991, 39, 1226-1228. (16) Stahl, R. Chromatographia 1993, 37, 300-302. (17) Batjoens, P.; De Brabander, H. F.; T’Kindt, L. Anal. Chim. Acta 1993, 275, 335-340.
3334 Analytical Chemistry, Vol. 69, No. 16, August 15, 1997
Figure 1. Calculated suppressed conductivity as a function of the first pKa of the weak acid used as eluent (λH+ ) 349.65 S cm2 mol-1, λOH- ) 198.3 S cm2 mol-1, λA- ) 32 S cm2 mol-1). Values of pKa for carbonic and cyanuric acids are indicated along with the resulting conductivity.
RESULTS AND DISCUSSION Background Conductance. The background conductance, resulting from the complete suppression of the conjugate base of a weak acid used as eluent, depends on the pKa of the acid, its concentration (H+ formed after acid dissociation), and the anion equivalent conductance. Figure 1 reports calculated background conductivities as a function of the eluent pKa, at various eluent concentrations, assuming for the equivalent conductance of the conjugate base a value similar to that of a large organic anion such as benzoate. The calculation shows that an increase of 1 pKa unit will lead to a roughly 3-fold reduction in the background conductance at constant total eluent concentration. However, the acid-base equilibrium of the weak acid sets an upper limit on the useful range of pKa. The use of bases conjugated with weak acids having pKa g 12 requires OH- concentrations of at least 10-2 M, irrespective of the concentration of the eluent. At concentrations lower than 0.01 M, the eluent is actually OH- and not the conjugate base of the weak acid. In this case, anionic impurities present in NaOH make it very difficult to obtain background conductivities lower than 1 µS cm-1 (without using EEP devices).7 For these reasons, it is not advantageous to choose weak acids with pKa > 10. The class of hydroxylated heterocyclic compounds containing nitrogen includes several examples of weak acids with the pKa in the required range. Uracyl could be a good selection (pKa ) 9.45). However, the monodissociated form is a very weak eluent (k′ ) 1.95, 4.68, and 23.8 for Cl-, Br-, and I-, respectively, using 5 mM uracyl + 5 mM NaOH as eluent), and the second dissociation step (pKa > 14) precludes the possibility for elution with the doubly charged anion. Besides the desired pKa, the conjugated bases of the weak acids have to have reasonable eluent strength, good chemical stability, ease of purification, and low cost. CA is a triprotic acid with pKa1 ) 7.2, pKa2 ) 11.1, and pKa3 > 14.18 It can be considered diprotic for the usual range of pH used in SAC. The species H2CA- and HCA2- will be referred to as hydrogen cyanurate and cyanurate, respectively. CA has excellent chemical stability toward oxidation,19 is very easy to purify from ionic impurities (recrystallization from hot, high-purity water), and (18) The Merck Index, 11th ed.; Merck & Co.: Rahway, NJ, 1989.
Figure 3. Relationship between measured and calculated capacity factors according to the multiple species eluent model21 (slope ) 1.044, correlation coefficient ) 0.984 for 151 data pairs).
Figure 2. Typical chromatograms with cyanuric acid as eluent. (A) NaOH, 5 mM; CA, 2.5 mM; flow, 1 mL min-1. (B) NaOH, 10 mM; CA 5 mM. Anion concentrations (µM): Cl-, 4.2 (0.15 mg L-1); NO3-, 5.3 (0.33 mg L-1); SO42-, 5.2 (0.50 mg L-1); I-, 12.5 (1.60 mg L-1); HPO42-, 10.5 (1.00 mg L-1); HAsO42-, 10.8 (1.50 mg L-1). Table 1. Measured Eluent Background Conductivities and pH with Cyanurate and Carbonate Eluents after Suppression
eluent
concn (mM)
background conductivity (µS cm-1)
pH
CA CA CA CA carbonate carbonate
2.5 5.0 7.5 10.0 5.0 10.0
5.2 7 9.5 11 19 27
4.85 4.70 4.65 4.55 4.28 4.12
manifests very low toxicity.20 The possibility of changing the prevailing species (from hydrogen cyanurate to cyanurate) makes the interval of eluent strengths very wide. From the pKa1 value, a 3-fold decrease in the suppressed background conductivity is expected with respect to that of the carbonate eluent at the same concentration. This was, indeed, experimentally observed (see Table 1). As a consequence, the baseline RMS noise is improved, leading to lower detection limits. Eluent Strength. Typical chromatograms of common anions are reported in Figure 2. Table 2 summarizes the measured capacity factors for this series of anions. The data reported show that, under the present experimental conditions, the doubly dissociated form of CA is stronger than carbonate. (19) Pelizzetti, E.; Maurino, V.; Minero, C.; Carlin, V.; Pramauro, E.; Zerbinati, O.; Tosato, M. L. Environ. Sci. Technol. 1990, 24, 1559-1565. (20) Canelli, E. Am. Soc. Public Health 1974, 64, 155-160.
