REPORT
TheU.K.'sContributions to IR Spectroscopic Instrumentation From Wartime Fuel Research to a Major Technique for Chemical Analysis Norman Sheppard School of Chemical Sciences University of East Anglia Norwich NR4 7TJ, U.K.
The first commercially available IR spectrometer was designed in 1913 by F. Twyman of Adam Hilger Ltd. of London. The optical layout was of the constant-deviation Wadsworth type and used a Nernst filament as the source, a thermopile as the detector, a 60° natural rock salt prism, and spherical mirrors made of nickel and steel. It cost £60. I made my first IR measurements in 1943 using a similar Bellingham and Stanley instrument. As I recall, it had a rock salt prism in the Littrow optical configuration. The re cording system was constructed in the laboratory and used a vacuum thermocouple with a charcoal getter. The wavelength cylinder was driven by an electric motor, the galvanome ter deflection was amplified by a photocell, and the spectrum was re corded on a rotating drum of photo graphic paper. (The spectrometer had been moved out of the laboratory and into the observatory to get away from the shaking caused by the Cam bridge buses outside.) This a u t o mated recording was a boon—I had seen others in my group recording spectra point by point (cell in; cell out; slide rule ratio; rotate the prism and repeat ad infinitum!). Before World War II, a number of 0003-2700/92/0364-877A/$03.00/0 © 1992 American Chemical Society
U.K. academic s p e c t r o s c o p i s t s worked in the IR region: F.I.G. Raw lins and Ε. Κ. Rideal, who together measured the spectra of sulfur in its various forms; C. P. Snow (who later achieved fame as a novelist), who measured the gas-phase spectrum of NO a n d s h o w e d t h a t it h a d a Q-branch because of the diatomic molecule's electronic a n g u l a r mo mentum; and C. R. Bailey, A.B.D. Cassie, and co-workers at University College in London, who were able to measure spectra within several hun dred yards of an underground train station. R. Robertson and J. J. Fox of the Government Laboratory used t h e Hilger spectrometer, with prisms of quartz, CaF 2 , NaCl, and KBr, to ob t a i n good v a p o r - p h a s e spectra of NH 3 , PH 3 , and SbH 3 . Subsequently
Fox and A. E. Martin made good use of t h e 3-μπι CH a n d OH b o n d stretching region and, with the aid of a laboratory-built diffraction grating spectrometer, distinguished between CH-containing groups in saturated and unsaturated hydrocarbons. After inheriting Snow's IR instru m e n t a t i o n in t h e early 1930s, G.B.B.M. Sutherland of Cambridge University attended the University of Michigan on a Commonwealth Fel lowship. At that time, the University of Michigan was at the forefront of developing IR techniques. S u t h e r land returned to Cambridge in 1935 with thermocouples and echelette gratings that had been manufactured at Michigan. He set up a lively re search group that used Hilger and laboratory-built grating spectrome ters. This group disbanded at the be ginning of World War II. In the late 1930s H. W. Thompson at Oxford University also formed a research group that made contribu tions to the IR spectra of polyatomic molecules using a Hilger spectrome ter.
