Product Review
Is there new life ahead for hyphenated IR? Despite a flat market,GC/FT-IRsystems continue to evolve. Fourier transform IR (FT-IR) spectroscopy has been used since the mid-to-late 1960s as a detector for GC. It has had a hard ttme competing with MS, however, largely because of its lower sensitivity. Despite its overwhelming acceptance, GC/MS does have certain limitations. MS detection relies heavily on library searches, which can sometimes lead to erroneous identifications; and although MS databases have become extremely large (close to a quarter million spectra), they are still finite. Perhaps the biggest drawback to MS is its inability to distinguish between certain isomers, and that is where FT-IR comes into play. "IR gives you information about isomers that MS doesn't," says Charles Wilkins of the University of Arkansas. In addition, IR spectra have characteristic frequencies that allow you to identify different functional groups. "You can get information about functional groups with MS, but it is much less direct. It is easier to identify functional groups and isomers with IR than with MS." Analytical Chemistry hah surveyed sevBritt Erickson
eral representative manufacturers to find out what is new in the area of hyphenated IR. Manufacturers say that the GC/IR market is small but steady. Although GC/IR appears to be a mature market, new systems continue to emerge. The specifications and features of selected commercially available GC/IR systems and accessories are given in Table 1. This review is not intended to be comprehensive and may only represent a fraction of what each manufacturer offers. Readers are encouraged to consult with the individual manufacturers for their complete product lines. The light pipe
The most common type of GC/IR interface involves passing the effluent from a GC column through a gold-coated flow cell, or "light pipe" while focusing an IR beam through windows at each end of the cell. Spectra are continuously obtained at about 1-s intervals. The windows on a light pipe are made of an IR-transparent material, such as KBr. The light pipe is generally operated at a temperature at least 10 degrees greater than the maximum GC oven temperature,
which prevents condensation of the sample components in the light pipe and on the KBr windows. Even repeated injections of aqueous samples do not interfere with the light pipe. light pipe interfaces are extremely robust and will last for many years. The light pipe itself can typically withstand temperatures up to about 325-350 °C. Occasionally, however, the light pipe windows become fogged and need to be replaced. The light pipes that are commercially available have very few differences. "One difference is how inconvenient the manufacturers make it to use with anything but their FT-IR systems," says Wilkins. "As far as the light pipes themselves, if they weren't so inconvenient to install, people could make their own rather inexpensively." Today, commercial light pipe interfaces sell for about $15,000-$25,000. Compatibility is a relative term. In principle, light pipe interfaces can be fit on any FT-IR system and any gas chromatograph. "What it actually takes to accomplish that is the hitch. It's all the other things you need to do to make it work—that's the tricky part," says Wilkins. The majority of manufacturers, including Bio-Rad, Bruker, Nicolet, and Perkin-Elmer, sell light pipe interfaces as an accessory to go with one of their FT-IR systems. Rarely, if ever, do they sell llght-pipe interfaces for another manufacturer's FT-IR system. One reason is that "a lot of effort goes into designing the light pipe to ensure
Analytical Chemistry News & Features, December 1, 1998 801 A
Product Review
Table 1 . Summary of selected GC/IR interfaces Model
Discrete Component GC/FT-IR
IRD
InfraRed Chromatograph
Manufacturer
Aabspec Instrumentation 2643 Grand Ave. Bellmore, NY 11710 800-783-9380
Bio-Rad Spectroscopy Division 237 Putnam Ave. Cambridge, MA 02139
Bourne Scientific 3 Post Office Square Acton, MA 01720 978-635-0011
800-225-1248 URL
None
www.bio-rad.com
www.BournelRC.com
Type
Light pipe
Light pipe
Direct deposition
Light pipe Composition Inner diameter Length Window composition Max. temperature (°C)
Solid gold 0.06-6 mm 6 cm KBr, ZnSe, or other 320
Gold-coated glass 1 mm 12 cm KBr (standard) 320
Sample deposition window Composition Temperature Deposition width
N/Aa
N/Aa
IR system compatibility
Any FT-IR system
N/Aa
N/Aa
Software
Requires automatic GC/IR software (#26G); automatic software not essential for #35G and #66G
Windows 95 and NT; HP MS Productivity Chemstation; IRD Software; GRAMS/32
Windows, GRAMS, and proprietary
Options
Light pipe can be packaged with PEAKMAX sample compartment
Searchable vapor-phase databases; ZnSe windows
Conversion kits for LC and SFC
Special features
Solid gold light pipe for good hightemperature optical behavior; transfer line contains by-pass valve allowing GC peaks to be "parked" in light pipe for increased scan times (#35G and #66G)
Interface to HP 6890 gas chromatograph with an optional configuration using an HP 5973 MSD
Picogram range detection limits; GC temperatures up to 420 °C; GC flow rates from 0.2 to 8 mL/min
Reader service no.
