Ion Chromatography The State of the Art - Analytical Chemistry (ACS

Ion Chromatography The State of the Art Purnendu Κ. Dasgupta Department of Chemistry and Biochemistry Texas Tech University Lubbock, TX 79409-1061

Much of the practice of ion chroma tography (IC) today is well-estab lished science transformed into eas ily practiced technology; little a r t remains. Since it was first described in 1975 (2), IC has matured rapidly. The number of articles on IC pub lished in ANALYTICAL C H E M I S T R Y

reached a peak in the mid-1980s. Nonetheless, judging by the number of articles abstracted by Chemical Ab stracts, IC is increasingly being ap plied to a wide variety of areas. U.S. manufacturers of IC instrumentation indicate that, despite the recession, worldwide sales of IC instrumenta tion are on the rise. The reliability of IC is often taken for granted. The technique has be come a routine tool for process analy sis and control, notably for trace 0003 - 2700/92/0364 -775A/$03.00/0 © 1992 American Chemical Society

analysis in the nuclear power and semiconductor industries. IC has be come so widespread that details such as modifications of vendor-supplied software and merits of vendor service contracts have been published i n peer-reviewed literature (2). Currently there are not many ven dors of IC columns, especially as compared with the large number of vendors for LC columns. (Major U.S.

REPORT vendors include Alltech/Wescan, Dionex, EM Science, Hamilton, Metrohm, and Waters.) The cost of IC columns continues to be higher than that of their LC counterparts; however, the gap in performance (ul timate efficiency and plates per unit time) between the respective best of ferings of the IC and the LC worlds is much smaller today than it was five years ago.

The basic principles of both sup pressed and single-column IC have been presented elsewhere and will not be discussed here. An A-page ar ticle by Fritz (3) presented the funda mentals and progress made u p to 1987. In this REPORT I will focus on developments t h a t have transpired over the past five years.

Cation determinations Because of the large differences in ion-exchange selectivities of alkali versus alkaline earth metals, tradi tionally it has been difficult to pro vide simple isocratic elution condi tions to allow the separation of both cation classes within a reasonable time period. Kolla et al. (4) intro duced a polyObutadiene-maleic acid) copolymer coated silica stationary phase t h a t made such separation possible for the first time, especially with the use of mildly acidic complexing eluents in the unsuppressed conductometric mode. Often referred to as "Schomburg columns," after the

ANALYTICAL CHEMISTRY, VOL. 64, NO. 15, AUGUST 1, 1992 · 775 A

REPORT senior inventor, these are commer cially available. Figure l a shows a separation of al kali and alkaline earth metals. For the alkaline earth metals there is a vast improvement in sensitivity over results obtained with previous sin gle-column conductometric proce dures. For example, the limits of de tection (LODs) for Mg 2 + , Ca 2+ , Sr 2 + , and Ba 2 + range from 500 pg to 10 ng for a 100 -\xL injection. A carboxylate functionality sta tionary phase of proprietary compo sition has also been developed for use in suppressed conductometric IC. Figure l b shows an isocratic separa tion of alkali metals as well as Ca 2 + and Mg 2 + on this column. Figure 2 shows a step-gradient elution of 10 alkali and alkaline earth metals on the latter column; with the use of trap columns to re move adventitious cation impurities, LODs for a 25-μΐ; sample in this an alytical system range from 30 pg to 5 ng for Li + to Cs + and from 170 to 550 pg for Mg 2 * to Ba 2 + . In 1971 Sickafoose introduced ion-

exchange chromatographic separa tion of heavy m e t a l s followed by postcolumn reaction with a chromogenic ligand for optical detection (5a). However, this technique gained little ground with practitioners of atomic spectrometry. One recent exception to the above is the separation and determination of t h e l a n t h a n i d e e l e m e n t s , pio neered by Cassidy (56). The most re cent version of this approach uses gradient elution with sodium octanesulfonate as an ion interaction re agent, which provides virtual ion-exchange sites by adsorption on a nonpolar (C 18 ) stationary phase with a-hydroxyisobutyric acid as the complexing eluting component. Quantitation is achieved by postcol umn reaction with Arsenazo III and optical absorbance detection at 658 nm (Figure 3). Typical detection lim its are < 1 ng for each element. The determination of trace levels of transition elements in complex en vironmental and biological samples has never been simple. Because of matrix interferences, direct accurate measurement of trace metals in a m a t r i x as common as seawater ranges from difficult to impossible w i t h atomic s p e c t r o m e t r i c t e c h niques. Selective preconcentration of desired analytes is difficult because of the overwhelming preponderance of alkali and alkaline earth metals in most real samples. Siriraks et al. (6) developed chela tion IC as a solution to this problem. The sample is preconcentrated on a macroporous poly(styrenedivinylben zene) (PSDVB) column with iminodiacetate functionalities. These groups have strong affinities for heavy met als. Ca 2 + and Mg 2 + are also captured to some degree. After the preconcentration step is completed, Ca 2 + and Mg 2 + are eluted with 2 M ammonium acetate while the