Developments during the war Sutherland worked on determining the location of unexploded bombs in London during the early years of the war. In 1941 the Ministry of Aircraft Production asked Sutherland at Cambridge and Thompson at Oxford to explore the possibility of using IR spectroscopy to identify the hydro carbons present in fuels from enemy
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REPORT sources and thereby determine whether they were using natural or synthetic petroleums. After a prom ising literature survey by Delia Simp son, IR measurements were done on the relevant pure hydrocarbons. The fuels were fractionated into the more and less volatile components a n d were sent to Oxford and Cambridge, respectively, for analysis. The IR an alytical method proved to be very successful, and techniques were soon developed to d e t e r m i n e chemical structures and reactions of natural and synthetic rubber as well as other polymers. Sutherland's first wartime group, which consisted of H. A. Willis, A. R. Philpotts, and P. B. Fellgett, concen trated on hydrocarbon analysis. His second wartime group developed IR techniques for the determination of polymer s t r u c t u r e s . A. J. H a r d i n g and I (as a wartime research stu dent) worked on n a t u r a l and syn thetic r u b b e r s , and o t h e r s in t h e group—including D. A. R a m s a y — worked on polystyrene, cellulose, car bohydrates, coal, polynuclear aromatics, polypeptides, and proteins. Later, the Sutherland group con centrated on new experimental tech niques. High-sensitivity photoconductive detectors made of PbS, PbSe, and PbTe led to spectral resolution, and Fellgett developed detector the ory. A fast IR spectrometer w i t h cathode ray presentation, based on the IR trading rules, was designed and built by Frank Daly. I derived much enjoyment and benefit from working with the approximately equal mix of chemists and physicists in Sutherland's group. In his 1950 Ph.D. thesis Fellgett at Cambridge University described the possibility of using IR interferometry, combined with Fourier t r a n s form (FT) analysis, to obtain a major multiplex (or Fellgett) a d v a n t a g e over dispersion spectrometry. This advantage is equal to Ν in terms of the speed of the measurement or to N1/2 in terms of signal-to-noise ratio (S/N), where Wis the number of res olution elements in a complete spec trum. The concept was not fully im plemented until two decades later, w h e n c o m p u t e r technology m a d e rapid FT calculations practical. The multiplex advantage is also at the heart of pulse FT methods in NMR spectroscopy t h a t e a r n e d Richard Ernst the 1991 Nobel Prize in Chem istry, and of high-performance devel opments in mass, microwave, UV— vis, and Raman spectroscopies. Thompson set up a similar war time research group t h a t included
D. H. Whiffen, R. E. Richards, and P. Torkington, who worked on hydro carbon analysis and the elucidation of polymer s t r u c t u r e s ; pioneered studies of fluorinated hydrocarbons; and contributed to experimental IR techniques, including work on heated samples. After the war, I. M. Mills, R. H. Williams, and other members of his group made extensive use of photoconductive detectors to obtain high - resolution v i b r a t i o n - r o t a t i o n spectra of gases. In J a n u a r y 1945 a Faraday Society general discussion (subsequently published in Volume 41 of Transac tions of the Faraday Society) was held on "The Application of Infrared Spec t r a to Chemical Problems." Many nonclassified s t r u c t u r a l analytical results of the Cambridge and Oxford groups were presented and, as one participant commented, the occasion had something of the air of an Ox ford-Cambridge sports match! Shortly after the war, W. C. Price set up a very successful experimentoriented research group at King's College in London. This group in cluded M. A. Ford, who described a successful double-beam p r i s m g r a t i n g s p e c t r o m e t e r in his 1956 Ph.D. thesis and later made major contributions to spectrometer devel opment at Perkin-Elmer Ltd.; and George R. Wilkinson and W. F. Sher m a n , who together advanced lowt e m p e r a t u r e and high-pressure IR techniques. There is no doubt that the influen tial contributions of the Sutherland, Thompson, and Price groups were largely responsible for setting the course of the instrumental aspects of a n a l y t i c a l IR spectroscopy in t h e U.K. after the war. They provided the climate within which, from the mid-1940s to the early 1950s, Adam Hilger, Grubb-Parsons, and Unicam began manufacturing IR spectrome ters. In the late 1950s these compa nies were joined by P e r k i n - E l m e r Ltd., t h e U.K.-based subsidiary of the U.S.-based Perkin-Elmer Corpo ration. A d a m Hilger Ltd.