401
402
403
N/Aa
ZnSe -150 to 100 °C ~ 0.1 mm
N/A = not applicable
that the optical throughput is optimized for the spectrometer," says Tim Johnson of Bruker. On the other hand, most light-pipe interfaces can be easily connected to any manufacturer's gas chromatograph, although some modifications may have to be made. "Some interfaces do not have a transfer line and require that a hole be drilled through the GC oven wall to connect the column directly to the light pipe," says Chris Thorne of Perkin-Elmer. In addition to providing a GC/IR accessory that is compatible with their FTS6000, Excalibur, and Century Series FT-IR systems, Bio-Rad also manufactures a unique light-pipe system, the InfraRed Detector (IRD). According to the manufacturer, the IRD was designed specifically as a chromatographic detector with an optimized light-pipe interface, rather than as a light-pipe accessory to a general-purpose FT-IR spectrometer. "The spectrometer is designed with a very small beam and a hot source. The optical path is short so there is no wasted light," says Norman Wright of Bio-Rad. Because 802 A
the IRD is application specific, it has no sample compartment. Originally provided by Hewlett Packard (HP), the IRD was sold to Bio-Rad about three years ago. Bio-Rad has maintained hardware and software compatibility with the HP product line. As a detector for the HP 6890 gas chromatograph, the IRD has a width of only 8 in. and is routinely integrated with an HP GC/MSD system. Aabspec offers GC/IR accessories designed to interface with any FT-IR spectrometer. According to the company's president, Val Rossiter, Aabspec's Discrete Component GC/FT-IR Systems can also be interfaced to any gas chromatograph. Three models are available with different requirements, depending on the type of FT-IR system and gas chromatograph. The light pipe is designed to go into the sample compartment of the FT-IR system and connects back to the gas chromatograph with the heated transfer line. "The light pipe itself is solid gold, so it is extremely inert and has unmatched high-temperature optical behavior" says Rossiter "In addition
Analytical Chemistry News & Features, December 1, 1998
the chromatographic resolution is good because the volume of the light pipe is very low—less than 20 uL in one capillary light pipe that we offer." The volume of the light pipe is determined by the width of the peaks in the chromatogram. "If your light pipe is too big, you can get two peaks present at the same time. If it is too small, you get only a very small fraction of a peak at any given time," explains Peter Griffiths of the University of Idaho. The key is to get as much as you can in the light pipe without losing chromatographic resolution. The volume of most commercially available light pipes ranges from 50 to 200 uL. Occasionally a makeup gas is added to the effluent coming out of the column when there are peaks in the chromatogram that are narrower than the light pipe, for example, with some very early eluting peaks. "Usually I would not recommend adding a makeup gas. You'd be better off to adjust the temperature program used on your chromatograph to make sure that ev-
GC/IR Accessory ( A - 5 8 6 )
GC/IR Interface
GC/IR
MI-TREK
Bruker Optics 19 Fortune Dr. Manning Park Billerica, MA 01821 978-667-9580
Nicolet 5225 Verona Rd. Madison, Wl 53711 800-232-1472
Perkin-Elmer 761 Main Ave. Norwalk, CT 06859 800-762-4000
Reedy R&D Services 197 West Harrsion St. Bourbonnais, IL 60914 815-933-2525
www.bruker.com
www.nicolet.com
www.perkin-elmer.com
www.rndserv.com
Light pipe
Light pipe
Light pipe
Matrix isolation
Gold-coated glass 1 mm 19.7 cm NaCI or KBr 350
Gold-coated glass 1 mm 15 cm KBr (standard) 325
Gold-coated glass 1 mm 12 cm KBr (standard) 350
N/Aa
N/Aa
N/Aa
N/Aa
Mirror-finished gold 10 K
-0.2 mm Bruker Equinox 55, Vector 22, IFS 66, and IFS 66v
Nicolet Magna 560, 760, 860, and Protege 460
Perkin-Elmer Spectrum GX, Spectrum 2000, and System 2000
Nicolet Magna or other
OPUS (OS/2 or Windows NT)
OMNIC Series (kinetics/data acquisition package)
Spectrum TimeBase
