Bon Chromatography The State of the Purnendu K. Dasgupta Department of Chemistry and Biochemistry Texas Tech University Lubbock, TX 79409-1061
Much of the practice of ion chromatography (IC) today is well-established science transformed into easily practiced technology; little art remains. Since it was first described in 1975 ( I ) , IC has matured rapidly. The number of articles on IC published in ANALYTICAL CHEMISTRY reached a peak in the mid-1980s. Nonetheless, judging by the number of articles abstracted by Chemical Abstracts, IC is increasingly being applied to a wide variety of areas. U.S. manufacturers of IC instrumentation indicate that, despite the recession, worldwide sales of IC instrumentation are on the rise. The reliability of IC is often taken for granted. The technique has become a routine tool for process analysis and control, notably for trace 0003- 2700/92/0364 -775A/$03.00/0 0 1992 American Chemical Society
analysis in the nuclear power and semiconductor industries. IC has become so widespread that details such as modifications of vendor - supplied software and merits of vendor service contracts have been published in peer-reviewed literature (2). Currently there are not many vendors of IC columns, especially as compared with the large number of vendors for LC columns. (Major U.S.
vendors include Alltech/Wescan, Dio n e x , EM S c i e n c e , H a m i l t o n , Metrohm, and Waters.) The cost of IC columns continues to be higher than that of their LC counterparts; however, the gap in performance (ultimate efficiency and plates per unit time) between the respective best offerings of the IC and the LC worlds is much smaller today than it was five years ago.
The basic principles of both suppressed and single-column IC have been presented elsewhere and will not be discussed here. An A-page article by Fritz (3)presented the fundamentals and progress made up to 1987. In this REPORT I Will focus on developments that have transpired over the past five years. Cation determinations Because of the large differences in ion-exchange selectivities of alkali versus alkaline earth metals, traditionally it has been difficult to provide simple isocratic elution conditions to allow the separation of both cation classes within a reasonable time period. Kolla et al. (4) introduced a polfibutadiene-maleic acid) copolymer coated silica stationary phase that 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
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senior inventor, these are commercially available. Figure l a shows a separation of alkali and alkaline earth metals. For the alkaline earth metals there is a vast improvement in sensitivity over results obtained with previous single-column conductometric procedures. For example, the limits of detection (LODs) for Mg2+,Ca2+,Sr2+, and Ba2+range from 500 pg to 10 ng for a 100-pL injection. A carboxylate functionality stationary phase of proprietary composition has also been developed for use in suppressed conductometric IC. Figure l b shows an isocratic separation of alkali metals as well as Ca2+ and Mg2+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 remove adventitious cation impurities, LODs for a 25-pL sample in this analytical system range from 30 pg to 5 ng for Li+ to Cs+ and from 170 to 550 pg for M$+ to Ba2+. In 1971 Sickafoose introduced ion-
exchange chromatographic separa tion of heavy metals followed by postcolumn reaction with a chromogenic ligand for optical detection (5~). 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 lanthanide elements, pioneered by Cassidy (5b). The most recent version of this approach uses gradient elution with sodium octanesulfonate as an ion interaction rea g e n t , which p r o v i d e s v i r t u a l ion-exchange sites by adsorption on a nonpolar (C18) stationary phase with a-hydroxyisobutyric acid as the complexing e l u t i n g component. Quantitation is achieved by postcolumn reaction with Arsenazo I11 and optical absorbance detection at 658 nm (Figure 3). Typical detection limits are I 1ng for each element. The determination of trace levels of transition elements in complex environmental 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 with atomic spectrometric techniques. 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 chelation IC as a solution to this problem. The sample is preconcentrated on a macroporous poly(styrenediviny1benzene) (PSDVB) column with iminodiacetate hnctionalities. These groups have strong affinities for heavy metals. Ca2+and Mg2+are also captured to some degree. After the preconcentration step is completed, Ca2+and Mg2+are eluted with 2 M ammonium acetate while the
heavier metals a r e retained. The heavier metals are eluted by an aliquot of 6 M HN03. The effluent is diluted with deionized water in line, and the metals are recaptured on a sulfonated PSDVB concentrator column. The metals on the concentrator column are subjected to chromatography 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 bovine 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 metals in t h e presence of large amounts of alkali and alkaline earths in matrices such as ocean water was developed in t h e USSR (7). The equipment setup, similar to that of suppressed anion chromatography with a carbonate-bicarbonate
La
Figure 1. Separations using a Schomburg column. (a) Separation of Li', Na+, NH,; K+, Mg2+, Can+, S?+, and Ban+(peaks 1-8, respectively), using a 0.1 mM EDTAl3.0 mM HNO, eluent. Amounts injected range from 25 ng f& Li' to 500 ng for Ban+. (Courtesy Waters Chromatography.) (b) lsocratic separation of Li+, Na+, NH;, K+, Mg2+,and Can+ (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 Can+. (Courtesy Dionex)
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Figure 2. Step-gradient elution (1640mM methanesulfonic acid at 10 min) of alkali/alkaline earth metals. Peaks 1-10 (amounts injected in ng in parenthe-
ses): Li' (25), Na+ (loo), NH; (250), K' (250), Rb+ (500), Mg2+ (125), Cs+ (500), Can+ (250), Sr2+ (EO), 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 octanesulfonatelx-hydroxybutyric acid gradient on a C,, stationary phase; postcolumn reaction detection with Arsenazo 111. (Adapted with permission from Reference 5b.)