In about 1940, Hilger produced a large D209 single-beam spectrome ter with a rock salt prism. This in strument was the basis of a ratio-re cording spectrometer produced toward the end of the war, which was developed by W. Zehden and A. C. Menzies. The detectors in the D209 double-beam spectrometer were a pair of evacuated H i l g e r - S c h w a r z thermocouples—at that time the most sensitive available—mounted
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one above the other. The two optical beams entered and exited the appro priate slits, one above the other, and their images were focused onto the corresponding elements of the ther mocouple. T h e o u t p u t from each thermocouple passed to a separate g a l v a n o m e t e r a n d w a s amplified photoelectrically, starting with a zero signal when the rectangular image from the galvanometer mirror was equally incident on two b a c k - t o back-coupled photocells. The output from the blank beam was used as the reference voltage in a potentiometric recorder; the output from the sample beam was measured against the blank beam as the spec t r u m was scanned by t u r n i n g t h e prism table. The output varied with spectral position, but the ratio was continuously recorded even as the rapidly fluctuating absorptions of the atmospheric water vapor vibrationrotation bands were scanned. A feed back system was incorporated in the electronic circuits to the galvanome ter to stiffen and speed t h e i r r e sponses. The prism table was turned by a cam-driven a r m so t h a t t h e spectra were presented linearly by wavenumber using CaF 2 , NaCl, and KBr prisms. In Cambridge, Fellgett coupled the electronic and optical systems of the double-beam instrument. To those who had long struggled to measure percentage absorptions from super imposed blank and sample singleb e a m s p e c t r a , it s e e m e d a l m o s t miraculous as the recorder automati cally drew the percent transmission spectra, particularly in the regions of complex atmospheric absorptions. It was difficult, however, to keep the D209 double-beam system in bal ance. Often a complete spectrum had to be r u n in several sections, with a d j u s t m e n t s in between. In some cases, purchasers found it simpler to operate in the single-beam mode. Hence, even though this was the first commercially produced double-beam IR spectrometer (and a ratio-record ing one at that), it was not successful enough to mass produce and there fore was available only as a special order. In 1950 a single-beam H668 spec trometer that was easier to operate appeared on the market and was suc ceeded in 1953 by the H800, a nulltype double-beam spectrometer. By then, despite strong competition from Perkin Elmer and Beckman in the U.S. and from G r u b b - P a r s o n s and Unicam in the U.K., the H800 proved to be the instrument of choice for numerous customers. It was spa-
REPORT told me that designing an accurate double-beam spectrometer with an empty sample well was not difficult; the real difficulty involved balancing the beams and retaining optical pre cision when the samples were in serted! However, a price had to be paid for the advantage of having an evacuable spectrometer: The sample well was rather narrow and only al lowed access from the top. This con figuration caused some difficulties when nonstandard accessories were used. There were also some problems as sociated with the manufacture and repair of the rather delicate Golay cell detector. Nevertheless, the per formance of this double-beam spec trometer made it competitive with the famous Perkin Elmer Model 21 produced in the U.S. at a slightly earlier date. The SP100 was also suc cessfully developed into a higher res olution prism grating spectrometer in the early 1960s, although not be fore Grubb-Parsons first pioneered such commercial spectrometers. In 1964 Unicam introduced the SP200—a moderately priced nonevacuable, double-beam spectrome ter with ordinate scale expansion. This model was available first as a prism instrument and then as a fil t e r / g r a t i n g spectrometer (the SP200G). Automated models with improved features followed over the y e a r s . A h i g h - p e r f o r m a n c e spec trometer with two diffraction grat ings was introduced as the SP1100 in 1973. The SP2000 and the SP4000 had three gratings, each used in first order, and high flexibility in spec trum presentation. All Unicam spec t r o m e t e r s from the SP200 to t h e SP4000 had optical null double-beam systems and used Golay-type pneu matic detectors. In 1979 Pye-Unicam was the first manufacturer to produce a moderately priced ratio-recording double-beam filter/grating spectrometer, the SP3, using triglycine sulfate pyroelectric detectors. When measuring intensity, high accuracy is easier to achieve with ratio recording than with the nulltype spectrometer using an optical comb. Ratio-recording transmission (or absorbance subtraction) methods ensure reliable measurement of slight differences (caused by trace impuri ties) among spectra of related sam ples. Ratio-recording spectrometers, with digital and automatic operation, optional PC control, and a variety of visual display unit spectral presenta tions, have been produced by Philips Analytical as the PU9500 and PU9700 series.