heavier m e t a l s are r e t a i n e d . The heavier metals are eluted by an ali quot of 6 M HNOg. The effluent is di luted with deionized water in line, and the metals are recaptured on a sulfonated PSDVB concentrator col umn. The metals on the concentrator column are subjected to chromatog raphy on the separation column with a complexing eluent such as pyridine-2,6-dicarboxylic acid. Detection is accomplished after postcolumn reaction with chromogenic ligands such as 4-(2-pyridylazo)resorcinol and measurement of optical absorption. Traces of heavy metals in a variety of matrices, including estuarine and ocean water as well as digested bo vine liver and oyster tissue samples, can be determined through the use of commercially available automated instrumentation. A related but unique approach for the determination of trace transition m e t a l s in t h e p r e s e n c e of l a r g e amounts of alkali and alkaline earths in matrices such as ocean water was developed in t h e USSR (7). T h e equipment setup, similar to that of suppressed anion chromatography with a carbonate-bicarbonate

Figure 1. Separations using a Schomburg column. (a) Separation of Li*, Na*, NH*., K*, Mg 2 *, Ca 2 *, Sr2*, and Ba 2 * (peaks 1-8, respectively), using a 0.1 mM EDTA/3.0 mM HN0 3 eluent. Amounts injected range from 25 ng for Li* to 500 ng for Ba2*. (Courtesy Waters Chromatography.) (b) Isocratic separation of Li+, Na+, NHJ, K*, Mg2*, and Ca2* (peaks 1-6, respectively) with 20 mM methanesulfonic acid on lonpac CS-12R. Amounts injected range from 25 ng for Li* to 250 ng for Ca 2 *. (Courtesy Dionex)

Figure 2. Step-gradient elution (1640 mM methanesulfonic acid at 10 min) of alkali/alkaline earth metals. Peaks 1-10 (amounts injected in ng in parenthe ses): Li* (25), Na* (100), NH* (250), K* (250), Rb* (500), Mg2* (125), Cs* (500), Ca2* (250), Sr2* (80), and Ba2* (250). (Courtesy Dionex)

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Figure 3. Separation of rare-earth elements after group separation from Hawaiian basalt with a sodium octanesulfonate-cc- hydroxy butyric acid gradient on a C 18 stationary phase; postcolumn reaction detection with Arsenazo III. (Adapted with permission from Reference 5b.)

eluent, features a stationary phase t h a t c o n t a i n s not only a n i o n - e x c h a n g e s i t e s b u t also c h e l a t i n g / cation-exchange sites t h a t preferen tially bind the transition metals. The sample (or a sequence of sam ples) is repeatedly analyzed for its anionic constituents, and the cations of interest preconcentrate on the top of the column. When a desirable level of preconcentration has been reached, a chelating eluting agent such as EDTA is injected and the anionic metal complexes are separated and detected conductometrically w i t h reasonably good sensitivity (LODs range from 10 to 50 ppb). The real power of the chelation IC technique is harnessed in combina tion with element-selective detection techniques such as inductively cou pled plasma optical emission spec trometry (ICP-OES) or inductively coupled plasma mass spectrometry (ICPMS). In such applications, the nitric acid eluate from the column is directly injected into the detection system. Purists may argue about whether these hyphenated systems are ion chromatographs with expensive de t e c t o r s or I C P - O E S a n d I C P M S u n i t s with the ion chromatograph serving merely as a sample cleanup tool. What is clear is that these tech niques result in an unprecedented combination of sensitivity and free dom from matrix interferences (8). Using IC at the front end greatly in creases the power of simultaneous multielement atomic absorption spectroscopy (9). A fivefold improve ment in LODs of individual elements is easily attained.

Anion determinations The mainstay of IC has been in the determination of anions. When more t h a n one anion is to be determined in a mixture and/or when there is no specific chemistry by which the analyte anion can be determined, IC is usually the only method capable of a c h i e v i n g s e p a r a t i o n . In s i n g l e column IC, the use of a poly (methyl methacrylate)-based anion-exchange s t a t i o n a r y p h a s e d o m i n a t e s . Al though favorable chromatographic performance has been demonstrated with very strong e l u e n t s such as n a p h t h a l e n e t r i s u l f o n a t e (10), t h e most common eluents used in singlecolumn IC are phthalate or b o r a t e gluconate. The repertoire of columns avail able for suppressed anion chromatog raphy has expanded in the past five years. Some columns offer greater speed (however, in my experience, for

