5 Ion Chromatography M. Ali Tabatabai Iowa State University, Ames, Iowa, U.S.A. Nicholas T. Basta Oklahoma State University, Stillwater, Oklahoma, U.S.A. Shreekant V. Karmarkar Lachat Instruments, Milwaukee, Wisconsin, U.S.A. I. INTRODUCTION Ion chromatography (IC) is a term that describes the advances made in the determination of ions. It has become a field of its own since its introduction by Small et al. (1975). Within the past 20 years, research in the area of IC has made significant advances in separation and determination of ionic species, and IC has become a rapid and sensitive technique for analyzing complex mixtures of ions. Now, ion chromatographs are available that feature high-speed separation, continuous monitoring by detecto r systems, and the instantaneous readout of analytical data. The IC technique, a type of high-performance liquid chromatograph y (HPLC), has gained popularity for accurate and precise determination of anions and cations in soils, plants, water, and other environmental materials, as well as samples from clinical, metal plating, power generation, semiconductor fabri cation, and other industrial sources. Several books have been published on IC, including the development and use of its components, and the potential of the technique as an analytical tool (Sawicki et al., 1978; Mulik and Sawicki, 1979a; Fritz et al., 1982; Smith and Chang, 1983; Weiss, 1986; Tarter, 1987; Small, 1989). TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. This is a revised version of the corresponding chapter in the previous edition (Tabatabai and Basta, 1991). It covers the basic principles of IC, the instruments and methods that have been developed, and the application of these methods to the analysis of soil, plant, water, and environmental samples. New approaches for application of IC to chemical speciation are described. Application of IC to soil and environmental analysis has been previously reviewed by Frankenberger et al. (1990), Tabatabai and Basta (1991), Tabatabai and Frankenberger (1996), and Karmarkar (1998). Several IC methods are available for the determination of ions other than those discussed in this chapter, but these methods have not been evaluated for soil analysis (Sawicki et al., 1978; Mulik and Sawicki, 1979a, b; Johnson, 1987). II. BASIC PRINCIPLES Ion chromatography has its roots in pioneering work in the area of ion exchange, including the development of synthetic ion-exchange resins. This technique falls under the broad category of liquid chromatography. A review of the work published on these topics is beyond the scope of this chapter, but information on the basic principles involved in the operation of ion chromatographs is presented. Typical components of an IC system (Fig. 1) include an optional autosampler, a high-pressure pump, and an injection valve with a sample loop of suitable size (typically 10–250 mL), a guard column (also called a precolumn), an analytical column, a postcolumn reaction system, a flow-through detector, and a data station ranging in complexity from a chart recorder to a computerized data system. A suitable Figure 1 Schematic diagram of components of a typical IC system. CD, conductivity detector; PAD, pulse amperometric detector. 190 Tabatabai et al. TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. mobile phase, the eluent, flows continuously through the columns and detector. Typically, all the components in contact with the eluent and sample are made fr om inert materi als, such as polyetheretherketone (PEEK), Teflon, or other polymers that are stable under acidic or basic solutions. After sample preparation, usually the sample is filtered through a 0.45 mm filter and diluted, and then a fixed volume is injected onto the guard column. It then passes on to the analytical column. The ions in the sample are separated as a result of differing affinities for the column packing material as the ions are swept along in the flowing eluent (Karmarkar, 1998). The packing material is selected on the basis of its ion selectivity and ion exchange