eluent, features a stationary phase t h a t contains not only anion-exchange s i t e s b u t also chelating/ cation-exchange sites that preferentially bind the transition metals. The sample (or a sequence of samples) 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 with reasonably good sensitivity (LODs range from 10 to 50 ppb). The real power of the chelation IC technique is harnessed in combination with element-selective detection techniques such as inductively coupled plasma optical emission spectrometry (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 detectors or ICP-OES and ICPMS units with the ion chromatograph serving merely as a sample cleanup tool. What is clear is that these techniques result in an unprecedented combination of sensitivity and freedom from matrix interferences (8). Using IC at the front end greatly increases the power of simultaneous multielement atomic absorption spectroscopy (9). A fivefold improvement in LODs of individual elements is easily attained. Anlon determinations The mainstay of IC has been in the determination of anions. When more than 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 achieving separation. I n singlecolumn IC, the use of a poly(methy1 methacrylate)- based anion - exchange stationary phase dominates. Although favorable chromatographic performance has been demonstrated with very strong eluents such as naphthalenetrisulfonate (IO),the most common eluents used in singlecolumn IC are phthalate or borategluconate. The repertoire of columns available for suppressed anion chromatography 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 counterparts), and small particle size (5 pm) high-efficiency stationary phases 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 a s humic acids from the columns. Previous PSDVB stationary phases with small amounts of cross linking were incompatible with organic solvents. However, a new generation of highly cross-linked supports made from 45%ethylvinylbenzene and 55% divinylbenzene has been introduced (11). These supports 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 challenging applications such as the determination of traces of other anions in concentrated acids, bases, and salts. The development of highefficiency stationary phases with moderate to high ion-exchange capacity is vital for establishing a successful solution to this problem (11). A minimum amount of sample must be injected on the column to determine the trace constituents. At the same time, a large capacity is necessary to prevent overloading the column with the major constituents, causing loss of chromatographic performance. Another unusual route to anion determinations has been developed by Lamb et al. (12).Adsorption of macrocyclic cryptands on a hydrophobic stationary phase produces virtual ion-exchange sites with any eluent bearing an alkali metal cation, because of the affinity of the adsorbed macrocycle for the cation. The effective ion-exchange capacity of such a column can be tailored by choosing the nature, composition, and temperature of the eluent. This method, which can be easily exploited for gradient elution, is described in more detail in a later section. IC/MS MS is one of the most powerful techniques for analyzing complex samples. 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 that it is the volatility of the suppressed effluent and not its electrical
conductivity that is of concern here. An NaCl eluent is just as acceptable in this case as a n NaOH eluent, because suppression (exchange of Na' for H') results in either HCI or water, and both are volatile. The first application of IC/MS was reported by Simpson et al. ( 1 3 ~for ) the analysis of carbohydrates separated in the anion-exchange mode with an NaOH eluent. Following the suppressor and a UV detector, a booster pump was used to direct the effluent through a thermospray interface into the mass spectrometer. Without the booster pump, the membrane suppressor cannot adequately handle the back pressure observed at the thermospray interface. The pump also allowed the simultaneous introduction of ammonium acetate to assist in the thermospray ionization process. Conboy et al. (13b) allowed most of the IC suppressor effluent to go to waste while a small amount was directed into a n ion spray interface and an atmospheric pressure ionization mass spectrometer. Both full-scan MS and MS/MS were performed on the eluites of interest: quaternary ammonium compounds and organic sulfates and sulfonates. The analytes were separated by ion interaction chromatography, and the ion interaction reagent was removed from the aqueous-organic matrix by the appropriate membrane