some of these the usable lifetimes are shorter than their standard counter parts), and small particle size (5 μπι) high-efficiency s t a t i o n a r y p h a s e s have been developed specifically for use with a hydroxide eluent. Mixed aqueous—organic solvents can be used effectively to alter ion-exchange selectivity and to remove tenaciously held material such as humic acids from the columns. Previous PSDVB stationary phases with small amounts of cross linking were incom patible with organic solvents. How ever, a new g e n e r a t i o n of highly c r o s s - l i n k e d s u p p o r t s m a d e from 45% ethylvinylbenzene and 55% divinylbenzene has been introduced (11). T h e s e s u p p o r t s allow t h e use of methanol, acetonitrile, or THF in the mobile phase. A solvent-compatible column for both single-column and suppressed IC use is also available. As IC matures as a technique, it may be used increasingly for chal lenging applications such as the de termination of traces of other anions in concentrated acids, bases, and s a l t s . T h e d e v e l o p m e n t of h i g h efficiency s t a t i o n a r y p h a s e s w i t h moderate to high ion-exchange ca pacity is vital for establishing a suc cessful solution to this problem (11). A minimum amount of sample must be injected on the column to deter mine the trace constituents. At the same time, a large capacity is neces sary to prevent overloading the col u m n with the major constituents, causing loss of chromatographic per formance. Another unusual route to anion de terminations has been developed by Lamb et al. (12). Adsorption of mac· rocyclic cryptands on a hydrophobic s t a t i o n a r y phase produces v i r t u a l ion-exchange sites with any eluent bearing an alkali metal cation, be cause of the affinity of the adsorbed macrocycle for the cation. The effec tive ion-exchange capacity of such a column can be tailored by choosing the nature, composition, and temper a t u r e of t h e eluent. This method, which can be easily exploited for gra dient elution, is described in more detail in a later section.

conductivity that is of concern here. An NaCl eluent is just as acceptable in this case as an NaOH eluent, be cause suppression (exchange of Na + for H*) results in either HC1 or wa ter, and both are volatile. The first application of IC/MS was reported by Simpson et al. (13a) for the analysis of carbohydrates sepa rated in the anion-exchange mode with an NaOH eluent. Following the s u p p r e s s o r and a UV detector, a booster pump was used to direct the effluent through a thermospray in terface into the mass spectrometer. Without the booster pump, the mem brane suppressor cannot adequately handle the back pressure observed at the thermospray interface. The pump also allowed the simultaneous intro duction of ammonium acetate to as sist in t h e thermospray ionization process. Conboy et al. (13b) allowed most of the IC suppressor effluent to go to waste while a small amount was di rected into an ion spray interface and an atmospheric pressure ionization mass spectrometer. Both full-scan MS and MS/MS were performed on the eluites of interest: q u a t e r n a r y ammonium compounds and organic sulfates and sulfonates. The analytes were separated by ion interaction chromatography, and the ion interac tion reagent was removed from the aqueous-organic matrix by the ap propriate membrane suppressor. R e c e n t l y H s u (13c) d e s c r i b e d IC/MS analysis of organic anionic compounds in a variety of matrices of practical interest (for example, haz ardous waste samples) with a parti cle beam interface following the sup pressor. He discussed the use of the approach for structural identification of eluites and its limitations, which were attributable primarily to con tamination problems with the inter face. Although there is little doubt t h a t MS detection following IC sepa ration can be advantageous for eluite identification, the LODs attainable thus far are not particularly impres sive; certainly they are far below the sensitivity levels for metallic ele ments t h a t can be achieved with IC/ ICPMS.

IC/MS

MS is one of the most powerful tech niques for analyzing complex sam ples. Since 1990 several papers have appeared on the use of mass spectrometric detectors for IC (13). In all cases, the suppressed mode has been used to avoid contamination of the ion source with salt deposits. Note t h a t it is the volatility of the sup pressed effluent and not its electrical

Ion-exclusion chromatography Ion-exclusion chromatography is su perior to ion-exchange chromatogra phy for the separation of weak ac ids—especially carboxylic acids. As applied to anionic analysis, a strong acid solution is used as the eluent. Eluite peaks can be detected by di rect optical absorption in the rela tively few cases in which the analyte


REPORT displays significant absorptivity. Even when a large counterion acid (e.g., octanesulfonic or perfluoroheptanoic acid) is used as eluent to re duce the background conductance, the background is still large and di rect conductivity detection in the sin gle-column mode is not particularly sensitive. The background conduc tance is reduced by exchanging the H + in the eluent for a low-mobility large cation such as tetrabutylammonium ("suppressed" detection mode). However, even in this case there are clear limitations on the upper limit of the eluent concentration. Okada (14a) described a different suppression technique: HI was used as eluent and reacted postcolumn with H 2 0 2 to produce nonionic, non conducting I 2 . Sensitive conductometric detection was possible; the highest HI concentration that could be used was ~ 4 mM because I 2 has limited solubility. T a n a k a and Fritz (14b) showed that for very weak acids (e.g., C0 2 ) water can be used as eluent, and a two-stage conversion of the eluite HX to MOH (M = K, Na) through two sequential ion-exchange columns (e.g., H 2 C 0 3 -» KHCO3 -» KOH) can be used to increase detection sensi tivity. Okada and Dasgupta (14c) de scribed a technique for optically de tecting eluite acids in the ρΚΆ range of 4.5-9.5. The strong acid eluent is

