SFC: the 'Sleeping Beauty' of separation science?  
 
Where previously we saw SFC only for chiral and polymer analysis, we now see more and more mainstream publications in which SFC replaces RPLC and NPLC. Agilent and Waters have taken SFC into their portfolio, and RIC, the company of Pat Sandra, has developed its own instrument. Why the interest in SFC?

Eric Lesellier: "Waters and Agilent see great opportunities because in preparative separations CO₂ has a much lower cost - 1 kilogram costs 1 euro - than organic phases."
Harbaksh Sidhu: “SFC has taken its place in purification due to its reduced solvent cost, faster dry-down and sample quality due to minimal exposure to additives among other reasons.  SFC/UV sensitivity is available to meet FDA needs.”
Larry Taylor: "It's not just about applications in chiral pharmaceuticals. There is  activity in nutrition and flavor for example, since CO2 is easier to remove and has less waste and health requirements, but also in, petrochemicals and biodiesel areas."
Hans-Gerd Janssen: "The green concept is indeed important, but the cost is always the deciding factor. Regarding CO₂ consumption SFC wins, but because the analyses are new and complex, it may create time loss, especially when something goes wrong, it costs many 'analyst'-hours.”
Eric Lesellier: "There are other advantages, like being able to work at atmospheric pressure. The current rapid methods such as working at extremely high pressures or high temperatures, are expensive because of the extreme conditions. In SFC, these performances are possible, but at lower pressures and normal temperatures. In comparison, SFC is actually a very simple method and also non-toxic."
Hans Kruisbergen: "Another advantage for preparative SFC is the short work-up time. In reversed-phase preparative LC large amounts of water are difficult to remove, in SFC CO₂ evaporates without effort at atmospheric pressure. As a modifier, typically an alcohol is used which is easy to remove. Moreover, SFC also is much faster than LC. The low viscosity of CO₂ can easily increase the laminar flow by a factor of 3 or 4 compared to LC. Also, because of the low viscosity we can work with the same particle size to upscale. And, it is possible to recycle the CO₂, so that consumption is reduced."

What are the blockades for SFC?
Hans-Gerd Janssen: "The current tremendous time and cost pressures implies that nobody is eager for a new technique, which usually brings lots of complications, especially if it's possible to still work with the "old" technology. The benefits must be very large, and the risks low for one to make the jump to SFC. Also the jump should be made as small as possible. It is important that SFC increasingly is being integrated into HPLC equipment and software. That makes the threshold lower, thus the techniques may still enter the areas where it should be applied.
Larry Taylor: "If there are formal methods then SFC will quickly grow. It is important at this point that methods are developed that can be implemented in industrial applications. For instance, a SFC method with UV for "trace analytes" is now in the process of gaining GMP approval. The instrument vendors have been vital in this regard.
Harbaksh Sidhu: “There have been validation packages available not just from Waters but other vendors.  However, there isn't enough uptake because the market is maturing and wrong perceptions of SFC exist such as instrument robustness. True there are issues, but lack of instrumentation familiarity is a major contributor.  That is what needs corrected.” 
Eric Lesellier: “Of course there are difficult issues such as poor solubility of polar compounds and ions in the mobile phase in particular. But the structural problem is the lack of well-trained chromatographers.which makes one afraid to enter the technique.”
Hans-Gerd Janssen: "What you see now is that in select companies some knowledge is gathered and disseminated. Funny thing is that much knowledge of SFC was in the Netherlands and still is. Peter Schoenmakers, John Uunk, and myself were among the pioneers of yesteryear, and have followed developments closely.”
Hans Kruisbergen: "For many, SFC is still unknown territory. Developments in recent years, however, have made SFC much cheaper and accessible. Once the opportunities and successes of SFC are apparent, clearly more people will become interested."

How do you see the chances of success?

Hans-Gerd Janssen: "It is remarkable that the same arguments we heard 25 years ago were also used to praise SFC as an attractive technique. What's different now? There are four major differences from the eighties. There are major suppliers who appear to be committee to the technology,."Green" is an important argument. Equipment is better and is more integrated into the routine environment. Moreover, lower expectations make a quiet entrance possible, without counter-productive reactions from disappointed users."
Larry Taylor: "The disadvantage is that there is little movement in the U.S. We have very few academically trained experts who can develop the technique, and there is no government support for development. For now, opportunities in European and Asian universities and large multi-national companies like MSD are significant. Further developments should cause SFC to be routine elsewhere.
Harbaksh Sidhu: “SFC will transition into regulated areas once there is enough movement in pharma and acceptance of SFC for Normal Phase analysis.  The SFC systems out there see tremendous use.  Until now, SFC was not available under major software platforms that customers are familiar with. That is changing.  The answers about SFC from 10 years ago do not apply today.  There has been growth in last 3 years.”  

Hans Kruisbergen: "SFC has the ability to grow into a vital tool in the laboratory. Preparative SFC has proven itself. Whether it will effectively become an indispensable technique will depend on the registered routine applications and the interests of the analysts to go ahead with SFC."
Eric Lesellier: "The opportunities are excellent, because it is a fantastic technique. If you look at the differences in HPLC with UV detection, it is better and faster without the burden of small particles and higher temperatures as in UHPLC. Increased throughput, no changes needed in the mobile phase, and low cost are additional advantages. I believe my Sleeping Beauty has now been kissed to awake!"

Frank van Geel wishes to thank the following persons for their contribution:

• Hans-Gerd Janssen, Unilever and Professor University of Amsterdam
• Larry Taylor, Professor at Virginia Tech.
• Eric Lesellier, Topic Owner SFC in Chromedia, teacher and researcher, University of Orleans
• Harbaksh Sidu, General Manager of the SFC division of Waters, Pittsburgh
• Hans Kruisbergen, MSD in the Netherlands, the former Organon
• Helene Boiteux, Business Development & Applications Manager
  SFC & Purification, Europe