Product
Review
The Stable World of
FT-IR
Fourier transform IR offers something for every level of analysis, from dedicated instruments for very particular applications to highend research instruments for which "flexibility" is the buzzword. Manufacturer representatives Analytical Chemistry talked to agree that the market is stratified into instruments intended for routine applications general research applications and advanced research applications General purpose research instruments are for those who use IR as a tool in their research. David Clark of PerkinElmer and Matthew Smith of Nicolet describe this work as "forensic". "Many analytical laboratories are doing forensic-like work, trying tofigureout what went wrong with a research instruments. The instruments in the table represent only afractionof each process or looking at a competitor's promanufacturer's offerings. Readers are encess to try tofigureout what it is" says couraged to contact the manufacturers Smith. The instruments have undergone a few directly for information on their complete changes since Analytical Chemistry pub- product lines. lished its first product review on FT-IR spectrometers (Anal. Chem. 1195, 67, 381 Instrumental considerations FT-IR instruments are built around an A-385 A). "There have been incremental interferometer, with about half of them changes," says Peter Griffiths, a professor being similar to the type that Michelson at the University of Idaho, "but nothing originally designed at the end of the past that I would call really major." The systems described in Table 1, not century. IR radiation from a broadband source, such as silicon carbide or highintended to be a comprehensive listing of temperature ceramic, is directed to a available FT-IR instruments, are predomibeamsplitter that divides the light benantly research-grade instruments that tween two optical paths, one or both of fall into the general research category. which can be changed. The mirrors reEach of the companies also provides inflect the light back to the beamsplitter, struments for the quality control market, where they are recombined and directed and several of them provide "high-end"
to the sample and detector. When the two paths are equal, the two beams are in phase. At every other position of the moving mirror, the beams are out of phase and create a repeating interference pattern, which is a sum of the cosine waves for all of the wavelengths. The interferogram, which is 3. plot of intensity optical path difference can be converted through a Fourier transform to a singlebeam spectrum In the classical Michelson interferometer, the mirrors are at right angles to one another, but Griffiths says there is no reason why they have to be. "Angles other than 90 degrees aren't bad, provided you can get high efficiency from your instrument that's great." The instruments in Table 1 include both 90° and 60° interferometers. The interferometer can move the mirror on either a mechanical or an air bearing each with its tradeoffs Air bearings are smoother and more accurate but they require a source of compressed air For many applications the improved performance cannot justify the inconvenience Spectral resolution is determined by the distance the moving mirror travels. Doubling the spectral resolution requires doubling the displacement distance of the moving mirror. The standard resolution achievable with research-grade instruments is 0.25 cm""1 to 1 cm"1, with even better resolution available. "If you're going to buy a top-of-the-line instrument, resolution doesn't cost verv much." says
The market for FT-IR spectrometers has become even more stratified.
Analytical Chemistry News & Features, April 1, 1998 273 A
Product
Review
Table 1. Summary of representa F T - i n instruments Product Diamond-20 Excalibur series
MB series
IFS 66/S series
Company
Analect Instruments 9420 Jeronimo Rd. Irvine, CA 92618 714-206-4100
Bio-Rad 237 Putnam Ave. Cambridge, MA 02139 617-868-4330
Bomem 450 St-Jean-Baptiste Quebec, Quebec G2E 5S5 CANADA 418-877-2944
Bruker Optics 19 Fortune Dr. Manning Park Billerica, MA 01821 978-667-9580
URL
www.analect.com
www.bio-rad.com
www.bomem.com
www.bruker.com
Optics Source Beamsplitters
Internal focused beam KBr or CaF 2
Ceramic or W-halogen KBr, Csl, CaF2, quartz, or Mylar 25000-50 cm" 1