subjected to exchange with Na + in a membrane suppressor, resulting in a p H - n e u t r a l effluent. In contrast, weak acids produce salts that are al kaline. This behavior is detected op tically via an a c i d - b a s e indicator (e.g., 4 - n i t r o p h e n o l ) i n t r o d u c e d through a membrane. Detection lim its as low as 50 pmol are possible, even w i t h 10 mM H N 0 3 as t h e eluent. Coupled and multimode separations For simultaneous determination of strong and weak acid anions, the coupled use of ion-exchange and ionexclusion modes increases the di mensionality of the separation. This greatly enhances the overall utility of the technique when it is essential to determine both classes of anions si multaneously. Jones et al. (15a) document how this can be achieved in the unsuppressed mode using sodium octanesulfonate as the eluent for the ionexchange mode and octanesulfonic acid for the ion-exclusion mode. Fur thermore, this system permits preconcentration of both analyte classes and t h u s allows p a r t - p e r - b i l l i o n level detection using direct conduc tivity detection. The ultimate in detection sensitiv ity is not required for many real sam ples. Indeed, in such cases singlecolumn u n s u p p r e s s e d o p e r a t i o n , with which different systems can be

coupled, may allow greater versatil ity. For example, Saari-Nordhaus et al. (15b) have shown that it is possi ble to perform simultaneous determi nation of cations and anions by cou pled but independent ion-exchange modes. This system is more practical than previous ones designed to si multaneously separate cations and anions on one column bearing both cation- and anion-exchange func tionalities. The latter columns have been commercially available for some time, but these types of mixed-mode separations are not popular. Stillian and Pohl (15c), however, have introduced a new type of sta tionary phase in which latex-bonded ion-exchange sites are attached to a n e u t r a l hydrophobic macroporous core. Both ion-exchange and hydro phobic interactions occur on such s t a t i o n a r y phases. Because up to 100% organic solvent can be used as eluent, this stationary phase is well suited for ion-exchange, reversedphase, or i o n - i n t e r a c t i o n s e p a r a tions, or any combination thereof. True multimode separations on a single column can be realized, as il lustrated in Figure 4. Suppression reactions and new suppressors Sato and Miyanaga (16a) have de scribed three new ways of achieving conductometric suppression. In one, the potassium ions in a dipotassium ethylenediamine diacetate eluent are

Figure 4. Two-dimensional separation on a column with both reversed-phase and anion-exchange properties. Elution during the first 10 min is isocratic, using 80:20 acetonitrile/water. Subsequently the eluent is switched to 20% acetonitrile and ramped over 10 min from 50 mM NaCI, 0.2 mM NaOH to 400 mM NaCI, 1.6 mM NaOH. Peaks 1-13: benzyl alcohol, diethyltoluamide, benzene, benzoic acid, benzenesulfonic acid, toluenesulfonic acid, p-chlorobenzenesulfonic acid, p-bromobenzoic acid, phthalic acid, terephthalic acid, p-hydroxybenzenesulfonic acid, 1,3,5-benzenetricarboxylic acid, and 1,2,4,5-benzenetetracarboxylic acid. (Courtesy Dionex)

Figure 5. Gradient elution with electrodialytically generated and suppressed NaOH eluent. The current program is 0-200 mA, resulting in 0-117 mM NaOH. Peaks 1-12:100 μΝ each of F~, IO3, HCOO-, BrOï, CI", NOi, NO5, SOf", HPOi", AsO?" CrOi", and Γ.


exchanged for copper to form a neutral nonconducting chelate. In a second scheme, ethylenediammonium carbonate reacts with resin-bound Cu 2+ to form the copper-ethylenediamine complex and carbonic acid. The third scheme involves postcolumn reagent introduction in stoichiometric amounts to reduce the background conductance. The superiority of any of these approaches over existing practices is unclear. Gjerde and Benson (16b) have described a method in which suppression is achieved by reacting the column effluent with a colloidal solution of ion-exchange resin in the suitable ionic form. The application of this method for trace analysis has been demonstrated by Robles (16c). It has long been claimed that application of a voltage of appropriate polarity across a suppressor membrane should aid the efficiency of chemical ion exchange. Tian et al. (16d) described an acid-regenerated suppressor to which voltage was applied. However, they made no performance comparisons in the presence and absence of applied voltage. Strong and Dasgupta (16e) have since shown that the application of

voltage in an acid-regenerated suppressor actually deteriorates suppressor performance. They report the successful use of water as régénérant in an electrodialytically operated suppressor, using two membranes to isolate the eluent channel from electrolytically evolved gas. Not only does such a suppressor obviate the necessity of preparing a régénérant; the penetration of the régénérant counterion can be completely eliminated. In one mode, the detector cell effluent can be used as the régénérant. (For practical purposes, this eff l u e n t is p u r e w a t e r c o n t a i n i n g traces of acids.) A simpler form of this suppressor based on sheet membranes has also been described (16e, 16f). A similar suppressor is expected to be commercially available soon. Columns of 2-mm i.d. and suitable hardware are available for IC. One of the advantages claimed for this "microbore" format is t h a t significantly larger eluent concentrations can be exchanged by the suppressors.

erating high-purity acids. They described how very high purity eluents (suppressed conductance of 0 - 1 7 5 mM NaOH is 340 ± 40 nS/cm) can be generated on line, either without gas in the eluent channel or with removal of the electrolytic gas through a porous hydrophobic m e m b r a n e . The latter method provides greater current efficiency and permits higher eluent concentrations. Such ultrapure eluents may be beneficial for performing trace analysis of common ions in concentrated acids, bases, and salts. Additionally, the eluent concentration generated is controlled by the current. Thus it has been shown that gradient elution can be performed by programming the eluent generator current (16f, 17) rather than by using mechanical valves. Figure 5 shows a typical gradient anion chromatogram generated by this technique. On-line electrodialytic generation of pure substances is still in its infancy and may hold great promise for many areas.

