SFC and HPLC: Together at Last

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By Todd Palcic and John Chen

After two decades of false starts and stops, the once-superstar-prospect turned falling star is now re-emerging as a more humble, yet very productive performer. Gifted with speed and power, the former future-star often misfired and tried to do more than it could. Although perhaps evoking the image of an ice-hockey player, this description is about supercritical fluid chromatography (SFC) — the former future of chromatography.

If you had listened to many analytical chromatography presentations in the late 1980s and early ’90s, you might have thought that SFC could do everything. However, after scientists found the instruments difficult to use and riddled with problems, analytical SFC fell out of favour. With analytical systems collecting dust, crucial preparative separations consequently became impossible without someone to develop a method. Therefore, SFC, possibly the best technique for automated preparative small molecule screening and preparative chiral separations of preclinical batches, became almost obsolete.

Thankfully, SFC is still around because of steadfast scientists who demand faster separations, serious cost-cutting managers who want to reduce solvent usage and new instruments from Mettler-Toledo International Inc. (Columbus, OH) and Thar Technologies Inc. (Pittsburgh, PA) that help the user finally take advantage of the copious benefits of the technology. While some folks ridiculed SFC in the 1990s as a make-believe technique, others disagreed. “The pendulum has swung too far,” wrote one author describing the importance of SFC for chiral separations and other pharmaceutical applications.

The critics almost won. SFC had reached its abyss in the 1990s as HPLC systems sales soared as the competing technology of choice. Even university research in the supercritical fields moved from SFC to reactions, extractions and particle formation. However, HPLC could not resolve all of the applications and in spite of the malaise, SFC survived. SFC companies still exist in North America because of some die-hard advocates such as Tom Chester, PhD of Proctor & Gamble Co. (Cincinnati, OH), Larry Taylor, PhD of Virginia Polytechnic Institute and State University (Blacksburg, VA) and Jerry King, PhD, formerly of the USDA and now at Los Alamos National Laboratory (Los Alamos, NM), who kept a series of symposiums going on their own. One should also not forget the primary reasons for its existence in the first place: lower solvent procurement costs, faster chromatography, longer lasting stationary phase, lower energy costs and low disposal costs. And primarily, as the president of PDR-Chiral Inc. (Lake Park, FL) says, “It’s a dry powder maker,” referring to the low concentration of solvent remaining in the separators after preparative separation because the primary component in the mobile phase (CO2) turns into a gas and disappears.

The Convergence

Pharmaceutical companies prefer to acquire as much flexibility in one instrument as possible, even if the price is higher. All of these companies want to obtain the greatest utility for their R&D space and overwhelmed researchers. Because time is the most critical issue, SFC has come to the forefront recently. However, because HPLC is more useful for a greater variety of compounds, it is typically the analytical chemists’ preferred choice.

Recently, when asked how her Gilson SFC unit performs, a scientist from a major pharmaceutical company in Denmark remarked, “It’s not an SFC anymore. We switched to large molecules, so I converted it to HPLC for now.”

This reply instantly brings to mind a visit to DuPont several years ago, when a scientist was asked a similar question upon seeing the Thar Technologies CO2 pump draw in solvent instead of CO2: “We aren’t running SFC on this campaign,” he replied. The scientist then said that he tried to develop methods in SFC first because it is faster and cheaper, but if the racemic mixture doesn’t separate, then they develop the method on their HPLC instruments. To them, converting their SFC system to HPLC and back was not only practical — it became absolutely necessary. They worked in a laboratory with limited space and like all companies these days, limited capital budgets. “We have saved probably hundreds of thousands of dollars using CO2 instead of solvents over the years,” he said.

In January 2000, the American Chemical Society published a book edited by Chester and J.F. Parcher, PhD, titled Unified Chromatography. In the book, authors specializing in different branches of chromatography — including gas chromatography, supercritical fluid chromatography, and high-pressure liquid chromatography — describe their fields while drawing out connections with other branches.

Chester states that one can use SFC and HPLC interchangeably with one piece of equipment. He cautions, however, that scientists should not change phases during an analysis or development of a method.

