Product
Reviews
When a substance is above its critical temperature and pressure (red area in the phase diagram above), it is said to be in its supercritical state. A supercritical fluid has both gas-like mass transfer and liquidlike solvating properties, and sometimes is referred to as a "super solvent." However, this statement is somewhat misleading. The solvating power of supercritical fluids approaches that of liquid solvents only as the density of the supercritical fluid is increased. Supercritical fluid mass transfer properties include solute diffusivities an order of magnitude higher and viscosities an order of magnitude lower than those of liquid solvents. Supercritical fluid solvent strength is easily controlled by adjusting temperature and pressure. Many supercritical fluids are gaseous at ambient conditions (which simplifies concentration procedures) and are relatively inert, nontoxic, pure, and inexpensive. The combination of these gas- and liquid-like properties with the other characteristics of
Available SFE and SFC systems span a wide range ofprice and sophistication. Before investing, determine which options suit your needs supercritical fluids makes them attractive for extraction and chromatography. Since its inception 30 years ago, the practical application of supercritical fluid technology has had its share of successes and disappointments. Little attention was
paid to analytical SFE prior to 1986. SFC was first reported in the late 1960s, but little commercial development took place until the early 1980s. As concern mounted over the problems of using conventional organic solvents, such as safety and cost, the techniques began to garner more attention from researchers and instrument manufacturers. We have opted to cover SFE and SFC together because of their common basis in supercritical fluid technology. Several companies offer SFE and SFC systems that span a wide range of price and sophistication. Analytical Chemistry enlisted the help of Steven B. Hawthorne of the Energy and Environmental Research Center at the University of North Dakota, Jerry W. King of the USDA, and Thomas L. Chester of Procter and Gamble Co. to guide potential purchasers in sorting through the options available and deciding when to consider investing in a system. This article is not intended to be a
Analytical Chemistry, Vol. 66, No. 6, March 15, 1994 369 A
Product
Reviews
comprehensive review of all the features of commercial systems, but rather a source of information to help the prospec tive customer make an informed choice. Specifications and features of selected SFE and SFC systems are listed in Table 1 and Table 2, respectively. SFE The desire to reduce the quantity of organic solvents used in the laboratory and to reduce sample preparation time has spurred the development of analyti cal SFE. However, the maturation of SFE depends on a number of factors. According to King, the development of SFE was for too long influenced and limited by researchers applying con
cepts that worked for SFC, but not nec essarily for SFE. King also notes that SFE instrumentation may not be as flex ible and adaptable as it could be and that SFE equipment of the future should embrace multiple options for collecting extracted analytes. King believes that the future com mercial success of SFE may depend on the development of standardized meth ods, in part mandated by government agencies such as EPA, USDA, and FDA. Broad federal mandates, such as EPA's pollution prevention acts and the re cently enacted Nutritional Labeling and Education Act, will require automated equipment capable of extracting multi ple samples overnight, which should
spur further commercial development. Sample size capability has been com promised on much of the currently available instrumentation, King says. "If the regulatory agencies decide on a par ticular sample size that is representative of a particular foodstuff or a hazardous waste, the analytical SFE industry must accommodate this parameter in the de sign of their equipment." He notes that many federal agencies have promoted the use and development of analytical SFE through intramural research pro grams and outside contracts. However, the development and acceptance of standard methods takes at least two to three years, partly because of the need for collaborative studies.
T a b l e 1 . Specifications for s e l e c t e d SFE a p p a r a t u s
Product
Spe-ed 680 BAR
SFE 723
7680T SFE
SFX 2-10
SFX 220
Manufacturer
Applied Separations
Dionex
Hewlett Packard
Isco
Isco
930 Hamilton St.
1228 Titan Way
2850 Centerville Rd.
4700 Superior
4700 Superior
Allentown, PA 18101
Sunnyvale, CA 94088
Wilmington, DE 19808
Lincoln, NE 68505
Lincoln, NE 68505
610-770-0900
408-737-0700
302-633-8000
402-464-0231
402-464-0231
$18,000
$23,000
List price"
$13,000
$40,000
$35,000
Dimensions" ( w X d X h , in.)