suppressor. Recently H s u ( 1 3 c ) described IC/MS analysis of organic anionic compounds in a variety of matrices of practical interest (for example, haz ardous waste samples) with a particle beam interface following the suppressor. He discussed the use of the approach for structural identification of eluites and its limitations, which were attributable primarily to contamination problems with the interface. Although there is little doubt that MS detection following IC separation can be advantageous for eluite identification, the LODs attainable thus far are not particularly impressive; certainly they are far below the sensitivity levels for metallic elements that can be achieved with IC/ ICPMS. Ion-exclusion chromatography Ion-exclusion chromatography is superior to ion-exchange chromatography for the separation of weak acids-especially carboxylic acids. As applied to anionic analysis, a strong acid solution is used as the eluent. Eluite peaks can be detected by direct optical absorption in the relatively few cases in which the analyte
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REPORT displays significant absorptivity. Even when a large counterion acid (e.g., octanesulfonic or perfluoroheptanoic acid) is used as eluent to reduce the background conductance, the background is still large and direct conductivity detection in the single-column mode is not particularly sensitive. The background conductance 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,O, to produce nonionic, nonconducting I,. sensitive conductometric detection was possible; the highest HI concentration that could be used was -4mM because I, has limited solubility. Tanaka and Fritz (14b) showed that for very weak acids (e.g., CO,) 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., H2C03 + KHCO, + KOH) can be used to increase detection sensitivity. Okada and Dasgupta (14c) described a technique for optically detecting eluite acids in the pK, range of 4.5-9.5. The strong acid eluent is
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subjected to exchange with Na’ in a membrane suppressor, resulting in a pH-neutral effluent. I n contrast, weak acids produce salts that are alkaline. This behavior is detected optically via a n acid-base indicator (e.g., 4-nitrophenol) introduced through a membrane. Detection limits as low as 50 pmol are possible, even with 10 mM HNO, a s 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 dimensionality of the separation. This greatly enhances the overall utility of the technique when it is essential to determine both classes of anions simultaneously. Jones et al. ( 1 5 ~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 thus allows part-per-billionlevel detection using direct conductivity detection. The ultimate in detection sensitivity is not required for many real samples. Indeed, in such cases singlecolumn unsuppressed operation, with which different systems can be
coupled, may allow greater versatility. For example, Saari-Nordhaus et al. (15b)have shown that it is possible to perform simultaneous determi nation of cations and anions by coupled but independent ion- exchange modes. This system is more practical than previous ones designed to simultaneously 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 stationary phase in which latex- bonded ion-exchange sites are attached to a neutral hydrophobic macroporous core. Both ion-exchange and hydrophobic interactions occur on such stationary phases. Because up to 100% organic solvent can be used as eluent, this stationary phase is well suited for ion-exchange, reversedphase, or ion-interaction separations, or any combination thereof. True multimode separations on a single column can be realized, as illustrated in Figure 4. Suppression reactions and new suppressors Sat0 and Miyanaga (16a) have described three new ways of achieving conductometric suppression. In one, the potassium ions in a dipotassium ethylenediamine diacetate eluent are
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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 1-1 3:benzyl alcohol, diethyltoluamide, benzene, benzoic acid, benzenesulfonic acid, toluenesulfonic acid, pchlorobenzenesulfonicacid, pbromobenzoic acid, phthalic acid, terephthalic acid, p hydroxybenzenesulfonic acid, 1,3,5-benrenetricarboxylicacid, and 1,2,4,5-benzenetetracarboxylic acid. (Courtesy Dionex)
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Figure 5. Gradient elution with electrodialytically generated and suppressed NaOH eluent. The current program is 0-200 mA, resultin in 0-1 17 mM NaOH. Peaks 1-12: 100 pN each of F-, IO:, HCOO-, BrO;, CI-, NO;, NO;, SOP-, “PO!-, As@-, CrOP-, and I-.