capacity. Ion chromatographic separation takes place by one of three separation modes: (1) ion exchange, examples of which include determination of common anions (e.g., Br À ,Cl À ,F À ,NO À 3 ,NO À 2 ,SO 2À 4 , and PO 3À 4 , and alkali and alkaline-earth cations (e.g., Na þ ,Li þ ,K þ ,Ca 2þ , and Mg 2þ ) and NH 4 þ ; (2) ion exclusion, which is used for the separation of low-molecular-weight organic acids (e.g., adipic, acetic, formic, malic, malonic, oxalic, succinic, and tartaric acids); and (3) ion pair separation, including separation of heavy metals and transition metal ions (e.g., Cd 2þ , Co 2þ ,Cu 2þ ,Fe 2þ ,Fe 3þ ,Pb 2þ ,Mn 2þ ,Ni 2þ , and Zn 2þ ). Details of each of these separation modes are described by Haddad and Jackson (1990). The first IC system developed in the early 1970s used conductimetric detection, but recent IC equipment features colorimetric (UV-VIS), pulse amperometric or spectroscopic detection systems, including inductively coupled plasma (ICP) spectrometry or hydride generation and atomic absorption spectrometry. The development of new detection modes has increased the capability of IC to measure a great number of analytes with improved detection limits. Depending on the analytical accuracy and precision required, ion chromatographs can be divided into two major groups: those that operate on the principle of eluent suppression (dual-column system) and those with no suppressor column (single-column system). Detailed comparisons between eluent-suppressed and nonsuppressed ion chromatography have been presented by Pohl and Johnson (1980) and Tarter et al. (1987). Both types employ conductimetric detection systems, based on the variation in electrical conductivity of a solution with the concentration of ions present. These detectors are used for the determination of all ionic species (inorganic anions and cations, and organic acids) in solution. Calibration graphs of specific conductance vs. ion concentration used in IC are usually linear at low concentrations of each ion (< 100 mg L À1 ). Ion Chromatography 191 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. A. Systems with Conductimetric Detectors 1. Eluent-Suppressed Ion Chromatography Until the late 1980s, the suppressed-type IC was only marketed by Dionex Corporation (Sunnyvale, CA). Figure 2 shows its basic components. For simplicity, the reservoirs of the eluent and water used for regeneration of the suppressor column and the valving system involved in the IC are not shown. The instrument employs the following components: 1. An eluent pump and reservoir 2. A sample injection valve (the sample loop can be adjusted from about 50 mL to several hundred mL) 3. An ion-exchange separation column 4. A suppressor column coupled to a conductivity detector, meter, and output device 5. A regenerating pump with electronic timer and controls Several types of column are commercially available for the ion- exchange separation of the common inorganic and organic anions via eluent-suppressed IC. The resin material used and the available columns were described by Weiss (1995). In the eluent-suppressed IC, the ion species are resolved by con- ventional elution chromatography followed by passage through an eluent stripper, or ‘‘suppressor,’’ column, wherein the eluent coming from the separating column is stripped or neutralized. Thus only the ion species of interest leave the bottom of the suppressor column; anions emerge in a background of H 2 CO 3 , which exhibits a low conductivity, while cations emerge in water. These ions are monitored subsequently in the conductivity Figure 2 Simplified schematic diagram of suppressed-type IC. 192 Tabatabai et al. TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. cell/meter/recorder (integrator) combination. The eluent flow rate can be varied by adjusting the pump pressure, but normally it is about 2–3 mL min À1 . An aliquot ($2 mL) of, for example, a suspension-free soil extract is injected by a plastic syringe into the injection valve of the IC. The sample loop on the injection valve can be adjusted, but