Separation Science Highlights From Pittcon® 2011

Robert L. Stevenson, Ph.D.

At Pittcon^® 2011, most new products appeared to be evolutionary, not revolutionary. Possibly, the recession sapped money for innovation and hence fewer new products resulted. But the meeting was successful: The consensus of the vendors was uniformly positive about the buying interest of conferees. The quality of leads was rated as good to high. I heard two “best ever”s—all very positive signs for the industry. The recession was bad in general, but this industry segment has seen much worse, especially in the 1970s and ’80s.

There were interesting developments this year. Seven new supercritical fluid chromatography (SFC) instruments were introduced. It’s been 30 years since there was this much commercial interest in SFC. In fact, there were decades when the new product flow was next to zero. GC-time-of-flight (TOF)-MS attracted significant attention. The mass analyzers are fast and hence compatible with the fastest chromatographic separations.

Tectonic-like industry consolidation is the third major development. It has many facets, depending on the situation of the stakeholder: Employees fear for their jobs since new owners often consolidate operations. Customers are also concerned about layoffs that may adversely impact instruments and support teams. Ultimately, users need reassurance that customer service, including tech support, will not suffer. This is especially important as the sophistication gap between the vendors and users widens. Some instruments are like high powered sports cars—They have powerful engines, but require experienced drivers. After all, society demands and values the results of our technology. But to maintain credibility and value, analysts must deliver accurate results. “Spinouts” can be deadly.

Chromatography

By all accounts 2011 was a very good year for LECO (St. Joseph, MI). The firm celebrated 75 years in business, and at Pittcon tied for the Gold Editors’ Award for the Citius™ LC-HRT (High Resolution TOFMS). The LC is from Agilent (Santa Clara, CA) and potentially other vendors. The mass analyzer is LECO’s Folded Flight Path™ design, where the resolution increases as more traverses are added to the flight path. The HRT comes in both an LC and GC version. The TOF analyzer is a briefcase-size box that contains five hollow aluminum rectangles on each end. These generate the gridless electric field that most would call a reflectron but that LECO calls a “mirror.” Between the two mirrors is another piece of aluminum with large vertical slots. These are called the periodic ion lenses, and they direct the ion beam from the source to the mirror and then to the other mirror. Two passes provide low-resolution TOF, which is useful in GC. High resolution of high mass number requires 8, 16, or 32 cycles. LECO claims the gridless feature is responsible for the high yield of ions. About 75% of incoming ions are detected in one example; 90% are detected in another. Reduced ion loss explains the improved detection limit of 1 pg. Other specs are impressive: resolution is 100,000 FWHM, mass accuracy is <1 ppm, and acquisition rate is 200 spectra per sec. (For a review of the Pittcon Editors’ Awards, see Hunt, J. Pittcon^® 2011 Editors’ Awards: A Two-Way Tie for the Gold Award. Am. Lab. 2011, 43(5), 4–6.)

Seven new SFCs! Who and why?

SFC showed surprising strength at Pittcon 2011, with seven instrument introductions plus a host of columns. After 30 years, market interest has reawakened from sort of a Rip Van Winkle slumber. Early Monday morning, the first product highlighted in the Waters (Milford, MA) press conference was the ACQUITY UPSFC™, which enables the analytical chromatographer to reduce the cost of analysis by up to 99%. Most of this can be attributed to the reduction in cost of the mobile phase. Food-grade CO₂ is about $1.5/lb compared to at least $50/lb for reversed-phase liquid chromatography (RPLC) eluents. Even HPLC-grade water is about $20/lb. Food-grade CO₂ is sufficiently pure for SFC since the CO₂ is purified as it is distilled from the cylinder into the pump head. Run times in SFC are reduced by 90% due to the lower viscosity and greatly improved mass transfer. The baseline noise of the diode array detector (DAD) is less than 10 uAU, which is good enough that analytes at the 0.01% level can be detected confidently. Interestingly, the P_max of the UPSFC is 6000 psi, which is only 40% of the P_max of other ACQUITY^® UPLC instruments. However, the low viscosity of SFC mobile phases and the short columns reduce the need for higher pressure.