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exchanged for copper to form a neutral nonconducting chelate. In a second scheme, ethylenediammonium carbonate reacts with resin- bound Cu2+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 e f f l u e n t w i t h 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 ( 1 6 ~ It ). has long been claimed that application of a voltage of appropriate polar ity 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 regenerant in a n 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 regenerant; the penetration of the regenerant counterion can be completely eliminated. In one mode, the detector cell effluent can be used as the regenerant. (For practical purposes, this effluent is pure water containing 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 that significantly larger eluent concentrations can be exchanged by the suppressors.
erating high-purity acids. They described how very high purity eluents (suppressed conductance of 0-175 mM NaOH is 340 k 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 membrane. 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 concentra tion generated is controlled by the current. Thus it has been shown that gradient elution can be performed by programming the eluent generator current (16i 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 Strong et al. (17) demonstrated the possibility of electrodialytically gen-
Gradient IC 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 single-column mode. They used a gradient between two eluents of equal conductivity. The strong eluent contained a greater eluent ion concentration but had a lower mobility counterion (Li') as the cation; therefore, 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 they used h a s much 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-
which is parallel to an increase in eluent strength. After suppression by a conventional membrane suppressor, the background conductance from the NaOH or LiOH eluent is the same. Background shift in the course of the gradient separation run is negligible. In another twist, the macrocycle cation binding decreases with increasing temperature; therefore, a decrease in column-exchange capac i t y for t h e same eluent can be brought about by increasing the column temperature. Examples of all three types of gradient IC are illustrated in Figure 6. The number of actual gradient IC applications described in the literature for real samples remains very limited. I n most cases, the user chooses a column-switching technique or at best a step gradient. The question of superiority among the above and electrodialytically or conventionally generated gradients may be moot. To some people, gradient IC is still the solution in search of the right problem. Detectors The conductometric detector remains the mainstay of IC. It has been shown that inexpensive directcurrent conductivity detectors of low cell volume can be easily constructed (19a).The performance of these detectors rivals that of commercial detectors when the background conductance is low (e.g., with a suppressed NaOH eluent). I t has been shown that, when an eluite acid peak is trapped in the cell, a chronoamperometric profile is generated that is characteristic of the anion; therefore, the mobility of the anion can be estimated from the data. This approach can confirm the identity of an anion (19b). Although the pulsed amperometric detector (20) was introduced in the early 1980s, it has become commercially available only recently. It is a particularly powerful detector, especially for the determination of various sugars and saccharides typically separated by anion - exchange chro matography. The repertoire of this detector continues to expand and now includes alcohols and alkanolamines.
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-borategluconate. (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.)
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Postsuppressor manipulations When the detector background is virtually devoid of ions-as with a n NaOH eluent in a suppressed system-there is always the alluring possibility that eluite ions can be exchanged 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 Ce3+, eluite anion for 0- aminobenzoate) with fluorometric detection. Subsequent work has also shown that W 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 replacement technique and have described in detail the sensitive detection of anions after replacement of the accompanying H’ with Li’ and flame photometry. Eluting cations were similarly detected in a cation chromatography system with a nitric acid eluent. A parallel technique of anion replacement with iodate and U V detection was also described. The replacement technique has been explored in the microcolumn format by Takeuchi et al. (21d).Nevertheless, the increased stability of electrical conductivity detectors and the subsequent improvement of S/N through better electronics, cell de sign, and thermostating are such that the meager gains in sensitivity-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 dimensionality. Thus, after conductometric detection of eluite HX in a conventional 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 previous work of Tanaka and Fritz (14b). Although the efficiency of the twostep conversion of HX to NaOH decreases 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 pK, of the eluite and approximate quantitation without specific calibration. Furthermore, ratioing the two detector signals (after appropriate correction for time delay and band dispersion) can yield several very ’
