Recent Developments in Ion-Exchange Columns for Inorganic Ions and Low Molecular Weight Ionizable Molecules

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32 LCGC LC COLUMN TECHNOLOGY SUPPLEMENT APRIL 2006 www.chromatographyonline.com Recent Developments in Ion-Exchange Columns for Inorganic Ions and Low Molecular Weight Ionizable Molecules

Ion exchange is one of the older of the chromatographic techniques yet each year new products continue to hit the market. In this paper, Chris Pohl of Dionex will summarize some of the stationary phases that have been developed for modern ion-exchange and ion chromatography. He will focus on phase design and then turn his attention to new anion and cation columns introduced in the last couple of years.

on-exchange chromatography is the years in the form of mixed-mode stationary chromatographic technique most phases. Early versions of this approach sim- I widely used for the separation of ply mixed particles of silica separately ionic and ionizable compounds. Classical bonded with reversed phase ligands or ion- ion-exchange techniques have been used exchanger ligands. Later versions used for many years for the separation of inor- simultaneous bonding of ligand mixtures. ganic cations and anions, amino acids, The newer generation of mixed mode organic acids, amines, and proteins. This phases makes use of a hydrophobic ligand review will discuss general stationary phase also containing an ionizable site (see the fol- architecture and covers new columns intro- lowing for more information about ligand duced in the last couple of years intended structure used in mixed-mode columns). for the analysis of inorganic ions and low Electrostatic-agglomerated films on molecular weight ionizable molecules. nonporous substrates: These were among the first types of materials to be employed Stationary Phase Architecture in separation of small ions. Developed in Stationary phase construction for columns the early 1970s and first described by in this category consists of eight basic Hamish Small of Dow Chemical (Midland, architectures: silane-based modification of Michigan), these materials have been the porous silica substrates; electrostatic- mainstay of suppressor-based ion chro- agglomerated films on nonporous sub- matography for many years. Originally strates; electrostatic-agglomerated films on developed as a convenient means of pro- ultrawidepore substrates; polymer-grafted ducing low-capacity hydrolytically stable films on porous substrates; chemically materials when first generation suppressors derivatized polymeric substrates; polymer- had limited capacity, this type of stationary encapsulated substrates; ionic molecules phase is now only used in guard columns adsorbed onto chromatographic sub- and concentrator columns where sample strates; and step-growth polymers on poly- capacity is not a major factor in column meric substrates. I will now delve into the design. Materials of this sort are constructed differences of the eight approaches. with a nonporous polymeric substrate. Silane-Based modification of porous sil- Early materials used a low cross-linked sub- ica substrates: Of the eight approaches, strate but modern materials of this type Chris Pohl silane-based modification of porous silica make use of high cross-linking to render the substrates, although one of the first stationary phase compatible with all com- Dionex Corp., Sunnyvale, California approaches to be employed is now rarely mon high performance liquid chromatog- employed in the separation of small ions raphy (HPLC) solvents. In principle, the Please direct correspondence to Chris when used in conjunction with conductivity substrate could be composed of inorganic Pohl at [email protected]. detection. However, this type of stationary materials such as silica, alumina, zirconia or phase has seen a resurgence in the last few titania but, to date, no commercial exam- www.chromatographyonline.com APRIL 2006 LCGC LC COLUMN TECHNOLOGY SUPPLEMENT 33

