Highlights of SFC 2014

Pages 2-5

The 8th International Symposium on Packed Column SFC (SFC 2014), which alternates between the United States and Europe each year, was held in Basel, Switzerland, on 8–10 October 2014. The symposium is dedicated to bringing scientists together to discuss the latest advances in supercritical fluid chromatography (SFC) and is organized annually by the Green Chemistry Group. SFC 2014 attracted more than 200 scientists from 15 countries. In addition 12 exhibitors displayed equipment and stationary phases for SFC use. The three-day conference included 21 oral presentations, 52 poster presentations, and one vendor workshop. A short course dedicated to SFC theory and achiral, chiral, and preparative SFC was held before the conference.

(PHOTO CREDIT: WESTEND61/GETTY IMAGES)

Analytical SFC

While purification is still a major application for supercritical fluid chromatography (SFC), the use of analytical SFC is rapidly increasing. This expansion has been helped by the introduction of advanced SFC equipment that allows the use of smaller particle size stationary phases and offers lower limits of detection and sensitivity equivalent to those seen in high performance liquid chromatography (HPLC). In the past analytical SFC has been performed only by skilled chromatographers. John Langley from the University of Southampton (Southampton, UK) discussed the use of analytical SFC in a walk-up environment. "Walk-Up" users are typically scientists who do not have extensive chromatographic training (organic chemists, students). He described the process, which has been used for open access support, and discussed how SFC is used. He also presented a number of analytical problems that were solved using SFC. The samples included sugars, biofuels, lipids, nucleosides and nucleotides, porphyrins, and molecular wires.

Davy Guillarme from the University of Geneva (Geneva, Switzerland) discussed the possibility of modern SFC coupled to mass spectrometry for life science applications such as drug metabolism, doping control analysis, and cannabinoid determination. For drug metabolism assays it was shown that SFC is complementary to HPLC, although sensitivity was lower in SFC–MS compared to LC–MS. In doping analysis, acceptable peak shape was observed for urine samples using SFC after dilution with 75:25 acetonitrile:water. The sensitivity of SFC–MS–MS was comparable to LC–MS–MS. For cannabinoid analysis, SFC–MS–MS was significantly more sensitive than LC–MS–MS.

Caroline West from Universite d'Orleans (Orléans, France) presented strategies for choosing an achiral SFC column. Her first recommendation was to choose a stationary phase that matches the polarity of the molecule to be analyzed. Using linear solvation energy relationships (LSER) she classified columns currently available for achiral SFC. Her final recommendation was to choose a stationary phase that reflects differences between the compounds to be resolved in the mixture. For example if the molecules to be separated differ based on polarity, the use of a polar phase is recommended. Hydrocarbon differences are best resolved on a non-polar phase and heterogeneous differences are best resolved with aromatic or mixed alkyl/polar phases.

Preparative SFC

For the past 20 years the main application of SFC has been the preparative resolution of enantiomers. While SFC is expanding into other areas, purification is expected to remain prominent because of the advantages of SFC over HPLC such as speed, lower costs, and reduced solvent usage. Eric Francotte reported on work at Novartis to replace reversed-phase chromatography with SFC for achiral purifications. Reversed-phase purification has a number of disadvantages relative to SFC. These include increased costs, use of toxic solvents, and slower purification and drying times. This initiative was highly successful as the percentage of purifications performed using reversed-phase chromatography decreased from >95% in 2009 to approximately 20% in 2013. For preparative applications SFC has the advantages of speed, high purity, absence of TFA salt formation, and low solvent volumes for evaporation. SFC is now the first choice for purifications at Novartis. Eric also reported on work to identify molecules that will not be successful in SFC purification. For high pKa compounds (>8) reversed phase is recommended for purification.

Craig White reported on facility and process changes required at Eli Lilly to allow unattended overnight operation of preparative SFC systems. Dedicated rooms designed for safe handling and storage of up to 100 L of flammable solvent have been built for modifier storage and fraction collection. Jimmy DaSilva from Merck discussed the use of non-conventional modifiers for chiral SFC purifications. Use of these solvents can provide increased solubility, which can ultimately increase purification productivities.

SFC Theory

There has recently been a large increase in theoretical research in many areas of SFC. Some of this latest research was presented during the conference. Two talks focused on the effects of column pressure drop and thermal conditions that impact efficiency in SFC. Abhijit Tarafder from Waters Corporation discussed estimation of temperature deviations in SFC using isenthalpic plots. He demonstrated how enthalpy can change with pressure and temperature differences within an SFC column and how these changes can lead to reduced efficiencies. For most of the experimental conditions currently used in SFC thermal effects are minimal. Only with high pressure drops or high temperatures is there a potential for detrimental thermal effects.

