FORTY YEARS OF FT-IR SPECTROMETRY
Strong-Men, Connes-Men, and Block-Buster or Peter R. Griffiths Department of Chemistry University of Idaho Moscow, ID 83843
The Third James L. Waters Annual Symposium Recognizing Pioneers in the Development of Analytical Instrumentation was held at the 1992 Pittsburgh Conference and Exposition in New Orleans. This year's symposium honored five pioneers in the field of infrared spectrometry: Paul LWilks, Jr., Foil A. Miller, Norman Sheppard, Peter R. Griffiths, and Bryce L Crawford. In our September issues, we present adaptations of the talks given by four of these innovators. In the Sept. 1 issue, Wilks described the evolution of commercial IR spectrometers and Miller discussed the infrastructure of IR spectrometry. Here Griffiths describes the remarkably circular development of commercial FT-IR instruments and Sheppard discusses the United Kingdom's contributions to IR spectroscopic instrumentation.
It is generally recognized t h a t the seminal discoveries in the development of FT-IR spectrometry have been the invention of the two-beam interferometer by Michelson and reco g n i t i o n of t h e m u l t i p l e x a n d throughput advantages by Fellgett and Jacquinot, respectively. In this article, however, I plan to skip over these momentous events. Instead, I will concentrate on the design of three fundamentally different types of interferometers by John Strong of J o h n s Hopkins U n i v e r s i t y (Baltimore, MD), P i e r r e Connes of t h e Centre National de la Recherche Scientifique (Orsay, France), and Larry Mertz at Block Engineering (Cambridge, MA). Each of these scientists played a vital role in developing FT-IR spectrometry into the sophisticated technique that we know today. I hope to show that the comm e r c i a l d e v e l o p m e n t of F T - I R spectrometry has occurred in a remarkably circular manner with many of the earliest innovations lying dormant for decades before being rediscovered and incorporated into the "latest" instruments.
Strong-Men Let us first consider the contributions of J o h n Strong, who has already been mentioned in Foil Miller's article (2). During his long tenure at J o h n s Hopkins University, Strong developed several types of interferometers, most of which were used in the far-IR region. Because the wavenumber range of the far-IR region is short, Fellgett's advantage is not as large as it is in the mid- or near-IR regions of the spectrum. Also, the maximum Jacquinot-allowed optical throughput is difficult to attain in practice, simply because it is too great for interferometers operating at medium resolution in the far-IR
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region. Nevertheless, the spectral ene r g y d e n s i t y e m i t t e d by f a r - I R sources is so low that the combination of Fellgett's and Jacquinot's advantages was crucial to opening up this spectral region. The far-IR interferometers developed in Strong's laboratory were the first to demonstrate the power of multiplex techniques for IR spectrometry. Two scientists who were i n t r o duced to Fourier spectrometry in Strong's lab proved to be particularly important in later development of t h e t e c h n i q u e . George V a n a s s e ' s group at the Air Force Cambridge Lab (subsequently Air Force Geophysics Research Lab) became a key force in the development of interferometers that could be carried in aircraft and spacecraft. Because this work was not intimately linked with t h e d e v e l o p m e n t of c o m m e r c i a l FT-IR spectrometry, I will—with regret—not discuss it further. The second m a n , a Scot by t h e name of Alistair Gebbie, played a key role in the early development of commercial far-IR Fourier transform spectrometers. Strong can be considered the original pioneering force; Gebbie—especially after accepting a position a t the National Physical L a b o r a t o r y in E n g l a n d — d e m o n strated a plethora of applications for these instruments and was the driving force behind their commercialization. It is interesting t h a t one of the first readily accessible articles on FT-IR spectrometry was co-authored in 1956 by Gebbie and Vanasse (2). Two types of interferometers were developed in Strong's lab: a standard Michelson-type i n s t r u m e n t and a lamellar grating interferometer. Each was subsequently incorporated into commercial far-IR Fourier t r a n s f o r m s p e c t r o m e t e r s in t h e 1960s. Initially, however, these ins t r u m e n t s o p e r a t e d in a "slowscanning" mode in which the moving mirror was typically translated at a speed of a few micrometers per second so that Fourier frequencies were well below 1 Hz. The IR beam had to 0003-2700/92/0364 -868A/$03.00/0 © 1992 American Chemical Society