Figure 2: Separation of ions using an anion- Figure 1: Separation of ions using an anion-exchange phase designed for use with carbon exchange phase designed for use with carbonate eluent systems. Column: IonPac AG22, AS22, 4 mm; eluent: 4.5 mM sodium carbonate–1.4 ples of such materials are known. The sub- control. Packing materials of this sort are mM sodium bicarbonate; flow rate: 1.2 mL/min; strate is then derivatized to introduce prepared through attachment of polymer injection volume: 10 ␮L; detection: suppressed charged groups onto the surface of the sub- strands to the surface of a substrate. To pre- conductivity, ASRS ULTRA II 4 mm, AutoSupres- sion recycle mode; temperature: 30 °C. Peaks: 1 strate. Following derivatization, the sub- pare such materials, the substrate is either ϭ fluoride (5 ppm), 2 ϭ acetate (20 ppm), 3 ϭ strate is brought into contact with a suspen- prepared with polymerizable groups on the chloride (10 ppm), 4 ϭ nitrite (15 ppm), 5 ϭ sion of oppositely charged colloidal surface or the surface is modified to intro- bromide (25 ppm), 6 ϭ nitrate (25 ppm), 7 ϭ particles to produce the final product. duce polymerizable groups. Resin, phosphate (40 ppm), 8 ϭ sulfate (30 ppm). Although this construction might sound monomer(s), and initiator are then allowed strained to the surface. Reactions that take like something that would be inherently to react to produce the grafted composite. place beneath the surface in the dense poly- unstable, in fact, such materials are nearly Incorporation of a cross-linking monomer mer matrix of the substrate will exhibit indestructible when constructed using a in the reaction mixture will produce a gel sluggish mass transport and relatively poor styrenic substrate with a colloidal film of with substrate particles suspended in the chromatographic performance. Early exam- vinyl–aromatic ion-exchange material. gel much like fruit cocktail suspended in a ples of this stationary phase architecture For the most part, materials of the sec- gelatin fruit salad. Because such a gel could exhibited relatively poor performance but ond type of architecture have been not be used as column packing material, newer materials such as the IC SI-52 4E replaced by a higher capacity version of this type of stationary phase synthesis pre- column (Showa Denko, Kawasaki, Japan) the material: electrostatic agglomerated cludes the use of cross-linking agents for illustrate that high performance materials films on ultrawide pore substrates. selectivity control (unless cross-linker is can indeed be constructed in this manner. Electrostatic-agglomerated films on added after the graft step). In theory, such Polymer-encapsulated substrates: Pro- ultrawide pore substrates: Using architec- materials could be prepared from either fessor Gerard Schomberg of the Max ture similar to that described previously, but polymer or silica substrates but in practice Planck Institute (Mulheim-Ruhr, Ger- making use of substrates with pore sizes in only polymeric substrates are in commer- many) pioneered this type of material as a the 1000–3000 Å range, it is possible to cial use. The IonPac CS18 column means of preparing reversed-phase materials construct substantially higher capacity mate- (Dionex) described in the following is a sta- using alumina as the base material. Synthe- rials. For example in a typical application tionary phase using this architecture. sis of polymer-encapsulated materials is with a ultrawide pore substrate with pore Chemically derivatized polymeric sub- accomplished by combining the substrate, a sizes large enough to accommodate a coat- strates: This type of material tends to preformed polymer with residual double ing of ion-exchange colloid on the interior involve proprietary chemistry, so the actual bonds and a suitable free radical initiator and exterior surfaces, the resulting material chemistry used for the derivatization reac- dissolved in solvent, stripping off the sol- will exhibit six to eight times the capacity tion is generally unknown. In general, pack- vent to leave a polymer film on the sub- achievable on an identical particle size non- ing materials of this sort are of rather substrate and then curing the film at elevated porous analog (that is, 30–150 mequiv/mL stantial capacity, so they have come into temperature to yield a cross-linked film per- for the ultrawidepore format vs. 5–30 vogue in recent years with the general shift manently encapsulating the substrate. The mequiv/mL for the nonporous format). toward materials of increasing capacity. The technique was later adapted by Schomberg’s Given the increasing importance of high Metrosep A Supp 8 column and the Met- group as a means of preparing