normally a volume of 100 mL is used. The 2-mL volume is convenient to ensure proper flushing of the injection valve loop and lines. The first suppression device introduced by Small et al. (1975) consisted of a column (dimensions ranging from 9 Â250 mm to 9 Â110 mm, or 2.8 Â300 mm) packed with a high-capacity resin material. The resin of the suppressor column had to be regenerated after about 50 analyses (8–10 h) by flushing the suppressor column with 0.5 M H 2 SO 4 (15 min) to remove anions or 1 M NaOH (15 min) to remove cations, followed by deionized water (25 min). In some early Dionex models (e.g., the Model 10) this could be accomplished without attending the instrument after each working day. This device had two disadvantages: (1) a relatively large volume of the suppressor column resulted in band broadening, which resulted in loss in chromatographic efficiency; and (2) the detector response to the ions of strong acids or bases decreased, whereas the response to ions of weak acids or bases increased as the active sites of the suppressor column were steadily depleted. The lack of a steady state resulted in poor precision. Despite these disadvantages, the packed-bed suppressor provided the foundation on which the suppress ed IC was developed. The background suppression (eluent, NaHCO 3 þNa 2 CO 3 ) was achieved according to R À ÀH þ þ Na þ HCO À 3 ! R À ÀNa þ þ H 2 CO 0 3 2R À ÀH þ þ 2Na þ CO 2À 3 ! 2R À ÀNa þ þ H 2 CO 0 3 The signal enhancement was achieved according to R À ÀH þ þ Na þ A À ! R À ÀNa þ þ H þ A À where R À is a functional group attached to the resin within the suppressor and A À is an anionic species in the sample. The batch-type or packed-bed column device was in used until 1975, when a continually operated fiber- based device was developed (Henshall et al., 1992). Presently, the following five suppression devices are commercially available: 1. A hollow-fiber membrane suppressor (Steven et al., 1981) and micro- membrane suppressor (Franklin, 1985; Stillian, 1985) that is generated Ion Chromatography 193 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. electrochemically (Henshall et al., 1992); commercially available with IC systems from Dionex. 2. The QuikChem small suppressor that is regenerated after every sample using chemicals (Karmarkar, 1996); commercially available with an IC system from Zellweger Analytics, Inc., Lachat Instruments Div. (Milwaukee, WI). 3. A set of two parallel small suppress or columns: one is regenerated electrochemically while the other is being used (Saari-Nordhaus and Anderson, 1996); commercially available with an IC system from Alltech Associates, Inc. (Deerfield, IL). 4. A device with postcolumn addition of a colloidal suspension of a high-capacity ion exchange material, also called solid phase reagent (Gjerde and Benson , 1992); available commercially from Sarasep, Inc. (San Jose, CA). 5. A self-regenerating suppressor (SRS), which utilizes autosuppression to enhance analyte conductivity while decreasing eluent conductivity, thus resulting in a significant improvement in analyte detection limits, is marketed by Dionex. The ions required for eluent suppression aregeneratedintheSRSbytheelectrolysisofwater.TheSRS combines the best features of micromembrane suppressor—high suppres- sion capacity, minimal peak dispersion, solvent compatibility, an d continuous use—with the added advantage of effortless operation and no maintenance. With the use of a hollow-fiber membrane suppressor or micromem- brane suppressor, the problems associated with the original packed-bed suppressor technique, such as band broadening, ion exclusion, and oxidation of NO À 2 , are eliminated. The disposable solid-phase chemical suppressor (SPCS) simplifies the instrumentation required to perform suppressed-based IC by eliminating the regeneration system and the complex postcolumn reaction system needed with other suppression techniques (Saari-Nordhaus et al., 1994). The lifetime of the SPCS cartridge is dependent on the ionic strength and flow rates of the eluent, varying from 7to12h. In a