16 X 20 X 36
1 4 X 2 2 X 17
18X21 X33
13X10X10
13X10X10
Automated
D
•
•
D
•
Autosampler capacity
NA
NA
8
NA
NA
Can be coupled to on-line analyzer/detector
α
D
•
•
•
Max. system pressure (psi)
10,000
10,000
5560
7500 or 10,000 e
7,500 or 10,000 e
Max. pump flow rate (mL/min)
500
18
4.0
90 or 40 e
90 or 40 e
Extractor vessel capacity (mL)
2.5-300
0.5-24
7.0
0.5-10
0.5-10
Max. number of simultaneous extractions
2
8
1
2
2
Max. oven temp. (°C)
200
150
150
150
150
Restrictor type
Variable
Fixed
Variable
Fixed, variable
Fixed, variable
Restrictor flow rate (mL/min)
100-5000"
250-1200"
0.5-4.0
0.5-10
0.5-10
Static and dynamic flow
•
•
•
•
•
Flow orientation
Vertical
Horizontal
Vertical
Vertical
Vertical
Collection mode(s)
Sorbent, solvent
Solvent
Cryogenic, solvent
Sorbent, solvent
Sorbent, solvent
s
List price represents a standard configuration or the average of a range
6
Dimensions may reflect stacking or side-by-side positioning of components
" Pressures and flow rates depend on the model of pump purchased with the extractor * Expressed as volume of expanded gas • = yes • = no
370 A
NA = not applicable
Analytical Chemistry, Vol. 66, No. 6, March 15, 1994
The advantages of SFE—the envi ronmental and biological matrices that can be analyzed, the nontoxic nature of many supercritical fluids, and the speed of extraction—have received a lot of attention recently, and many people be lieve that SFE is the wave of the future and that getting involved with it now may result in a competitive edge. How ever, according to Hawthorne, the big gest mistakefirst-timepurchasers make is assuming the technology is mature because it is possible to buy "a system." SFE has not yet reached the "black box" stage in which the analyst can put a sample in at one end and get a result out at the other (without necessarily understanding all that goes on in be
tween) that, for example, GC and AA spectroscopy have reached. For SFE neophytes, Hawthorne rec ommends purchasing a simple, manual system (or assembling one from modu lar components that can be obtained from several suppliers) and experiment ing to find out how to successfully ex tract a certain type of sample. Extract ing a bottom sample from a severely polluted waterway will beget a set of problems entirely different from those created by a clean, dry sample such as polymer beads. Hawthorne, who has taught SFE for many years, firmly be lieves that this approach is the best way to truly learn the technique and eventu ally apply it successfully. After such ex
perimentation, researchers and lab managers will be in a better position to judge which system meets their needs. Restrictors and options
According to the experts, one of the most important components of the in strument is the restrictor. The role of the restrictor is to allow depressurization of the supercritical solution and separate the solvent from the dissolved analytes—this process stops when the restrictor plugs. Rapid improvements in restrictor technology have made plug ging problems lessen, and undoubtedly they will continue to improve. Three types are available:fixedlinear, auto matic variable, and manual variable.
Table 1 . Specifications for s e l e c t e d SFE a p p a r a t u s (continued)
Product
SFX 3560
DGMS
SFE-400
AutoPrep 44
PrepMaster
Manufacturer
Isco
Marc Sims SFE
Supelco
Suprex
Suprex
4700 Superior
1012 Grayson St.
Supelco Park
125 William Pitt Way
125 William Pitt Way
Lincoln, NE 68504
Berkeley, CA 94710
Bellefonte, PA 16823
Pittsburgh, PA 15238
Pittsburgh, PA 15238
402-464-0231
510-843-1306
814-359-5459
412-826-5200
412-826-5200
List price*
$50,000
$23,000
$10,000
$80,000
$24,000
Dimensions* (w X d X h, in.)
