Many wonder if the time has come to merge ion chromatography (IC) with HPLC. They ask, “If IC were a country, would it really deserve to be an independent micro state such as Singapore, Liechtenstein, or Monaco?” To me, the short answer is still an emphatic “yes!” IC technology is really much more advanced than HPLC, plus the users and most of the applications are unique. Sweeping IC into the much larger HPLC country would put IC at risk of losing the identity that is responsible for its remarkable, innovative technology and corresponding growth.
Indeed, innovation in both instrumentation and separations chemistry is what distinguishes IC from the other modes of liquid chromatography. During the last decade, detection limits for challenging ionic analytes have improved by about 1000 times. In 1990, chromatographers struggled to achieve limits of detection of low ppb. Today, the detection limits for the same analytes are low ppt, where “t” = trillion, not thousand. Now, mobile phases can be prepared automatically with the aid of the Dosino from Metrohm (Herisau, Switzerland) and Reagent Free™ eluents (RFIC™) from Dionex (Sunnyvale, CA). No other segment of HPLC enjoys anything similar in concept, convenience, or performance.
In terms of applications, where would the world be without the exquisite assays of seven common anions in water; ppt assays of chloride and corrosion inhibitors in the steam cycle of power plants; or quick assay of carbonate, phosphate, and carbohydrates in soft drinks? These were significant topics at the 22nd International Ion Chromatography Symposium (IICS), which attracted over 100 scientists to the classic Hilton Netherlands in Cincinnati, OH, September 19–22, 2010 (www.CASSS.org). First, let’s look at new advances in technology.
Ion Chromatography Award for 2010
Prof. Brett Paull, 2009 chair of IICS in Dublin, Ireland, received the IICS Award for his work on monoliths for stationary phases in IC. His title, “Polymeric Monolithic Phases: The Future or a Fading Novelty?” is certainly provocative. It seems to reflect that despite intense efforts from several research teams, problems persist. He cited columnto- column reproducibility as one, and low column efficiency as another. Monoliths do offer very low pressure drops, but have difficulty competing with the column efficiency provided by columns packed with sub-2-μm porous particles. Monoliths do compete with columns packed with 3-μm particles, and give very low pressure drop. Apparently, as one reduces the unit cell size of monoliths to get more efficiency, the throughpores are choked off, and thus the backpressure increases very rapidly. Another issue is the lack of multiple vendors. Potential customers are cautious about relying on single-source vendors. The primary patents on monoliths start to expire in 2012, so more competition can be expected.
Going back to the performance issues, monoliths are amenable to making engineered structure along the bed by photografting bonding sites and then attaching ligands. Masks can control the bonding of the surface chemistry, providing varying type and density. This facilitates the creation of novel devices such as open-tubular capillaries with different monolithic films along the surface. Another possibility is taking advantage of the low pressure required for existing monolithic columns to instruments made with common plastics. Prof. Paull used the SIChrom™ from FIAlab (Seattle, WA) as an example. The instrument is rated to 700 psi, which should be more than sufficient for today’s monolithic columns. He concluded that monoliths will certainly find a niche in separation science.
The long list of chromatographic modes got longer again with the addition of zwitterion chromatography (ZwIC), with presentations by Prof. James S. Fritz of Iowa State University (Ames). With ZwIC, the stationary phase consists of zwitterions bonded or sorbed to the stationary phase. The mechanism involves the differential partitioning of the analyte in the stationary phase based on differences in hydrophobic and electrostatic interactions. For best results, the zwitterion needs to have the charges separated by three carbon atoms. If the charges are separated by fewer or more than three, the chromatographic selectivity declines rapidly. Most frequently, ions can be eluted with distilled water. One separation showed that the common anions separated to baseline with water as the eluent. ZwIC is compatible with high-ionic-strength matrices such as seawater. One example showed the direct assay of iodide in seawater with UV detection.
At last year’s IICS meeting, Chris Pohl of Dionex described a prototype capillary ion chromatograph that was formally introduced at Pittcon® 2010 as the ICS- 5000. The design is much more than a size reduction dictated by a 100-fold reduction in the operating sweet spots. The column diameter shrank tenfold from 4 mm to 0.4 mm. The scale factors are generally related to the area of the column, or D2, which explains the 100-fold reduction.
Many chromatographers hear the word “capillary” but immediately translate this to “difficult,” “bleeding edge,” or something similar. Simply making the connections can be problematic, since a little dead volume produces noticeable loss of resolution. However, Dionex introduced a novel design called the Cube that efficiently connects the individual components (eluent degasser, injection valve, and guard and separation column with thermostat, carbonate removal device, and electrolytic suppressor) in a highly engineered assembly. The individual components snap into place. Plumbing connections are cut to fit and color coded. The number of fittings has been cut in half. The ICS-5000 holds one or two Cubes. The latter option doubles throughput in the same space. Better yet, the thermostats on each column compartment can be set at different temperatures, such as 30 ºC for the assay of anions, and at 60 ºC for cations.
Traditionally, IC is used for specific applications where it is clearly the technique of choice. About half the market is involved in the separation of the common anions using some variant of the technology described in U.S. EPA Method 300, even if the need for the data is not environmentally driven. This application is quite mature and receives little attention. At the other end of the spectrum is interest in novel methods for the assay of disinfection by-products (DBPs), including bromate, perchlorate, and haloacetic acids. Another related, high-interest topic is speciation. This is driven by the variable toxicity of certain analytes that precludes setting definitive control specifications based on elemental assays. For example, chromium 3 is an essential nutrient, but chromate and dichromate are feared as potent carcinogens. Dr. Kannan Srinivasan of Dionex pointed out that, with all the attention on DBPs, more than half the mass remaining after disinfection is still not characterized. Perhaps it was the 1930s décor of the hotel, but this warning reminded of The Shadow, a radio mystery from the days before television.
Herb Wagner of Shaw Environmental (Cincinnati, OH) discussed the U.S. EPA’s requirements for method development with a focus on analytical methods. He explained that the composition of the matrix can favor or confound most analytical methods. The chemistry is understandable, but still must be worked through, almost case by case. For method development, the U.S. EPA has developed very stringent requirements for synthetic matrices. Proposed new or improved methods need to be tested against these formulations as well as samples from the field. He cited as an example the use of specified synthetic matrices for assaying bromate and perchlorate in drinking water along with the detection limits. Up to this point, the lecture was clear, but then he introduced some acronyms for minimum reporting level, method blanks, laboratory fortified blanks, calibration check standards, and so on.
For example, Stephen Winslow of Shaw Environmental described the new statistical calculator developed by the U.S. EPA for setting the lowest concentration minimum reporting levels (LCMRLs) for drinking water. Ten times the limit of detection is out. According to Winslow, “The LCMRL calculator is a multiconcentration procedure where the lowest true concentration for which recovery is predicted with a high confidence (99%) to be within 50 and 150% recovery.” The calculator evaluates thresholds for precision and accuracy. Outliers are down-weighted, but not rejected. The calculator is available as a free download at http://water.epa.gov/scitech/drinkingwater/labcert/analyticalmethods_ogwdw.cfm.