variant of the suppressor column system, the resin in the suppressor column is replaced by an ion-exchange membrane in tubular form to condition the eluent continuously (Stevens et al., 1981). This membrane (sulfonated polyethylene hollow fiber) acts exactly like the suppressor resin in that ions are exchanged from the membrane for ions in the eluent system. The innovation is that for the analysis of anions the membrane is regenerated continuously by a gravity-fed (or low-pressure) flow of low-concentration H 2 SO 4 that continuously replaces the ions that 194 Tabatabai et al. TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. are exchanged onto the fiber with ions from the regenerant. Thus, separate regeneration steps are eliminated. The replac ement of the conventional ion-exchange resin bed suppressor column with the hollow fiber suppressor allows continuous operation of an IC without varying interference from baseline dips, ion-exclusion effects, or chemical reaction. Stevens et al. (1981) concluded that conventional suppressor column systems had less band spreading than those using hollow-fiber suppressor, and this resulted in slightly poorer resolution of early eluting ions with the latter type of eluent suppression technique. However, our experience is different. Furthermore, work by Weiss (1986) showed that because of the low dead volume of a membrane suppressor, mixing and band broadening effects are minimized and the sensitivity is generally enhanced compared with the more traditional packed bed suppressor. Details of the theory of operation of the hollow-fiber suppressor were discussed by Stevens et al. (1981), Hanaoka et al. (1982), Small (1983), Weiss (1986), and Dasgupta (1992). The reactions involved in the separator column and suppressor column (or one of the devices listed above) in the determination of anions, alkali metals, and alkaline earth metals are shown in Table 1. In the determination of anions, the IC is equipped with a separator column packed with a low-capacity anion-exchange agglomerated resin in the HCO À 3 form, and the suppressor column contains a strong acid high-capacity cation- exchange resin in the H þ form or one of the other suppression devices listed above. The eluent used normally is a mixture of dilute NaHCO 3 and Na 2 CO 3 , although other dilute mixtures (e.g., Na 2 CO 3 þNaOH) are also used (Johnson, 1987). The anions are separated and converted to their strong acids in a background of H 2 CO 0 3 , which has no charge and low conductivity. The presence of strong acids in H 2 CO 3 is measured by a conductivity cell and reported as peaks on a stripchart recorder or integrator. The peak height is directly proportional to the concentration of ions in solution. From calibration graphs prepared for peak height versus concentration of ions in standard solutions containing the ions of interest, the concentrations of the ionic species in the sample are calculated. Because of the excellent signal-to-noise ratios, when equipped with a suppressed conductivity detector the IC system can achieve detection limits two orders of magnitude lower than those obtained in a nonsuppressed IC system. The mixture of the standards can be prepared from reagent-grade chemicals. Figure 3 shows a typical chromatogram of a standard solution containing 2mgL À1 each of F À ,Cl À ,PO 3À 4 -P, NO À 3 -N, SO 2À 4 -S. The separation of PO 3À 4 from several other oxyanions is shown in Fig. 4. Recent developments by Dionex involve the use of an autosuppression with the anion self-regenerating suppressor, which uses water as a regenerant. In this system, water undergoes electrolysis to form oxygen Ion Chromatography 195 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. Table 1 Reactions in Separator and Suppressor Columns in Determination of Anions and Alkali/Alkaline Earth Metal Cations by Ion Chromatography Component Reaction Anion a Alkali metal cation b Alkaline earth metal cation c Eluent 3 mM NaHCO 3 þ1.8 mM Na 2 CO 3 5mM HCI 2.5 mM HCI þ2.5 mMm-PDA.2HC1 d Displacing ion HCO À 3 H þ m-PDH 2þ 2 