Electrochemical eluent generators

Gradient IC

Strong et al. (17) demonstrated the possibility of electrodialytically gen-

Significant new advances have been made in gradient IC for single-

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REPORT column and suppressed systems. Jones et al. (18) have devised a way to dispel the myth that gradient elution cannot be performed in the sin gle-column mode. They used a gradi ent between two eluents of equal conductivity. The strong eluent con tained a greater eluent ion concen tration but had a lower mobility counterion (Li+) as the cation; there fore, the conductivity was the same as that of the weaker eluent in the

higher mobility Cs + form. The work of Lamb et al. (12) on the use of adsorbed macrocyclic ligands to provide virtual anion-exchange sites was previously mentioned. The macrocycle t h e y used h a s m u c h greater affinity for Na + than for Li + . Therefore a gradient between the same concentrations of LiOH and NaOH (starting with Na + and ending with Li + ) results in a column with gradually decreasing capacity—

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which is parallel to an increase in eluent strength. After suppression by a conventional membrane suppres sor, the background conductance from the NaOH or LiOH eluent is the same. Background shift in the course of the gradient separation run is neg ligible. In another twist, the macrocycle cation binding decreases with in creasing temperature; therefore, a decrease in column-exchange capac ity for t h e s a m e e l u e n t c a n be brought about by increasing the col umn temperature. Examples of all three types of gradient IC are illus trated in Figure 6. The number of actual gradient IC applications described in the litera ture for real samples remains very limited. In most cases, t h e u s e r chooses a column-switching tech nique or at best a step gradient. The question of superiority among the above and electrodialytically or con ventionally generated gradients may be moot. To some people, gradient IC is still the solution in search of the right problem. Detectors

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The conductometric detector remains t h e m a i n s t a y of IC. It h a s been shown t h a t inexpensive directcurrent conductivity detectors of low cell volume can be easily constructed (19a). The performance of these de tectors rivals that of commercial de tectors when the background conduc tance is low (e.g., with a suppressed NaOH eluent). It has been shown that, when an eluite acid peak is trapped in the cell, a chronoamperometric profile is generated t h a t is characteristic of the anion; therefore, the mobility of the anion can be esti mated from the data. This approach can confirm the identity of an anion (1%). Although the pulsed amperometric detector (20) was introduced in the early 1980s, it has become commer cially available only recently. It is a particularly powerful detector, espe cially for the determination of vari ous sugars and saccharides typically separated by anion-exchange chro matography. The repertoire of this detector continues to expand and now includes alcohols and alkanolamines. Postsuppressor manipulations

Figure 6. Three types of gradient IC. (a) Single-column gradient separation of 14 anions (F~ to SCN~) using an isoconductive gradient between cesium-borate gluconate and lithium-borate gluconate. (Adapted from Reference 18.) (b) Separation of 14 anions (F~ to SCN-) on a macroporous hydrophobic stationary phase loaded with Cryptand D-2.2.2., linear gradient from 20 mM NaOH to 20 mM LiOH over 20 min. (c) Separation of 15 anions (F~ to phthalate); same stationary phase as part b, 20 mM NaOH, linear gradient from 30 °C to 80 °C. (Parts b and c adapted with permission from References 12a and 12b.)


When the detector background is vir tually devoid of ions—as with an NaOH eluent in a suppressed sys tem—there is always the alluring possibility that eluite ions can be ex changed with some other more easily

detectable species. Following the original work of Downey and Hieftje (21a), Shintani and Dasgupta (21b) explored membrane-based conver sion of an eluite (e.g., H + for Ce 3+ , eluite anion for o-aminobenzoate) with fluorometric detection. Subsequent work has also shown that UV absorption detection of benzoate exchanged for the eluite anion via an anion-exchange membrane may actually lead to better detection limits under the proper experimental conditions. Galante and Hieftje (21c) have stressed the universal calibration characteristics of this ion replace ment technique and have described in detail the sensitive detection of anions after replacement of the ac companying H + with Li + and flame photometry. Eluting cations were similarly detected in a cation chro matography system with a nitric acid eluent. A parallel technique of anion replacement with iodate and UV de tection was also described. The replacement technique h a s been explored in the microcolumn format by Takeuchi et al. (21d). Nev ertheless, the increased stability of electrical conductivity detectors and the subsequent improvement of S/N through better electronics, cell de sign, and t h e r m o s t a t i n g are such that the meager gains in sensitivi ty—if any—obtained by the replace ment techniques probably cannot justify the added complexity of the process. The merit of postsuppressor ion exchange has been viewed in a differ ent light by Berglund and Dasgupta (21e). They have exploited such a procedure to enhance the informa tion output by increasing its dimen sionality. Thus, after conductometric detection of eluite HX in a conven tional NaOH eluent-suppressed IC system, two sequential membranebased converters transform HX to NaOH, which is detected by a second conductometric detector. This procedure is similar to previ ous work of Tanaka and Fritz (14b). Although the efficiency of the twostep conversion of HX to NaOH de creases with increasing pK& of HX, the sensitivity for detection of weak acids or linearity of response are nevertheless much better after such an ion-exchange step. The method allows an estimation of the ρΚΆ of the eluite and approximate quantitation without specific calibration. Furthermore, ratioing the two de tector signals (after appropriate cor rection for time delay and band dis p e r s i o n ) can yield s e v e r a l v e r y

