The past and future development of analytical supercritical fluid extraction (SFE) is traced in terms of experimental strategies, applications, vendor support, and timely acceptance of the existing technology. The current state of the art is compared with research activity in the 1980s. New vendor activity, in terms of automation and hyphenation with chromatographic separations, is discussed.
A sample preparation survey in 2002 suggested that SFE was being used by less than 2% of respondents. A similar survey repeated in 2012 showed that the use of SFE had only doubled, despite the obvious overall advantages of supercritical carbon dioxide-based fluids (1).The situation now is different from the 1980s because instrumentation is more robust and engineering applications in food, pharmaceuticals, bioanalytical, and materials are more plentiful. On the other hand, analytical SFE has not changed very much because there is a lack of support from major vendors and newcomers to the technology are forced to re-learn SFE history and protocols that have been around for years.
To put it in perspective, consider a report of the 9th Annual Waste Testing and Quality Assurance Symposium held in Crystal City, Virginia (USA) dated 23 July 1993 by Robert Stevenson, entitled "SFE in Purgatory". He writes: "SFE generates high interest in surveys, but low sales. All the necessary groundwork seems to be there, but where are the orders? Where are the users?"
Parenthetically, at about this same time, it was reported by experts in the field that "SFE is over the hump and that it is rapidly developing into the extraction method of choice for the 21st century with more and more laboratories around the U.S. and the world embracing it for sample cleanup and sample preparation" (2). No doubt this assessment was partly based upon the fact that between 1987 and 1989 more than 100 papers were published concerning the use of supercritical fluids for extraction (3).
Possible explanations for the perceived current lack of enthusiastic growth among analytical SFE practitioners are the limited number of suppliers that market the technology today and the exodus of vital vendors such as Hewlett Packard, Suprex, Dionex, ISCO, and Lee Scientific from the field around the turn of the century. A somewhat similar vendor exodus occurred slightly earlier in time in the field of SFC but, thanks to the efforts of Terry Berger and associates, the drought was not as long-lived, and SFC has survived to "live" again. Hopefully many of the developments that benefited the SFC community will now foster further development of analytical SFE. Currently, there appears to be a small core of established vendors, as well as relatively new vendors, that are committed to making analytical SFE a viable method for sample preparation prior to chromatographic analysis. Today these vendors are Jasco, Inc., Supercritical Fluid Technologies, Inc., Waters Corp., Applied Separations, and Taiwan Supercritical Technology.
The problem of long-term analytical SFE exploitation may also be in part an education issue as evidenced by the failure of academicians to both learn, explore, and apply the technology in junior-senior chemistry laboratory courses at the university level even when new instrumentation is made available. Furthermore, the perceived remarkable speed and versatility of SFE often tempts beginners to look for shortcuts rather than to examine the application systematically (4). A hasty approach to method development often leads to unexpected problems and analytical SFE is a sophisticated technology that is best mastered by adhering to the rigour required when developing any analytical method. It should be remembered that while there has been and continues to be a plethora of applications in engineering, SFE was only developed as an analytical technique in the mid-1980s. To gain some appreciation for past developments (5–10), the reader is encouraged to read the numerous reviews that have appeared recently as well as in the older literature. Look for details concerning the pros and cons of: (a) On-line or off-line coupling with a variety of separation techniques; (b) dynamic versus static extraction protocols; (c) solid-phase or liquid-phase extract trapping; (d) modifier addition to the matrix versus modifier addition to the fluid; (e) adsorbent in the extraction thimble to retain unwanted compounds such as water; (f) experimental strategies for extracting analytes from solids, gels, creams, sludges, liquids, and so on; and (g) polar-modified, high density CO2.
The use of SFE in modern process engineering applications was initiated in Germany during the late 1960s. These early studies showed that SFE was a viable alternative to distillation and solvent extraction processes. Furthermore, it allowed the processing of substances whose extraction could be adversely affected by high temperatures and the presence of solvent residuals. SFE using CO2 is now an established industrial process for the production of high-value natural products such as hops, decaffeinated tea and coffee, herbs and spices, medicinal herbs, seeds, and marine oils. Further examples include extraction processes where an undesirable component is removed from the matrix, such as pesticides from medicinal herbs. In ancillary fashion, SFE processing continues to find applications as shown by the variety of lipophilic extracts available as commercial products such as polyunsaturated fatty acid esters derived from fish oils, neat and roasted sesame seed oil, cranberry seed-based oils, oils high in n-3 and n-6 fatty acid content, and pumpkin seed extracts coupled with more traditional SFE-derived products such as decaffeinated coffee (11).
The scope of this report is therefore limited to advances concerning analytical SFE. The goals of this article are to both describe the current state of the art in relation to new instrumentation and to address unique extraction strategies, theories, and applications. In addition, interesting hyphenations of analytical SFE with chromatography and spectroscopy that have been reported during the past three years, as well as new aids for achieving successful extraction with CO2 will be considered.