a weak cation capacity chromatographic materials and the rosep A Supp 10 column (Metrohm, exchange phase using a preformed butadi- increasing use of high-capacity suppressor Herisau, Switzerland) (see the following) ene–maleic acid copolymer as the encapsu- devices for ion chromatography, this station- are both believed to be examples of materi- lating polymer. Introduction of this mate- ary phase architecture has seen wide use. als of this type, although, no specific infor- rial fundamentally changed the focus of Polymer-grafted films on porous sub- mation on the preparation chemistry is stationary phase design for inorganic strates: This type of material is widely used available from the manufacturer. The criti- cations, shifting the emphasis from strong to prepare high capacity packings where cal difficulty with such materials is the cation-exchange materials to weak cation- cross-linking is not required for selectivity requirement that the derivatization be con- exchange materials for most applications. 34 LCGC LC COLUMN TECHNOLOGY SUPPLEMENT APRIL 2006 www.chromatographyonline.com

2 3 7

4 8 1

9 Figure 4: Separation of petrochemical indus- try analytes. Column: IonPac CS18, 2 mm; elu- Figure 3: Separation of common anions ent: 0.5 mM MSA, gradient to 1 mM at 20 min, 0 5 10 15 20 25 along with disinfectant by-product anions. gradient to 4 mM at 28 min, gradent to 11 mM Time (min) Columns: IonPac AG23, AS23, 4 mm; eluent: at 34 min, isocratic to 40 min, back to 0.5 mM 4.5 mM sodium carbonate–0.8 mM sodium at 40.1 min; eluent source EGC II MSA; flow bicarbonate; flow rate: 1.0 mL/min; injection Figure 5: Separation of transition metals and rate: 0.3 mL/min; temperature: 50 °C; injection volume: 25 L; detection: suppressed conduc- nonmetal cations. Column: 100 mm ϫ 4.6 mm ␮ volume: 5 ␮L; detection: suppressed conductiv- tivity, ASRS ULTRA II 4 mm, AutoSupression Universal Cation; mobile phase: 2 mM tartaric ity, CSRS ULTRA II 2 mm, AutoSuppression recy- recycle mode; temperature: 30 °C. Peaks: 1 acid, 1 mM oxalic acid; flow rate: 1 mL/min; ϭ cle mode. Peaks: 1 ϭ lithium (0.05 ppm), 2 ϭ fluoride (3 ppm), 2 chlorite (10 ppm), 3 detection: conductivity. Peaks: 1 ϭ lithium (0.5 ϭ ϭ sodium (0.2 ppm), 3 ϭ ammonium (0.25 ppm), bromate (20 ppm), 4 chloride (6 ppm), 5 ppm), 2 ϭ sodium (0.5 ppm), 3 ϭ ammonium ϭ ϭ 4 ϭ ethanolamine (3.0 ppm), 5 ϭ methy- nitrite (10 ppm), 6 chlorate (25 ppm), 7 (0.5 ppm), 4 ϭ potassium (0.8 ppm), 5 ϭ ϭ ϭ lamine (3.6 ppm), 6 ϭ diethanolamine (3.6 bromide (25 ppm), 8 nitrate (25 ppm), 9 nickel (5 ppm), 6 ϭ zinc (5 ppm), 7 ϭ cobalt (5 ϭ ϭ ppm), 7 ϭ potassium (0.5 ppm), 8 ϭ ethyl- phosphate (40 ppm), 10 ϭ sulfate (30 ppm). ppm), 8 ϭ magnesium (0.7 ppm), 9 ϭ calcium amine (3.0 ppm), 9 ϭ dimethylamine (1.4 (0.7 ppm). (Courtesy of Grace Alltech.) An example of this architecture is the Uni- ppm), 10 ϭ N-methyldiethanolamine (3.0 ppm), 11 ϭ mopholine (3.2 ppm), 12 ϭ 1- versal Cation column (Grace Alltech, Deer- dimethylamino-2-propanol (3.7 ppm), 13 ϭ N- tage of the column selectivity is that the field, Illinois) described in the following. methylmorpholine (7.5 ppm), 14 ϭ butylamine position of carbonate relative to other com- Ionic molecules adsorbed onto chro- (1.5 ppm), 15 ϭ magnesium (0.25 ppm), 16 ϭ mon anions can be readily adjusted by alter- matographic substrates: A number of calcium (0.5 ppm), 17 ϭ strontium (0.5 ppm), ing the ratio of carbonate and bicarbonate in 18 ϭ barium (0.5 ppm). such columns were developed in the early the mobile phase. While carbonate is typi- 1990s for anion-exchange separations. Such to the resin surface. Finally, in a repetitive cally not an analyte of interest in drinking columns have the advantage of providing series of reactions, this polymer-coated sub- water analysis, its proximity to other anions exceptional resolution of highly hydrated strate is allowed to react with first an epoxy of interest can often compromise analytical anions such as fluoride from the column monomer and then an amine monomer. By performance. With the IonPac AS22, car- void volume. However, because the station- using a primary amine for the amine bonate can be moved from an early portion ary phase is an adsorbed coating, it slowly monomer, it is possible to introduce branch of the chromatogram to just after nitrate, leaches from the substrate through