Besides minimizing the time required for method development, models able to predict the retention behavior of analytes and evaluate the values of exchange constants (stationary/mobile phase) can help to compare the strength of CA eluent with respect to the common hydrogen carbonate/carbonate buffer. In the CAbased eluent, up to three species (OH- and mono- and disodium cyanurate) are present. Recently, a model for eluents consisting of multiple species was proposed.21 This model allows calculations for eluent and eluted anions that undergo several protonation equilibria under the frame of the stoichiometric models. Although more physically consistent, the approach based on the GouyChapman theory for the electrical double layer, complemented with the possibility for specific adsorption of the counterions and analytes ions to the chromatographic surface,22 exhibits the same linear dependence of the logarithmic capacity factor on the logarithmic counterion concentration provided by ion exchange models.23 The Stahlberg model22 shows that, in addition to specific interactions, the exchange constants depend on the surface charge density of the chromatographic surface, which in turn is dependent on the eluent concentration. For 1:1 electrolytes at concentrations below 30 mM, the surface charge density is constant. Under these conditions, the stoichiometric and the electrostatic/hydrophobic models are formally equivalent. However, besides the computational difficulties and the neglect of ion correlation effects for multiply charged ions in the Poisson-Boltzmann equation, a detailed theory for analytes subject to acid-base equilibria, accounting for electrostatic/specific interactions and changes of the distribution of conjugated species according to the variation of their pKa in the presence of charged interfaces,24,25 has not yet been presented. Table 3 reports the values of the global exchange constants (including electrostatic and specific interactions) with respect to OH- obtained from data of Table 2 through the stoichiometric exchange model given in ref 21. The exchange constants were gathered by using the steepest descent method26 through minimization at the same time of the sum on all the experimental data (21) Hajos, P.; Horvath, O.; Denke, V. Anal. Chem. 1995, 67, 434-441. (22) Stahlberg, J. Anal. Chem. 1994, 66, 440-449. (23) Haddad, P. R.; Jackson, P. E. Ion Chromatography; Elsevier: Amsterdam, 1990. (24) Minero, C.; Pelizzetti, E. Pure Appl. Chem. 1993, 65, 2573-2582 (25) Minero, C.; Pelizzetti, E. Adv. Colloid Interface Sci. 1992, 37, 319-334.
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Table 2. Measured Capacity Factors (k′) for Various Anions Using Cyanuric Acid-Based Eluentsa concn (mM)
pH
F-
Cl-
Br-
NO3-
2.5 2.5 2.5 2.5 5.0 5.0 5.0 5.0 7.5 7.5 7.5 10.0 10.0 10.0
9.04 10.72 11.04 11.51 9.09 10.82 11.25 11.71 10.89 11.38 11.9 10.9 11.47 12.03
0.47 0.35 0.25 0.21 0.26 0.19 0.14 0.13 0.15 0.13 0.10 0.13 0.10 0.12
1.42 0.98 0.83 0.61 0.80 0.58 0.44 0.39 0.44 0.36 0.32 0.37 0.30 0.30
3.48 2.37 2.05 1.49 1.96 1.43 1.11 0.97 1.08 0.88 0.74 0.92 0.72 0.72
3.91 2.74 2.29 1.72 2.20 1.63 1.29 1.09 1.22 1.00 0.86 1.03 0.78 0.80
5.0 10.0
10.95 11.12
0.54 0.39
1.35 1.02
1.55 1.19
a
HPO42-
CNS-
CrO42-
S2032-
ClO4-
Cyanurate Eluents 16.90 18.01 11.39 9.46 10.16 6.18 7.30 3.46 9.61 6.12 7.11 3.19 5.34 1.99 4.80 1.31 5.04 1.89 4.35 1.15 3.87 0.76 4.44 1.34 3.64 0.78 3.54 0.59
28.19 13.89 11.92 8.76 8.72 4.54 3.50 3.22 2.45 1.96 1.78 1.59 1.32 1.27
30.98 20.75 18.59 13.41 17.56 13.07 10.27 8.80 9.49 7.97 7.17 8.09 6.68 6.50
50.91 25.78 17.77 9.23 16.68 8.82 5.38 3.54 5.12 3.12 2.07 3.65 2.15 1.56
53.21 25.55 17.39 9.17 16.32 8.73 5.29 3.55 5.04 3.09 2.06 3.59 2.13 1.56
52.12 34.77 28.70 22.57 28.68 21.94 17.41 14.61 15.70 13.30 12.15 13.36 11.11 10.79
Carbonate Eluentsa 6.91 2.46 5.62 1.19
3.21 1.75
16.63 14.24
6.74 3.41
6.91 3.46
23.13 18.96
I-
Values for the carbonate eluent are reported for comparison.