Ken Broeckhoven from Vrije Universiteit Brussel (Brussels, Belgium) presented the trade-off between viscous heating and decompression cooling in SFC. His work showed that both viscous dissipation and decompression cooling are relevant in SFC but only a small effect on efficiency can be observed.

Jacob Fairchild presented recent work from Waters Corporation that seeks to explain retention time drift in achiral SFC columns. It has been observed by Waters and Bristol-Meyers Squibb that retention times decrease over time for first generation SFC columns. This decrease had been attributed to poor flow control for early models of SFC instrumentation, but it has also been seen with the latest generation of SFC equipment. In addition to retention shift, changes in selectivity have been observed over time. Waters now attribute the retention time and selectivity changes to silyl ether formation (SEF) caused by interactions of surface silanols and methanol. Approaches to minimize SEF as well as to regenerate columns impacted by SEF were presented.

New Uses of SFC

Analytical and preparative SFC has been used for almost 25 years in the pharmaceutical industry. Introduction of robust instruments with increased sensitivity as well as improved stationary phase design now allow SFC use to expand beyond the traditional fields. At SFC 2014 a number of talks discussed the latest fields to explore SFC as an analytical tool. Eric Lesellier from Universite d'Orleans presented a number of cosmetic analyses performed using SFC and concluded that SFC performs very well for the separation of natural compounds and cosmetic formulations. Frederic Begnaud from Firmenich SA found SFC to be ideally suited for molecules used in the flavour and fragrance industry.

Gilles Goetz presented an SFC technique developed at Pfizer to measure intramolecular hydrogen bonds. Polarity is an important attribute for potential drug candidates in pharmaceutical discovery. This technique allows Pfizer scientists to predict in vitro cell permeability as well as provide a readout of hidden/exposed polarity. Use of this technique has enabled the design of compounds (both small molecules and peptides) with improved permeability.

Yun Huang, also from Pfizer, discussed a method development approach in SFC–MS–MS to eliminate matrix effects in bioanalysis. There are numerous matrix components in bioanalysis such as endogenous biological components, endogenous metabolites, or residual formulation components that can interfere with MS ionization efficiency and impact method accuracy and robustness. While some of these matrix components can be removed during sample preparation, it is more efficient to separate the matrix from the compounds of interest using chromatography and eliminate an involved sample preparation step. Multiple examples of resolving phospholipids and PEG 400 from compounds of interest using SFC were shown.

Poster Sessions and Best Poster Awards

As with previous SFC meetings, many attendees chose to present their work in poster format. Topics covered all areas related to SFC including analytical and preparative SFC, and stationary phase development and evaluation. Awards consisting of cash prizes were given to the top two posters as judged by the Scientific Committee.

This year the two best posters were selected for recognition. Sam Colgate from the University of Florida and Terry Berger from SFC Solutions were acknowledged for their poster entitled "Explaining Thermal Gradients in SFC: What the Dissipation Function, Φ, in Navier-Stokes Really Means". According to the authors, the dissipation function is widely believed to represent heating of the column because of friction between the mobile phase and the stationary phase; this is sometimes called "restrictive heating".

The authors stated that the dissipation function is a measure of the gain in net entropy of the expansion. Consequently either fluid dynamics or thermodynamics can be used to calculate temperature change occurring across a packed column in SFC resulting from pressure drop and expansion.

Elise Lemasson of the Universite d'Orleans was recognized for her poster titled "Development of an Achiral SFC Screening Strategy for Impurity Profiling of Active Pharmaceutical Ingredients". The author discussed impurity profiling of drug candidates using SFC. Twenty-three columns and a series of modifier additives were explored with the goal of identifying the best stationary phases for providing orthogonal separations, and to maximize the probability of resolving all impurities. This work generated a ranking of columns, providing guidance to users on which stationary phases to explore for achiral SFC impurity profiling.

SFC 2015

The 9th annual Symposium on Packed Column SFC (SFC 2015) will be held next summer in Philadelphia, Pennsylvania, USA, on 22–24 July. Additional information on SFC 2015 will be posted as it becomes available on the Green Chemistry Group website: www.greenchemistrygroup.org. In addition to SFC 2015 in Philadelphia an additional SFC meeting is being planned for Shanghai, China. As details are finalized for SFC 2015 in China, information will also be posted on the Green Chemistry Group website.

Larry Miller is with Amgen in Cambridge, Massachusetts, USA.

Larry Taylor is with the Department of Chemistry at Virginia Tech in Blacksburg, Virginia, USA.

LCGC magazine attended SFC 2014 in Basel. The programme and many of the oral and poster presentations from SFC 2014 and previous SFC conferences can be found at: www.greenchemistrygroup.org/index.html

E-mail: millerl@amgen.com

Website: www.greenchemistrygroup.org

This article is from The Column. The full issue can be found here>>