How Mertz Raised the Hertz be modulated by a chopper, as was typically done with standard disper sive spectrometers. After demodula tion of the chopper frequency, the re sulting signal or interference record was composed of a dc component (which contained no useful informa tion) and an ac component or interferogram. The dc signal was sub tracted from the interference record as the first step in computing the spectrum. Gebbie convinced b o t h G r u b b Parsons, a well-recognized manufac turer of IR spectrometers, and Re search and Industrial I n s t r u m e n t s Company (RIIC), a small company t h a t had previously made only acces sories for IR spectrometers, to fabri cate instruments based on one of his designs: t h e G r u b b - P a r s o n s I S - 3 and the RIIC FS-520. In 1964 the prototype instrument was installed in the laboratory of Harold "Tommy" Thompson at Oxford University, and I was one of the two graduate stu dents who had the dubious privilege of trying to measure spectra with it. In the prototype interferometer, the moving mirror was mounted on the end of a piston t h a t was dragged by a cable being wound onto a slowly ro tating drum. The lubricant between the piston and the cylinder was a thick grease. Specks of dust would cause the piston to stick until enough pressure was applied by the cable for the mirror to j u m p to its correct posi tion. In the production model, the RIIC FS-520, the cable drive was re placed by a lead screw that worked far more efficiently. Gebbie's group h a d also built a smaller, simpler interferometer that was the basis of the interferometer in the G r u b b - P a r s o n s Mark II intro duced in 1966. This instrument had a stepping-motor drive t h a t could gen erate an optical path difference of > 5 cm (see Figure 1). Both the RIIC FS-520 and the G r u b b - P a r s o n s Mark II were modular instruments with source, sample, and detector units t h a t could be bolted onto the i n t e r f e r o m e t e r c o m p a r t m e n t . Be
cause both interferometers were de signed for long-wavelength opera tion, mechanical tolerances did not have to be very severe and the instru ments could be readily fabricated. These instruments were developed before the s t a r t of the minicomputer era, so they produced an interferogram rather t h a n a spectrum. Inter ferograms were recorded on punched paper tape at a rate of about one data point per second. Computation of the spectrum was a very lengthy opera tion, because these instruments were developed before Cooley and Tukey had reported the fast Fourier t r a n s form (FFT) algorithm (3). At Oxford, Thompson's "FT-IR Group" of two graduate students, who rarely mea sured interferograms of more t h a n 256 data points, was the second larg est computer user in the university. Two other i n s t r u m e n t s , the F S 720 a n d the FS-820, followed the RIIC FS-520. The FS-720 incorpo rated a smaller, more reliable Michelson interferometer than that of the FS-520 but otherwise had similar specifications. The FS-820 was de-
REPORT signed for very far-IR measurements (frain = 8 cm - 1 ) using a lamellar grat ing interferometer. RIIC instruments were sold in the United States under the company trade name of Limit In struments. Other far-IR instruments were built in the late 1960s by Coderg in France and Polytec in Ger many. Although none of these instru ments were sold after about 1975, many of the more influential spectroscopists in the world used them and realized the potential of FT-IR for chemical spectrometry. Connes-Men Pierre and Janine Connes were the second major influence in the devel opment of Fourier spectrometry in the early 1960s. Pierre Connes de signed and fabricated several inter
ferometers capable of giving optical p a t h differences of > 1 m for very high resolution spectrometry in the near-IR region. If a standard Michelson interferometer were used for the type of measurements achieved by Connes, a simple calculation would show a maximum allowed mirror tilt of < 2 μτείά over the entire length of travel if the resolution were not to be d e g r a d e d . To a c h i e v e t h i s goal, Connes replaced the plane mirrors of the standard Michelson interferome ter with cat's-eye retroreflectors that compensated optically for the effect of tilt (4, 5). A simple representation of a c a t ' s - e y e i n t e r f e r o m e t e r d e signed by Connes is shown in Figure 2. (The work of Janine Connes, who contributed to the calculation of the spectrum from these long interfero grams, will be discussed later in this article.) In combination, these two remark able scientists really stood the world of conventional vibrational spectrom etry on its ear. I remember Pierre Connes visiting Thompson's labora tory and rolling out a five-yard-long, high-resolution near-IR spectrum of methyl iodide. One of my co-workers had j u s t finished an overnight scan where he measured one band in the mid-IR spectrum of the same mole cule at lOx lower resolution and at much lower signal-to-noise ratio (S/N), u s i n g a P e r k i n E l m e r 125 grating spectrophotometer. Connes then told us t h a t the spectrum had been measured with a source having the intensity of the solar radiation reflected from Jupiter. Even though Thompson had been skeptical about the merits of Fourier spectrometry a few months previously, I think this experience opened his eyes to the true capability of this technique. Although no contemporary FT-IR spectrometer is equipped with cat'seye retroreflectors, many of them in clude some other type of optical tilt compensation. The first company to provide such compensation was prob ably Analect I n s t r u m e n t s . Analect developed an unusual type of inter-