Table 3. Exchange Constants Obtained by Fitting the Retention Data Reported in Table 2 to the Multiple Eluent Species Model21 a anion A HCyanCyan2HCO3CO32FClBrNO3ISO42HPO42PO43CNSCrO42S2O32ClO4a
SO42-
CA eluent KA/OH
error (%)
carbonate eluent KA/OH
error (%)
2.6 6.4 0.20 0.59 1.44 1.65 7.1 6.9 10 152 13 18 18 22
9.8 7.4 7.3 7.2 6.3 14 9 5.8 14 13 5.1
2.7 5 0.18 0.62 1.62 1.89 7.9 6.4 8.1 167 16.4 20 21 26.8
5 0.2 2.2 3.2 6.3 7.7 6.8 4.4 2.8 2.4 5.9
Exchange constants refer to the exchange with OH-.
of the squared percent relative deviations between the calculated and measured retention factors. This procedure is faster than simplex routines and, when a large number of parameters are involved, assures a fast convergence on the minimum around the starting-point values. The simultaneous best-fit search allows more reliable values to be obtained for the exchange constants of the eluent species. The calculated exchange constants allow retention factors to be estimated with good precision. As Table 3 reports, the mean percent errors are quite low for singly charged solutes. However, the prevision is less precise for doubly charged anions. For phosphate, the fit is better, since two parameters can be adjusted. The absolute values for the exchange constants are partially dependent on the starting values of the exchange constants of the eluent. With absolute minimization, the best values of solute (26) Vetterling, W. T.; Teukolsky, S. A.; Press, W. H. Numerical Recipes (FORTRAN); Cambridge University Press: Cambridge, UK, 1985.
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exchange constants with respect to those of hydroxyl compare well for cyanurate and carbonate eluents and roughly with results obtained with a different ion exchange column.21 The comparison between exchange constants obtained for CA and carbonate eluents shows that cyanurate is somewhat stronger than carbonate. However, as the eluent species are singly and doubly charged, the eluent exchange constant values may suffer the same model weakness exhibited for doubly charged solutes. In addition, the values for hydrogen carbonate and carbonate calculated here are far from those reported previously (0.71 and 9.70, respectively).21 Also, the exchange constants evaluated for solutes and those measured with OH- as eluent are markedly different. In this case, linear plots of the logarithm of the normalized retention time against the logarithm of the eluent concentration are obtained, and the exchange constant is proportional to the retention factor. Using 50 mM sodium hydroxide as eluent, we obtained 1.7, 4.4, 5.3, 25, 48, and 67 for the exchange constants of chloride, iodide, nitrate, thiocyanate, and thiosulfate anions, respectively. Thus, although the model is useful for predicting capacity factors, it needs some improvement, mainly in the treatment of hydrophobic interactions and in the management of doubly charged species and species subject to acid-base equilibria. It can provide only a relative comparison of exchange constants, as done above for quantifying the eluent strength. The absolute values of exchange constants must be considered with care. The solubility of CA sets an upper limit to the useful range of eluent concentrations. At ambient temperature, its solubility is 38 mM.18 This value is comparable to the saturation concentration of CO2 (33 mM at 25 °C and 1 atm).27 Then, the CA advantage over carbonate can be ascribed to either the slightly higher eluent strength or solubility. In addition, the lower background conductivities achieved by CA-based eluents allow the use larger eluent concentrations. This makes the cyanuric acid-based eluent well-suited for the analysis of strongly retained anions like chromate, thiosulfate, iodide, thiocyanate, and perchlorate. (27) Hammond, C. R. Handbook of Chemistry and Physics, 71st ed.; CRC Press: Boca Raton, FL, 1990; Section IV.