important clues about the purity of the eluite peak. Measurement of atmospheric gases Since its inception, IC has played a key role in environmental analysis. The application of IC to determine trace levels of aqueous sulfate in po table water and the determination of sulfate in atmospheric particles (af ter collection on a filter and aqueous extraction) have been pivotal devel opments. Determination of gaseous S 0 2 , arguably the most important of t h e major gaseous p o l l u t a n t s , is achieved after collection in a dilute H 2 0 2 absorber and preremoval of particulate sulfate by a filter. Recently we have made a concerted effort to d e t e r m i n e a t m o s p h e r i c gases of interest by using IC without discrete manual collection, transport, and analysis steps. The attractive ness of such an approach is t h a t many of the atmospheric gases of in t e r e s t (e.g., HCOOH, CH3COOH, HC1, HONO, HNO3, S 0 2 ) lead to characteristic anions that can be eas ily collected in an aqueous scrubber liquid. Thus a number of gases can be simultaneously collected and de termined by anion chromatography. Lindgren and Dasgupta (22a) de scribed the use of a porous hydrophob i c - m e m b r a n e - b a s e d diffusion s c r u b b e r . S c r u b b e r liquid flows through a membrane tube, and air is sampled through a jacket tube around it. The liquid effluent for the scrubber is aspirated through the in jection loop of an ion chromatograph. Every few m i n u t e s t h e injection valve is switched briefly to inject the sample onto the chromatographic system. Simultaneous determination of several gases at part-per-trillion levels is possible with inexpensive equipment. The aqueous anion chromatogram of a typical atmospheric sample is shown in Figure 7. Similar systems have since been used aboard sampling aircraft and by S w e d i s h a n d Swiss r e s e a r c h e r s . When the analyte ion of interest does not occur at significant levels in the aerosol phase (e.g., N 0 2 ) , a helical membrane diffusion scrubber can be used; this permits the use of a longer membrane and thus greater collec tion efficiency. Vecera and Dasgupta (22b) showed the facile determination of part-per-trillion levels of HONO with such a device and used UV de tection, which is more sensitive than conductometry for determining N 0 2 . Since t h e n , these a u t h o r s have demonstrated the simultaneous col lection and analysis of HONO and V

HNO3. Following low-pressure ion chromatographic s e p a r a t i o n on a minicolumn and reduction of nitrate to nitrite, the Griess-Saltzman reac tion was used to produce a purple dye t h a t was detected colorimetrically (22c). This work was carried out with a wet dénuder, consisting of a glass tube and highly wettable wall, as the collecting element instead of t h e m e m b r a n e - b a s e d diffusion scrubber. Scrubber liquid flowed down the wall and was collected at the bottom while sample gas flowed upward. Dasgupta and co-workers (22d) were the first to use such a dénuder. They showed t h a t particles are efficiently transmitted through such a device and that single-digit part-per-trillion-level detection limits for S 0 2 are easily achieved. We have continued to explore this technique. Newly designed parallel plate or annular wet denuders permit n e a r - q u a n t i t a t i v e collection of gases of interest at much higher flow rates than single-tube wet denuders and, with inexpensive equipment, permit detection limits below the p a r t - p e r - t r i l l i o n level. After t h e gases are removed, soluble constituents of t h e aerosol can be determined. Ongoing work shows t h a t "wet impactors" for collecting atmospheric aerosols can be designed, and thus a new method for continuous automated analysis of the soluble

iJUl l/vk\ l/ SO4"

( N0 2

NO;

F

Γ

mGil 1

Time

f-

Figure 7. Ion chromatogram of an ambient air sample compared with a liquid-phase standard. Top trace is an ion chromatogram ot ambient air scrubbed into dilute H z 0 2 . Lower trace is an ion chromatogram of a standard liquid phase. Peaks a and b correspond to 700 pptr HN0 3 and 320 pptr S0 2 , respectively. The scale bar next to the upper trace represents 100 nS/cm; the scale bar next to the lower trace represents 3 μβ/οππ. (Adapted from Reference 22a.)