Separator column Eluent R-HCO 3 þNaHCO 3 R-H þHCl ,R-H þHCl R-PDAH 2 þPADH 2þ 2 þ2Cl À , ,R-HCO 3 þNaHCO 3 R-PDAH 2 þPDAH 2þ 2 þ2C1 À Sample R-HCO 3 þMA! R-H þMA!R-H þMA R-PDAH 2 þMA þ2C1 À ! R-A þMHCO 3 R-M þPDAH 2þ 2 þ2C1 À þA R-A þNaHCO 3 ! R-M þHCl!R-H þMC1 R-M þPDAH 2þ 2 þ2C1 À ! R-HCO 3 þNaA R-PDAH 2 þMC1 2 Suppressor column Eluent R-H þMHCO 3 ! R-OH þHCl!R-Cl þH 2 O 2R-OH þPDAH 2þ 2 þ2C1 À ! R-M þH 2 CO 3 2R-C1 þPDA þ2H 2 O Sample R-H þNaA! R-Na þHA R-OH þMCl!R-C1 þMOH 2R-OH þMC1 2 !R-C1 þM(OH) 2 a M ¼Na, A ¼anion. b M ¼alkali metal, A ¼associated anion. c M ¼alkaline earth metal, A ¼associated anion. d m-PDA.2HCI ¼m-phenylenediamine dihydrochloride. 196 Tabatabai et al. TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. gas and hydronium ions in the anode chamber, and hydrogen gas and hydroxide ions in the cathode chamber. The hydroxide ions generated at the cathode are excluded from the eluent chamber by Donnan exclusion. Cation exchange membranes allow hydronium ions to move from the anode chamber into the eluent chamber to neutralize hydroxide eluent, while Na þ ions in the eluent move across the membrane into the cathode chamber, maintaining the charge balance. In this process, the eluent (e.g., NaOH, Na 2 CO 3 /NaHCO 3 , or boric acid/Na tetraborate) is converted to water. The result is a dramatic improvement in signal-to-noise ratio due to three factors: (1) eluent background conductivity decreases as the eluent is suppressed to a less conductive medium, water, (2) analyte conductivity Figure 3 Typical chromatogram of anions separated in a suppressed-type IC system. Ion Chromatography 197 TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. increases because the analyte anions associate with the more conductive hydronium ions, and (3) sample counter ion peaks typical of nonsuppressed IC are eliminated. In the determination of the alkali metal ions (Li þ ,Na þ ,K þ ,Rb þ ,and Cs þ ), the separator column is a low-capacity cation-exchange agglomerated polystyrene divinylbenzene copolymer cation resin in the H þ form, and the suppressor column contains a strong base high-capacity anion-exchange resin in the OH À form. The alkali metals are separated and converted to their hydroxides in a background of H 2 O, which has a very low conductivity. The conductivity of the metal hydroxides is measured by a conductivity cell and reported as peaks on a stripchart recorder or integrator. The reactions involved in the separator and suppressor columns are shown in Table 1. The separation and detection of the alkaline-earth metal ions (Mg 2þ ,Ca 2þ ,Sr 2þ ,andBa 2þ ) are similar to the procedures for the alkali metals, except that a mixture of 2.5 mM HCl þ2.5 mM m-phenylenediamine (PDA) dihydrochloride is used as the eluent (Table 1). Suppression of the 5 mM HCl eluent used for measuring monovalent cations, or of PDA dihydrochloride eluent used for determining divalent cations, is achieved according to R þ OH À þ HCl ! R þ Cl À þ H 2 O 2R þ OH À þ PDA ð2HClÞ!2R þ Cl À þ PDA þ2H 2 O: Figure 4 Typical chromatogram of oxyanions separated in a suppressed-type IC system. (A) 0.05 mM; (B) 0.5 mM with respect to each of the oxyanions. (Karmarkar and Tabatabai, 1992.) 198 Tabatabai et al. TM Copyright n 2004 by Marcel Dekker, Inc. All Rights Reserved. [...]... equipped with a precolumn (AG-3, 3 Â 50 mm) and a separator column (AS-3, 3 Â 250 mm), both packed with a low-capacity anion-exchange resin, and a suppressor column (9 Â 100 mm) packed with a high-capacity cation-exchange resin in the Hþ form The injection valve was attached to a 50 -mL sample loop, and the eluent was 3.0 mM NaHCO3 þ 2.4 mM Na2CO3 at a flow rate of 2.3 mL minÀ1 and pump pressure of 4.8 MPa... injection loop was used, this single-column technique had detection limits, expressed in mg LÀ1 of soil extract, as follows: ClÀ, BrÀ, NOÀ -N, and SO2À -S: 0.0 25; ClOÀ and IÀ: 0 .5; and NOÀ -N and 3 4 2 4 SO2À -S: 1.0 This technique proved reproducible for determination of 3 TM Copyright n 2004 by Marcel Dekker, Inc All Rights Reserved Ion Chromatography 217 