Globar, quartz KBr, Csl, ZnSe, BK7, KCI
0.25 cm" 1 standard 0.1 cm" 1 maximum
1.0 cm" 1 maximum
Glowbar, W, Hg, Xe KBr, CaF2, Quartz I, II, and III, Mylar 7500-370 cm" 1 standard >40,000-20 cm ~1 optional 0.25 cm ~1 standard 0.1 cm " 1 optional
Spectral range Resolution
7000-400 cm" 1 12,000-1,200 cm" 1 1.5 c m ' 1
14,000-200 cm" 1
Interferometer
Transept IV
60 °, dynamically piezoaligned Michelson; mechanical bearing
Michelson with corner-cube optics
High-precision air-bearing interferometer
Detectors
DTGS, MCT, InGaAs, or InAs
Peltier-cooled or ambient DTGS, MCT, PbSe, InSb, PbSe, Si, Si bolometer, linearized MCT
MCT, DTGS, InGaAs, Si
DTGS, MCT, InSb, Ge, InGaAs, Si, GaP, bolometer
Sample chamber
INA
9 x 11.25 x 7.125 in.; accommodates sampling accessories from all major vendors
17.6x17.6x15.4 cm
25 x 27 x 16 cm; kinematic or fixed baseplate; side or top access
Data system
Pentium 75 MHz or faster, MS-DOS 6.22 or higher with Windows 95
Windows 95-compatible Win-IR Pro, Merlin, or Win-IR
Windows-based software; GRAMS; Win-Bomem Easy; AIRS
Windows NT and OS/2 compatible software
Options
Mid-IR, extended mid-IR, or near-IR
Right- and left-side external beam; GC/IR; TGA/IR; FTRaman; FT-IR microscopy and mapping; emission; fast kinetics; photoacoustic spectroscopy; up to 5 beam ports
Side port; dedicated accessories
Step-scan; ultra-fast rapid scan; up to 3 external beam ports and two input ports; module for VCD, VLD, and PM-IRRAS; microscopes; FT-Raman; GC, TGA, TLC, and emission accessories
Special features
Permanently aligned optics
ACE (accessory control electronics); USB interface; full validation; interactive CDROM
Reader service no.
401
402
Griffiths. "Because computer systems are so big, you never worry that the computer isn't going to handle that amount of data." Various detectors are available for the mid-IR. The most common of these is made of deuterated triglycine sulfate (DTGS), a pyroelectric detector with a very linear response across the IR Because the detector can operate at room temperature, it is used for standard applications. Another popular detector is mercury cadmium telluride (MCT), which has a wavelength-dependent response. The cut274 A
Instrument diagnostic routines; digital electronics for precise scanner control; vacuum version available with spectral range >40,0005 cm" 1 403
off wavelength can be adjusted by changing the ratio of the elements. MCT is much more sensitive than DTGS, which can translate into either lower detection limits or faster data acquisitions. However, it must be cooled to liquid nitrogen temperatures. Another possible drawback to MCT, but one that doesn't manifest itself very often, is that it doesn't cover the entire IR. "If you think of the usual definition of the mid-IR as 4000 to 400 cm-1, all MCT detectors get out to 4000 cm-1," says Griffiths. "The narrow-band ones go down to
Analytical Chemistry News & Features, April 1, 1998
404
about 750 cm"1, the mid-band ones go to about 600 cm"1, and the wide-band ones go to about 450 cm"1. The narrow-band ones are the most sensitive but have the least linear response; the wide-band ones are only about four times more sensitive than DTGS but are more linear than the narrowband detectors." Griffiths finds the midband detectors the most useful because they are sensitive over the region usually required for organic IR spectroscopy. "There are probably some people who say they have to look at the region below 600 cm-1 to interpret the spectra of organic
FT/IR 6 0 0 series
Infinity series
M Series
MagnaIR series
Spectrum GX series
JASCO 8649 Commerce Dr. Easton, MD 21601 410-822-1220
Mattson 5225 Verona Rd. Madison, Wl 53711 608-276-6300
MIDAC 17911 Fitch Ave. Irvine, CA 92614 714-660-8558
Nicolet 5225 Verona Rd. Madison, Wl 53711 608-276-6100
Perkin- Elmer 761 Main Ave. Norwalk, CT 06859 203-762-4000
www.jascoinc.com
www.mattsonir.com
www.midac.com
www.nicolet.com
www.perkin-elmer.com
Ceramic; W or Hg Ge-coated KBr, CaF2, Csl, or Mylar 7800-350 cm"' standard 15000-50 cm"' expanded 0.5 cm" 1 standard; 0.25 cm - 1 maximum
SiC or W KBr, Quartz, CaF2, Mylar, or Csl 7000-350 cm' 1
1550 K air-cooled Ge-coated KBr or CaF2
Mid-IR or quartz halogen Quartz, CaF2, KBr, Csl, FIR grid, Mylar 15000-30 cm"'
0.125 c m " 1
