Product
Review
Fattening up SFE sales Fat-labeling requirements place heftydemandon extraction market. Since its commercial development in the early 1990s, supercritical fluid extraction (SFE) has undergone somewhat of a roller coaster ride. Many analysts were quick to try the technique but found themselves giving up, frustrated with early SFE systems that did not live up to their expectations. Others stood by SFE through thick and thin, knowing that one day their instrument would deliver what it promised. Today SFE is well-established in several areas, including the environmental, pharmaceutical, and polymer industries but, above all, has found a strong niche in the food analysis In the beginning many thought the environmental industry would profit the most from SFE; however nearly a decade later despite the introduction of several EPA SFE-based methods SFE has not had the impact that was anticipated Instead the SFE market is becoming increasingly dominated by food-related applications particularly the determination of fat content for compliance with FDA nutritional labeling requirements
A summary of selected SFE systems that are commercially available is provided in Table 1. This review is not intended to Since Analytical Chemistry last reviewew be a comprehensive summary of all manuthe SFE market (Anal. Chem. 1194,66, facturers in the SFE business; rather it is designed to represent the types of SFE sys369 A), the number of SFE instrument tems that are currently available. manufacturers has declined. According to the experts, market forces have weeded out the high-quality products from the unWhat SFE can and can't do desirable—something they all agree was One of the biggest advantages of SFE over direly needed in the early days of SFE. "Ex- other sample preparation techniques is that tremely poor systems were initially sold, it can be automated, making it well-suited which is one thing that hurt the field quite for fast, routine analyses. But what exactly a bit," says Steven Hawthorne of the Endoes it mean to be automated? "The definiergy and Environmental Research Center tion of automation varies," warns Hawat the University of North Dakota. Improve- thorne. According to Hawthorne, some ments have been made on some of the SFE systems are automated, but only in the original SFE systems, making them better sense that you can load eight or nine samsuited for today's SFE market; but for the ples at a time. Others, however, can handle most part the changes don't reflect "new" significantly more samples—some as many developments adds Jerry King of the U S as 44. The distinction lies in whether the Department of Agriculture (USDA) mode of operation is simultaneous or se-
quential. Not all systems perform more than one extraction at a time—those that do are referred to as simultaneous or parallel extractors. Sequential systems, on the other hand, can be automated, but they extract one sample after another in a serial mode. More samples can be loaded onto a sequential system than onto a simultaneous one, making sequential extraction ideal for overnight runs. However, as many as nine samples (depending on the instrument) can be extracted simultaneously in the same amount of time as one sample can be extracted sequentially. One advantage to sequential automation is greater method flexibility different programmed methods be run for each sample. Another big advantage of SFE over other common extraction techniques is that "benign", inexpensive solvents—such as supercritical C02—are used, which greatly cuts down on the consumption of
Analytical Chemistry News & Features, May 1, 1998 333 A
Product
Review
Table 1 . Summary of selected SFE instruments Model
Spe-ed SFE-4
Soxtec System N-S
HP 7 6 8 0 T
SFX 3 5 6 0 DM
Manufacturer
Applied Separations 930 Hamilton St. Allentown, PA 18101 610-770-0900
Foss Tecator Box 70 SE - 263 21 HSganas Sweden 46 42 361500
Hewlett Packard 2850 Centerville Rd. Wilmington, DE 19808 800-227-9770
Isco 4700 Superior St. Lincoln, NE 68504 800-228-4250
URL or e-mail
www.appliedseparations.com
www.fossnorthamerica.com/ fossgroup.htm
www.hp.com
www.isco.com
Type
Analytical and process scale
Gravimetric fat analysis
Analytical
Analytical and gravimetric fat analysis
Dimensions (W x D x H, in.)