contin- sites through subsequent reaction with improving the analytical performance of the ued use and is rapidly removed when even additional epoxy monomer. The resulting column when utilized for determination of low percentages of solvents are incorporated surface composite can be exceptionally trace levels of nitrite, chlorate, and bromide. in the eluent. The latter disadvantages sig- hydrophilic, containing only aliphatic sub- The high-capacity column based upon nificantly have limited the popularity of stituents and yet it is completely compatible ultrawide pore substrate (210 ␮equiv/col- such phases. The Metrosep A Dual 4 col- with high-pH mobile phases which tend to umn in the case of the 250 mm ϫ 4 mm umn (Metrohm) described in the following damage most hydrophilic stationary phases. column) allows most drinking water sam- is the most recent commercial product to ples to be directly injected without overload- utilize this stationary phase architecture. New Anion-Exchange ing the column. In addition, the column Step-growth polymers on polymeric Chromatography Columns exhibits an unusually wide application range substrates: This simple yet versatile synthe- The analysis of trace anions in drinking with good chromatographic performance sis method has seen wide use in recent years. water is an important area in environmental for analytes ranging from fluoride to per- Over the last few years, six anion-exchange analysis and is the focus of the United States chlorate, which generally does not exhibit columns (see below) have been introduced Environmental Protection Agency’s Method good chromatographic performance with using this stationary phase architecture. It is 300.1 Part A. In the area of anion-exchange columns designed for analysis of the com- essentially a hybrid of the third and fourth column development, a new anion- mon anions. architectures described previously. Station- exchange phase for analysis of drinking In the area of disinfectant byproduct ary phase preparation begins with sulfona- water, the IonPac AS22 column (Dionex), analysis, two new columns, the IonPac tion of a wide-pore substrate to introduce was introduced this year at the Pittsburgh AS23 (Dionex) and the Metrosep A Supp anionic surface charges. Then, an epoxy- Conference. This is the first column based 7 column (Metrohm) have been intro- amine copolymer is formed in the presence upon Type 8 stationary phase architecture duced to add to the field of commercially of this material, producing an amine rich (see Figure 2) designed specifically for use available anion exchange columns designed “basement” polymer electrostatically bound with carbonate eluent systems. An advan- for this application available from Dionex, www.chromatographyonline.com APRIL 2006 LCGC LC COLUMN TECHNOLOGY SUPPLEMENT 35

Figure 7: Adjustment of solvent content using Figure 6: Separation of ionic analytes using a mixed-mode column to affect elution order of an LC–MS-compatible eluent system. Column: analytes. Column: 150 mm ϫ 3.2 mm Primesep 150 mm ϫ 4.6 mm Primesep 200; mobile B2; flow rate: 0.5 mL/min; detection: UV phase: 20:80:0.1 methanol–water–formic absorbance at 210 nm; eluent (left): 20:80:0.2% acid; flow rate: 1.0 mL/min; detection: UV acetonitrile–water–trifluoroacetic acid; eluent absorbance at 250 nm. Peaks: 1 ϭ DOPA, 2 ϭ (right): 20:80:0.1% acetonitrile–water–trifluo- tyrosine, 3 ϭ phenylalanine. Courtesy of SIELC roacetic acid. Courtesy of SIELC Technologies Technologies, Prospect Heights, Illinois. from anions commonly found in drinking Showa Denko, and Grace/Alltech. The water, the analysis of perchlorate at low Figure 8: Separation of ions obtained using a IonPac AS23 column, constructed in a part-per-billion levels in a high total dis- monolithic column at two flow rates. Column: manner analogous to the IonPac AS22 solved solids groundwater matrix tends to 100 mm ϫ 4.6 mm Metrosep Dual 4; eluent: 12 described previously, can accomplish the be a demanding application. Analysis of mM p-cyanophenol, 5 mM potassium hydrox- separation of the common anions along perchlorate at such levels in the presence of ide; temperature: 30 °C; injection volume: 5 ␮L; detection: suppressed conductivity, ASRS ULTRA with the disinfectant byproduct anions more than 3000 ppm total dissolved solids II 4 mm, AutoSupression recycle mode. Peaks: 1 mandated