Product
Review
FT-IR SPECTROSCOPY If s Allume With Mirrors
Fourier transform IR spectroscopy has had a long history of trial, error, abandonment, and rediscovery. The key piece of instrumentation, the interferometer, was invented in the late 1800s by Albert Michelson, at a time when the spectral properties of light began to be discussed in terms of waves rather than particles. The Michelson interferometer was a simple apparatus composed of a beamsplitting prism and two mirrors, set at right angles to each other. The mirrors, one fixed and one moving, reflected the two portions of the beam back through the beamsplitter, where they were recombined. As long as the mirrors were set at equal distances from the beamsplitter, the intensity remained at a maximum for each wavelength. But as the movable mirror traveled along its axis, the difference in pathlength created repeating interference patterns between the recombined beams. Lord Rayleigh realized that by using a Fourier transform method to break down the interference pattern into sine waves
analysis. Dispersive (wavelength-scanning) IR spectrometers dominated research and the commercial instrument market. However, with the advent of microprocessors in the 1960s and the introduction of the "fast" Fourier transform, which cut the number of calculations considerably for a given data set, FT-IR spectroscopy became feasible and the first commercial instruments were introduced. By the mid-1980s, FT-IR spectroscopy was well established, and with its advanthat represented the contributions from in- tages of speed and resolution had more than caught up with conventional disperdividual wavelengths of light, the intersive methods. ferogram could be converted back into wavelength absorbances. The method Today, FT-IR spectrometers have filtered out much of the noise that plagued moved out of the laboratory and are being dispersive spectroscopy in the far-IR used extensively on production lines for range and made it easier to obtain spectra a wide range of samples, including soft from the weak-intensity sources that drinks, extracted polymers, and films were available. and coatings. Chemical imaging and miThrough the 1950s, calculation tools re- croscopy, toxic gas monitoring, and remained too crude to handle the number of mote spectral sampling are a few of the common uses for dedicated routine instruinterferogram data points needed to obments. At the same time, the boost in tain a high-quality spectrum for chemical
Advances in spectrometer design are making FT-IR more versatile and affordable
Analytical Chemistry, June 1, 1995 381 A
Product
Re νi e w
Table 1 . Summary of representative products
Product Company
Genesis Series ATI Mattson 1001 Fourier Dr. Madison, Wl 53717 800-423-6641
Spectrum 2000 FT-IR Perkin Elmer 761 Main Ave. Norwalk, CT 06859 203-762-1000
Price Dimensions (h χ w χ d; cm) Type Optics Source
$18,500-$28,500 43 χ 46-cm footprint Rapid-scanning Michelson
$29,900 and up 50 χ 50 χ 50 Rapid-scanning Michelson
Filters
IR ceramic emitter conductively cooled for mid-IR; high-energy tungsten-halogen source for near-IR Selectable low-pass electronic filters
Beamsplitter
Ge/KBr, extended-range Ge/KBr or CaF2
Spectral range (cm"1)
Mid-IR system: 6000-350; near-IR system: 10,000-2000 ± 1.0 maximum; software selectable to 64
Mid-/far-IR; high-energy tungsten-halogen near-IR; dual internal source available Cut-on, cut-off, and bandpass filters; validation filters; up to 7 filters in software-controlled filter wheel Choice of 10 interchangeable barcoded beamsplitters for near-IR, mid-IR, far-IR, and special applications 15000-30
Resolution (crrf1) Interferometer Features
Scan modes
Rapid
Dynascan Michelson interferometer with self-compen sation for dynamic alignment changes due to tilt and shear Rapid
Modulation frequency range
6.25-25 kHz, selectable
15Hz-100kHz
1.5 NA High-linearity 16-bit ADC with floating point 32-bit FFT calculation Choice of LiTa03; DTGS; narrow-, mid-, and wide band MCT; InSb
0.05-5.0 NA DSP; parallel processing at up to 100 MIPS
1 collimated external beam
4 output beams; up to 8 detectors; emission port and parallel beams available Compatible with full range of sampling accessories
Mirror scan velocity (cm/s) Mirror stepping rate (s/step) Demodulation and signal processing Detectors
Sample chamber No. samples/data channels Cell types Interfaces Data system
Special features
High-throughput 60° geometry with corner-cube reflectors; autotune standard on all models
Variable to < ± 0.3 standard; ± 0.15 optional
Transmission, ATR, DRIFTS, specular reflectance, gas IR microscopy, fiber-optic, GC Built-in notebook PC or desktop PC with Windows 3.1 and WinFirst software for data acquisition, analy sis, and reporting; simplified routine user mode; ApPro method development and formatting software; PLS Quant Portable configuration; remote modem spectrometer operation; diamond-turned optics; complete system validation package for GLP, GALP, GAMP, and ISO9000 registered laboratories
Variety of cooled and room-temperature detectors; optimizing preamplifier and circuitry; sealed and desic cated far-IR detector compartments; PAS available
GC, TGS, FT-IR microscopy, near-IR FT-Raman spec troscopy, fiber-optic PC with Windows-based Spectrum software with cus tomizable interface for each user; optional Expert Search, QuantC, Quant+ Expert, QC-IR, time-resolved IR, and Spectrum OBEY macro language software; validation protocols Validated software, automated S/N and ASTM level D validation routines
Options and accessories
Fiber-optic sampling accessories for QC and reac tion monitoring
Optional software modules, sampling accessories, and interfaces
Scope and applications
QC in mid-IR and near-IR; routine analysis
Reader Service No.