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 potable water and the determination of sulfate in atmospheric particles (after collection on a filter and aqueous extraction) have been pivotal developments. Determination of gaseous SO2, arguably the most important of the major gaseous pollutants, is achieved after collection in a dilute H202 absorber and preremoval of particulate sulfate by a filter. Recently we have made a concerted effort to determine atmospheric gases of interest by using IC without discrete manual collection, transport, and analysis steps. The attractiveness of such a n approach is that many of the atmospheric gases of interest (e.g., HCOOH, CH,COOH, HC1, HONO, HNO,, SO,) 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 determined by anion chromatography. Lindgren and Dasgupta (22a) described the use of a porous hydrophobic - m e m b r a n e - b a s e d d i f f u s i o n scrubber. Scrubber liquid flows through a membrane tube, and air is sampled through a j a c k e t t u b e around it. The liquid effluent for the scrubber is aspirated through the injection loop of an ion chromatograph. Every few minutes the 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 Swedish a n d Swiss researchers. When the analyte ion of interest does not occur a t significant levels in the aerosol phase (e.g., NO-,),a helical membrane diffusion scrubber can be used; this permits the use of a longer membrane and thus greater collection efficiency. Vecera and Dasgupta (22b)showed the facile determination of part - per - trillion levels of HONO with such a device and used U V detection, which is more sensitive than conductometry for determining NO,. Since then, these authors have demonstrated the simultaneous collection and analysis of HONO and
HNO,. Following low -pressure ion chromatographic separation on a minicolumn and reduction of nitrate to nitrite, the Griess-Saltzman reaction was used to produce a purple dye that was detected colorimetrically (22~). This work was carried out with a wet denuder, 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 denuder. They showed that particles are efficiently transmitted through such a device and that single-digit part - per - trillion -level detection lim its for SO, are easily achieved. We have continued to explore this technique. Newly designed parallel plate or annular wet denuders permit near-quantitative 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 - per - trillion level. After the gases are removed, soluble constituents of the aerosol can be determined. Ongoing work shows that “wet impactors” for collecting atmospheric aerosols can be designed, and thus a new method for continuous automated analysis of the soluble
Figure 7. Ion chromatogram of an ambient air sample compared with a liquid-phase standard. Top trace is an ion chromatogram of ambient air scrubbed into dilute H202.Lower trace is an ion chromatogram of a standard liquid phase. Peaks a and b correspond to 700 pptr HNOBand 320 pptr SOa, respectively. The scale bar next to the upper trace represents 100 nS/cm; the scale bar next to the lower trace represents 3 pSlcm. (Adapted from Reference 22a.)
\
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REPOR7 fraction of the atmospheric aerosol composition is possible. Ion analysis and IC The vast majority of IC applications involve simultaneous determination of more than one analyte. Most users regard the ease with which IC can determine analytes other than the principal species of interest a s a great asset. Therefore it is not surprising that IC has survived commercially amid competition from continuous-flow analyzers, whether segmented or unsegmented. Increasingly it is claimed, however, that IC has finally met its match in capillary zone electrophoresis (CZE). Several vendors provide buffer recipes that permit analysis of cations or anions via indirect photometric detection. The number of plates (or plates per unit time) that can be generated by this technique is much greater than that attainable by IC. Concentration detection limits, however, are less attractive than those obtained with IC, and preconcentration by various electromigration/ stacking techniques is less straightforward than the type of uniform preconcentration for all ions that can
be attained on an IC preconcentrator column. These hurdles may not be impossible to overcome. A greater challenge for many real samples concerns analytical reproducibility in routine use. Because CZE is acutely dependent on the stability of electroosmotic flow, and because the latter can be significantly affected by the adsorption of sample constituents to the walls of the capillary, a ready and universally applicable solution to such a problem is not immediately apparent. Regardless of the fact that separation selectivities in electrophoresis and ion exchange are quite different and IC would therefore always remain in some form, IC is hardly threatened at the moment. Nevertheless, the next time another overview on ion separation techniques is described in this JOURNAL, CZE will doubtlessly demand much greater attention than 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 Chromatography.
References (1) Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975,47,1801-09.
(2) Lynch, G. J.Process Control Qual. 1991, I. 249-63. (3) Fritz,-J . S. Anal. Chem. 1987, 59, 335 A-344 A. (4) Kolla, P.; Kohler, J.; Schomburg, G. Chromatographia 1987,23, 465-72. ( 5 ) a. Sickafoose. J. P. Ph.D. Dissertation, Iowa State University, 1971. b. Cassidy, R. M. Chem. Geol. 1988, 67, 185-95. ~ _ _ (6) Siriraks, A.; Kingston, H. M.; Riviello, J. M.Anal. Chem. 1990,62,1185-93. ( 7 ) Dolognosov, A. M.; Grishinova, T. V.; Timerbaev, A. R. Abstracts of Papers, International Ion Chromatography Symposium, Denver, CO, 1991; Abstract 16. (8) a. Riviello, J.; Siriraks, A.; Manabe, R. M.; Roehl, R.; Alforque, M. LC-GC 1991,9, 704-11; b. Kawabata, K.; Kishi, Y.; Kawaguchi, 0.;Watanabe, Y.; Inoue, Y. Anal. Chem. 1991, 63, 2137-40; c. Heithmar, E. M.; Hinners, T. A.; Rowan, J. T.; Riviello, J. M. Anal. Chem. 1990, 62. 857-64. - ?