in EPA Method 300.1 Part B in which can be found in groundwater sam- ϭ fluoride (2 ppm), 2 ϭ chloride (3 ppm), 3 ϭ less than 23 min (see Figure 3). The IonPac ples necessitated the development of cus- sulfate (5 ppm), 4 ϭ iodide (20 ppm), 5 ϭ thio- AS23 column also shares with the IonPac tomized anion-exchange materials exhibit- sulfate (10 ppm), 6 ϭ thiocyanate (20 ppm). AS22 the unique feature of being able to ing good chromatographic performance for phosphoric acid in soft drinks: Metrosep A adjust the selectivity of the column for car- perchlorate while still providing high load- Supp 1 HS (Metrohm) is a special column bonate over a wide range. The Metrosep A ing capacity. In recent years, several new for the rapid determination of phosphoric Supp 7 column, another column designed products have been introduced to address acid using a carbonate-based eluent system. specifically for this application is based this application area. The IonPac AS20 It is prepared from a styrene divinylben- upon chemical modification of a polyvinyl (Dionex) is constructed using Type 8 archi- zene copolymer which has been derivatized alcohol substrate (Type 5 architecture). tecture. The high capacity of the stationary chemically (Type 5 architecture). This col- The Metrosep A Supp 7 column offers an phase (310 ␮equiv/column in the case of umn enables the analysis of phosphate in exceptionally high-efficiency separation of the 250 mm ϫ 4 mm column) combined cola drinks in the presence of chloride and the standard anions along with the oxy- with the highly hydrophilic polymer back- sulfate in less than 3 min. The other recent halides. The two columns each have some- bone result in an ideal material for the entry to this product category is the IonPac what different selectivity with regard to the analysis of perchlorate in environmental Fast Anion IIIA column (Dionex) (4). This oxyhalides and carbonate. Another column samples. The column is specified as the con- column, based upon Type 8 architecture, is recently added to this category is the IC SI- firmation column in EPA Method 314.1 designed for use with hydroxide eluents. It 91 4C column (Showa Denko, Kawasaki, for the determination of perchlorate in allows rapid elution of both phosphate and Japan) specifically designed for analysis of drinking water samples because its selectiv- citrate with total analysis time being less bromate when used in conjunction with ity is significantly different from the IonPac than 4 min for samples of minimal com- EPA Method 317, which utilizes a postcol- AS16 (Dionex), specified as the primary plexity. Citrate is commonly found in soft umn reaction and UV detection for a column in EPA Method 314.1. In addition, drinks and represents an analytical chal- determination of bromate. Because of the a lower capacity version of this the AS20 lenge when using conventional carbonate specificity of the method, it is not necessary column, the IonPac AS21 (Dionex) has eluent systems due to its high selectivity to use a high-resolution column for this been introduced for the analysis of perchlo- relative to phosphate. This column chem- application. Thus, the IC SI-91 4C col- rate in EPA Method 331.0 via electrospray istry is compatible with carbonate eluent umn is only 100 mm in length. LC-MS. The lower capacity of the column systems as well, permitting the rapid analy- Another relatively new area of environ- renders it compatible with a volatile eluent sis of both phosphate and citrate with mental interest is the analysis of perchlorate. system based upon methylamine suitable either eluent system. Perchlorate was generally thought to be rare for use in electrospray applications while In addition, Metrohm has introduced in the environment but the relatively recent the high sensitivity of the detection tech- two new high capacity anion columns for discovery of perchlorate in food crops (1), nique allows achievement of the necessary use in high ionic strength matrices. The dairy products (2) and human milk samples detection limits with much smaller injec- Metrosep A Supp 8 (Metrohm) allows (3) has elevated greatly the interest in this tion volumes, thus, avoiding column over- determination of nitrite, bromide, and drinking water contaminant, especially in load for this relatively low capacity column. nitrate in concentrated salt solutions. UV United States. While perchlorate is a highly Two new anion-exchange columns have detection at 215 nm allows the determina- retained anion and typically well resolved been introduced recently for the analysis of tion of the above analytes in the single digit 36 LCGC LC COLUMN TECHNOLOGY SUPPLEMENT APRIL 2006 www.chromatographyonline.com

