product review
The Endearing FTIR Spectrophotometer More sophisticated software and imaging capabilities have enhanced these indispensible instruments. James P. Smith and Vicki Hinson-Smith
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hemists really love their FTIR spectrophotometers. In fact, biologists, materials scientists, process engineers, pharmaceutical scientists, and many other professionals love them, too. Why shouldn’t they? FTIR instruments are relatively inexpensive, sturdy, stable, flexible, and fast. And the information they provide is often indispensable. Through the years, these instruments have steadily evolved, and new applications are continually being developed. Although the basic instruments have not significantly changed since Analytical Chemistry last reviewed them (1998, 70, 273 A–276 A), expanded computer power, the trend towards miniaturization, and more sophisticated imaging have inspired some important new innovations. Instrument manufacturers currently offer a wide variety of FTIR systems, and each product line usually includes a researchgrade instrument, a flexible laboratory instrument, and, as target markets become more specific, one or more special-purpose instruments. Table 1 describes selected commercial instruments; this is not meant to be a comprehensive list of available instruments or manufacturers.
The basic FTIR system FTIR measurements are conducted in the time domain. But because no detector is fast enough to record a complete time course at the high frequencies of IR radiation, an interferometer modulates the IR
signal to a manageable frequency. This is accomplished by directing the radiation from a broadband IR source to a beam splitter, which divides the light into two optical paths. Mirrors in the paths reflect the light back to the beam splitter, where the two beams recombine, and this modulated beam passes through the sample and hits the detector. In a typical interferometer, one mirror remains fixed, and the other retreats from the beam splitter at a constant speed. As the mirror moves, the beams go in and out of phase with each other, which generates a repeating interference pattern—a plot of intensity versus optical path difference—called an interferogram. The interferogram can be converted into the frequency domain via a Fourier transform, which yields the familiar singlebeam spectrum. The resolution of this spectrum is determined by the distance that the moving mirror traveled. Analyses generally fall into three categories, which are determined by the wavelengths of the radiation. Midrange IR covers the wavenumbers 4000– 400 cm–1 (which correspond to wavelengths of
2.5–25 µm), where strong absorptions from fundamental molecular vibrations are measured. Near-IR (NIR) ranges from 4000 cm–1 to the visible region, where weaker absorptions due to second harmonics are prevalent. Far-IR can go beyond 50 cm–1 in some instruments. Because of the FT step and because multiple interferograms are typically averaged to achieve a good S/N, performing FTIR requires a lot of computer power. But Richard Jackson of Bruker Optics points out that this resource is plentiful today. “The biggest change in FTIR is probably the gradual migration
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product review
Table 1. Summary of representative FTIR spectrophotometers.1 Product
MB154
Equinox 55
FTS 7000 series
Spectrum One
Company
ABB Bomem, Inc. 585 Charest Blvd. East Ste. 300 Quebec City, G1K9H4 Canada 418-877-2944 www.abb.com/analytical
Bruker Optics, Inc. 19 Fortune Dr. Manning Park Billerica, MA 01821 978-439-9899 www.brukeroptics.com
Digilab LLC 68 Mazzeo Dr. Randolph, MA 02368 800-225-1248 www.digilabglobal.com
PerkinElmer Instruments 710 Bridgeport Ave. Shelton, CT 06484 800-762-4000 www.perkinelmer.com
From $36,000
From $58,000
From $20,000
Dual source: bore-sighted SiC and tungsten halogen
Air- or water-cooled SiC for mid-IR; tungsten lamp for NIR/ vis; mercury arc for far-IR
Water-cooled mid-IR or NIR tungsten are standard; other water-cooled sources optional, including UV
Long-life, mid-IR, prealigned tungsten halogen
Beam splitter
Permanent installed ZnSe beam splitter
A range of nine beam splitters is available
KBr, CsI, CaF, quartz, Mylar, or metal mesh
Wide-range KBr or CaF2; CsI
Spectral range
15,800–500 cm
25,000–30 cm
50,000–10 cm
7800–225 cm–1 (mid-IR); 14,700–2000 cm–1 (NIR)
Resolution
1–64 cm–1
Better than 0.2 cm–1
0.1 cm–1
0.5 cm–1
Interferometer
Cube-corner, wishbone swing arm
Proprietary Rocksolid wishbone design
Mirrors arranged at 60°; 50-mm aperture; air bearing; rapid scan
Proprietary rotary Dynascan; selfcompensating for tilt and shear
Detectors
Extended mid/near DTGS with thallium bromoiodide window
25 different detectors available
Full range of mid-, near-, and far- Temperature-stabilized DTGS or IR InGaAs; LiTaO3; MCT
Options
