Chemical Instrumentation
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S. 2. LEWIN, New York University, Washington Square, New York 3, N. Y.
T h i s series of articles presznts a suraey of Me basic principles, characterisli,,s and l i m i l a t i m of th,ose instruments which find important applications i n chemical work. The emphasis is on commercially available equipment, and appro.cimate prices are quoted to show the order of magnitude of cost of the oarious tjper qf d ~ s i g nand constrz~cti~m.
77. Infrared Spectrometers Although the optical principlvs involved in the design of spectrometric instnimentr are the samr whether the instrument is intended for use in thc ultraviolet, visible, or infrared rrgions of bhe spectrum, the last-named r e ~ i o nprreents certain special problems that justify the treatment of infrared spertrometers a8 a special category of instrnmcntatian. These problems arise from t h ~ rharacteristies of the ( A ) sourcc of mdiation, (B) optical mstcrials for windons, lenses and prisms, and (C) det,ectors.
length on either sidc of this maximum. The total radiation omitted by the glower varies a8 the fourth power of its temperature (Stcian-Boltzmann Law). Consequent&, tho spocial problems created by this typo of source arc: ( 1 ) the rapid variation in intensity with wavelength, and ($) the high sensitivity to thermal effcets. Two different sources arc commonly employed in commercial i n f r a ~ dspec-
ULTRAVIOLET
Radiation Source
ABSORPTIVITY,CM:'
The sources ~mplayedto provide the rsngr of radiant cnergy required for work in the infmrrd canaist of refractory materials heated to glowing temperature. The radiation cmitted from such sources has a n intcn~ity distribution that is a rharsrt,eristic function of the temperature, similar to the curves shoum in Figure 1 for
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02
VISIBLE
transmitting, and dispersing materials to achieve tho desired information about the interaction of selected umdengths with the specimen under study. In the infrared region it is particularly difficult to find materials having suitable reflectivitie~, transmissivities and dispersive powers over the range of wnvelengths of interest. Figure 2 shows the ahsorptivities of a number of optical materials as a function of n.avclength. I t is evident that glass, which ~ e r v r so s admirably for optical components in the visihle region, cuts off all infrarpd heyond 2 microns. For the most common infrared work (coverinp the range from 2 to 15 microns), one is forced to use such strurturall\- unstable and difficult-to-fabricate materials as singlecrystsl NaCI, KBr, K R S d (thallium broma-iodide), and other ionic, coordinstion latticr crptals.
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06
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0 8 10
WAVELENGTH, Figure 2.
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MICRONS
Figure 1. Radiant energy emitted per unit wavelength or o fundion of the wavelength for a black-body radiator at severoi temperoturer.
a n ideal blsrk body emitter. The emission is a maximum s t a. wavelength that depends upon the temperature of the glower, and the radiant energy intensity falls off rapidly as a function of the wave-
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NEAR-INFRARED
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feature
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iI
INFRARED
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p
Absorptivity or a function of wavelength for several optical material..
trometors; they are the Globw, and the Ncrnst glower. The Glohar is a rod of sintered silicon carbide t,h;~tis rleet,rieally heated to about 1000-1400°K hy means of a current flowing through the rod in the direction of its long axis. The Globar cannot be heated to a significantly highcr temperature than this without seriously shortening its life due to oxidation reartions. The Nernst glowcr is generally constructed in the shape of n hollow tube and is composed of a mixture of sireanium and yttrium oxides. I t is heated electrically to about 2000°K. The Nernst glowcr is thus a more intense source than the Glohar, but its higher operating temperature makes it somewhat more delicate and short-lived in use. A third source ocrasionallp enrountercd is a. coil of Niehrome wire, which is raised to incandescence by resistive heating.
The dispersion characteristics of several types of prism materials are shown in Figure 3. I t will be notell that those suhstanrrs whieh show good transmission
Infrared Optics
properties have poor dispersive powers in the infrared; those which show good dispersion are usable only over very limited portions of this spectral region. Hence, infrared spectrometers based upon
Betwecn the source of the inirared radiation and the find detector it is neressary to employ n varirt,y of reflecting,
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Figure 3. Dispersion os o function of wovelength for several optical moteriolr.