Figure 1. Evolution of commercial IR spectrometers. DB, double beam; N, null; F/G, filter/grating; dashed lines indicate periods of limited commercial exploitation and usage.
d In 1988 Philips Analytical entered ;1 t h e F T - I R field w i t h t h e Model PU9800 series, which used a Nicolet ;t interferometer. The U.Redesigned, 1, low-cost PU 9600 Model 1990 (since :e renamed the Mattson 1020)—which h features a rotating frame interferom ιeter and internal digital signal pro )cessing—followed. The development it of an actual spectrum, in S/N terms, 3, could now be monitored in real time. 3. Perkin-Elmer Ltd. The sequence of events t h a t led the ie Perkin-Elmer Corporation to set up ρ an IR design and production facility y a t Beaconsfield probably s t a r t e d with a visit by Van Zandt Williams is from the Norwalk, CT, office of Per k i n E l m e r in a b o u t 1 9 5 0 . He described the newly designed Model û
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21 d o u b l e - b e a m s p e c t r o m e t e r to Sutherland's group and to representatives of the organic and inorganic chemistry laboratories in Cambridge, where the usefulness of IR spectroscopy for molecular structure deter mination was highly appreciated. The audience was very impressed by the presentation, and a Model 21 was ordered for the new chemical laboratory then under construction. We al ways understood in Cambridge t h a t we received No. 1 of the Model 21 production line, and indeed Perkin Elmer made many subsequent modifications and improvements to our particular spectrometer. As other U.K. laboratories ordered more Model 21s, Ralph Gilson, laboratory superintendent for the new chemistry building, agreed to provide
REPORT ther IR spectrometer development has been assigned to Perkin-Elmer Ltd. in Beaconsfield. A general perspective Developmental stages of commercial IR s p e c t r o m e t e r s from t h e m i d 19408 through the present are summarized in Figure 1. U.K. contributions are discussed in relation to these in the box at right. Overall, U.K. manufacturers have made many contributions to the international production of IR spectrometers, and Perkin-Elmer Ltd. as well as the U n i c a m - o r i g i n a t e d companies continue to do so today. It is my understanding that Perkin-Elmer Ltd. alone h a s manufactured and sold worldwide more than 13,500 IR spectrometers since 1958, when the company produced its first spectrometer, the PE Model 137. This figure does not include the substantial sales of the larger 580 series ratio-recording spectrometers and their successors. Competition with other firms—particularly those outside Europe and the U.S. (e.g., Shimadzu)—remains strong in the low- and moderate-cost dispersion field. However, the cheaper FT instruments will soon take over even the moderate-cost field. Other contributions Reflecting microscopes are widely used in conjunction with IR spectrometers for obtaining spectra from very small samples or from small p a r t s of h e t e r o g e n e o u s s a m p l e s . Some of the first applications were developed in the early 1950s in the U.K., using a reflecting microscope designed in 1947 by C. R. Burch. Another widely used accessory is the v a r i a b l e - p a t h l e n g t h sealed liquid cell, designed by R. R. Gordon and H. Powell of British Petroleum. U.K. firms have long been suppliers of a wide range of IR accessories. The Research and Industrial Instruments Company was very successful, as was Specac Ltd. These companies have competed well with overseas m a n u f a c t u r e r s such as H a r r i c k , SpectraTech, and Wilks Scientific. Two organizations have contributed much to the dissemination of IR techniques and to effective interactions among academic and industrial IR spectroscopists in the U.K.: the Spectroscopic Panel of the Institute of P e t r o l e u m ( 1 9 4 5 - 7 5 ) a n d t h e IRDG, which is still active and typically meets three times a year. IRDG originally stood for the InfraRed Discussion Group but subsequently became the Infrared and Raman Discussion Group.
