Product
Review
Ion Chromatography
For many applications in environmental, pharmaceutical, and industrial metals analysis, ion chromatography (IC) continues to be a reliable and sometimes the only effective method. Although IC is relatively familiar and well established for routine analysis, the instrumentation for it has undergone significant changes in the past few years. Sophisticated techniques such as gradient IC are now being used by more researchers; process applications are also growing in importance and may start to influence the direction of IC instrument design in the next several years. We asked Purnendu (Sandy) Dasgupta of Texas Tech University (Lubbock, TX) for help in identifying current trends in IC and advice on buying IC components and systems. Table 1, although not intended to be comprehensive, lists some representative research-grade IC systems. IC vs. HPLC
Should you buy a dedicated ion chromatograph or use IC columns and a conductivity detector with a standard HPLC system? "IC is basically HPLC with a column
exchange column. However, most of the New separation and LC systems on the market have stainless steel flowpaths, which make them unsuitdetection capabilitiesable for trace metal determinations." These applications demand a dedicated IC continue to refine thissystem or at least an LC system with a flowpath made of polyetheretherketone (PEEK) or some other suitable metal-free mature method material. Joe Romano of Waters (Milford, designed for separating ionic species and an appropriate detection system," Dasgupta says. "Whether you need a dedicated IC system depends first of all on how frequently you plan to use it and secondly on the sensitivity and dynamic range required for your analyses." A basic IC system consists of the appropriate IC column, LC hardware, and a suitable detector, most commonly a conductivity detector (often available as an option for LC). Where IC and LC systems differ is in the need for inert flowpath materials. "For anion determinations," Dasgupta says, "there's usually no problem with using a standard LC setup and an anion-
MA) says that 316-grade stainless steel systems can also be made sufficiently inert for metal determinations by passivating the flowpath with nitric acid. Suppressors
IC can be performed either with or without suppression of background ions. "Generally, the performance of a suppressed system is better," says Dasgupta. "It used to be that if you said this, you were supporting one company [Dionex], but the original patent for column-type suppressors has run out, and other companies such as Alltech/Wescan are selling them now. It's clear that suppression has merit; suppressed systems still have significantly better sensitivity and dynamic
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Product
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range than nonsuppressed or singlecolumn (SCIC) systems, even though better columns and thermostatted detectors have improved the performance of SCIC." Placed directly after the main separation column, a suppressor works by neutralizing background ions in the eluent through ion exchange with another solution. In column suppressors, the stationary phase and column dimensions must be selected in tandem with the elution medium for the main column so as to neutralize the eluent ions without affecting the analyte ions and to provide sufficient working time. The solid phase can be regenerated with a separate régénérant solution that does not come in contact with the eluent. The first commercial packed-column ion-exchange suppressor was made by
Dionex in 1975. Tubular membrane suppressors were introduced in the early 1980s, but they required a significant length of membrane tubing for effective background ion suppression. "The challenge was to minimize the dispersion caused by the length of the membrane," Dasgupta explains. One temporary solution was to pack the tubing with glass or resin beads, but the elasticity of the membrane caused the beads to pack down over time and create voids. Dionex replaced the tubular membrane suppressors a few years later with sheet membrane-based units. Sheet membrane suppressors contain a sandwich of two membrane sheets alternated with three ion-exchange screens. Eluent passes down the middle of the sandwich, and régénérant fluid flows along the outside layers. In the most current de-
sign, introduced in 1992, electrical current rather than a régénérant solution is used to regenerate the sheet membranes. Dasgupta says this method has some significant advantages. 'You don't need an extra solution to regenerate the membranes; you can use the detector waste instead. Also, electrical regeneration is cleaner— traces of régénérant solutions can leak into the eluent in chemical suppressors." Dasgupta says another type of suppressor used in his lab is a filament-filled design that is sold in Sweden by SciTech but, because of U.S. patent issues, is unavailable in this country. The choice of a column suppressor versus a membrane suppressor may depend on the balance between analytical and practical needs. Packed columns tend to cause more band broadening than do
Table 1 . Summary of representative products
Product Company
Universal IC Alltech Associates 2051 Waukegan Rd. Deerfield, IL 60015 708-948-8600
DX 500 Dionex 1228 Titan Way Sunnyvale, CA 94088-3603 408-737-0700
QuikChem 8000 Lachat Instruments 6645 W. Mill Rd. Milwaukee, Wl 53218 414-358-4200
Models 616 and 626 Waters 34 Maple St. Milford, MA 01757 508-478-2000
Price Dimensions (h χ w χ d, cm); mass (kg) Separation system Flowpath
$9000-$15,000 Varies (modular system)
$17,000-$45,000 50 χ 46 χ 42; 38 kg
$15,000-130,000 30 χ 88 χ 30; 25 kg
$21,000 and up 7 8 x 2 9 x 6 1 ; 50 kg
PEEK standard; stainless steel optional
PEEK for IC; stainless steel for HPLC
Stainless steel or PEEK
Elution modes Mobile phases Gradients
Isocratic, gradient Up to 4 Step, linear, high-pressure
Gradient, isocratic Up to 4 Linear, concave, convex
Isocratic 1 NA
Stainless steel (Model 616) or PEEK (Model 626) Gradient, isocratic Up to 4 Step, linear, concave, convex
Suppressed, nonsuppressed Disposable column cartridge Not required
Suppressed, nonsuppressed Self-regenerating flat membrane Electrical; automatic or external water Conductivity with micro processor control; combined ECD/conductivity; tunable or PDA UV-vis Optional membrane reactor for UV detection Gradient or isocratic pumps; standard or microbore flow rates; optional switching valves
Suppressed, nonsuppressed Column with automatic in-line regeneration Chemical
Detection system Background eluent ion modes Suppressor type Regeneration Detectors
Postcolumn reactor Special features
Reader Service No.