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REPORT ferometer in which the substrate and the compensator plate of the beamsplitter are wedged, and the optical path difference is introduced by scanning the compensator plate. Two stationary cube corners are used as the retroreflectors in this interferometer (6). This device, which is becoming known as the Doyle interferometer, is remarkably resistant to vibrations. Mattson Instruments has installed cube corner retroreflectors in all of its instruments and has eliminated the air bearing from its drive system. Perkin Elmer uses three different types of tilt compensation optics on its Models 1600, 1700, and 2000 interferometers, whereas Bomem uses yet another type in its Michelson series of instruments. Another compensation approach for a conventional Michelson interferometer is to dynamically align either the fixed mirror or the beamsplitter to compensate for the effect of tilt in the moving mirror drive. This solution was first described by Henry Buijs of Bomem, Inc. (7). Beckman Instruments' short-lived foray into mid-IR Fourier spectrometry involved the use of a dynamically aligned interferometer. Unfortunately, this instrument proved that the response of the feedback signal to the piezoelectric transducer had to be much higher than the frequencies at which the radiation is modulated; otherwise, the approach does not work correctly. Recently, Nicolet Instruments and
the Digilab Division of Bio-Rad Laboratories introduced excellent dynamically aligned interferometers in their top-of-the-line FT-IR spectrometers. Although none of the instruments mentioned above were directly derived from the original Connes model, it is fair to say that the success of the cat's-eye interferometer for high-resolution spectrometry led several instrument companies to consider the use of optical or optoelectronic tilt compensation in the drives of their interferometers. The mirror speed on Connes' early interferometers, like that of the far-IR instruments derived from the work in Strong's lab, was very slow. The early slow-scanning interferometers all used a chopper to modulate the amplitude of the IR beam. The use of a chopper for amplitude modulation resulted in problems because of the high dynamic range of the interference record and the effect of source drift. To circumvent these problems, Connes developed an alternative— and far superior—technique for modulating the detector signal. He introduced a sinusoidal dither on the position of the "fixed" retroreflector, thereby modulating the phase of the interferogram sinusoidally and yielding the first derivative of the interference record on demodulation (8, 9). Phase modulation has proved to be an excellent way to improve the performance of slow- and step-scanning interferometers, and it has been in-
Figure 1. Far-IR spectrometers in Thompson's lab at Oxford in 1964. In the foreground is a Grubb-Parsons Mark II "cube" with 1 -m heatable gas cell bolted between the interferometer and the detector modules. In the background on the right is the RIIC FS-520, and on the left is a large vacuum chamber that housed a far-IR grating monochromator.
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corporated into all subsequent commercial FT-IR spectrometers that allow step-scanning as an option. Block-Busters
Despite the seminal developments resulting from work initiated in the laboratories of John Strong and Pierre Connes, I believe that the most important contributions to today's state of the art of commercial FT-IR spectrometry came from a remarkable group of physicists and engineers at Block Associates (later Block Engineering) in Cambridge, MA, a small company founded by Myron Block. It may be invidious to single out one man from the staff, but I believe that many of the company's significant developments were the brainchild of an optoelectronic engineer named Larry Mertz. Although many benefits were derived from slow- and step-scanning interferometers, there were some fundamental problems for mid- and near-IR spectrometry. The maximum allowed throughput of interferometers operating at low resolution in the mid-IR region is very high because of Jacquinot's advantage, so that the S/N of the interferogram in the region of the centerburst could easily exceed the linear range of the detector and/or the electronics and the dynamic range of the analog-todigital converter (ADC). As noted above, interferograms measured by amplitude modulation were susceptible to the effect of source drift. Choppers modulated the beam at only a few tens of Hertz; thus, the effect of 1//noise could be very high. For instruments used in remote sensing (including measuring the spectra of stars and planets), atmospheric scintillation could limit the S/N in an analogous manner. Finally, the time required for measuring interferograms with a slower a step-scanning interferometer is usually several minutes and sometimes several hours. Mertz recognized the enormous gains that could be derived by scanning the moving mirror of a two-beam interferometer fast enough to modulate each wavelength in the spectrum of interest at its own characteristic audio frequency. By scanning the moving mirror rapidly, only the ac component of the interference record is measured. Thus the interferogram is measured directly; the S/N is kept to a level at which the dynamic range of relatively inexpensive ADCs is not exceeded; and the effects of source drift, \lf noise, and atmospheric scintillation are minimized.