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Chemical instrumentation s siugle prism do not have high resolution; nohievement of high resolution in this field involves either thr us? of a number of interchangeable prisms, enrh best sl~itrd for n restricted portion of the spectrum, or n grating combined with a f o r e p r i m for separating out the various spectral orders of diffraction.
Detectors The det,ectors employed in rommerrial infrared speetromcters are all based upon the thermal effect produeetl when tllc infrarcd radiation is absorbed. Sinrc oven with a spectrometer of low rceolution, the total energy falling on the detertor a t any wavelength setting is v u y small, the requirements for a mtisfactory detector are very stringent. I t m l r t have a small sensitive area, a l o r heat capacity, a high and "on-srlective :%I,sorpt,ivity for infrared of all wavel~ngth.;, a rapid bime constant, a l o r noise lpvel. and a high thermal sensitivity. The detectors commonly used are the 1.4) Ibermoeoaple, (B) bolomelu, and (C) Oolnz, pne~~malic cell.
Figure 4. Design of a thermocouple u3eful os on infrared detector.
A typical thermacauplr detector is shown in Figure 4. A tine gold foil (as thin as 0.3 micron, in some eases, and with a n area of about 2 mm X 0.5 mm) is welded t o two thermoelectric materials. The infrnred radiation is focussed onto the gold foil, which serves as the thermal cont a r t for the thermoelectric substanre.;. The surface of the gold is hlaekened t o improve the absorptivity, and the thermocorqdp is mounted in nn evnnrstr,cl envelope with a n infrared-t,mnsmitti~~g windon, t,o minimize heat losses by cow dnction and convection. A power input to this type of detector ss small as 10-'" watts can he detected over the inherent, electrical noise.
Figure 5. Plan of thetherrnktor bolometer mode by Olympic Development Co., Stomford, Connecticut.
.4 bolometer is a temperature-sensitive resistor. Thc active element may he a thin nohlc-metal foil, or a flake of n ther(Continued on page A784)
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Chemical Instrumentation mistor material. Two of these elements are generally mounted close to each other, with one shielded from the impinging radiation to serve as a reference, or eompensating resistor, as shown in Figure 5. The sensing and reference elements constitute two arms of a. balanced Wheatstone bridge circuit, so that radiation absorbed by the sensor creates an unbalance signal that is amplified and measured. The sensitivity of a bolometer detector can be as great as that of the thermocouple previously described. PNEUMATIC CH4MBER
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ABSORBING FILM
Figure 6.
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Journal of Chemical Education
Background Bnother factor t,llat p o s e a srriuus problem for infrared spwtrometrr desipn is t.he presence of strong absorbers of infrared radiation in normal laboratory air. Figure 7 shorn s part of the absorp
FLEXiBLE MIRROR
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// LlNE GRID IMAGE
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GRII
Constructionof the Golay pneumoticinfrored detector.
The Golay detector, shown schematieally in Figure 6, employs the expansion of a gas as the sensing element. The gas is confined in a chamber one wall of which
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is a Hexiblc membrane. As radiant energy is absorbed by thc gas, it expands, and displaces the membrane. Light focussed onto the outer, mirrorod surface of thifi memhrane produces an image of a line grid an the plane of that grid ( c j . light coming from lamp a t extreme right, in Figure 6). When the image of t,he grid superimposes exactly on the actual grid, the intensity of light transmitted to the photocell (lower right, Figure 6) is a maximum. If the image reflected from the Hexible memhrane moves, as a consequence of heating of the enclosed gas, the transmitted light intensity falls. I t has heen estimated that displacements as small as 10-Qm arc
detectable by this arrangement. Power inputs to the detertor as small as 5 X 10V" watts are drtertable over the inherent noise.