Table I:

Column name Company Ionic retention mode pKa of ionic site Primesep A SIELC Technologies Cation exchange 0 Primesep 100 SIELC Technologies Cation exchange 1 Primesep P* SIELC Technologies Cation exchange 1 Primesep 200 SIELC Technologies Cation exchange 2 Primesep C SIELC Technologies Cation exchange 3.5 Primesep 500 SIELC Technologies Cation exchange 5 Primesep D SIELC Technologies Cation exchange 10 Primesep B2 SIELC Technologies Cation exchange 5 Primesep AB SIELC Technologies Anion and cation exchange not stated Acclaim Surfactant Dionex Anion exchange 10

*Utilizes a phenyl group in the hydrophobic portion of the ligand parts-per-billion range. A sodium chloride simultaneously resolve mono-, di-, and tri- real environmental samples requires the use eluent is used for these applications. The ethanolamine all along with the common of a colorimetric metal complexing post- column which appears to be an example of inorganic cations single analysis. The col- column reagent such as pyridylazoresorci- Type 5 architecture with a styrene–divinyl- umn can be used in isocratic mode with nol to boost sensitivity. benzene backbone, has an extremely high either suppressed or nonsuppressed conduc- capacity (700 ␮mol/column in the case of tivity detection and can readily separate a New Mixed-Mode Columns the 150 mm ϫ 4 mm column). The other wide variety of amines without resorting to One product category which has seen a new column in this category is the Met- solvent containing eluents (see Figure 4). In fairly large number of new products in rosep A Supp 10 column, apparently, also addition, the high hydronium selectivity of recent years is silica-based mixed-mode sta- an example of Type 5 architecture with a the column makes it a good choice for tionary phases. While commercial prod- styrene–divinylbenzene backbone (210 analysis of multiply charged species. Separa- ucts with mixed-mode stationary phases ␮mol/column in the case of the 250 mm ϫ tions of diamines as well as biogenic amines have been around since the late 1980s, 4 mm column) is suitable for high-ionic- were both demonstrated using the new col- there has been considerable new-product strength samples. The column can be oper- umn, neither separation requiring any activity in this area based upon new syn- ated at elevated temperature using simple organic solvent in the mobile phase. thetic approaches. The company leading carbonate eluent systems or at ambient A significant fraction of cation-exchange this new wave of activity is SIELC Tech- temperature with a carbonate–bicarbon- columns in use today for the analysis of nologies. Table I lists a number of columns ate–perchlorate eluent system. The addition inorganic cations utilize the stationary from SIELC Technologies as well as a new of perchlorate to the eluent system helps phase architecture developed by Professor entry into this product category from mask high-energy sites often found in Schomberg (Type 6 architecture). While Dionex. SIELC Technologies utilizes an styrenic stationary phases. the most common application for such embedded ionizable group in their ligand columns is determination of alkali metal design. Their approach borrows from the New Cation-Exchange Chro- and alkaline earth metal cations, this sta- architecture of polar embedded reversed matography Columns tionary phase architecture also finds fre- phase ligands. Polar embedded phases are In the area of cation-exchange columns, quent use for the determination of transi- widely utilized to overcome the problem of Dionex recently introduced a new cation- tion metal cations. Although spectroscopy stationary phase dewetting in reverse phase exchange column (the IonPac CS18) opti- remains the preferred methodology for the HPLC. For such columns, a polar group is mized for the separation of common inor- analysis of transition metal cations in envi- incorporated into the ligand situated near ganic cations along with polar amines (5). ronmental samples, chromatographic tech- the silica