22X18X30
36 X 20 X 28
19X19X23
38 X 26 X 29
17X22X22
Automated
•
G
α
•
•
Autosampler capacity
25
ΝΑ
ΝΑ
44
NA
Can be coupled to on-line analyzer/detector
•
•
Π
D
•
Max. system pressure (psi)
7500 or 10,000 e
6000
6000
7400 or 10,000"
7450 or 10,000"
Max. pump flow rate (mL/min)
90 or 40 e
15
1700"
7
7 or 14"
Extractor vessel capacity (mL)
0.5-10
5-300
1-10
0.5-10
0.150-50
Max. number of simultaneous extractions
1
2
4
1
2
Max. oven temp. (°C)
150
100
200
150
150
Restrictor type
Fixed, variable
Variable
Fixed
Variable
Variable
Restrictor flow rate (mL/min)
0.5-10
0-15
0-2
1.0-7.0
1.0-7.0
Static and dynamic flow
•
•
•
•
•
Flow orientation
Vertical
Vertical
Vertical
Vertical
Vertical
Collection mode(s)
Solvent
Cryogenic, solvent
Solvent
Sorbent, solvent
Sorbent, solvent
a
List price represents a standard configuration or the avera je of a range
" Dimensions may reflect stacking or side-by-side positionin3 of components c
Pressures and flow rates depend on the model of pump pi rchased with the extractor
" Expressed as volume of expanded gas • = yes
Π = no NA = not applicable
Analytical Chemistry, Vol. 66, No. 6, March 15, 1994 371 A
Product
Reviews
The most common type is the fixed linear restrictor, which is usually made of fused silica or stainless steel tubing. Fixed linear restrictors are inexpensive and allow the extracted analytes to be collected directly on a trap or in solvent, but they are prone to plugging. Fusedsilica restrictors suffer an additional drawback because co-solvents added to the extraction fluid tend to make the tubing prone to fractures. The newer linear restrictors are heated, which alle viates many of the plugging problems. Automatic variable restrictors are the most expensive; manually con trolled variable restrictors are interme diate in cost. It is important to realize, says Hawthorne, that a particular re strictor may work better for difficult samples and that testing of restrictors should be part of the instrument evalua tion. According to Hawthorne, the manu facturer should be willing to run some test extractions on the instrument un der consideration. (Most manufacturers that we spoke to will run a test extrac tion; however, they reserve the right to refuse extremely hazardous samples.)
He suggests sending in your worst nightmare of a sample to get an answer to "Can your restrictor handle my nasti est sample?" Also send in an average sample, spiked, and ask "Can your in strument efficiently collect my analytes after the sample is extracted?" The an swers to these two questions should help you determine which system is right for your laboratory. A wide variety of options, including extraction cells that can be hand tight ened, fraction collectors, cryogenic trap ping units, repair kits, upgrade kits, and printout capability, are available in SFE systems. Some systems are modular, and some are fully contained. Standard features on one apparatus may be op tions (at extra cost) on another. SFC
Many people consider SFC to be a hy brid, or a middle ground, between LC and GC because it combines the advan tages of both. This dual nature adds a great deal of control and flexibility in developing separations. In LC, the com position of the mobile phase needs to be adjusted to effect the separation de
sired; in GC, the temperature needs to be regulated. In SFC, the density (or pressure), temperature, and composi tion of the supercritical fluid all have major influences on solute retention. SFC can offer speed, solvent savings, and efficiency not available with HPLC; it uses lower temperatures than does GC and can be used to analyze a much higher molecular weight range than is possible with GC. A myriad of SFC ap plications already exists, and more ap pear in the literature each month. The configuration of the instrument to be purchased will depend primarily on its analytical application and the detector required. Instruments can accommo date open-tubular (capillary) columns, packed columns, or both. Packed-column SFC seems to be favored by those with an HPLC back ground. Its advantages include greater efficiency per unit time and the ability to inject a larger amount of sample be cause the amount of stationary phase per theoretical plate is greater, and therefore column overload is less of a problem. Packed column lengths can be much longer in SFC than in HPLC, pro-
Table 2. Specifications for selected Sri< instruments
ι
Product
600 SFC
600 SFC/GC
SF3
HPG120SA
MPS 225
Manufacturer
Dionex
Dionex
Gilson Medical
Hewlett Packard
Suprex
1228 Titan Way
1228 Titan Way
3000 W. Beltline Hwy.
2850 Centerville Rd.
125 William Pitt Way
Sunnyvale, CA 94088
Sunnyvale, CA 94088
Middleton, Wl 53562
Wilmington, DE 19808
Pittsburgh, PA 15238
408-737-0700
408-737-0700
608-836-1551
302-633-8000
412-826-5200
List price3
$31,000
$33,000
$26,000
$64,000
$48,000
Dimensions" (w X d X h, in.)