Integrated, including HP MS Chemstation, Nicolet OMNIC Series, and Ml-TREK software
Window materials; detectors
Window materials such as ZnSe
ZnSe windows
Interface to existing GC/MS or FT-IR
Small dead volume; high optical throughput
Complete vapor-phase spectral library obtained using GC/IR for good hit quality
Software includes real-time Gram Schmidt reconstruction, real-time 3D displays, and real-time display of functional group profiles; software ineludes pre-run data collection Simulation under user-selected conditions
Highly definitive IR bands; picogram range sensitivity; consistent spectra for low- and high-volatility samples; concurrent MS and IR; modularity; automation
404
405
406
407
erything elutes with about the same width," advises Griffiths. The addition of a makeup gas somewhat raises GC/FT-IR detection limits. Sensitivity has always been an issue with light-pipe GC/IR interfaces. "The early instruments had very low sensitivity, but they didn't really need that much because that was before the era of capillary columns," says Griffiths. "Nonetheless, even now it is difficult to get a decent spectrum from less than 10-20 ng of most components on a light pipe system." Another limitation with light pipe interfaces is that there is only a limited amount of time to obtain a spectrum because the sample flows through the cell. "Once it's gone, you can't go back and measure it a second time. You'd have to put more sample in," says Wilkins. Matrix isolation/direct deposition
The desire to reduce the minimum identifiable quantity (MIQ) to subnanogram levels spurred the development of a fundamen-
tally different type of GC/IR interface that involves trapping each analyte onto a cooled substrate window. In thefirstsystem, a matrix-isolation approach was used, in which each sample component is entrained in an argon matrix and condensed onto a moving metal substrate. Shortly thereafter, another type of interface emerged in which the sample is no longer diluted in any type of matrix and hence is termed a direct-deposition system. Thefirstcommercially available matrixisolation GC/IR system, the Cryolect, was developed in the 1980s by Gerald Reedy of Cryolect Scientific, which was later sold to Mattson Instruments. Although the system is no longer commercially available, it undoubtedly had a large impact on the field. With the Cryolect, a small amount of argon is added to the column effluent, and the resulting mixture is passed onto the surface of a metal substrate that has been cryogenically cooled to about 10 K. Each sample component is trapped in the argon matrix and deposited on the substrate,
which slowly moves so that the entire chromatogram is recorded. Once the sample has been deposited, the substrate is moved into the path of an IR beam, and spectra of each GC peak are obtained. The matrix-isolation approach provides high sensitivity because the area over which the sample is deposited is small (—200 um). The intensity of the IR bands is inversely proportional to the cross sectional area. In addition, the reduced temperature of the matrix results in greater absorbance. MIQs for compounds with average absorptivities are in the 100-200 pg range for matrix-isolation systems, 10-fold lower than for light-pipe systems. One advantage of the matrix- isolation approach is that the IR bands are very sharp, particularly for small molecules. "Compounds are diluted in solid the molecules do not associate among themselves Thev vibrate independently of their neighbors" says Reedy "If you are interested in the structural details of the molecule then matrix isolation is the way to go"
Analytical Chemistry News & Features, December 1, 1998 803 A