ΒΕΡαπτ f r a c t i o n of t h e a t m o s p h e r i c c o m p o s i t i o n is p o s s i b l e .

aerosol

Ion analysis and IC T h e v a s t m a j o r i t y of I C a p p l i c a t i o n s involve s i m u l t a n e o u s d e t e r m i n a t i o n of m o r e t h a n o n e a n a l y t e . M o s t u s e r s r e g a r d t h e e a s e w i t h w h i c h IC c a n determine analytes other t h a n the p r i n c i p a l s p e c i e s of i n t e r e s t a s a g r e a t a s s e t . T h e r e f o r e it is n o t s u r p r i s i n g t h a t IC h a s s u r v i v e d com mercially a m i d competition from continuous-flow analyzers, whether segmented or u n s e g m e n t e d . I n c r e a s i n g l y it is c l a i m e d , h o w e v e r , t h a t I C h a s finally m e t i t s m a t c h i n c a p i l l a r y zone e l e c t r o p h o r e s i s (CZE). S e v e r a l v e n d o r s p r o v i d e buffer r e c i p e s t h a t p e r m i t a n a l y s i s of c a t i o n s or anions via indirect photometric de t e c t i o n . T h e n u m b e r of p l a t e s (or plates per unit time) t h a t can be gen e r a t e d by t h i s t e c h n i q u e is m u c h g r e a t e r t h a n t h a t a t t a i n a b l e by IC. Concentration detection limits, how ever, a r e l e s s a t t r a c t i v e t h a n t h o s e obtained w i t h IC, a n d preconcentration by various electromigration/ s t a c k i n g t e c h n i q u e s is l e s s s t r a i g h t f o r w a r d t h a n t h e t y p e of u n i f o r m p r e c o n c e n t r a t i o n for all i o n s t h a t c a n

b e a t t a i n e d o n a n IC p r e c o n c e n t r a t o r column. T h e s e h u r d l e s m a y n o t be i m p o s s i ble to o v e r c o m e . A g r e a t e r c h a l l e n g e for m a n y r e a l s a m p l e s c o n c e r n s a n a lytical reproducibility in routine use. B e c a u s e C Z E is a c u t e l y d e p e n d e n t o n t h e s t a b i l i t y of e l e c t r o o s m o t i c flow, a n d b e c a u s e t h e l a t t e r c a n be signifi c a n t l y affected b y t h e a d s o r p t i o n of s a m p l e c o n s t i t u e n t s to t h e w a l l s of t h e capillary, a r e a d y a n d universally applicable solution to such a problem is n o t i m m e d i a t e l y a p p a r e n t . R e g a r d l e s s of t h e fact t h a t s e p a r a tion selectivities in electrophoresis a n d ion e x c h a n g e a r e q u i t e different and IC would therefore always re m a i n in s o m e form, IC is h a r d l y threatened at the moment. Never t h e l e s s , t h e n e x t t i m e a n o t h e r over v i e w o n ion s e p a r a t i o n t e c h n i q u e s is d e s c r i b e d i n t h i s JOURNAL, CZE will doubtlessly d e m a n d m u c h greater at tention t h a n the casual reference made herein. Research on ion chromatography at the author's laboratory is supported by the Office of Basic Energy Sciences, U. S. Department of Energy, through DE-FG05-84ER13281. However, this article has not been subject to review by the DOE, and no endorsement should be inferred.

The author also gratefully acknowledges the help of Dionex Corporation and Waters Chro matography.

References (1) Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975, 47, 1801-09. (2) Lynch, G. J. Process Control Qual. 1991, 1, 2 4 9 - 6 3 . (3) Fritz, J. S. Anal. Chem. 1987, 59, 335 A - 3 4 4 A. (4) Kolla, P.; Kohler, J.; Schomburg, G. Chromatographic 1987, 23, 4 6 5 - 7 2 . (5) a. Sickafoose, J. P. Ph.D. Disserta tion, Iowa State University, 1971. b. Cassidy, R. M. Chem. Geol. 1988, 67, 185-95. (6) Siriraks, Α.; Kingston, Η. Μ.; Riviello, J. M. Anal. Chem. 1990, 62, 1185-93. (7) Dolognosov, A. M.; Grishinova, T. V.; Timerbaev, A. R. Abstracts of Papers, In ternational Ion Chromatography Sym posium, Denver, CO, 1991; Abstract 16. (8) a. Riviello, J.; Siriraks, Α.; Manabe, R. M.; Roehl, R.; Alforque, M. LC-GC 1991, 9, 7 0 4 - 1 1 ; b. Kawabata, K.; Kishi, Y.; Kawaguchi, O.; Watanabe, Y.; Inoue, Y. Anal. Chem. 1991, 63, 2 1 3 7 - 4 0 ; c. Heithmar, E. M.; Hinners, Τ. Α.; Rowan, J. T.; Riviello, J. M. Anal. Chem. 1990, 62, 857-64. (9) Dulude, G. R. Can. Chem. News 1991, 43(6), 3 1 - 3 5 . (10) Maki, S.A.; Danielson, N. D. Anal. Chem. 1991, 63, 699-703. (11) a. Pohl, C. Α.; Saini, C. Abstracts of Papers, International Ion Chromatogra phy Symposium (IICS), Denver, CO,