The first rapid-scanning interferometers made at Block Engineering were only about 10 cm along any side. A photograph of one of them, the Model 196, is shown in Figure 3. The moving mirror was driven with a voice-coil transducer, similar to the drives used on most modern interferometers. Despite the absence of a laser reference, successive interfere g r a m s were c o h e r e n t over s h o r t retardations and could be effectively signal-averaged. In the early Block Engineering instruments, interfere grams were collected on a Fabri-Tek signal averager. (Ironically, FabriTek later became part of Nicolet Instruments.) At the end of the measurement, the interferogram was played repeatedly through a digitalto-analog converter into an audio frequency wave analyzer so that the spectrum could be obtained without the need for a digital Fourier t r a n s form. The small size of this instrument, together with its remarkably high sensitivity, meant t h a t it could be used for military and civil remote sensing in the mid- and near-IR regions as well as for the spectrometry of s e v e r a l a s t r o n o m i c a l s o u r c e s . Manfred Low at Rutgers University and at New York University published almost 100 papers showing the many chemical applications for which this instrument could be used. Despite Low's yeoman efforts, however, most chemical spectroscopists believed that the resolution of this instrument (16 c m 1 in the mid-IR region and 32 crer 1 in the near-IR region) was too low for their applications. Mertz had a marvelous feeling for multiplex methods in spectroscopy, as anyone who has read his 1965 monograph, Transformations in Optics, will testify. Around this time, he derived the multiplicative method of phase correction, in which the phase spectrum is computed at low resolution from a short double-sided region of the interferogram around the centerburst (10). This method of phase correction is still used in most of today's commercial FT-IR spectrometers. Mertz's final seminal contribution to FT-IR spectrometry was the idea of measuring the optical retardation by using a small H e - N e laser and by using this reference interferogram both to generate the trigger to the ADC and to stabilize the velocity of the moving mirror with a feedback circuit. More than 25 years after he came up with this concept, all commercial FT-IR spectrometers still in-
Figure 2. Schematic of a cat's-eye interferometer. Only one beam path is shown; however, note that in practice the input beam fills the beamsplitter.
corporate a H e - N e laser for these purposes. Amazingly, the application for a patent for laser fringe referencing was denied on the grounds that it was "obvious to anyone versed in the state of the art." I have been told t h a t Mertz was so disgusted with this decision that he opted to leave the "instrument business" and move to the Smithsonian Astrophysical Institute. Block Engineering had a remarkably innovative staff, and their innovation extended beyond FT-IR spectrometry. For example, a paper they submitted to the Journal of the Optical Society of America in 1959 was r e jected, apparently because the authors were from a commercial organization. Instead, the authors decided to publish it in JOSA as a paid advertisement. Because the advertisement was well received by the readers, the authors decided to follow it up with new advertisements every month proposing some wayout (and often patentable) idea. Among the first of these advertisements (March and December 1960) were the first records of the small, rapid-scanning interferometers that were to prove so vital for the later commercial development of FT-IR spectrometry. B r a i n s t o r m i n g sessions were held each month to come up with the next wacky idea for the JOSA advertisement. Although the journal's editorial staff was less than happy with this approach, they could do nothing to prevent it from happening until the pressure of coming up with 12 truly novel ideas a year finally led to its demise. Today, these advertisements are difficult to find; many libraries bind only the technical parts of scientific journals and not the advertisements, but a search
Figure 3. Block Engineering Model 196 rapid-scanning interferometer. (Reprinted from a 1968 Block Engineering sales brochure.)