Conductivity with two electrodes and electronic offset of high- or low-con ductivity eluents Optional for UV detection Column heater for nonsuppressed mode; temperature-controlled detector cell housing; standard or microbore flow rates 401
402
NA = Not applicable
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Conductivity with temper ature controller; UV-vis
Derivatization for color formation Performs FIA and IC simultaneously and inde pendently; automatically dilutes off-scale samples
403
Nonsuppressed NA NA Conductivity with five electrodes; tunable or PDA UV-vis; ECD; Rl; scanning fluorescence None Gradient pump with column-switching valve; standard or microbore flow rates; single-key board control 404
The membrane suppressors, and some analytes undergo a shift in retention time during the column lifetime. Switching suppressors costs re-equilibration time, but at under $10 each, they are generally inexpensive enough for vendors to market as disposable products. Shree Karmarkar of Lachat (Milwaukee, WI) adds that his company has developed and applied for a patent on a chemically regenerated short column suppressor, and Lakshmy Nair of Alltech (Deerfield, IL) says her company's newer disposable columns are designed to alleviate the typical column-based problems. Membrane suppressors, on the other hand, are continuously regenerated but typically cost $750-$1000 apiece. In daily use, they have a life span of about six months in Dasgupta's lab. "Some suppressors have lasted more than a year in my lab," Dasgupta says, "and a few have gone bad in three months." Dionex (Sunnyvale, CA) expects at least a oneyear lifetime for the suppressors in typical routine applications work. Generally suppressors can fail in one of two ways. The first is temporary failure from high back pressure in the system. If the source of pressure is removed, the suppressor will usually work well again. Poisoning of the membrane, usually from chemical adhesion by hard-to-remove species, is more serious. If the poisoning species can be removed without damaging suppressor components, sometimes the suppressor can be revitalized. Solutions containing organic solvents can be used with some membranes to strip off the residues. Electrical suppressors have temperature-sensing circuit breakers built in to shut them off if there's an overheating problem.
ume and flow rate. Because the suppressor is so pressure-sensitive, its design and the detector's internal design must be adjusted to accommodate these changes so they don't become the limiting factors in instrument performance. Another recent advance is the introduction of stationary phases for fluoride separation. Fluoride is among the most poorly retained anions in IC, says Dasgupta, and its peak tends to overlap with the water dip. Columns that can clearly resolve fluoride from the water dip are now available for use in applications such as drinking water analysis. Gradients
Columns and separation media
Since he last reported on the status of IC (Anal. Chem. 1992, 64, 775 A-783 A), Dasgupta says, gradient elution methods for IC have gained popularity among users. Most variations used for HPLC, including linear, convex, and concave continuous gradients, are now in use for IC as well. However, he notes, there are some tradeoffs to using gradients in IC. For one thing, background conductance and noise increase during gradient elution in most cases. Waters has developed an isoconductive gradient system for nonsuppressed IC, Dasgupta says, but for most applications, gradient elution is practical only in suppressed mode. Even with a suppressor, though, gradient IC isn't suitable for every application. Although it gives better resolution than isocratic separation does in many cases, Dasgupta says, it isn't the best technique for sensitive detection. "If your goal is to resolve closely eluting peaks and you aren't trying to monitor trace concentrations, gradient elution is worthwhile. But if you only have two or three target compounds to monitor for," he says, "it's better to optimize your isocratic protocol."