2, 1, or 0.5 cm" 1
Ever-Glo or quartz-halogen KBr, quartz, CaF2, Csl, solid substrate, XT-KBr, or ZnSe 7800-350 cm" 1 standard 25000-20 cm"' expanded 0.5 cm" 1 standard; 0.125 cm"' optional
28° incident Michelson; corner cube mirror; mechanical bearing; electromagnetic drive; auto-alignment and rapid-scan features
60° geometry; corner-cube optics; Laser Quadrature scan control
Michelson with dual mechanical bearings
Frictionless electromagnetic drive; digital dynamic alignment; autotune for optimization of system throughput; rapid scan
Dynascan Michelson, selfcompensating for dynamic alignment changes due to tilt and shear; frictionless, electromagnetic drive
DLATGS, MCT, or InSb
DTGS, MCT, InGaAs, Ge, InAs, InSb, Poly DTGS, and
DTGS, MCT, or InSb
DTGS, TE-cooled DTGS, Si, PbSe, InSb, MCT, InGaAs, Si bolometer
InSb, DTGS, MCT, photoacoustic and custom options
20 x 30 x 21 cm with removable front, back, top, and base to accommodate oversized accessories
7800-350 cm" 1
Si
0.15 cm" 1 maximum
INA
6.63 x 8.63 X 6.28 in.
8 x 1 0 x 6 in.
21 x 26 x 18 cm with snap-in baseplate; transparent slide door for easy viewing and sample access
Spectra Manager, 32 bit for Windows 95 or Windows NT
MS-DOS 6.22 or higher; Windows 3.11 or Windows 95
GRAMS/32 data- processing software
Microsoft Windows compatiSpectrum 2 software, comble software platform; OMNIC patible with Windows 95 and E.S.P. software Windows NT
Attenuated total reflectance; IR reflection; IR polarization; Micro-20 IR microscope; FTRaman; emission; diffuse reflectance
Microscopes, GC, TGA
Attenuated total, external, specular, grazing angle, or diffuse reflectance; microscopes; fiber-optic probes; gas cells
Dual-channel spectroscopy; step-scan spectroscopy; microscopes; auxiliary experiment module; FT-Raman; GC/IR; TGA/IR
Microscopes; fiber optics; autosamplers; GC; TGA; Raman
Inexpensive fully evacuable system available for removal of H 2 0 and 0 2 without purging
Fully automated spectral range changing capability
Industrial or application-specific packaging and software
Computer automatic system recognition, diagnosis, and optimization
Vibration isolation; system recognition; diagnostics and automatic validation software; multiple password-protected software interfaces; interactive multimedia microscope software
405
406
407
408
409
compounds, but not many people do in fact." Changes at the high end
Flexibility is the word most often used to describe research-grade instruments. "Flexibility is why most people buy research-grade instruments," says Griffiths. " [They want] the capability of running in different spectral regions, changing detectors, changing sources, getting a beam that comes out of the instrument in different ways." In fact, research instruments are so flexible that drawing the line between mid-
dle- and top-end instruments can be difficult. "A lot of instruments are actually designed to be upgraded from a mid-price, mid-performance instrument to a top-end, highly flexible instrument." Step-scan techniques in their current incarnation have been around for about 10 years, but they have grown in popularity in the past few years. Bio-Rad, Bruker, and Nicolet have been successful with their respective step-scan instruments. The companies have implemented more software control of the step-scan features and experiments, making it easier for us-
ers to carry out sophisticated experiments. The major advance has been in adding computer control of ancillary accessories. The three major areas for step-scan techniques are in measurements of modulated samples, imaging, and time-resolved spectroscopy. An example of a modulated sample is a polymer that is subjected to a modulated strain. In rapid-scan FT-IR experiments, the modulations of the interferometer and the sample would result in cross-talk. The mirror is rapidly "dithered" about a single mirror position, pro-
Analytical Chemistry News & Features, April 1, 1998 2 7 5 A
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[email protected] Product
Review