Figure 5. Analytical sensitivity as a function of the concentration of analytes. A linear calibration function results in a flat line. Conditions: CA, 2.5 mM; NaOH, 5 mM; except for acetate (2.5 mM CA, 2.5 mM NaOH, retention time 4.81 min) and arsenate (5 mM CA, 10 mM NaOH).
Figure 4. Gradient chromatograms with CA as eluent. (A) Concentration gradient (5 mM NaOH, 2.5 mM CA for 2 min, to 20 mM NaOH, 10 mM CA in 10 min). (B) pH gradient (7.5 mM NaOH, 5 mM CA for 5 min, to 20 mM NaOH, 5 mM CA in 12 min). The two runs are baseline subtracted (see the insets for baseline profile). Anion concentrations (µM): F-, 21 (0.40 mg L-1); Cl-, 17 (0.60 mg L-1); Br-, 12.5 (1.00 mg L-1); NO3-, 16 (1.00 mg L-1); SO42-, 26 (2.50 mg L-1); HPO42-, 52 (5.00 mg L-1); I-, 39 (5.00 mg L-1); CrO42-, 43 (5.00 mg L-1); HAsO42-, 72 (10.0 mg L-1); CNS-, 340 (20.0 mg L-1); ClO4-, 250 (25.0 mg L-1).
Gradient Elutions. As with other multiply charged weak acid anion eluents, CA offers a wide eluting power by varying both the concentration and the pH of the eluent. In both cases, baseline drifts are observed: concentration gradients lead to an S-shaped baseline, whereas pH gradients (at constant total eluent concentration) give a broad baseline peak roughly related to the “elution” of the singly charged eluent species by the doubly charged one. When baseline drifts are concerned, OH- is indisputably better. However, owing to its low eluent strength, relatively high concentrations of OH- (100-150 mM) are required in order to elute multiply charged anions. For example, hydroxide is not a good eluent with the Dionex AS4-SC columns used in this work. In this case, laboratory practice12 suggests to using either hydroxide-selective phases (like the Dionex AS11) on which hydroxide ion is a very powerful eluent, giving limited baseline shifts when properly used, or (tetra)borate gradients. Typically on an AS4-SC column, a gradient from 5 to 28 mM tetraborate will separate in 8 min fluoride, acetate, and formate ions while eluting sulfate ion at a retention volume of about 25 mL. The baseline rise is 8 µS cm-1. Figure 4 gives two examples of gradient chromatograms with CA as eluent. Both runs are baseline corrected (see the insets
for the baseline profiles). The reproducibility of the baseline is excellent: by subtracting two different baseline records obtained under the same set of conditions, the long-term drift is under 0.1 µS cm-1. This indicates that the eluent can be easily obtained free from ionic impurities. The total baseline rise for a concentration gradient from 2.5 to 10 mM total CA (Figure 4A) is 5.5 µS cm-1. Under these conditions, fluoride is distinct from water dip, whereas perchlorate is eluted at a retention volume less than 20 mL. A good separation of weakly retained anions requires only a lower starting eluent concentration (about 1.5 mM) or pH. Under the last condition, the baseline rise is under 8 µS cm-1. If elution of the strongly retained anions, such as thiocyanate or perchlorate, is not required, then the gradient could be stopped at lower cyanurate concentration; the elution of phosphate takes place when the gradient is at about 6 mM cyanurate, and the baseline rise is halved. The pH gradient under the conditions reported in Figure 4B leads to a baseline peak of 1.8 µS cm-1. Also in this case, the baseline change shows good reproducibility. The retention values are analogous to those recorded in the concentration gradient, except for HPO42-, which elutes earlier due to the lower initial eluent pH. Linearity of Calibration and Limits of Detection. Due to the dissociation of the weak acid eluent formed after chemical suppression, the pH of the suppressed eluent may be comparable to the pKa of carboxylic acid and other weak acid analytes. This negatively affects the detectability, the sensitivity, and the linearity of the calibration curves. When considering strong acid anions, nonlinearities in calibration are theoretically predicted and observed under a definite set of experimental conditions.13 The linearity of response in SAC with carbonate eluents at low sample concentrations has been a matter of some concern.13,14,28-31 The use of micromembrane suppressors at low carbonate concentra(28) Small, H. Ion Chromatography; Plenum: New York, 1990. (29) Tian, Z.; Hu, R.; Lin, H.; Hu, W. J. Chromatogr. 1988, 439, 151-157. (30) Polite, L. M.; McNair, H. M.; Rocklin, R. D. J. Liq. Chromatogr. 1987, 10, 829-838. (31) ACS Committee on Environmental Improvement. Anal. Chem. 1980, 52, 2242-2249.