401
Educational, QA, routine, and research applications; built to order from customer specifications for sam pling, applications, and performance 402
NA = Not applicable ΙΝΑ = Information not available at press time
382 A
Analytical Chemistry, June 1, 1995
Diamond-20 KVB/Analect 9420 Jeronimo Rd. Irvine, CA 92719 714-587-2322
Equinox Series Bruker Instruments 15 Fortune Dr. Manning Park Billerica, MA 01821
Magna-IR Nicolet Instruments 5225-5 Verona Rd. Madison, Wl 53711 608-276-6100
508-667-9580 $28,000 20 χ 46 χ 43 Rapid-scanning non-Michelson
$35,000 and up 20 χ 70 χ 55 Step-scanning Michelson
$25,000-$65,000 34 χ 67 χ 63
Internal air-cooled high-efficiency Reflex sphere Computer-selectable gain; high-pass and lowpass filters; delay compensation KBr, CaF2
Globar
Ever-Glo for mid-IR; optional quartz-halogen source for dual-source operation Neutral density; far-IR, near-IR, and visible filters KBr, Csl, Solid-Substrate silicon, CaF2, quartz
Standard: 4400-400; optional extended mid-IR: 7400-450; near-IR: 12,000-1200 Keyboard-selectable; ±1.6-2
15,000-20
Transept IV refractively scanned interferometer with precision cross-roller bearings and cubecorner mirrors; hermetically sealed Rapid NA
0.1-2.5 NA 18-bit ADC Internal DTGS standard; optional detectors include DTGS, LN2-cooled MCT, TE/DTGS, TE/MCT, TE/NAS 2 detector channels; 1 focused internal beam and 1 collimated external beam Compatible with full range of sampling accessories NA 66-MHz 486-based data system with DOS 6.22 and Windows 3.1 ; optional advanced chemometrics software
Low-pass analog filter KBr
Step-scanning Michelson
25,000-50 ± 0.2 apodized
± 0.35 standard; + 0.09 optional
Patented mechanical bearing design; digital signal processing control mechanism
Vectra-Plus Michelson interferometer; DSP mirror control; dynamic alignment; autotuning
Rapid, low-velocity rapid, phase-modulation step, amplitude-modulation step 1.6-160 kHz for rapid scan; 5-1000 Hz dithering frequency for step scan; 0.1-100 λ amplitude modulation for step scan at all frequencies 0-40 0.02 DSP demodulation; no lock-in amplifier required; in-scan co-adding DLATGS, MCT, InSb, photovoltaic MCT, PAS
Rapid, phase-modulation step, amplitudemodulation step 250 Hz-120 kHz for rapid scan; 25-1000 Hz phase modulation range for step scan
Up to 6 beam channels and 7 data channels
Center-focused sample compartment beam, 2 collimated external beams, emission port Gas, liquid, solids; ATR, DRIFTS, PAS, specular reflectance IR microscopy, GC, TGA, FT-Raman spectroscopy, fiber-optic PC with Windows-based Omnic software for data management and analysis; Quick-IR; optional validation, kinetics, Quant-IR, InterpIR, Macros Basic, Macros Pro, and spectral database software DSP system control and processing; dia mond-turned, pinned-in-place optics; userserviceable design; step scanning without lock-in amplifiers
Compatible with full range of sampling
accessories TGA, IR microscopy, FT-Raman spectroscopy, GC PC-based data system with OS/2 Warp oper ating system
Small footprint with full-sized sample compart ment; vibration-tolerant Transept wedges; optics to direct IR beam to external sampling accessories; permanently aligned and hermetically sealed optics DRIFTS, ATR; external automated and heated sampling systems; process development configuration QC, QA, process development; process monitoring
50 ms rapid scan; 50 ns step scan; slow scan option down to 100 Hz
403
404
ΙΝΑ
Research or routine analysis
0.0158-8.22 0.001-10 DSP; simultaneous in-phase and quadrature spectra without the use of lock-in amplifiers Prealigned and pinned-in-place;TGS, MCT-A, MCT-B, Csl/TGS, PE/TGS, PbSe, PbS, InSb, Si
Optional sample cells; IR microscopes, midIR and near-IR fiber-optic probes, long-path gas cells; auxiliary experiment module Research methods for rapid and step scanning 405
Analytical Chemistry, June 1, 1995
383 A
Product
Review
computing power provided by advances in personal computer and software design has made possible an ever more complex repertoire of FT-IR methods. We asked Charles Wilkins of the University of California, Riverside, for his comments on trends in FT-IR spectroscopy and his advice for instrument buyers. Table 1, although not comprehensive, presents a representative selection of commercially available FT-IR spectrometers. Optics and detection