Pumping SFC eluents, including CO₂, is much more difficult than most LC mobile phases such as water, acetonitrile, and methanol. CO₂ is a liquid with a significant vapor pressure at room temperature. If it is cooled and compressed too much, it will form a solid, aka dry ice. The ACQUITY UPSFC uses Peltier cooling elements attached to the pump heads to cool and hence liquefy the CO₂ so that it works in metering pumps. This approach has traditionally been difficult to implement since the flow rates were higher. But UPLC is usually performed with 2-mm-i.d. columns or smaller, which flow at about 250 μL/min. Peltier cooling appears to be adequate. Waters also developed a new line of UPLC columns specifically for SFC called Viridis™ columns.

Aurora SFC (Redwood City, CA) and Selerity Technologies Inc. (Salt Lake City, UT) both introduced SFC modules designed to convert a legacy HPLC to SFC. This allows the chromatographer to explore the efficacy of SFC with only a modest investment in instrumentation, assuming, of course, that one has a gradient HPLC sitting idle. One requirement is that the CO₂ be conditioned to pump well, and another is that the backpressure needs to be controlled. The Aurora SFC Fusion™ A5 is designed to sit to the left of an Agilent 1100 or 1200 dual-pump gradient HPLC, and needs only 12 in. of bench space. CO₂ is supplied to the A5 as a vapor at 40–60 bar. This is de facto distillation of the mobile phase, which helps purify it. Thus, food-grade CO₂ is usually sufficient for UV detection. This may not be sufficient, however, if flame ionization detection (FID) for hydrocarbons is contemplated. The A5 has a condenser and booster pump to manage SFC compressibility. Pumps on the HPLC meter the flow. After the column and detector, the A5 also controls the system pressure.

Selerity’s 5000 Super Critical Fluid Pumping Module was designed in collaboration with the Research Institute for Chromatography (Kortrijk, Belgium) to expand HPLC instruments into SFC. Speed and cost are major benefits, but sometimes selectivity is also different. Selerity is the leader in characterizing fuels by hydrocarbon type. The 5000 contains both cooling and heating zones plus a backpressure regulator to control the retention. It operates from 80 to 400 bar with a flow of 0.5 to 10 mL/min. The system uses a small electronic pressure regulator, like the SFC Fusion A5, but a spokesperson for Selerity says she prefers a long capillary tube on the outlet of the detector to add a bit of pressure to keep the mobile phase condensed. Bubbles in the detector are unwelcome. Of course, long capillaries worked well in HPLC before degassers and helium sparging became the norm.

Sepiatec (Berlin, Germany) introduced two new SFC systems. The Sepmatix 8× Parallel SFC is designed for rapid analytical screening of stationary phases and run conditions (primarily temperature, density, and % modifier). Crossing chiral analytes with suitable chiral stationary phases (CSPs) is largely empirical, and many CSPs exist. The columns work in parallel simultaneously, which saves time. A chiral column screening wizard displays thumbnails of up to 80 chromatograms simultaneously. Simple inspection leads to quick selection of the most favorable candidates. The fluidics of the Sepmatix involves two metering pumps (for CO₂ and modifier). After the mixer, an electronic valve distributes the flow to the eight independent flow paths. A multiplexed DAD records each chromatogram. Although it was not shown at the meeting, the company also makes a compact eight-column HPLC analyzer fed by an eight-channel autosampler. This seems to raise the possibility of adding an autosampler and converting the Sepmatix SFC to a very high-throughput SFC analyzer.

On a larger scale, Sepiatec introduced the compact Prep SFC operating at flow rates of 100 mL/min. This should be adequate for columns as large as 30 mm i.d. Prep SFC of chiral compounds has a huge cost advantage, plus reduced concern about residual solvents. Those concerned about residual methanol can try ethanol. P_max is 300 bar, which should be more than adequate. The system pressure is controlled with a fast electronic valve. Detection is with a UV/VIS detector with λ range of 190–740 nm.