16x17x36
16x17x36
18x20x33
22x24x29
Max, oven temperature (°C)
250
250
150
150
Max. pressure (psi)
10,000 or 15,000
10,000
5560
7500 or 10,000
Max. flow rate (mL/min) (compressed gas flow)
50 (10,000 psi)
50 (10,000 psi)
4
90 (7500 psi) or 50 (10,000 psi)
Mode of extraction
Parallel
Parallel
Serial
Serial
Extractor vessel capacity
0.5-50 ml_ (up to 4 vessels); up to 2.3 L (single vessel)
0.5-50 mL; 300, 500, and 1000 mL (single vessel)
7 mL
0.5-10 mL
Max. no. of samples per run
4
4
8
24
Options
Additional oven modules; modifier addition pump module; recirculating cooling bath; in-line trapping capabilities
Modifier pump; thermostated bath; additional oven modules
Modifier pump
On-line FT-IR analysis; ESE capability (hot, pressurized solvent extraction)
Special features
Large sample capacity; 3 modes of collection (vial, SPE cartridge, or solvent)
Large sample capacity; application support
Reader service number
401
402
toxic organic solvents. Small quantities of cosolvents are occasionally required to enhance the solubility of an analyte in the supercritical fluid state. "For environmental samples, raising the C02 temperature to 150-200 CC is often as effective as adding modifiers. Avoiding modifiers has the advantage in that pure C02 extracts generally do not need additional cleanup before GC analysis," says Hawthorne. One of the criticisms of SFE is that it requires a significant amount of sample preparation. King disagrees. "In my opinion, it doesn't take that much more sample prep or method development than any of the competing methods or even the older Soxhlet-based methods." Despite more than a decade of development, SFE is not at the stage where just anyone can operate a system. 'To say that SFE is totally at the 'black box' stage would be a bit of overkill. I think it still requires 334 A
Disposable, bar-coded sampie cartridges; instruments designed for plant use
403
consciousness of what you're doing," says King. Part of the reason why SFE has had problems catching on is that it does require the operator to understand the extraction process. Some people just don't want to spend the time to learn how the technique works, says Hawtiiorne. Because there is no universal method that works for all analytes in every matrix out there, people tend to give up on SFE. Hawthorne emphasizes that if people would just read the literature, they would find SFE methods that work for 80-90% of the environmental matrices. "For semivolatiles, polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and about half the pesticides the methods are available." As with other extraction methods, one of the problems with SFE is that the resulting extracts are not always free from unwanted matrix components. Numerous sample cleanup methods have been investi-
Analytical Chemistry News & Features, May 1, 1998
404
gated, with and after the extraction step. Sorbent materials are often used to trap the interfering substances, and in some cases the composition of the solvent is varied for greater recovery. "Several years ago we looked at SFE simply as extraction. Now we are seeing an integration of the cleanup step into the extractor cell—the extraction as well as the sample cleanup are done totally within the SFE unit," explains King. Hawthorne re-emphasizes that cleanup is particularly necessary for fat-soluble analytes; however, for environmental samples, extracts obtained using pure C02 are often clean enough for direct injection onto the GC. "I think SFE has come a long way, ,ut a lot of fine-tuning is still needed to create more versatile instruments," says King. He would like to see more collection options, such as different trapping modules to accommodate more applications. Typical extract
L C - 9 0 0 SFE/SFC modular system
FA-100
D e n s e Gas M a n a g e m e n t System
SFT-1000
SFE-1 L or 10 L
Jasco 8649 Commerce Dr. Easton, MD 21601 800-333-5272
LECO Corporation 3000 Lakeview Ave. St. Joseph, Ml 49085 800-292-6141
Marc Sims SFE 1012 Grayson St. Suite A Berkeley, CA 94710 510-843-1306
Supercritical Fluid Technologies 1 Innovation Way Suite 302H Newark, DE 19711 302-738-3420
Thar Designs 730 William Pitt Way Pittsburgh, PA 15238 412-826-3939
www.jascoinc.com
www.leco.com
[email protected] [email protected] www.TharDesigns.com
Analytical
Gravimetric fat analysis
Analytical to mini-pilot plant
Analytical-to-pilot scale
Pilot-to-process scale
12x18x18
25x23x22
40x22x30
36x24x30
Varies with configuration
80
150
150
300
200
5078
15,000
5803
10,000
10,000
c\J
t ^
f \J\J
1 kg/mm
Parallel or serial
Parallel
Parallel or serial
Parallel or serial
Serial
1-50 mL
10 mL
5-300 mL
up to 4 L (standard); larger sys- 200mL-10 L tems available on custom basis
8
9 (3 per module)
9 (with 2 add-ons)
4
Varies with configuration (up to 4 extractors)
Detector options, including FID, MS, and evaporative light scattering
3-channel add-on modules (maximum of 9 channels); with or without printer
Cosolvent addition system; on-line UV detector; magnetic drive stirred extractor; continuous liquid extractor (membrane)