What sets FT-IR apart from dispersive or scanning IR spectroscopy is its ability to detect essentially all the wavelengths within a range simultaneously, and to do it without reducing the amount of light available for analysis and detection. Instead of using a monochromator and entrance and exit slits for wavelength dispersion, FT-IR instruments are designed so that the broadband source (usually a globar) sends light with mixed wavelengths straight to the interferometer. The light power available is therefore much greater for FT-IR than for dispersive IR spectroscopy at comparable resolution. The moving mirror, which scans at up to several centimeters per second, converts the IR frequencies of the light spectrum to audio frequencies for detection. The simultaneous acquisition of data at all wavelengths often called the "multiplex advantage," makes spectral acquisition much faster for FT-IR than for dispersive spectroscopy, so the instrument can be used either to cut analysis times or to acquire more spectra for a given time per sample, thereby enhancing sensitivity. Several varieties of interferometer are used in commercial instruments. The most common, says Wilkins, is the relatively simple Michelson design. Others include the Fabry-Pérot (or "étalon") model, which sandwiches two imperfectly reflective mirrors with a variable gap between them to achieve the path difference effect needed for interferometry. One commercial instrument uses an interferometer with no moving mirrors, originally designed for field-use military instruments. The introduction of affordable lasers has made calibration easier to integrate into the Michelson interferometer design, Wilkins says. "Many instruments use a He-Ne laser for calibration and reference, because the laser's output frequency is known [i.e., the light is monochromatic] and can be used to calculate the moving mirror's position." The interferometer 384 A
design is like having two interferometers back to back: It consists of a double-sided moving mirror shared by twofixedmirrors on either side of it, one for the broadband source and the other for the reference laser. Typical detectors include triglycine sulfate (TGS) and deuterated triglycine sulfate (DTGS) pyroelectric detectors and mercury cadmium telluride (MCT), a semiconductor alloy. Because IR is essentially heat, says Wilkins, "For highsensitivity applications, detectors must be cooled with liquid nitrogen or liquid helium. Usually cooled MCT detectors are used for GC/IR or LC/IR." Routine instruments operating in the mid-IR region tend to have less expensive detectors that operate at room temperature; these are generally TGS or DTGS. What makes FT modulation and demodulation feasible is the increase in computer power used for commercial instruments. Wilkins notes that the biggest
GC/FT-IR, where losses in spectrometer performance can create an apparent loss of chromatographic resolution. "Not only do you need to use cooled detectors to get maximum sensitivity for the small samples that reach the spectrometer," explains Wilkins, "you need high enough mirror speed to collect several spectra across each chromatographic peak. One way to increase spectral acquisition speed is to have the mirror not move very far for each scan, but high resolution depends on long mirror distances." Typical resolution for qualitative analysis and routine work is ~ 1-2 cm-1, he says, whereas very high resolution is in the range of 0.5 cm-1 or less. That level of resolution takes longer to achieve because the mirror scans a greater distance at a constant frequency. Mirror speeds are limited by the performance of the controller. Up to four or five years ago, analog-to-digital converters (ADCs) used to digitize the mirror position had rates of 110 kHz, which corresponds to a top mirror speed of 3.5 cm/s, but the 16-bit, 200-kHz ADCs introduced on the market at that time boosted the maximum to ~ 6 cm/s. Several companies have also started to use digital signal processing controllers for both mirror control and signal demodulation. All routine and most research-grade instruments feature rapid-scanning interferometer mirrors that move at a constant, usually selectable, speed within a given range. In the past few years, however, another mirror mode has been introduced change in the past few years has been the