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Table 4. Limit of Detections (at S/N ) 3, 25 µL Injection, flow 1 mL min-1) for Various Anions Obtained Using CA- or Carbonate-Based Eluents with Similar Eluting Power (Compare the Retention Times) on Dionex AS4-SC Columna CA eluent anion
carbonate eluent
DL DL tR DL DL tR (min) (µg L-1) (µM) (min) (µg L-1) (µM) DLratiob
Cl2.35 NO34.03 2SO4 8.51 I7.54 HPO425.43 HAsO4210.02 CH3COO- 5.06 ClO414.20 CNS9.08
5 10 20 60 90 100 30 1000 200
0.12 2.50 0.16 4.79 0.25 10.40 0.48 9.35 1.0 5.09 0.7 9.12 0.5 4.82 10 23.20 3.4 17.80
10 25 60 190 150 200 250 3600 750
0.28 0.40 0.62 1.5 1.7 1.4 4.2 36 13
2.0 2.5 3.0 3.2 1.7 2.0 8.3 3.6 3.7
a Conditions: 2.5 mM CA/5 mM NaOH or 1.8 mM Na CO /1.7 mM 2 3 NaHCO3 for Cl-, NO3-, SO42-; 5 mM CA/10 mM NaOH or 5 mM 22Na2CO3 for I , HPO4 , HAsO4 ; 2.5 mM CA/2.5 mM NaOH or 3.0 mM NaHCO3 for CH3COO-; 10 mM CA/20 mM NaOH or 10 mM Na2CO3 for CNS- and ClO4-. b DLratio ) DLcarbonate/DLCA.
tions leads to calibration curves that are linear over 4 orders of magnitude.30 Despite other limitations, it is generally recognized that OH--based eluents give excellent calibration linearity, especially with EEP.7 The eluents using conjugate bases of weak acids having large pKa values, at least from a theoretical point of view, may produce high calibration linearities29 and improved sensitivity for weak acid anion solutes. Figure 5 shows the analytical sensitivity as a function of the sample concentration obtained with CA-based eluents. Good linearities are obtained for all examined anions
3338 Analytical Chemistry, Vol. 69, No. 16, August 15, 1997
up to 300 µM (25 µL injection, linear coefficient of determination r2, given by the peak area vs the injected concentration, is always >0.999). This is also observed at 5 mM, a relatively high concentration in the case of carbonate-based eluents. It is worth noting the linearity and sensitivity found for acetate. Table 4 shows the limits of detection attainable with the cyanurate eluent under the reported isocratic conditions. Values are based on signals equal to 3 times the standard deviation of the background signal.31 These figures are at least 2-3 times better than the limits of detection observed using carbonate eluents with similar eluting power. Again, it is worth noting the figure for acetate. The DLratio (2.7 ( 0.8, except for acetate; see Table 4) compares well with the ratio of the background conductivities measured with carbonate or CA-based eluents under the same conditions (2.6 ( 0.15, see Table 1). Thus, the lower background conductivity translates directly into lower detection limits, indicating a first-order baseline noise of mechanical and hydrodynamical nature. For acetate, the detection limit is reduced by 2-3 times because of the lower background conductivity and is further lowered by 3 times because of the higher pH of the suppressed eluent. ACKNOWLEDGMENT The authors are very grateful to MURST and CNR for financial support.
Received for review February 20, 1997. Accepted May 27, 1997.X AC970205Z X
Abstract published in Advance ACS Abstracts, July 1, 1997.