of JOSA issues published in the early 1960s will prove rewarding to anyone willing to make the effort. Commercial development Chemical spectroscopists did not take readily to the Block Engineering "cube" because of its low resolution. The development of three key components that took place in parallel during the late 1960s changed this situation. The first was the H e - N e laser, as noted above; the second was the minicomputer, which will be covered in greater detail in the next section; and the third was the triglycine sulfate (TGS) pyroelectric bolometer. For interferometry to be commercially viable for mid-IR spectrometry, a r o o m - t e m p e r a t u r e detector with a response time of < 1 ms was needed. The thermocouple and the thermistor bolometer detectors typically installed in grating spectrophotometers in the 1950s and 1960s did not meet this criterion. It was partic-
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REPORT ularly serendipitous that the response time of TGS detectors was < 1 ms and that their sensitivity kept the S/N of an interferogram of an unattenuated Globar source just within the dynamic range of the 14bit ADC used in the early FT-IR spectrometers. In addition, the response of these detectors was linear across the centerburst. The laser-referenced, computercontrolled FT-IR spectrometer, introduced in 1969, signaled the start of the modern era of Fourier transform spectrometry. The driving force behind its development was a remarkable character by the name of Tom Dunn. In the mid-1960s, Dunn ran a small company in Silver Spring, MD, serving as a sales representative for several high-tech electrooptics businesses—including Block Engineering. Despite intense and often very innovative marketing, Dunn Associates was never able to convince the chemical community that the Block Engineering "cube" was the answer to its prayers. Dunn recognized that Block's recent development of a rapid-scanning interferometer with a 2-cm retardation and a 2-in. open aperture (the Model 296) would change this state of affairs. Block Engineering and Dunn Associates merged to form the short-lived Dunn Analytical Instruments Division of Block. With $1 million in venture capital, the company developed in one year the FTS-14 and introduced it at the 1969 Ohio State Molecular Spectroscopy Symposium. The most popular computer of this era was the Digital Equipment PDP-8. Unlike most computerized instruments of the day, however, FT-IR spectrometers required a word length of at least 16 bits, making the PDP-8 inappropriate. Block located a small manufacturer of a computer that looked suitable for its purpose and thus became the first original equipment manufacturer (OEM) customer of Data General Corporation. Making an FT-IR data system in the late 1960s proved to be a daunting task (11). Minicomputers contained ferrite core memories that were bulky and expensive relative to contemporary technology. Disk drives were small enough to be rackmounted but typically occupied about 5 cu. ft. The standard data system of the FTS-14 incorporated a Data General Nova with 8 kB of memory and a 512-kB fixed head disk of dubious reliability. Booting up the computer required toggling in a series of octal instructions. Alphanumeric in-
Figure 4. Nicolet Model 7199 FT-IR spectrometer with a GC/FT-IR interface on the right. As shown here, the 1977 instrument had a footprint exceeding 100 sq. ft. (Reproduced courtesy of the Nicolet Instrument Co.)
put and output were done through a Teletype ASR-33, a loud mechanical monster that was actually one of the more reliable components of the entire data system. All spectra had to be plotted on a digital plotter rather than on a CRT display. Nevertheless, to anyone raised on an RIIC FS-520, obtaining spectra in this way rather than carrying a roll of punched paper tape to the computer center was a real luxury. The FTS-14 was developed under exceptionally high pressure so that the prototype instrument could be built before the venture capital expired. As a result, a few changes had to be made after the prototype was introduced. One of these has always struck me as particularly amusing. The source of the FTS-14 was powered by a standard ac power supply that worked fine in Cambridge, MA, but caused glitches at Ohio State University because of 60-Hz harmonics that appeared at 200-cm -1 intervals across the entire spectrum. It should have been a simple matter to replace the ac power supply with a dc supply. However, the Ohio State meeting started the day after Labor Day, and neither the Physics Department stock room nor any electronics stores in Columbus were open the two days before the meeting. Suddenly a first-year student from MIT, Brough Turner, who had been brought along as a "go-fer" (although he subsequently became the director of the software group), came up with the brilliant idea that a 3.5-V dc power supply could be made by using a battery charger in combination with a toaster whose heating elements had been appropriately rewired. Both a battery charger and a toaster were available from department stores having Labor Day sales, and a potentially disastrous situation was averted. It was a good thing
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that conferees could not see under the curtained table so conveniently provided by the meeting organizers! In 1970 the Dunn Analytical Division of Block changed its name to Digilab and, under its leadership, the FT-IR market developed rapidly. Although there were a few other players in the FT-IR field in the early 1970s, such as Idealab and EOCOM, none had a significant impact on the chemical spectroscopy market and Digilab had a virtual monopoly for almost five years. Instrument performance improved dramatically during this time, but there were some difficult problems. The field of digital electronics was evolving very rapidly, and faster, more reliable components were being introduced each year. Maintaining instruments in an evolving state of the art while keeping them serviced presented an unenviable problem for Digilab. By the middle of the decade, enough people had become convinced of the merits of FT-IR that serious competition arrived in the form of Nicolet Instruments. Nicolet already had a data system with its computer (the NIC 1180), which was well suited for FT-IR. The company purchased the line of interferometers from EOCOM (and hired a promising young engineer named David Mattson) and incorporated it into the Nicolet Model 7199. By producing a reliable instrument backed up by a competent group of applications scientists, Nicolet soon matched Digilab's sales, and by the end of the decade it had assumed the dominant position in the field. Other companies also got into the act in the late 1970s and early 1980s, including Analect Instruments, which had a scanning wedge interferometer, and Bomem, which was the first to incorporate dynamic alignment into a high-resolution in-
Gas chromatograph
Power supply board
Laser detector Detector
Laser
Detector preamp
Flow cell assembly
IR source Interferometer
Velocity amplifier
Figure 5. Interior of a present-day Hewlett-Packard IRD showing the interferometer at the lower right corner. The entire GC/FT-IR system has a footprint of about 10 sq. ft. (Reproduced courtesy of Hewlett Packard, Scientific Instrument Division.)