into some research-grade systems. Step scanning is a better controlled and more move from integral computers, which were specially designed for the spectrome- sophisticated version of the slow-scanning motion of interferometers from the early ter and usually incompatible with other 1960s. In step scanning, the moving mircomputer systems or analytical software, to the use of PCs and generic worksta- ror "dithers" or oscillates its position sinusoidally at a preset modulation fretions for instrument control, data acquisition, and interferogram demodulation and quency about each of a series of fixed distances from the source. This feature, analysis. The amount of memory available to PCs and workstations these days is which allows simultaneous modulation of all wavelengths at the same frequency, is approaching the gigabyte range, and disks or boards with 1 GB of memory are used for 2D FT-IR spectrometry of procompact enough to fit on the palm of the cesses such as polymer stretching and for depth profiling in photoacoustic spechand. Spectral libraries and analytical software for FT-IR methods have kept pace trometry (PAS) for characterization of layered solids. The only significant drawwith the growth in computer sophisticaback to step scanning relative to rapid tion. scanning, says Wilkins, is its higher sensitivity to external vibrations in the lab. Scanning Scanning rates and distance ranges for the Buy for the application moving mirror are central to the trade-off between speed and resolution for FT-IR in- Wilkins' general advice to potential instrustruments. A common example is the efment buyers, whether for routine or refect of these parameters on resolution in search-level experiments, is to try to
The future ofFT-IR instrumentation may be a higher degree ofspecialization for routine use.
Analytical Chemistry, June 1, 1995
match the instrument capabilities to the expected applications. Instrument speed and resolution are the first parameters to consider. They generally depend on the control of interferometer mirror speed and the scan range. Sensitivity depends largely on the choice of detector, whether cooled or room temperature. However, instrument noise may depend on the region of the spectrum being acquired. 1T-IR presents the· greatest noise-reduction advantages in the mid-IR and far-IR regions of the spectrum. The advantage isn't as great in the near-IR region, Wilkins says, because in addition to detector noise there is significant source noise. Source noise can actually be made worse by FT modulation because the interferometric conversion can spread the error from a narrow band all over the derived spectrum. The UV-vis region is strongly limited by source noise. Another consideration is the type of sample that will be run in the instrument and what kinds of sample cells, interfaces, or accessories will be needed. GC/FT-IR requires a gas flow-through tube; liquids may require various types of specialized sample cells for techniques such as attenuated total reflectance, which is often used for analyzing aqueous solutions. Solids may be accommodated either by the traditional method of grinding them with KBr and pressing the mixture into pellets or by methods such as diffuse reflectance, PAS, or IR microscopy, all of which may be used to analyze an intact sample in situ. These three methods are growing in popularity, especially for products such as semiconductors and coatings. IR microscopy has also been used to increase sensitivity for GC detection. The eluent is trapped on ZnSe, an IR-transparent and water-inert support material, and the IR beam scans down and through the sample for chemical imaging. Wilkins says the future of FT-IR instrumentation may well be a higher degree of specialization to accommodate the increase in routine use, particularly on industrial process lines. Dedicated instruments for a particular routine application are cheaper to make and require fewer adjustments than general-purpose spectrometers that may be required to perform a large variety of hyphenated or advanced techniques. For routine methods, he says, "Eventually the trade-off will be cost versus performance. Specialized is the way to go—there's no point in paying for extra features that you aren't going to use." Deborah Noble
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