anions in soil extracts, as is evident from... and Tabatabai, 1979) Since then, many papers have appeared in the soil science literature on the use of suppressed and nonsuppressed IC systems for the determination of anions and cations in soil solutions and exchangeable bases in soils Some of these methods have been applied successfully to plant and water analysis The IC system should be useful for a variety of methods used in soil, plant, and environmental. .. of 2 .5 mL minÀ1 and a Figure 14 Diagram of oxygen combustion flask and filter paper used for enclosing plant sample (Busman et al., 1983.) TM Copyright n 2004 by Marcel Dekker, Inc All Rights Reserved Ion Chromatography 2 15 pump pressure of 3.1 MPa ( 450 psi) The average total S values of 15 plant samples by the IC method and by the methylene blue method after digestion with NaOBr were 0. 255 and 0. 259 %,... of NHþ 4 and alkali and alkaline earth metals in soil and plant extracts and water samples, provided that the aliquot analyzed is free from interfering substances such as organic materials, high concentrations of soluble salts, and extreme pH values Both the eluent-suppressed and single-column IC systems have been evaluated for the determination of these metals in soils, plant materials, and natural... using a 50 0-mL loop were Naþ, Ca2þ, and Mg2þ: 0. 05 mg LÀ1; Liþ and Ba2þ: 0.1 mg LÀ1; NHþ : 0 .5 mg LÀ1; Kþ and Sr2þ: 1 mg LÀ1 Results obtained 4 by this single-column IC system agreed closely with those obtained by other methods: steam distillation for NHþ (r ¼ 0.996***), AES for Naþ 4 (r ¼ 0.992***), AES for Kþ (r ¼ 0.999***), AAS for Ca2þ (r ¼ 0.996**), and AAS for Mg2þ (r ¼ 0.9 95* **) (Nieto and Frankenberger,... sample preparation and was not subjected to matrix interferences This use of HPAEC-PAD was applied to the analysis of organic materials (plant residues, animal wastes and sewage sludge) and soil Glycuronic Acids Acid hydrolysis (0. 25 M H2SO4) coupled with enzyme catalysis (pectolyase and -D-glucuronidase) was employed by Martens and Frankenberger (1990c) to extract galacturonic and glucuronic acids... cetyltrimethylammonium p-hydroxybenzoate Reference Karmarkar (19 95) Saari-Nordhaus et al (1994) Slingsby and Pohl (1996) Medina et al (1996) Mattusch and Wennrich (1996) Zerbinati (19 95) Adjusting pH Saari-Nordhaus et al Solid-phase extraction using a cartridge packed with (1994) H - or OHÀ-saturated resin for basic or acidic samples, respectively Ion collection and dissolution for airborne samples Collection of... determination of ClÀ, NOÀ , and SO2À in 3 min by using 15 mM phthalic acid (pH ¼ 2 .5) as an 3 4 eluent and a guard column as an analytical column The equipment configuration that they used consisted of a Perkin-Elmer Series 10 Liquid Chromatograph pump, a 50 mL sample injection loop, a Wescan ion-guard anion cartridge (26 9-0 03, 4.6 Â 30 mm), a Wescan ion-guard holder (269002), a Jasca Uvidec 100-V UV spectrophotometer... All Rights Reserved Summary of Off-Line and On-Line Techniques for Sample Preparation in IC Sample preparation objective Recommended technique Off-line techniques Removal of particulates Filtering through 0. 45 mm membrane Removal of high Solid-phase extraction using a cartridge packed concentration of chloride with Agþ-saturated cation exchange resin Removal of high Solid-phase extraction using a cartridge . R-PDAH 2 þPADH 2þ 2 þ2Cl À , ,R-HCO 3 þNaHCO 3 R-PDAH 2 þPDAH 2þ 2 þ2C1 À Sample R-HCO 3 þMA! R-H þMA!R-H þMA R-PDAH 2 þMA þ2C1 À ! R-A þMHCO 3 R-M þPDAH 2þ 2 þ2C1 À þA R-A þNaHCO 3 ! R-M þHCl!R-H þMC1 R-M þPDAH 2þ 2 þ2C1 À ! R-HCO 3 þNaA R-PDAH 2 þMC1 2 Suppressor. R-PDAH 2 þMC1 2 Suppressor column Eluent R-H þMHCO 3 ! R-OH þHCl!R-Cl þH 2 O 2R-OH þPDAH 2þ 2 þ2C1 À ! R-M þH 2 CO 3 2R-C1 þPDA þ2H 2 O Sample R-H þNaA! R-Na þHA R-OH þMCl!R-C1 þMOH 2R-OH þMC1 2 !R-C1 þM(OH) 2 a M ¼Na,. Na 2 CO 3 5mM HCI 2 .5 mM HCI þ2 .5 mMm-PDA.2HC1 d Displacing ion HCO À 3 H þ m-PDH 2þ 2 Separator column Eluent R-HCO 3 þNaHCO 3 R-H þHCl ,R-H þHCl R-PDAH 2 þPADH 2þ 2 þ2Cl À , ,R-HCO 3 þNaHCO 3 R-PDAH 2 þPDAH 2þ 2 þ2C1 À Sample