Figure 7. a. Infrared spectrum drawn by single beom spectrometer that hos been purged thoroughly with dry nitrogen; b. some, far unpurged instrument.
tion produced by the water vxpor present in the usual opbicsl path b ~ t w o ninfrared source and detector. Flushing the in(Conlinued o n page A786)
Chemical Instrumentation strument with dry nitrogen serves to eliminate most of this interfering absarption, but this expedient is generally undesirable, and it can he avoided by the use of the dual ahann4 instrument design. The several unique features of infrared spectroscopy that have been discussed :$hove h a w ixrrt,ed a controlling influence aver the form infrnn:d spectrometers hsvc taken. Because the rlotector responds to n small thcrmsl ellcct, it has been absolutelg cssentinl to employ modulated ("chopped") rndintion beams, so that random variations in the t,hermal environment of the drtrrt,or will not interfere xith t,he infrared sign:il. I t hxa been necessary to use high-gain, low-noise amplifiers ttmed sharply to the chopping frequency, so t,hnt only the dcsirerl signal is amplified, :mrl noiscnnd drift a m rejected. Bermme of t,he mxvni1:~hilityof infrared transparent mnt,erialfi that can he ground into lcnsrs, it has lheen necessary to employ mflcrtion optirn in these speetromvters. 'l'h,~r, nearly all collimation and focussing is neromplished hy the use of suitably mrved, front,-surfaced mirrors. The hrge variation in henm intensity n p :L funrt,ion of wnvclcngth makes it necessary to vary t,hr manochromator slit widths l~ntwrt:n wide limit,% when smnning n spectntm, if the signal-to-noise ratio a t thr dr.tcot,or is to be kept fairly rnnst~ntt,. The ~,wmnceof the H.0 and COz a h sorption 1,ands in tho instrnment atmosphrre neaessitntcs ~pocinlrarrective meae urns if the spact.romct,cris to yield spectra
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L Figure 8. Optical layout of the Boird-Atomic double beam, null balance recording spectrometer [Model NK-I, and previous modelr).
eharacteristio of the sample alone. Since the source intensity may show considerable variation between runs, and since it is difficult to reproduce exactly the large number of different slit openings employed during a run, single beam instruments, in which it is necessary to run two successive spectra (a blank, then a sample run) and then obtain the sample spectmm by pointby-point subtraction, have never heen popular. The use of the double beam principle, either in the form of s, nullbalance or a ratio-recording arrangement,
has been brought to its finest degree of development in inirared speetrometers. Tho field of infrared spectrometry has proved to be almost as profitable commercially as it is indispensable scientifically, and a substantinl number of highly dcvrloped instntments is currently available.
Baird-Atomic One of the major prodnrrrs of double-
(Continued o n page A788)
Chemical Instrumentation heam spectrometers far t,he infrared region is Rxird-Atomic, h e . , Cambridge, Massachusetts. Their current instrument is the Model NK-1 (810,950) which supplants thpir earlier Models KM-1 and 4-55. The optird layout of all these models is essentially the same, and is shown in diagram in Figure 8. A Glohar radiildion source (lower emtcr of Figorc, in rentcr of wheel with eirrulnr cutouts) is mnintsined a t 1 100DIi,and is ~urroundedby a metal shield that i~ wnt.cr-coaled, t o minimize attack of the Glohar s ~ ~ r f a ahy e heated air. Radiation from the source emerge: through two ports in the shield (see Figure 9), and is directed a t two focussing mirrors, whieh in torn direct h a m s through the samplc and rcfcrence cells respectively. Aftcr passing through these erlls, these beams impingo on the interrupter rotat,ing mirror. This consists of s semicircular mirror, which fills half of the circular area, of the interrupter. This component is so positioned t h a t when light from the sample crll is strilcing the mirrored surfxco and being reflected h~t,o the entrance slit of the monoehromstor, the light from the rrfermoe cell ia bloekcd off by t,he rrar of thzt mirror. Ondmlf cycle later, light from the refercnre crll is passing through the empty half of the interrupter circle and cnt,ering the manoehromstor, while light from the snmplc crll is passing through the empty xrca and being ahsorbed on the right-hand wall of the instrument cnsc. Thus, t h e interntptcr cnusrs thc light rrhirh enters the monochromator t o nltornnto betmen t h a t which came through t,hp sample nnd t,hat whieh came through the rofcrmrr. In the manorhromator srrtion, t,he diverging light beam is reflected t o the collimator mirror, which