Product Review Today there is only one commercially available matrix-isolation GC/IR system, the MI-TREK, which is manufactured by Reedy R&D Services. The MI-TREK is similar to the Cryolect, except the cooling source remains stationary. "There is a special bearing that transfers the heat between silver and copper to get the collection surface down to cryogenic temperature," says Reedy. The MI-TREK is typically interfaced with an HP 6890 gas chromatograph and a Nicolet Magna IR spectrometer. In principle, however be interfaced with other systems as well. The interface alone sells for $79 000. Another major advance in GC/IR occurred in the late 1980s with the introduction of the Bio-Rad Tracer, the first realtime direct-deposition GC/IR system. As sample components elute from a GC column, they are sprayed onto a window— basically a microscope slide made of ZnSe and placed inside a vacuum chamber at about 120 K. The carrier gas is pumped away in the vacuum system, leaving the column eluent frozen on the window. "The window moves continuously and very slowly so the chromatogram is deposited in a track along the surface of the window," explains Sidney Bourne president of Bourne Scientific who helped develop both the Cryolect and the Tracer "During the run the track passes through the optical beam so the FT-IR is scanning continuously.There is about a 10-15 s delay from the time any molecule freezes on the window to the time it passes through the optical beam " savs Bourne The sample comnonents are rleposited as a verv small spot C—100 urn") resulting in even Greater sensi tivity than the matrix-isolation systems. The sensitivity is comparable to that of a quadrupole GC/MS but not quite as sen•f
r* -tell.
"\r
sitive as ion traps, says Griffiths. You probably need 100-500 pg to get a reasonLi
,.
"
T,
,1,1
able spectrum, says Bourne. However, 1 •
x
1
1
1 -x 1
some users claim to have pushed it down to 20 pg. Bourne has now started his own company and introduced a new system, the InfraRed Chromatograph (IRC). There are not many differences between Bio-Rad's Tracer and Bourne Scientific's IRC. "As far as the bottom line performance, they are the same. The sample is deposited in the same sample width, so the absorbance you get is the same. The S/N is the same, and the sensitivity is the same," says Bourne. The alignment of the optical beam with the deposited sample is a critical aspect of 804 A
direct-deposition systems. Because the sample is deposited as a very small spot, it is easy for the IR beam to miss it. "Temperature changes tend to shift things around," says Bourne. The IRC is designed in such a way that alignment adjustments can be made through the wall of the vacuum chamber while under operating conditions. An advantage of the IRC is that it is easy to switch from GC to LC or supercritical fluid chromatography (SFC). "For $10,000, we sell a conversion kit to switch from GC to LC. The only difference is the sample delivery. All of the rest the optics and the software are identical" S3.Vs Bourne. The key to LC/IR is eliminating the solvent and depositing the solute on a moving substrate Switching to SFC is much simpler "It is basically just a change in the restrictor and some other minor parts" Bourne's SFC kit sells for $1000 "Capillary SFC uses
"Certainly a combined GC/IR/MS system gives you much less equivocal answers than individual IR or MS detection." the same interface as our GC interface," says Bourne. "A number of people have used the Tracer for SFC/IR and LC/IR, but it is a little trickier," says Griffiths. "Essentially what Bourne has done is develop the first on-line LC/IR system." Direct-deposition systems are more expensive than light-pipe systems. "They are more complicated optically; they have moving parts—a window moves back and forth with a microstepping motor; and they operate under a pretty decent vacuum," says Griffiths. An entire system (without the gas chromatograph) costs $110,000-$150,000. GC/IR/MS
For even greater accuracy in identifying the components of a complex mixture, it is common to use IR as a complementary technique to MS. "Certainly a combined
Analytical Chemistry News & Features, December 1, 1998