Cosolvent addition; stirred vessels; weight scales; mass flow meters; windowed vessels with camera options
Cosolvent addition; fractionation; UV detector; stirred vessels
Back-pressure regulator; high-pressure cells for diode array, UV-vis, and circular dichroism detectors
Integrated balance and printer; integrated refrigeration system for pump head cooling; removable, directly weighable traps; dedicated for fat analysis only
Recovery of solvent-free extract; conventional trapping solvents not required
Manual to fully automated controlled models; customization to specific application needs
Computer controlled for repeatable results and sensors integration; "finger-tight" vessels and cyclone separators
405
406
407
408
409
collection options, which can be performed with or without cryogenic cooling, include open vessel, sorbent cartridge, and liquid collection; however, not many commercial instruments are capable of handling all of them, says King. "The way SFE systems are now, the collection options are pretty well welded in. I think that is something that could be improved." King and co-workers have taken matters into their own hands and have modified commercial SFE systems to allow for various collection options. Analytical versus process SFE
SFE was first applied in food and agricultural analysis as a processing technique more than 25 years ago. King believes the success that he and his co-workers have had in developing analytical SFE systems stems from knowledge gained in the process world. He recommends that all analytical chemists interested in SFE method de-
I O
velopment familiarize themselves with the basic engineering principles behind supercritical fluids. According to King, there is a significant amount of synergism between analytical and process development. Today, most commercial analytical and process SFE systems can operate at pressures as high as 10,000 psi and temperatures up to 150 °C. For the extraction of fat-containing samples, King recommends a minimum flow rate of 4-8 mL/min (measured as compressed gas flow) to ensure that time-intensive extractions, such as defatting, are completed as quickly as possible. "For environmental samples, which are usually low in concentration, the rate of extraction of the analytes going into the fluid is much more limiting than the solubility. A higher flow rate doesn't help it actually hurt with volatile compounds because it makes them harder to trap " tions Hawthorne.
The biggest difference among SFE systems—what differentiates analytical from process scale—is the sample-size capacity. SFE systems can handle a range of sample sizes in volumes ranging from 0.5 mL to several liters. When you start getting into the liter range, processing usually comes to mind. However, the line between analytical and process scale is often drawn arbitrarily. Some analytical SFE instruments are capable of handling 2-L samples, although the extraction rate may be compromised even in these esses says King. SFE versus hot, pressurized solvent extraction
Not to be confused with SFE is a similar technique referred to as enhanced solvent extraction (ESE) or accelerated solvent extraction (ASE, a registered trademark of Dionex). "People draw somewhat false boundaries for SFE, but by definition SFE
Analytical Chemistry News & Features, May 1, 1998 3 3 5 A
Product
Review
systems operate in the supercritical fluid state. Enhanced solvent systems use conventional solvents that are hot and pressurized, but they are not above the critical conditions and are still a liquid solvent extraction system," says Hawthorne. "If you want to draw the lines in a phase diagram and make these distinctions, they are certainly valid from the physical chemistry. If you want to draw them based on what actually happens during an extraction, they are much less valid." Tom Chester of Procter and Gamble emphasizes that there are no real boundaries that define the supercritical fluid region {Anall Chem. 1997 69 165 A). He advocates that it is often advantageous to move around the phase diagram—outside the "boxed" supercritical fluid region as long as you continue to work in a region with only one phase To confuse the issue even further, there are now units available that are capable of doing both SFE and high-pressure liquid extraction. According to King, these systems to some extent combine the features of an ESE unit with those of SFE modules. "You're simply seeing the manipulation of pressure and temperature, both as a liquid or quasiliquid state with the supercritical fluid state to get the job done," he explains. "Conventional liquid solvent extraction is done with a variety of liquids. The results can very much depend on the choice of the extracting liquid." This is particularly true for the analysis of fat. "The AOAC [Association of Official Analytical Chemists] lists 28 different methods for fat analysis with dffferent solvents and even pre-extraction hydrolysis techniques to handle different foodstuffs by liquid extraction" Falling in between SFE and ESE is an interesting variation, referred to as enhanced fluidity extraction, which has been largely pioneered by Susan Olesik and her colleagues at Ohio State University. According to Olesik, the term "enhanced fluidity" is generic and can be used for anything, not just extractions. Increasing the fluidity simply means you are decreasing the viscosity. Most of the enhanced fluidity extraction work done by Olesik's group involves mixtures of large quantities of liquid C0 2 and liquid methanol. However, they work on all three types of extractions SFE, enhancedfluidityextractions, and ESE. "Our whole point is to look at different environmental systems and to try to figure out where the best place [in the phase diagram] is to do the extraction Quite often the issue with enhanced fluid-