terferometer. Bomem also fabricated a high-speed vector processor to reduce the time required for FFT of very long interferograms. BrukerPhysik built an evacuated FT-IR spectrometer, based on the Genzel interferometer, that allowed the operator to switch from one spectral region to another without breaking the vacuum. Led by Nicolet (with the MX-1) and Analect (with the fX-6250), FT-IR companies all expanded their product lines to include smaller, less expensive spectrometers. IBM Instruments made a brief foray into the FT-IR field by repackaging the smaller Bruker spectrometers and interfacing them to IBM scientific computers. In 1982 several Nicolet employees, led by David Mattson, started a second FT-IR company in Madison, WI, and Mattson Instruments has maintained a significant
presence ever since. The impact of these companies on the overall market in IR spectroscopy made the "dispersive giants" of the time, Beckman and Perkin Elmer, finally sit up and take notice. Beckman Instruments made an ill-starred entry into FT-IR spectrometry with a truly innovative instrument, but it had too many flaws to satisfy most spectroscopists. Like IBM Instruments, Beckman left the FT-IR business in less than five years. The story with Perkin Elmer was quite different. The company had considered an entry into FT-IR as early as 1953 and, throughout the 1970s, made exceptionally highperformance instruments that were mounted in spacecraft. Perkin Elmer took an excruciatingly long time to get into the commercial FT-IR business, however. Following a sales
agreement with Analect, the company repackaged the fX-6250 as the Perkin Elmer 1500. Within two years, it introduced the Model 1800—an FT-IR spectrometer designed and made in Norwalk, CT. The performance of this instrument was superb, but its price was too high and its optical design far too inflexible for it to gain a major part of the FT-IR market. Shortly thereafter, Perkin Elmer introduced the 1700 series, an instrument conceived and built in England. Although tremendously successful in Europe, the 1700 did not make a significant impact on the North American market. In 1986 Perkin Elmer introduced the compact and inexpensive 1600 series FT-IR spectrometers. Although these instruments did not perform as well as their two predecessors, their low cost and excellent reliability led to high market acceptance. Perkin Elmer finally arrived on the FT-IR scene. The Perkin Elmer story has been reflected in many other FT-IR companies, all of which now market spectrometers that sell for under $20,000. These instruments have for all intents and purposes replaced grating spectrometers in the North American marketplace, and there is an intense and ongoing competition among all the major players in the field. The potential of FT-IR is reflected in the number of takeovers and mergers of FT-IR companies: Analect is now owned by KVB, Bomem by Hartmann and Braun, Digilab by Bio-Rad Laboratories, Mattson by Orion and, most recently, Nicolet by ThermoElectron. The Fourier transform and other software
The theory of FT-IR spectrometry was well understood 30 years ago and was elegantly described in 1961 by Janine Connes, the wife of Pierre Connes. In many respects, Janine Connes' contributions to FT-IR spectrometry have been as important as her spouse's. Her Ph.D. dissertation, translated into English in the early 1960s (12), introduced the computational aspects of FT-IR to many novices, including myself. At that time, the thought of performing Fourier transforms of interferograms consisting of more than a few thousand data points was too daunting for most spectroscopists to consider. The longest interferogram ever transformed before the mid1960s contained only 12,000 points, yet the computation took more than 12 h. When transforming long inter-
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REPORT ferograms by use of t h e classical Fourier transform, even the Connes tended to output only a very small fraction of the spectrum. Probably the single most important argument against widespread use of Fourier spectrometers before 1965 was the length of time needed to perform a Fourier transform. The paper by Cooley and Tukey (3) in 1965 describing the FFT and the subsequent paper by F o r m a n (13) showing its application to interfere metric spectrometry changed t h i s situation completely. In the space of a couple of years, the FFTs of interferograms containing tens or h u n dreds of t h o u s a n d s of d a t a points were being carried out routinely in m a n y labs. On the other hand, it could still take a long time to per form FFTs of 1 million data points on a large mainframe computer. In 1970 J a n i n e Connes reported the time taken for FFTs of interfere grams of various lengths, using large mainframe computers (14). On only two of these computers had a 1024K transform been performed, and the time for a single-precision