U.K. contributions in more general perspective Single-beam recording prism spectrometers 1945 The Hilger D209 was often used as a single-beam instrument and competed with the earlier Perkin Elmer Model 12 and Beckman IR-2. Double-beam null-type prism spectrometers ca. 1950 The first reliable double-beam spectrometer was probably made by Baird Associates and was of an optical null design. The highly successful Perkin Elmer Model 21 followed. However, U.K. manufacturers produced good quality competitors in the double-beam variants of the Grubb-Parsons S3 and S4, the Unicam SP100, and the Hilger H800 models. Double-beam prism/grating spectrometers 1956 The Grubb-Parsons GS-2, with Merton replica gratings, led the field of double-beam grating IR spectrometers for five or six years. The company was also successful with the Spectromaster. Later, the Perkin Elmer X21 (from the U.S.) and X25 (from Germany) series, the Beckman IR-7, and models from other companies gave Grubb-Parsons strong competition. Low-cost prism spectrometers 1957 The U.S.-produced Perkin Elmer Model 137 was the pioneer ultra lowcost instrument of this type and formed the basis of a long series of lowto medium-cost spectrometers from Perkin-Elmer Ltd. Perkin Elmer in the U.S. and in Germany concentrated on double-beam prism/grating research spectrometers. Double-beam filter/grating spectrometers 1961 These instruments were pioneered by Perkin Elmer with the joint U.K./U.S. Model 237 and the U.K.-produced Model 337. Filter/grating spectrometers became the standard. FT far-IR spectrometers ca. 1961 Because of Fellgett and Gebbie's work on the multiplex concept, GrubbParsons and Beckman (U.K.) had a strong lead in FT spectrometers for the far-IR region. For higher resolution work the FT still had to be performed on a mainframe computer, which was slow and inconvenient but nevertheless effective.
The Spectroscopic Panel consisted of industrial spectroscopists and representatives from U.K. oil companies, and related firms such as ICI. The panel invited leaders of selected university spectroscopy groups to meet with them twice a year to report on their research work. In recompense, small sums of money were donated to the research groups, to be used in any way that the group leaders considered appropriate. No specific programs of research were supported or requested. These biennial meetings provided opportunities for industrial members to learn about recent advances in spectroscopic techniques developed in academia. Specific contacts could be followed up later by the industrial firms, if desired, t h r o u g h consultations or joint projects with individual university members of the panel. Approximately every three years, the panel organized meetings on recent advances in chemical spectroscopy, which were open to everyone. The proceedings of these meetings provided timely accounts of recent ad-
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vances and were published as special volumes in 1956, 1959, 1962, 1968, 1971, and 1977, mostly by the Institute of Petroleum. The original motivation for assembling the panel was to discuss the importance of IR methods, but later topics covered all the atomic and molecular spectroscopies. It has always seemed to me t h a t this panel was an ideal industry-university scientific forum. I benefited a great deal from p a r t i c i p a t i n g in these discussions and formed some lifelong friendships with industrial colleagues. The IRDG provides a forum for spectroscopists from university, industrial, and government laboratories as well as for instrument manuf a c t u r e r s . R e m a r k a b l y , since i t s inception in 1950 the group has been under the chairmanship of only two persons—the late A. E. Martin and the late H. A. Willis—both of whom died in 1990. The IRDG meetings have very low registration fees, and membership costs only a few pounds every five y e a r s or so, mainly to cover mailing costs.
FT mid-IR spectrometers ca. 1970 FT in the mid-IR region was a major breakthrough, pioneered in the U.S. by Digilab. The U.K.'s contributions to this area from Perkin Elmer and Unicam occurred later. Ratio-recording, double-beam spectrometers 1975 Early pioneers in ratio-recording instruments were the Perkin Elmer Model 13 and the Applied Physics Corporation Model 90. Ratio recording was also used in the Perkin Elmer Model 180 produced in the U.S. However, mass production of instruments was initiated by Perkin-Elmer Ltd. with the successful Model 580. Microprocessor-based dispersion spectrometers 1975 The advantages of digitally recording spectra and replotting the same data with different ordinate units (transmission or absorbance) and different wavenumber scales became apparent because of the need to incorporate computers into FT instruments. Perkin-Elmer Ltd. and Pye-Unicam have since been at the forefront of developing microprocessor or PC control of spectrometer parameters. Low-cost ratio-recording dispersion spectrometers 1979 Low-cost ratio recording was introduced with Pye-Unicam's SP3 spectrometer and was followed by the Perkin Elmer 680 series. Both have since undergone further development, and such dispersion double-beam spectrometers are now standard. Low-cost mid-IR FT spectrometers ca. 1980 Early FT mid-IR instruments were exciting in terms of their performance but costly relative to quality dispersion spectrometers. This discrepancy allowed enough room for the "late-flowering" of ratio recording and microprocessor operation in dispersion instruments. However, as instrument versatility increased and the costs of microprocessors and PCs decreased, FT spectrometers could be produced at prices comparable to those of dispersion instruments. In the future, FT spectrometers will be the IR workhorses; simple, inexpensive dispersion spectrometers will be required only by laboratories short of money or trained technical staff.