28 X 23 X 28
28 X 23 X 28
42X18X24
78 X 22 X 25
38 X 22 X 31
On-line SFE capability
•
•
•
G
•
Injection mode
Direct, split, timed split
Direct, split
Direct, split, timed split
Direct, split
Direct, variable speed
Autosampler capacity
80 c
80 '
80 or 400"
100
64
Number of pumps
1
1
2
2
1
Column type
Packed, open tubular
Packed, open tubular
Packed
Packed, open tubular
Packed
Number of column inlets
2
2
5
2
1
Restrictor type
Fixed
Fixed
Variable
Variable
Fixed
Number of detector inlets
2
2
1
3
3
Detectors
FID, UV, FTIR, MS
FID, UV, FTIR, MS
UV, MS, ELSD"
FID, UV, NPD, ECD
FID, UV, NPD, SCD
* List price represents a standard configuration or the average of a range * Dimensions may reflect stacking or side-by-side positioning of components 0
Depends on the model of autosampler
" ELSD = evaporative light-scattering detector • = yes
372 A
G = no
NA = not applicable
Analytical Chemistry, Vol. 66, No. 6, March 15, 1994
viding well over 100,000 theoretical plates. Packed-column SFC usually requires the addition of an organic modifier or co-solvent to the main mobile phase component (C0 2 in most applications). Programming with packed columns is usually done with composition gradients, as in HPLC, with the pressure and temperature held constant. The newest instruments are both sophisticated and easy to use. Open-tubular SFC is often preferred by those with more GC experience. Open-tubular columns, because of their permeability and length, usually have greater efficiency than packed columns.
onto an open-tubular column, four modes are commonly used: direct injection using a four-port rotary valve containing a smaller internal sample loop; split injection, in which the flow from the injector is split with only a small portion of sample entering the column; time-split injection, in which the rotor is rapidly activated pneumatically to allow only a small fraction of the sample onto the column; and, of course, injection by direct, on-line SFE. (A variation of the time-split injector is the variablespeed injector in which the speed of actuation can be controlled, allowing a small or large amount of sample onto the column.)
The configuration of the instrument will depend primarily on the analytical application and the detector required.
Injection volume can be a problem with capillary columns. For liquid samples, effective injection volumes with open-tubular columns are often just a few nanoliters. Trace analysis is very difficult or impossible with such small volumes. Researchers have reported the use of several injection techniques that provide larger effective volumes, but none are commercially supported at present. The FID is the most commonly used detector for open-tubular SFC; the UV detector is the most popular for packedcolumn SFC. Virtually all chromatographic detectors (UV, electron capture, flame photometric, nitrogen and phosphorus thermionic, fluorescent, chemiluminescent, evaporative light scattering, and photoionization) can be used on line. Many spectroscopies (mass, FT-IR, atomic emission, ICP, and NMR) are also being investigated as modes of detection.
They are most often used with neat C 0 2 and with pressure or density programming. This allows easy, subnanogram, universal detection of organic compounds. Despite many reports of impressive results, commercial development of open-tubular instrumentation has not proceeded as quickly in recent years as that of packed-column instrumentation. "There seems to be a chicken and egg problem," says Chester. "The manufacturers don't seem interested in developing new [open-tubular] instruments until there is more market demand, but the more advanced users have reached the limits of the current designs, now about 10 years old. Independent researchers can't generate the interest leading to market expansion without better equipment." Injection mode is another concern. Direct injection using a six-port rotary valve with an external sample loop is the standard mode used for packed columns, just as in HPLC. For injection
Researchers have proved that SFE and SFC techniques can be used to extract and analyze a broad range of matrices and determine a variety of compounds; as research proceeds, individual methods will certainly be improved and refined. The desire to reduce the quantity of organic solvents used in the laboratory and the eventual adoption of SFE and SFC standardized methods will probably be responsible for the push toward maturation of these techniques. As with all analytical techniques, as instrument manufacturers endeavor to keep their finger on the pulse of the analytical consumer, instrument design will evolve and improve. All of these factors will contribute to the development of SFE and SFC as powerful tools for analytical chemists. Felicia Wach
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Analytical Chemistry, Vol. 66, No. 6, March 15, 1994 373 A