surface to prevent stationary This column utilizes hydrogen abstraction niques still play an important role when phase dewetting in highly aqueous eluent grafting technology (Type 4 architecture) simultaneous determination of transition systems. The proprietary ligands utilized by with a two layer graft coating on a highly metals along with non-metal cations such SIELC Technologies make use of a similar cross-linked high surface area substrate as ammonia or small aliphatic amines is architecture but substitute an ionizable based upon styrenic monomers. The resin is required. A representative separation utiliz- embedded group. This combination over- first grafted with a hydrophilic monomer ing the Universal Cation column from comes the reproducibility issues associated system to block hydrophobic interactions Grace Alltech is shown in Figure 5. Under with mixed ligand architectures employed with the underlying substrate and then sub- the conditions shown, lithium, sodium, in first-generation mixed-mode stationary sequently grafted with a carboxylic acid potassium, ammonium ion, nickel, copper, phases by assuring a homogeneous distri- monomer system to incorporate cation zinc, magnesium, and calcium are sepa- bution of the two retention sites utilized in exchange sites. This new column is useful in rated using a complexing eluent system mixed-mode stationary phases. It also the analysis of a wide variety of polar amines and detected via conductivity detection. allows for better control of stationary phase commonly used in various industrial appli- Generally, however, detection of transition selectivity through direct control of the lig- cations. Especially notable is the ability to metal cations at levels likely to be found in and architecture. www.chromatographyonline.com APRIL 2006 LCGC LC COLUMN TECHNOLOGY SUPPLEMENT 37

Mixed-mode ion-exchange phases offer a New Monolithic Columns Conclusions number of advantages over conventional An emerging area in ion exchange station- New ion-exchange columns for small ioniz- ion-exchange media or conventional ary phases is the use of monolithic struc- able molecules and inorganic ions continue reversed-phase media. For samples contain- tures. Three academic groups are actively to be introduced each year, as improvements ing mixtures of analytes, some of which are pursuing research in this area: Dr. Charles in column selectivity continue at a steady ionic and some of which are neutral, it is Lucy’s group (6) at University of Alberta, pace. The growth in ion exchange is spurred often difficult to arrive at chromatographic Dr. Paul Haddad’s group at the University by environmental regulations and food conditions suitable for retention of all ana- of Tasmania (7,8) and Dr. Brett Paull’s safety analysis. Due to their ruggedness, lytes. While the addition of an ion-pair group at Dublin City University (9–13). most new columns have been packed with reagent is commonly used to solve this sort While the three groups have investigated a polymeric-based materials and this trend of problem, ion-pair reagents cause a num- wide variety of different monolithic materi- will undoubtedly continue. Ion exchangers ber of significant problems including: long als for use in ion chromatography, until based upon polymeric monoliths have made equilibration times, reagent purity problems recently, there have not been any commer- their appearance, mainly for biomolecule that frequently result in chromatographic cial products available for use in ion chro- separations, and with the appearance of the artifacts, low loading capacity, abnormal matography based upon their research. first such column based upon a silica mono- chromatographic peak shapes under over- However, one such material recently has lith, the promise of monolithic media in ion load conditions, incompatibility with been introduced: the Metrosep Dual 4 col- chromatography has been demonstrated. A LC–MS and limited ability to tailor the umn (Metrohm). Although the supplier significant fraction of future ion chromatog- selectivity of the system. Mixed-mode does not reveal the nature of the stationary raphy stationary phases will likely include columns allow retention of both neutral and phase, the monolithic structure (based polymer-based monolithic media. ionic analytes under conditions that over- upon silica with 2-␮m macropores and 13 come most of the problems mentioned pre- nm mesopores), the eluent system (based References viously with ion-pair reagents. For example, upon cyanophenol) and the maximum sol- (1) C. Sanchez, K. Crump, R. Kreiger, N. Khan- Figure 6 shows an example separation on a vent constraint (Ͼ5% acetonitrile) all daker, and Gibbs, J. Environ. Sci. Technol. 