ity is that if you get it just slightly out of the supercritical realm, everything is easier to handle. You get higher precision, and the instrument works better," she explains. O n - o r off-line
SFE can be directly coupled with other techniques, but tandem on-line approaches have traditionally required highly skilled personnel to configure and operate the equipment. In recent years, however, some advances have been made, particularly in the area of SFE-IR. Phil Liescheski and co-workers at Isco have developed a commercial FT-IR interface for direct on-line coupling to an SFE unit, which can be used to determine free fatty acid content even in the presence of triglycerides. This measurement is important because fatty acids affect the W3.V food tastes and also can serve as an indicator for the age of some food products. For example older grains usually have higher concentrations of free fatty acids than fresher grains Fatty acids have characteristic carbonyl bands in the IR region and can be quantified bv monitoring the shape of the IR carbonvl band Many analysts prefer off-line approaches, rather than on-line techniques in which SFE systems are coupled with GC, SFC, GC/MS, or FT-IR. King is no exception. "A lot of the early work regarding in-line techniques continues to be very interesting, but it is still more academic in its applications. Off-line SFE techniques reign supreme," he says. SFE combined with an independent method, such as enzyme immunoassay, often yields a more rapid— yet still low-solvent approach to sample preparation.
Hawthorne and King, subcritical water extraction has great potential for extracting PAHs, PCBs, and pesticides from soil samples and fruits and vegetables {Anal. Chem. 1994,66,2912). The food and agricultural industries have undoubtedly profited from SFE. At the same time, the environmental industry has resorted to more traditional (and less environmentally friendly) methods. Stuart Cram, worldwide environmental marketing manager for Hewlett Packard, believes that SFE has not been cost-effective and does not have sufficient throughput for environmental samples. He sees many logistical problems with sampling and would like to see all sample preparation and matrix isolation done in the field rather than in the laboratory. What does the future hold for fieldportable SFE systems? The Department of Defense has developed and is currently testing one system that is about the size of a big shoebox and weighs only about 30 lb, according to Lieutenant Colonel Dennis Perry, program manager for the Defense Special Weapons Agency's Chemical-Biological Arms Control Technology Program. "It uses dry ice, so we don't need a compressed C0 2 tank," says Perry. The portable SFE unit is undergoing tests at Pacific Northwest National Laboratory to determine how various modifiers added to the C0 stream affect the extractions. According to Perry the system works well for the extraction of nonpolar analytes but polar analytes present a greater extraction challenge Additionally if the right modifiers are identified some of the more difficult analytes from soils may be extracted Britt Erickson
On the horizon
At the USDA's National Center for Agricultural Utilization Research in Peoria, IL, King and co-workers are developing an instrument for subcritical water extraction, a method originally pioneered by Hawthorne's group for environmental samples. According to Hawthorne, under subcritical conditions, water has a very low dielectric constant, allowing it to dissolve analytes that traditionally require nonpolar solvents. King's system is a modified commercial SFE unit that uses pressurized hot water along with CO, to give both ends of the solvent polarity spectrum. Unlike ESE-SFE units that use conventional organic solvents this system only environmentally compatible solvents. According to
336 A Analytical Chemistry News & Features, May 1, 1998
Upcoming product reviews for 1 9 9 8 August 1: Charge-transfer device detectors September 1: Nitrogen, phosphorus, and sulfur detectors October 1: Ion trap mass spectrometers November 1: MALDI-TOF (matrixassisted laser desorption timeof-flight) MS
If your company manufactures any of these instruments, please let us know. E-mail (
[email protected]) or call us (202-8724570) at least three months prior to the listed date of publication.