An increasing number of vendors have entered the IC column market, where small manufacturers of specialty columns can compete successfully with larger manufacturers. More high-capacity and organic solvent-compatible columns are being offered, and column durability has improved in the past few years. The biggest recent trend in IC instrumentation has been the continuing reduction in column bore size. This trend has a significant effect on IC hardware design, perhaps more important than its effect on conventional HPLC design, Dasgupta says. The move from 4- to 2-mm bores, for instance, requires hardware changes to accommodate the fourfold reduction in vol-
Although the traditional conductivity detector is still the mainstay of IC systems, other detectors used in conventional LC and CE, such as UV absorbance, electrochemical, and fluorescence detectors, have made their way into ion chromatographers' laboratories. UV detection modes for IC include direct detection with or without suppression and indirect detection for nonsuppressed IC. As in conventional LC, direct detection, where the analyte absorbs light but the eluent medium doesn't, is the least complicated choice and is generally best for sensitive detection if it works. Use of a sup-
Detectors
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registered trademark of LACHAT Instruments, Inc. CIRCLE 3 ON READER SERVICE CARD
Analytical Chemistry, Vol. 67, No. 5, March 1, 1995 207 A
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pressor helps reduce the background and extends the range of application for UV detection in cases where an eluent such as carbonate absorbs UV more strongly before suppression than it does afterwards. Indirect detection is useful in a number of cases where the analytes don't absorb UV light but the eluent ions do; this method doesn't require suppression. Again, as for LC, absorbance and fluorescence detection are often enhanced by the use of a postcolumn reactor between the suppressor, if used, and the detector. In addition, says Dasgupta, the pulsed amperometric detector, introduced commercially a decade ago, is now being made by several companies and is particularly useful, for example, in carbohydrate analysis. This detector works by switching rapidly between working and self-cleaning modes to avoid the inactivation from buildup on the working electrode that is typical of amperometric methods. Hyphenated techniques
In the past few years, hyphenated IC techniques have crossed the boundary from
"homebrew" setups in individual labs to commercially available instrumentation. For instance, Dionex and Thermo Jarrell Ash have collaborated to produce dedicated instrumentation for IC/ICP-AES, and "chelation chromatographs" that use IC are also commercially available. Because ICP-AES requires a different matrix from the IC eluent, the adapted IC hardware elutes analytes as a plug in a more compatible solvent (often a suppressed eluent that has very little electrolyte) , analogous to analyte concentration by SPE. ICPMS, on the other hand, is compatible with standard IC fluids, at least in principle. If the eluent has high background salt, however, it is still preferable to operate in suppressed mode. With the burgeoning popularity of ICPMS and the introduction of several commercial instruments in the past year, systems that perform IC/ICPMS may be a commercial possibility. IC/MS is still in the exploratory stages; three or four research groups are actively pursuing it, Dasgupta says. Two persis-
Suppressor-Based and Single Column in One Reliable System Alltech's Universal Ion Chromatograph! Suppressor-Based is Best for Anions
Single Column is Best for Cations
0 6 12 18 24 Mitl. 1. 2. 3. 4. 5. 6.
1. 2. 3. 4. 5. 6. 7.
Lithium (0.2ppm) Sodium (1.5ppm) Ammonium (1.5ppm) Potassium (2.5ppm) Magnesium (2ppm) Calcium (2ppm)
Fluoride (2ppm) Chloride (20ppm) Nitrite-N (2ppm) Sromide (2ppm) Nitrate-N E10ppm) Phosphate-P (2ppm) Sulfate (60ppjn)
» Analyze Anions, Cations, Organic Acids, Metais and More! » Compatible with all IC Methods - No More Compromises • Solid-Phase Chemical Suppressor Eliminates Regeneration - Meets EPA Method 300.0 Requirements 1
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Waukegan Road · Deerfield, IL 60015 Alltech 2051 Phone: 1-800-255-8324· Fax: 708-948-1078 AUI01
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Analytical Chemistry, Vol. 67, No. 5, March 1, 1995
tent problems are the composition of the eluent from the chromatograph, because MS doesn't tolerate high salt concentrations, and flow incompatibility between IC and MS. The split-flow interfaces developed for commercial LC/MS might solve the second problem, but so few researchers are working with IC/MS that, Dasgupta says, "At this point, it's not clear that a sufficient market exists for commercial development." ProcessIC
Dasgupta notes that there is a growing need for commercial instrumentation in process monitoring. Nuclear power plants, for instance, require continuous on-line monitoring of high-purity water in the system. There's no substitute for suppressed IC because any off-line handling would contaminate the samples past the extremely low tolerance threshold. Dow Chemical also uses IC for a variety of processes but, says Dasgupta, the company often assembles its process IC equipment in house. Commercially available process ion chromatographs are elaborate and fairly expensive; as such, they may be out of range for small industrial users' budgets, but less sophisticated versions could fill a niche. Dasgupta says affordable instruments could readily be developed for many process applications. "There are always tradeoffs, but in most applications, you don't need the ultimate in resolution or sensitivity. With a larger particle-size column, there's a smaller pressure drop in the system and less maintenance is required. You could produce reasonably lowcost moderate pressure process IC systems." Outlook In the next few years, Dasgupta says, IC instrumentation is likely to move in the direction of improved column durability, increased separation speed, and the introduction of on-line protective methods that might include removal of undesirable matrix species from samples before separation. He adds that vendors should look at the possibility of making field-portable instruments with low solvent consumption for environmental and other on-site applications. "IC is a mature technique and is not changing very much," Dasgupta says. "However, its reliability and its ability to handle complex matrices are very good and, for routine use, better than any competing technique available today." Deborah Noble