Broad selection of detectors; proprietary center-focus Arid Zone sample compartment
Raman; Raman microscope; IR Up to five external beams; tropimicroscope; micro and macro hy- cal enclosure available perspectral imaging; step-scan; thermogravimetric analysis/IR; GC/IR; and full range of NIR accessories, including fiber optics and integrating sphere
Price Optics Source
–1
–1
–1
Sampling accessories include universal ATR, horizontal ATR, diffuse reflectance, liquid sipper, NIR reflectance, fiber probes, tablet autosampler, FTIR microscopes, and FTIR and FT-NIR imaging systems
1
Some companies offer multiple instruments. Contact the vendors for their full product lines. ATR: Attenuated total reflectance DTGS: Deuterated triglycine sulfate
of vibrational spectroscopy [including IR] into the biological sciences,” he says. “The spectra of biological systems are very complex, but advances in computers and chemometrics software mean that these spectra can now be analyzed.”
Expanded capabilities As computers have become more powerful, so has software. Typical spectrometer software these days is designed to simplify quality assurance/quality control (QA/ QC) measurements and ensure compliance with the U.S. Food and Drug Administration’s 21 CFR Part 11 standard for electronic record keeping. Packages also may be designed with advanced diagnostics, “good laboratory practice” validation, and certification protocols. Some 38 A
MCT: Mercury cadmium telluride
software packages contain the built-in ability to diagnose problems, suggest possible causes, and take corrective action. For example, real-time atmospheric vapor correction is meant to remove a source of uncontrolled variation that has been the bane of IR spectroscopists for years. Built-in intelligence, especially for sample handling, is another innovation from the past five years, says David Clark of PerkinElmer Instruments. The type of intelligence depends on the application: “In routine quality control measurements, the customer does not need higher raw performance, but is now looking for a system that simplifies analysis and reduces uncontrolled variation,” Clark explains. On the other hand, “In problem solving and research applications, most users are looking
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to solve problems faster, [with the goal of] decreasing ‘time to market’ for new products or reducing product defects.” Ken Kempfert of Thermo Nicolet adds that the continued growth of the FTIR field has been fueled by intuitive, easy-to-use systems, which open the door of FTIR analysis to a wider population of users. “Development of an FTIR system which works in concert with the sampling accessory and software is vital,” says Kempfert. In practice, this means that the system identifies the sampling accessory being used, tests the sample, and reports the results. The entire analysis can be completed within 1–2 min by a modern FTIR spectrometer that uses purge sealed accessories, smart electronics, and software, he notes.
product review
Dedicated systems During the past five years, a major trend toward dedicated systems has developed, according to Scott Little of Pike Technologies, a supplier of FTIR spectrometer accessories. “There is a push toward near-IR dedicated systems by most of the manufacturers—[that is,] systems packaged in a more industrial, more rugged way,” he says. “FT-NIR is now routinely used for incoming materials quality control testing.” For example, instruments with built-in, dedicated sampling systems might sit at the railroad tracks, ready to sample and analyze industrial materials as they arrive. These systems operate out to 10,000–15,000 cm– 1 (1000–670 nm). Dedicated systems for process analytical technology (PAT), which has been adopted by several chemical and pharmaceutical process industries, are notable. PAT involves the use of raw material properties, manufacturing parameters, process monitoring, and chemometric techniques to produce finished products of acceptable quality. The central goal of PAT is to generate information about product quality in real time. Traditionally, process monitoring involved temperature, pressure, flows, pH, and other physical parameters, but PAT focuses on the use of in-line testing using FT-NIR, Raman, or other physiochemical techniques. This trend represents a revolutionary change in control philosophy, and FTIR is an ideal technique for its application. Continual monitoring, however, requires very stable instrumentation. Some companies offer permanent (one-time) factory alignment of the interferometer and validation procedures designed to ensure precisely reproducible spectra. But stability of the instrument isn’t the only consideration, notes Henry Buijs of ABB Bomem; the stability of the sampling accessories and the reliability of the calibration also need to be considered. “The emphasis has been on FTNIR, which is more demanding on calibration,” he explains.