computa tion was either nine or 22 min, de pending on the amount of memory available. Today, these figures can be matched with commercially available software written for PCs at a cost of less t h a n 1% of the cost of the com p u t e r s used by J a n i n e Connes in 1970; furthermore, they do not need to be housed in their own large, tem perature-conditioned rooms. For example, w i t h t h e GRAMS software of Galactic I n d u s t r i e s , a real 1024K transform performed in double precision takes only 55 s u s ing a 33-MHz 486 PC equipped with a m a t h coprocessor. For t h e FFT lengths t h a t most of us need, the computation is over within a second of the instant we press the RETURN key! Even faster computations are on the horizon. Exceptionally small dig ital signal-processing chips are available on which a IK FFT can be performed in 3 ms. When these chips are either expanded or ganged to gether, the time for even a 1024K FFT will be reduced to < 1 s. This improved computational per formance has led to a great change in the way most potential customers shop for an instrument. Most chem ists in the market for an FT-IR spec trometer are now far less concerned about optical performance (e.g., S/N and baseline stability) t h a n about about speed, ease of use, and soft ware versatility. Indeed, it could be argued t h a t the evolution of truly high-performance software (e.g., au
tomated spectral subtraction, spec tral searching, multicomponent anal ysis, derivative spectrometry, deconvolution, and maximum likeli hood methods), which operates on in creasingly fast and inexpensive PCs, has probably been the main reason for the huge rise in popularity of FT-IR spectrometry during the 1980s.
The future D u r i n g t h e p a s t 30 or 40 y e a r s , FT-IR spectrometers have developed from instruments found only in labo ratories where measurements could not be made in any other way to rou tine, inexpensive instruments used by experts and neophytes alike. At this point, commercial FT-IR spec trometers appear to be evolving in two directions, both of which reflect past developments. First, very small low-resolution interferometers are being used for an increasing number of dedicated ap plications, such as process or reac tion monitoring. An instrument pro viding 2 c m - 1 resolution is more than adequate for this task. These inter ferometers can be interfaced to an in expensive and versatile data system (often simply a PC) to provide a labo ratory IR spectrometer whose level of performance would h a v e a m a z e d anyone working in the field 10 years ago. It is quite instructive to compare the size of a GC/FT-IR system that was state of the art only 15 years ago with a contemporary system. Figure 4 shows a Nicolet 7199 spectrometer (vintage 1977) equipped with a GC/ FT-IR interface. For the purpose of comparison, a contemporary Hewlett-Packard IRD (infrared de tector) is shown in Figure 5. The size of the interferometer in the IRD dif fers little from t h a t of the Block En gineering Model 196 shown in Figure 3, although the performance is dra matically different. Obviously the as sets of small i n t e r f e r o m e t e r s are once again starting to be recognized. At the other end of the (price) spec t r u m , top-of-the-line i n s t r u m e n t s provide either very high resolution (Av > 0.002 cm" 1 ) or medium resolu tion (Av > 0.2 cm - 1 ) combined with considerable flexibility. The interfer ometer can perform rapid-, step-, and slow continuous scanning, and the optics can permit several differ ent experiments (e.g., GC/FT-IR, mi croscopy, and FT-Raman spectrome try)—often necessitating different spectral ranges—to be installed on the same spectrometer. After a period of time when the in struments from every manufacturer
874 A · ANALYTICAL CHEMISTRY, VOL. 64, NO. 18, SEPTEMBER 15, 1992
seemed to have about the same spec ifications and performance, spectres copists have a real choice of instru ments. For process monitoring and other applications in which high res olution is not required, small, inex p e n s i v e , h i g h - p e r f o r m a n c e spec t r o m e t e r s are available. Benchtop instruments suitable for the microscale revolution in teaching organic chemistry can be purchased for well under $20,000. Scientists requiring high-resolution spectra can choose among i n s t r u m e n t s equipped with cube corner retroreflectors or dy namic alignment. Other instruments incorporate rapid-, step-, and slow continuous scanning options. Authors of the other papers in this series have described the difficulties faced by the pioneers of dispersive IR spectrometry. It is probably fair to say t h a t these pioneers could not have dreamed that IR spectrometry would be capable of the m e a s u r e ments being carried out routinely in laboratories across the world. It is al most scary to think where this field will be by the turn of the century.