ducing a phase modulation that acts as a carrier signal for the polymer modulation signal. The high-frequency modulation of the mirror can be separated from the lowfrequency modulation of the polymer. Another application of modulation experiments is photoacoustic depth profiling. Bruker has recently introduced a digital signal processor into the electronics that has greatly expanded the available range of modulation amplitudes and frequencies for the polymer stretching and photoacoustic, as well as other phase-modulated experiments. Step-scan capability is also particularly useful in imaging applications. A 128 x 128 array has more than 16,000 pixels. Interferograms could not be measured simultaneously from all the pixels at the data rate required. By stopping the mirroi motion, the signal can be read from each pixel before the mirror is stepped to the next position. Sostek says, "You need step-scan for imaging right now because the dynamic range of the arrays and the speed of transfer out of the array and into the computer is not fast enough to beat the cycle of the interferometer." Currently Bio-Rad is the only company that offers imaging capabilities. Step-scan methods also permit timeresolved spectroscopy for the monitoring of very fast, reproducible, and triggerable reactions. A reaction is allowed to run while the interferometer is held stationary. The mirror is stepped through the positions, and the reaction is repeated as many times as required to achieve the desired spectral resolution. Time-resolved spectroscopy is often used in kinetic studies. According to Janice Hellman of Bruker Optics, time-resolved spectroscopy is a particular strength of the company's instruments which can achieve time resolution as
good as 500 ps The Nicolet instrument has two independent balanced digitizers, which permit the simultaneous digitization of the signal from two detectors. Thii sapability is used in polarization modulation IR reflectionabsorption spectroscopy (PM-IRRAS), which is used to look at monolayers on surfaces, such as lubricants on disk drives. The digitizers can also be used for circular or linear dichroism experiments. Getting the job done
In the area of low-end instruments (those intended for primarily industrial applications) , there is a move toward tailoring
276 A Analytical Chemistry News & Features, April 1, 1998
instruments that are easier to use for specific applications. "People are not really so interested in having the most performance and the most features. They are interested in having a system that does exactly the job they want to do," says David Clark of Perkin-Elmer. That capability requires an array of options but not necessarily flexibility in the final product. "We have to offer a range of simple but customizable systems," says Clark. "Validation and regulatory compliance are also becoming increasingly important for many markets." Bio-Rad is changing its approach to the low-end and middle segments of the market. Although the company has always had instruments for those segments, ease of use was not necessarily a priority. The Excalibur line of instruments combines the flexibility and performance of the research-grade instruments with ease of use. 'This new series is really the first in which we've put together what I consider a complete low-end system with diagnostics, validation, and ease-of-use features," says Sostek. "Performance has always been the keynote "We really put 3. lot of focus on reliability and that [means] redesigning the instrument." Nicolet is making its instruments easier to use by building accessory recognition into the software. When accessories are changed, the system recognizes the component and changes the operating parameters as necessary. Smith says that the systems try to provide "positive feedback when things are going right." Although flexibility is important, it isn't everything. "If you're going to do a lot of a particular application, you are better off getting a low-end instrument and dedicating it to that application," explains Griffiths. "If I have four graduate students working on four different projects, I would much rather they have their own instruments." Celia Henry Upcoming product reviews July 1: Microwave digestion August 1: Charge-transfer device
detectors September 1: Nitrogen, phosphorus, and sulfur detectors If your company manufactures instruments for these techniques, please let us know. E-mail (
[email protected]) or call us (2028724570) at least three months prior to the listed date of publication.