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1991;Abstract 10; b. Toofan, M.;Stillian, J.; Pohl, C.; Saini, C. ZZCS Abstracts, Abstract 15; c. Woods, J. P. ZZCS Abstracts, Abstract 21;d. Siriraks, A.; Pohl, C.; Toofan, M. ZZCS Abstracts, Abstract 40. (12)a. Lamb, J. D.; Smith, R. G. J. Chromatogr. 1991, 546, 73-88; b. Smith, R. G.; Drake, P. A.; Lamb, J. D. J. Chromatogr. 1991, 546, 139-49; c. Lamb, J. D.; Drake, P. A.; Woolley, K. In Advances in Ion Chromatography, Jandik, P.; Cassidy, R. M., Eds.; Century International: Medfield, MA, 1990;Vol. 2, pp. 215-31; d. Lamb, J.D.; Drake, P.A. J. Chromatugr. 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. J. Chromatogr. 1989,473, 171-88; b. Saari-Nordhaus, R.; Nair, L.; Anderson, J. M., Jr. Abstracts of Papers, International Ion Chromatography Symposium, Denver, CO, 1991;Abstract 4; .c. Stillian, J. R.; Pohl, C. A. J. Chromatogr. 1990,499,249-66. (16)a. Sato, H.; Miyanaga, A. Anal. Chem. 1989,61,122-25; b. Gjerde, D. T.; Benson, J. V. Anal. Chem. 1990,62,612-15; c. Robles, M.N. Abstracts ofPapen, Inter-
national Ion Chromatography Symposium, Denver, CO, 1991;Abstract 55; d. Tian, Z.W.; Hu, R. Z.; Lin, H. S.; Hu, W. L. J. Chromatogr. 1988,439, 151-57; e. Strong, D. L.; Dasgupta, P. K. Anal. Chem. 1989,61,939-45;f. Strong, D. L.; Joung, C-U.; Dasgupta, P. K. J. Chromatogr. 1991,546,159-73. 17) a. Strong, D. L.; Dasgupta, P. K.; Friedman, K.; Stillian, J. Anal. Chem. 1991,63,480-86;b. Strong, D. L.; Dasgupta, 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 20306;b. Larew, L.A.; Johnson, D. C. Anal. Chem. 1988, 60, 867-72; c. LaCourse, W. R.; Jackson, W. A.; Johnson, D. C. Anal. Chem. 1989,61,2466-71; d. Andrews, R. W.; King, R. M. Anal. Chem. 1990,62,2130-34;e. Lacourse, W. R.; Johnson, D. C.; Rey, M. A.; Slingsby, R. W. Anal. Chem. 1991,63,134-39. (21)a. Downey, S.W.; Hieftje, G. M. Anal. Chim. Acta 1983, 153, 1-13; b. Shintani, H.; Dasgupta, P. K. Anal. Chem. 1987, 59, 1963-69; c. Galante, L. J.; Hieftje, G. M. Anal. Chem. 1987, 59,2293-2302and Anal. Chem. 1989,60, 995-1002; d. Takeuchi, T.; Suzuki, E.; Ishii, D. Chromatographia 1988,25, 58284;e. Berglund, I.; Dasgupta, P. K. Anal. Chem. 1991,63,2175-83.
(22)a. Lindgren, P. F.; Dasgupta, P. K Anal. Chem. 1989,61,19-24;b. Vecera, Z.; Dasgupta, P. K. Environ. Sci. Technol. 1991, 25, 225-60; c. Vecera, Z.; Dasgupta, P. K. Anal. Chem. 1991,63,221016; d. Simon, P. K.; Dasgupta, P. K.; Vecera, Z. Anal. Chem. 1991,63,123742.
Purnendu IC ‘Sandy” Dasgupta, a native of India, received his B. Sc. degree (1968) and his M.Sc. degree in inorganic chemistry (1970) fiom the University of Burdwan, India, and his Ph.D. (1977) in analytical chemistry from Louisiana State University at Baton Rouge. His research focuses on atmospheric analysis, ion chromatography, automated flow analysis (in the laboratory and for the chemical process industry) and, most recently, suppressed conductometric detection in capillary electrophoresis. He was recently appointed Paul Whifteld Horn Professor at Texas Tech University.
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