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1991; Abstract 10; b. Toofan, M.; Stillian. J.; Pohl, C; Saini, C. IICS Abstracts, Abstract 15; c. Woods, J. P. IICS Ab stracts, Abstract 21; d. Siriraks, Α.; Pohl, C; Toofan, M. IICS Abstracts, Abstract 40. (12) a. Lamb, J. D.; Smith, R. G. /. Chromatogr. 1991, 546, 73-88; b. Smith R. G.; Drake, P. Α.; Lamb, J. D. /. Chro matogr. 1991, 546, 139-49; c. Lamb J. D.; Drake, P. Α.; Woolley, K. In Ad varices in Ion Chromatography, Jandik, P. Cassidy, R. M., Eds.; Century Interna tional: Medfield, MA, 1990; Vol. 2, pp 215-31; d. Lamb, J. D.; Drake, P. A /. Chromatogr. 1989, 482, 367-80. (13) a. Simpson, R. C; Fenselau, C. C. Hardy, M. R.; Townsend, R. R.; Lee Y. C; Cotter, R. J. Anal. Chem. 1990, 60. 248-52; b. Conboy, J. J.; Henion, J. D Martin, M. W.; Zwigenbaum, J. A. Anal. Chem. 1990, 62, 800-07; c. Hsu, J. Anal. Chem. 1992, 64, 434-43. (14) a. Okada, T. Anal. Chem. 1988, 60, 1666-69; b. Tanaka, K.; Fritz, J. S. Anal. Chem. 1987, 59, 708-12; c. Okada T.; Dasgupta, P. K. Anal. Chem. 1989, 61 548-54. (15) a. Jones, W. R.; Jandik, P.; Swartz M. T. / Chromatogr. 1989, 473, 171-88 b. Saari-Nordhaus, R.; Nair, L.; Ander son, J. M., Jr. Abstracts of Papers, Inter national Ion Chromatography Sympo sium, Denver, CO, 1991; Abstract 4; c. Stillian, J. R.; Pohl, C. A. / Chromatogr. 1990, 499, 249-66. (16) a. Sato, H.; Miyanaga, A. Anal. Chem. 1989, 61, 122-25; b. Gjerde, D. T.; Ben son, J. V. Anal. Chem. 1990, 62, 612-15; c. Robles, M. N. Abstracts of Papers, Inter

national Ion Chromatography Sympo (22) a. Lindgren, P. F.; Dasgupta, P. K. sium, Denver, CO, 1991; Abstract 55; d. Anal. Chem. 1989, 61, 19-24; b. Vecera, Tian, Z. W.; Hu, R. Z.; Lin, H. S.; Hu, Z.; Dasgupta, P. K. Environ. Sci. Technol. W. L. / Chromatogr. 1988, 439, 151-57; 1991, 25, 225-60; c. Vecera, Z.; Das e. Strong, D. L.; Dasgupta, P. K. Anal. gupta, P. K. Anal. Chem. 1991, 63, 2210Chem. 1989, 61, 939-45; f. Strong, D. L.; 16; d. Simon, P. K.; Dasgupta, P. K.; Joung, C-U.; Dasgupta, P. K. /. Chro Vecera, Z. Anal. Chem. 1991, 63, 1237matogr. 1991, 546, 159-73. 42. (17) a. Strong, D. L.; Dasgupta, P. K.; Friedman, K.; Stillian, J. Anal. Chem. 1991, 63, 480-86; b. Strong, D. L.; Das gupta, P. K.J. Membr. Sci. 1991, 57, 32136. (18) Jones, W. R.; Jandik, P.; Heckenberg, A. L. Anal. Chem. 1988, 60, 197779. (19) a. Qi, D.; Okada, T.; Dasgupta, P. K. Anal. Chem. 1989, 61, 1383-87; b. Okada, T.; Dasgupta, P. K.; Qi, D. Anal. Chem. 1989, 61, 1387-92. (20) a. Newburger, G G.; Johnson, D. C. Anal. Chem. 1987, 59, 150-54 and 203Purnendu K. "Sandy" Dasgupta, a native 06; b. Larew, L. Α.; Johnson, D. C. Anal. ofIndia, received his B. Se. degree (1968) Chem. 1988, 60, 867-72; c. LaCourse, W. R.; Jackson, W. Α.; Johnson, D. C. and his M. Se. degree in inorganic chem Anal. Chem. 1989, 61, 2466-71; d. An istry (1970) from the University ofBurddrews, R. W.; King, R. M. Anal. Chem. wan, India, and his Ph.D. (1977) in an 1990, 62, 2130-34; e. LaCourse, W. R; alytical chemistry from Louisiana State Johnson, D. C ; Rey, Μ. Α.; Slingsby, R. W. Anal. Chem. 1991, 63, 134-39. University at Baton Rouge. His research (21) a. Downey, S. W.; Hieftje, G. M. focuses on atmospheric analysis, ion chro Anal. Chim. Acta 1983, 153, 1-13; b. matography, automated flow analysis (in Shintani, H.; Dasgupta, P. K. Anal. the laboratory and for the chemical pro Chem. 1987, 59, 1963-69; c. Galante, L. J.; Hieftje, G. M. Anal. Chem. 1987, cess industry) and, most recently, sup 59, 2293-2302 and Anal. Chem. 1989, 60, pressed conductometric detection in capil 995-1002; d. Takeuchi, T.; Suzuki, E.; Ishii, D. Chromatographia 1988, 25, 582- lary electrophoresis. He was recently appointed Paul Whitfield Horn Professor 84; e. Berglund, I.; Dasgupta, P. K. Anal. Chem. 1991, 63, 2175-83. at Texas Tech University.

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