Two books by U.K. authors have done much for IR spectroscopy since 1945. One is Lionel Bellamy's twovolume set, The Infrared Spectra of Complex Molecules, which is famous for its outstanding account of spectra and structure correlations in organic chemistry. The first and second editions were published by Methuen in 1954 and 1958; the third edition was published by Chapman and Hall in 1975. Overall, Bellamy's book became one of the most cited references of the Science Citation Index. The second book is Laboratory Methods in Infrared Spectroscopy. The third edition, by H. A. Willis, J. H. van der Maas, and R.G.J. Miller, was published by Wiley. This book is perhaps less well known in the U.S. but has been widely used in the U.K. and in Europe, and it is a gold mine of information on practical techniques. Some personal reminiscences I would like to conclude with some personal comments on t h e g a i n s brought about by FT spectroscopy in the mid-IR region. In the early 1970s
my research group was concentrating on the IR spectra of chemisorbed monolayers on finely divided, oxidesupported, metal catalysts. This work required the measurement of very weak spectra of adsorbed species, usually superimposed on much stronger backgrounds from the catalyst itself, and with limited total radiation throughput because of scattering by the porous catalyst discs. I had heard of Pierre and Janine Connes' pioneering FT work in France, used to measure the very weak IR spectra reflected from the planets, and had convinced myself that mid-IR FT methods were what I needed. I was therefore amazed and delighted when Digilab put such a mid-IR instrument on the market in the early 1970s. I had a ready-made case for a large research grant and was successful in obtaining it. It transformed my group's work. Also, visits from several other researchers from m a i n l a n d Europe, who came to use the instrument on t r a n s i t i o n m e t a l oxide c a t a l y s t s , opened up another area of research
i n t e r e s t a n d made me some new friends in the field of spectroscopy. Subsequently, my colleague Michael Chester s developed FT methods capable of obtaining IR spectra from literally a single monolayer of adsorbed species on a flat metal surface, using the reflection-absorption method. By studying chemisorption on a number of specially cut crystal surfaces, it was possible to isolate the spectrum of one adsorbed species at a time. Only then could the more complex spectra from the finely divided m e t a l c a t a l y s t s — w h i c h we knew had overlapping features from several surface species—be confidently analyzed into contributions from separately identified chemisorbed complexes. Sometimes the simplified s i n g l e - c r y s t a l s p e c t r a proved to be interprétable in terms of the presence of quite unexpected species. Of course the digital transmission ratioing (or absorbance subtraction) of a spectrum obtained after chemisorption against another of the bare catalyst, obtained beforehand, proved to be v e r y v a l u a b l e . The m u c h improved S/N resulting from the use of FT methods also greatly widened our horizons and enabled the very important extension to flat surfaces of metal single crystals. A considerable number of individuals helped me write this article by filling in the gaps in my personal knowledge and memory. I am particularly appreciative of help from Richard Aslet, Delia Agar, Sarah Barker, Michael Cudby, Donald Powell, Christopher Barrett, Frank Daly, Francis Dunstan, Mick Ford, Charles Perkins, W. C. (Bill) Price, and Jack Shields.
Norman Sheppard received his B.A. degree in chemistry (1943) and his Ph.D. (1947) from Cambridge University, and he is a fellow of the Royal Society. He was assistant director of research in spectroscopy at Cambridge University until 1964, when he moved to the newly established University ofEastAnglia to kelp found the School of Chemical Sciences. Throughout his research career, he has used IR and other forms of vibrational spectroscopy to study molecular structure and dynamics. His most recent work focuses on the identification of surface intermediates on catalyst surfaces.
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