renders i t parallel and dirrrts i t onto the prism. I t is dispersed in passing through the prism, is reflected hack ilrto thc prism by n Littrow mirror, and suffers 5 s c ~ o n ddi8persion. I t rctums t,o thc rollimitting mirror as a psrdlcl heam, hence upon rrflection i t is iocussed (thc path now is the obverse of thc first reflection path) and is directed through t,he monor:hromntor w i t slit onto the receiving mirror, whieh focusses i t onto thr holomrtrr detector. The halomotor aomirts of s platinum st,rip i mm long X 0.3 mm wid? X 0.1 micron thick. I t alternately sees t,he radiation n . h i ~ h has come through t,hr snmple and reference paths respertivdy. If thpse two hennu am cqrlnl in intensity, st,esdg signal rvould ho pradnrt:d a t t,hr Irolometcr, and hcnco from thc W h ~ a t stone hridgo cirntit of whirh i t is a part; if therc is m y unbnlnnro in the t,wo beams, the output from the lhrirlge rimxiit contains nn ac component (having the frequenry of the intcrruptrr). This ;Ir romponrnt is amglifiod, and the amplified outl)ut is fed int,o n mechanical synchronous rectifier (employing n rommltt:ttor mounted on the int,orruptcr shaft) that ronverts it into a dc signal, the polnrit,y of nhich dcprnds upon whether the sample hcnm was stronger or weaker thnn tho rrfrrener (Continxed on page A790)
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Chemical Instrumentation ~
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beam. This dc voltage is in turn amplified, and serves to drive a. servo motor which moves the shutter comb that is mounted in the path of the reference beam. The detailed arrangement of the sourre and equalizing shutter is shown in Figure 9. As this shutter moves into the reference beam, the light intensity falls; as i t is retracted, the intensity rises. Thus, the shutter reaches a n equilibrium position such t h a t the signal driving it is reduced t o zero; is., such t h a t the reference beam intensity has been adjusted t o exact equality with the sample beam.
Figure 9. Radiotion murce ond balancing comb shutter of the Baird-Atomic spectrometer. Reference cell c o m p d m e n t i s a t right; romple cell comportment a t left rear.
The recorder pen is mechanir:ally linked to the shutter comb, and is moved across the chart paper as the equalization position is sought. The record traced on the chart paper is, therefore, a rarord of the extent t o which the reference beam had to he attenuated to achieve equality x d h the sample beam, and is equivalent to a record of the relative transmission of the sample compared to the reference.
Figure 10. Program cams of Baird-Atomic Ipertrometer. upper cam controlr movement of Littrow mirror behind prism; lower cam controir opening of riitr.
The wavelength scale is determined by the rotation of the Littrow mirror, and is programmed by means of a specially cut.
(Continued on page A79a)
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Chemical Instrumentation cam so that the chart record is linear either in wavelength or in rmve numbers (frequency), as desired by the user. The slit program is determined by mother cam. These cams, and the arms that transfer their p r a g r m s to the Littrow mirror and the slit system respectively, are shown in Figure 10. The Model PjK-1 includes several features of scanning control that were not standard on previous models. A constant scanning rate a t any value betu-een 0.1 minute/micron and 1 hour/mieron can he chosen by means of a variable resist snce control, as in the eerlier models. I n addition, an accelerated rate of seeming can be chosen, which causes the rate of sean to increase automatic all^ during the run a t a, predetermined rate. That is, a. run can start a t a scanning rate of 5 minute/micron and accelerate continuously so that the run ends a t a scanning rate of 0.5 minute/micron. This type of acceleration is useful in speeding up many runs, since the resolution usually desired is less a t longer wavelengths, and fester scan times e m be tolerated. Another useful feature that permits much saving of time is the automatic speed suppression control. The shaft of the pen and comb balancing motor is provided m-ith a signal generator that produces a volhge vhich is proportional to the rate of rotation of the shaft. This voltage is then fed back to the sean motor drive system, serving as a. brake on the motion of that motor. Thus, the scan motor can be set to scan through the wavelength range rapidly when no absorption bands are present; as soon as a band is encountered, the motion of the balancing motor thereby initiated generates a signal which brakes down the scan motor to a slow rate of scan. As soon as the absorption band has heen traced, and the braking signal drops to zero, the scsn motor speeds up again, until the next absorption hand is approached. A six-position switch permits control of the minimum speed during the band; the speed between bands can be either a t the constant or accelerated rates dearrihed previously. The standard prism and cam assembly is far NitCl, and covers the range 2 to 16 microns. Other prisms and cams can be interchanged for these to permit use of the instrument out to 0.2 micron in the ultraviolet (quartz prism) or to 38 microns (KBr prism). The resolution with an NaCl prism is 0.015 micron; accuracy of navelength setting is f 0 . 