GC/IR/MS system gives you much less equivocal answers than individual IR or MS detection," emphasizes Griffiths. There are two ways to configure a GC/ IR/MS system. The first, referred to as parallel linkage, is to split the GC effluent, sending a small amount to the MS detector and the majority of it to the IR spectrometer. "Because IR is so much less sensitive than MS, you have to compensate for that difference. In effect what you are doing is decreasing the sensitivity of MS to be the same as that of IR," says Wilkins. The other way, referred to as serial linkage, is to send the effluent through the light pipe first, and then send as much as you want to the MS detector. "You can place a light pipe before any detector because it is totally nondestructive," adds Griffiths. It is even possible to have more than two detectors (e.g., IR MS, and a flame-ionization detector). "In a directdeposition system you do not destroy the sample; you trap it. However, you can't readily evaporate it and pass it on to another detector. Typically, you have to use an effluent splitter if you are going to use a directdeposition system," says Griffiths. Searching spectral libraries
Once IR spectra have been obtained, identifying each of the components is extremely time consuming without the aid of a computerized spectral search program. With a light-pipe interface, spectra are measured in the vapor phase, and therefore a library of vapor-phase spectra is necessary. On the other hand, direct-deposition and matrixisolation systems yield spectra that are almost identical to KBr disk spectra or condensed-phase spectra. There are many more condensed-phase IR spectra (>150,000) available than vaporphase spectra (—15,000). Libraries can be purchased from several sources, including many of the GC/IR manufacturers that are listed in Table 1. They are usually provided as an extra cost option and are often specific for particular applications (e.g., flavors and fragrances, solvents, hazardous chemicals). One vapor phase library was compiled by the U.S. Environmental Protection Agency (EPA)—it contains 3300 spectra, including most of the EPA targeted compounds. The Coblentz Society and the National Institute of Standards and Technology are collaborating on another IR spectral library, which is expected to be available to the public in the fall of 1999.
According to Kathryn Kalasinsky of the Coblentz Society, the library will be offered for a minimal price and will include both vapor and condensed phase spectra (total of —20,000 spectra), including spectra from the EPA library. Vapor phase spectra are much different than condensed-phase spectra for polar compounds that contain many hydrogen bonds. However, there are fewer differences between the two types of spectra for compounds that are less polar and do not contain hydrogen bonding groups because the intermolecular interactions are weaker. In some cases, spectra obtained by a directdeposition system can be successfully searched against a library of vapor-phase spectra. What's ahead
Manufacturers don't expect to see dynamic growth in the GC/IR market over the coming years. The market is steady, however, and not expected to decline. "Anybody who is dealing with complex mixtures is a potential customer for GC/IR," one manufacturer told Analytical Chemistry. The forensic market has traditionally been very strong for GC/IR, as have the flavors/fragrances, food, and petroleum industries. "Much of the noncommercial development of GC/IR systems is being done through EPA" says Griffiths. The environmental area will likely continue to make use of GC/IR, primarily for compounds with polar functional groups, he says. Of course, one cannot leave out the pharmaceutical companies who are mandated to identify all of the side products and metabolites of potential drugs, adds Wilkins. With the pharmaceutical and biotechnology industries capturing a substantial part of the instrumentation market some manufacturers expect to surge of interest in LC/IR similar to that being seen now with LC/MS Britt Erickson is an Assistant Editor of Analytical Chemistry. Upcoming product reviews for 1999 April 1: Gas chromatographs May 1: Chromatography data systems June 1: Benchtop GC/MS systems
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.
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