Acknowledgments and apologies I would like to pay tribute to all the scientists and engineers over the past 40 years who have contributed to the development of Fourier spec trometry to its present state of the art. By singling out three or four peo ple in the way t h a t I have done here, and because of space limitations, I have failed to acknowledge the many people who made critical contribu tions. When I read through the text and found that I had not even men tioned Tomas Hirschfeld, Jack Koenig, or Bill Fateley, I realized the impossibility of giving credit where credit is due. Space limitations also prevented me from discussing the vi tal contributions of the accessory manufacturers, especially Spectra Tech and Harrick Scientific, to the development of the popularity of FTIR. To all my colleagues whose con tributions have gone unrecognized in this paper, I apologize.
References (1) Miller, F. A. Anal. Chem. 1992, 64, 824 A-831 A. (2) Gebbie, Η. Α.; Vanasse, G. Nature 1956, 178, 432. (3) Cooley, J. W.; Tukey, J. W. Math. Cornput. 1965, 297, 978. (4) Connes, J.; Connes, P.J. Opt. Soc. Am. 1966, 56, 896. (5) Connes, J.; Connes, P.; Maillard, J. P. /. Phys. Radium 1967, 28, C2:136. (6) Doyle, W. M.; Mcintosh, B. C. Proc. SPIE (Proceedings of the 1981 International Conference on FT-IR Spectroscopy) 1981, 289, 71.
(7) Buijs, H. Presented a t the 1977 Inter national Conference on FT-IR Spectros copy, Columbia, SC, J u n e 1977; paper TH.B.8. (8) Connes, J.; Connes, P.; Maillard, J. P. I. Phys. Radium 1967, 28, C2:120. (9) Connes, J.; Delouis, H.; Connes, P.; Guelachvili, G.; Maillard, J . P.; Michel, G. Nouv. Rev. Opt. Appl. 1970, 1, 3. (10) Mertz, L. Infrared Phys. 1967, 7, 17. (11) Curbelo, R.; Foskett, C. In Proceed ings of the Aspen International Conference on Fourier Spectroscopy, 1970; V a n a s s e , G. Α.; Stair, A. T.; Baker, D. J., Eds.; 1971, AFCRL-71-0019, p. 221. (12) Connes, J.; Recherches sur la Spectros copic par Transformation de Fourier, trans lated as Document AD 409 869, Defense D o c u m e n t C e n t e r , A l e x a n d r i a , VA (1963); also in Rev. Opt. 1961, 40, pp. 45, 116, 171, 231. (13) Forman, M. L.J. Opt. Soc. Am. 1966, 56, 978. (14) Connes, J. In Proceedings of the Aspen International Conference on Fourier Spec troscopy, 1970; Vanasse, G. Α.; Stair, A. T.; Baker, D. J., Eds.; 1971, AFCRL71-0019, p. 83. Suggested reading The following books contain much useful information on t h e history of FT-IR spectrometry. Mertz, L. Transformations in Optics; John Wiley: New York, 1965. Griffiths, P. R. Chemical Infrared Fourier Transform Spectroscopy; W i l e y - I n t e r science: New York, 1975. Griffiths, P. R.; De Haseth, J . A. Fourier Transform Infrared Spectrometry; WileyInterscience: New York, 1986. Johnston, S. F. Fourier Transform Infrared: A Constantly Evolving Technology; Ellis Harwood Ltd.: Chichester, E n g l a n d , 1991.
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Grandma Called It Roughage Fiber Facts and Fallacies
T Peter R. Griffiths received his bachelor's and doctoral degrees from Oxford Univer sity, where he used the two early far-IR in terferometers shown in Figure 1 of this ar ticle. As part of his postdoctoral research at the University of Maryland, he worked with the Block Engineering Model 196 in terferometer (see Figure 3). He then joined the development team for the Digilab FTS-14 as a product specialist. Since 1972 he has been a member of the chemis try faculties of Ohio University, the Uni versity of California-Riverside, and most recently the University of Idaho, where he is chairman of the department of chemis try. His primary research interests lie in the field of molecular spectroscopy, and he has published more than 140 research pa pers, 23 book chapters, and three books on various aspects of FT-IR spectrometry.
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ANALYTICAL CHEMISTRY. VOL. 64, NO. 18, SEPTEMBER 15, 1992 · 875 A