FTIR imaging microscope Another important new development is IR spectrochemical imaging, which combines the capabilities of FTIR spectrometers, FTIR microscopes, and IR array detectors, says Mark Druy of Digilab. “This
Table 1. Representative FTIR spectrophotometers (continued).1 Product
IR Prestige
Nexus
Company
Shimadzu North America 7102 Riverwood Dr. Columbia, MD 21046 800-477-1227 www.shimadzu.com
Thermo Nicolet Corp. 5225 Verona Rd. Madison, WI 53711 608-276-6100 www.thermo.com
Price Optics Source
$25,000–$60,000 Air-cooled ceramic (standard); tungsten halogen for NIR (optional)
Ever-Glo or quartz halogen
Beam splitter
Germanium on KBr plate (standard); germanium on CsI (far-IR); silicon on CaF2 (NIR)
KBr; quartz; CaF2; CsI; extended KBr; ZnSe
Spectral range
350–7800 cm–1 (standard); 240–12,500 cm–1 (optional)
–1 350–7800 cm (standard); 20–27,000 cm–1 (expanded)
Resolution
0.5–16 cm–1 (selectable)
0.5 cm–1 (standard); 0.09 cm–1 (optional)
Interferometer
Michelson interferometer with mirrors at 30°; advanced dynamic alignment; sealed and desiccated; automatic drier
Electromagnetic drive; digital dynamic alignment; rapid scan; step scan (optional)
Detectors
DLATGS with temperature control (standard); MCT with liquid nitrogen (optional); InGaAs (optional for NIR)
DLATGS; thermoelectrically cooled DLATGS; silicon; PbSe; InGaAs; InSb; MCT IR microscopes; IR imaging; auxiliary experiment module; FT-Raman; GC/IR; TGA/IR; surface analysis module
Options
1
Some companies offer multiple instruments. Contact the vendors for their full product lines. DLATGS: L -alanine-doped deuterated triglycine sulfate MCT: Mercury cadmium telluride
technique . . . enables the individual to perform chemical analyses of medical, biological, industrial, and other samples with a previously unobtainable level of spatial resolution, speed, and ease of analysis,” he explains. Experts predict that these systems will evolve to provide larger and larger image sizes with a greater degree of spatial and IR resolution. The drawback to current IR microscopes is that they can be relatively slow. But recent IR microscope innovations are improving ease of use and speed of analysis, making IR imaging “an ever more practical analysis tool,” notes Kempfert. For example, using dichroic optics within the IR microscope provides simultaneous data collection and viewing of the sample—greatly speeding up sample analysis. The driving force, he adds, is a researcher’s desire to analyze sample composition over an expanded area and obtain detail from any specific point.
Kempfert believes that within the next 5–10 years, the cost of the pixel-based detectors required for such analyses will decline further and lead to faster area and point analyses by FTIR. An even bolder prediction is that future imaging systems combined with smaller, dedicated analyzers will bring IR spectrometers closer to providing answers to scientific questions, not just IR spectra. This will occur through a combination of miniaturization and the development of expert system software that analyzes the IR spectrum without the intervention of a spectroscopist, says Druy. Whether or not that vision is ever realized, chemists have had a fruitful 20-year relationship with FTIR spectroscopy, and that relationship will, no doubt, continue to grow. James P. Smith and Vicki Hinson-Smith are freelance writers based in Amherst, Mass.
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