39, Primesep 200 column illustrating the ability match an experimental material first 24, 9391–9397 (2005). of a mixed-mode column to retain ionic described by Dr. Lucy (14). The column (2) A.B. Kirk, E.E. Smith, K. Tian, T.A. Ander- analytes with good retention and selectivity, exhibits excellent efficiency (8000 plates for son, and P.K. Dasgupta, Environ. Sci. Technol. utilizing a LC–MS-compatible eluent sys- nitrate with a 100 mm ϫ 4.6 mm column 37, 4979–4981 (2003). tem. Figure 7 illustrates the ability to tailor operated at 2 mL/min) along with excellent (3) A. Kirk, P. Martinelango, K. Tian, A. Dutta, the selectivity of a mixed-mode separation permeability (5.4 MPa at 2 mL/min). Fig- and P. Dasgupta, Environ. Sci. Technol. 39, 7, through independent control of the two ure 8 illustrates the minimal performance 2011–2017 (2005). retention modes. By adjusting the amount penalty associated with operation of mono- (4) B. De Borba, C. Pohl, J. Rohrer, C. Saini, and of trifluoroacetic acid in the mobile phase, it lithic materials at elevated flow rates. Only B. Thompson, paper 130-3, 2005 Pittsburgh is possible to adjust the retention time of an very minor changes in resolution can be Conference. anionic component (in this case seen with a twofold change in flow rate. (5) M. Rey, A. Bordunov, and C. Pohl, poster bromide),which is retained via an anion- Figure 8 also illustrates two other interest- 750-9P, 2006 Pittsburgh Conference. exchange retention mechanism. At the same ing properties of this architecture. First, the (6) P. Hatsis and C.A. Lucy, Analyst 127, (4) time, adjusting the amount of trifluoroacetic selectivity of this column is unusual in 451–454 (2002). acid in the mobile phase has the opposite terms of the relatively modest separation (7) P. Zakaria, J.P. Hutchinson, N. Avdalovic, Y. effect on the retention of a cationic solute. factor for fluoride and chloride. This selec- Liu, and P.R. Haddad, Anal. Chem. 77(2), The combined effects of these two retention tivity is distinctly different from most aion 417–423 (2005). modes allow control of the elution order. In exchange materials, which exhibit a much (8) J.P. Hutchinson, P. Zakaria, A.R. Bowie, M. the case of systems containing mixtures of larger separation factor for these two ions. Macka, N. Avdalovic, and P.R. Haddad, ionic and neutral components, it is possible Second, the fluoride peak shows greater Anal. Chem. 77(2), 407–416 (2005). to adjust the retention of the neutral compo- asymmetry than other analytes in Figure 8, (9) E. Sugrue, P.N. Nesterenko, and B. Paull, nents through adjustment of the solvent even when compared with an analyte with Anal. Chim. Acta 553(1–2), 27–35 (2005). content without affecting the retention of similar retention time (for example, chlo- (10) E. Sugrue, P.N. Nesterenko, and B. Paull, J. ionic components. Likewise, changing the ride). This is no doubt a consequence of Chromatogr. A 1075(1–2) 167–175 (2005). ionic strength allows control of the retention chemical interaction between fluoride (11) B. Paull and P.N. Nesterenko, Trends in Anal. of ionic components without affecting the anion and the underlying silica substrate Chem. 24(4), 295–303 (2005). retention of neutral components. As a result, because fluoride is prone to reacting with (12) E. Sugrue, P. Nesterenko, and B. Paull, J. Sep. a wide variety of different elution orders can silicate to form fluorosilicate. Nonetheless, Sci. 27(10–11) 921–930 (2004). be achieved with a mixed-mode system. this first commercial product illustrates the (13) E. Sugrue, P. Nesterenko, and B. Paull, Ana- This sort of selectivity control is difficult to potential of this technology. No doubt this lyst 128(5) 417–420 (2003). achieve using only reversed-phase or only column represents just the first of a whole (14) P. Hatsis and C.A. Lucy, Anal. Chem. 75(4) ion-exchange retention mechanisms. new class of ion chromatography materials 995–1001 (2003). Ⅲ which can be expected in the coming years.