John Blackwell, PhD, manager, Separations Group at Rhodia Pharma Solutions (Cranbury, NJ) agrees, stating in a recent talk that his older Gilson analytical system is “very flexible because it allows both HPLC and SFC” and that the Thar Technologies preparative SFC/HPLC system provides a cost-effective alternative to SMB separations.

Why SFC?

The irony here is that on a recent search of HPLC and SFC instrument manufacturers, no company currently marketing a “unified” instrument was found. Until recently, Gilson Inc. (Middleton, WI) offered such an analytical instrument, but Instrumenta reported in late 2001 that Gilson “ponders move out of SFC development” by shifting its development team into automated HPLC applications for genomics and proteomics. Conversely, Thar Technologies was noted as planning to focus more on preparative chiral separations using SFC and HPLC in the same instrument. “Researchers have been quietly using these instruments interchangeably for years,” declares Lalit Chordia, PhD, president of Thar Technologies. “If you can make it easy for them to switch techniques in a matter of minutes, then the user won’t have a difficult decision when choosing either SFC/HPLC or HPLC only.”

As described by Chordia, changing the SFC system over to HPLC on a Thar Technologies instrument involves very few mechanical changes. For example, the user typically connects a stainless steel tube from the CO2 tank to the CO2 pump, then pumps the CO2 into a higher pressure tube where a more polar modifier enters through a Valco T. The user can then heat the fluid to 32 C or above for running in SFC mode. The fluid then reaches a point where a mixture injects onto a column and then out of the column into a high-pressure UV cell, then onto the back pressure regulator (unique to SFC) and finally into the appropriate separators. “When changing over to HPLC, it is essentially the same as SFC, except we set the back pressure regulator to full-open mode and pump solvents rather than CO2,” he says. Examples of chromatogram readouts by Thar Technologies are shown in Figure 1.

Dauh-Rurng Wu, PhD of Bristol-Myers Squibb Co. (New York, NY), one of the most experienced scientists in large preparative SFC, holds the same opinion. “Using SFC equipment allows me to switch over to HPLC, but you cannot do the same with an HPLC system,” he says.

In 2002, Cheryl Harris wrote an article in Analytical Chemistry following Chordia’s and Chester’s line of thinking called “The SFC Comeback.” The article also suggests that hydrocarbon solvents, liquid CO2 and supercritical CO2 are used interchangeably. However, it cautions that “HPLC applications can be run on an SFC instrument, but not the other way around.”

By the early 1990s, copious instruments lay idle and preparative SFC remained untried in most countries, even by the die-hard CO2 lovers. When one could find the equipment, it was too expensive, overly complicated and unreliable. Instrument companies ignored the need for simple, inexpensive instruments in favour of increased sophistication, automation and profit. This business strategy doomed the main players in SFC as most of them switched the focus to HPLC.

Scientists in the mid-1990s officially buried the product line by declaring in a scientific journal that not only had SFC ceased to expand, but the instrumentation of the mid ’90s did not offer more technology and still remained at an uncompetitive price.

Re-emergence

In 1993, pharmaceutical scientists might have asked, is anyone still using SFC? Conversely, some of the top pharmaceutical companies such as Pfizer Inc., GlaxoSmithKline Inc., Merck & Co. Inc., Bristol-Myers Squibb, Pharmacia Corp., AstraZeneca, Johnson & Johnson as well as service firms like Chiral Technologies Inc. and Rhodia-ChiRex use and promote the use of this environmentally friendly technology today. “Thar’s SFC is a powerful tool for chiral and a-chiral separations,” states Bing Shi, PhD of Pfizer. Jiejun Wu, PhD of Johnson & Johnson Pharmaceutical Research & Development LLC (Raritan, NJ) comments similarly.

“The SFC-50 allowed us to purify enantiomers in grams scale with high purity, along with huge solvent savings. I found it to be a great tool complementary to prep HPLCs,” Wu says.

In 2003, the new question is: who isn’t using SFC?

Todd Palcic is sales and marketing manager at Thar Technologies Inc. (info@thartech.com) and John Chen is national sales manager at Rose Scientific Ltd. (Edmonton, AB).