0 1 5 w ; reproducibility is f 0.005 p. The reprodueibility of transmission readings is f0.17,. The Model NK-la ($8500) consists of the basic instrument described above, but without several of the special features, such ns the accelerated and the automatic scsn controls. An auxiliary recorder can attached to these spectrometers to be used for drawing a duplicate trace while the spectrometer recorder is producing its primary trace. The transmission (ordinate) signal can be expanded or contracted, to permit a d j u s t ment of the size of the auxiliary record. (Continued on page A794)
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Chemical instrumentation Beckman A group of three spectrometers covering a wide range of capabilities is manufactured by Beckman Instruments, Inc., Fullerton, California. Model IR-5 ($5460) is a nun-balance, double beam instrument similar in design principles to the Baird-Atomic Model NK-la just described. A schematic representation of the former is shown in Figure 11. The radiation source is a coiled Nichrome wire; two light paths are produced by the mirrors A, A, B, B, C, C. The sample beam is E; the reference beam, D, passes through the comb shutter, which is adjusted automatically to achieve equality with the ssmple beam intensity. As in the previous case described, the prism disperses the radiation twice, because of the Littrow mounting. A chopper alternates the light seen by the thermocouple detector between the ssmple and reference paths. A flat-bed type recorder is used, and the chart presentation is linear in transmittanee and wavelength. A single scanning speed is provided, chosen to suit average conditions in routine use. Wavelength resolution and accuracy are both +0.030 r ; repeatability is +0.01 a. Reprodueibility of transmittance readings is +1% T. A double monochromator, double beam spectrometer is manufactured as the Model I R 4 ($15,120). The optical schematic of this instrument is given in
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Figure 11. Schemetic representation of the layout of the Beckman Model IR-5 null balance, double beam infrored spectrometer.
Figure 12.
Optical diagram of the Beckman Model IR-4 double monochromator spectrometer.
Figure 12. The null-balance principle is incorporsted in this design, so that when employed as a double beam instrument, i t is similar to the spectrometers dready described, except that the presence of two
prisms yields dooble the resolution a& tainable with the single prism instruments. A variety of scanning speeds is possible, and push-button controls are provided. (Continued on page AA797)
Chemical Instrumentation The radiation p o l m e is a Nprnst dower. a i t h elertroni~:ally regulated heating input. The instrument can hc operatcd as :I singlr heam spectrometer, when dcsired for g r a t e r qumtitative accuracy. A still higher resolution instrument. is the prism-grating spectrometer, Model IR-7 ($18,000), the optical plan of which is shown in Figure 13. This has three t,imcs grextcr resolution than the double henm, double prism instrument, eorresponding tn 0.3 wave number (em-') over the rangc 4000 to 6 i 0 cm-I. Thc inslrumrnt may be opcrnted eit,hpr douhle Iwam or singlr hcsm; scanning s:~erd is veriahle from 0.8 cm-'/mimite t o 200 rm-llminote. Photometric reprodnrihility on single beam is +O.lf/, T. The instntment is constructed of gas-bight compartment.+ and can he p11r~1x1a i t h dry nitrogen when required.
preliminary run, and thcn programmed the slits to cancel orat the background ahsorptian during the sample r i m ) , and IR-6 (iiimplified single heam inst,rument). These are no longer h e i q manofartnrrd.
SMITH,R. A,, JOSEG, F. E., and C ~ a s n n n R. P., "The Detecdion and Measuremtmt of Infra-red Radiation," Oxford Uuivrrsity Press, London, l!l5i.
Bibliography GOLAY,hl. J. E., "The Throretirxl and Practical Sensitivitv of the Pneumatic Infm-red Detector," Rev. Sri. Inst,r., 20, 81&820 (1!l49). I
