Chemical htrumentation 5. 1 LEWIN, N e w York Universiiy, Washington Square, New York 3, N. Y.
T h i s series of articles presents a survey of the basic principles, characteristics, and limitalions of those i n s t r u m t s which jind important applications i n chemical work. The emphasis is on commercially available equipment, and approximate prices are quoted to show the order of magnitude of cost of the various types of design and construction.
9. Spectrophotometers (Continued) In solerting a speetl.ol,hotonrctct., onr of the primary eonsider.ztians should in general bc to favor that instrument that is as simple in construction and employs as few components as possible, consistent with the required resolution, sensitivif, and reproducibility. The absorption spectrum of the sample determines the required resolution; i.e., if the spectrum contains sharp peaks (narrow hands), s. high resolution instrument may be required; if the peaks itre broad, the resolution of the spectrometer (monochromator) may be correspondingly low. The extinction coefficient and concentration (or thickness) of the sample determine the required sensitivity; i.e., only if the sample absorbs most of the incident radiation need the photometer be capable of meitsoring and comparing low intensities. If the purpose for which the spertrophotometer is intended is the determinat,ion of the concentration of it colored eonstituent in a medium in whieh no other constituent has an ahsorption hand dose to that of the desired sobstrtncc, a low redution instrument i~ in gcneml to be preferred. I t tends to be quicker, aimpler, adequately reproducible, and more available (because it is less expensive) than the high resolution instruments. On the other hand, a high resolution monochromator is necessary. if ( A ) it is important to distinguish between aihstances having closely situated absorption bands, (B) it is necennary to test for deviations from the Beer-Bouguer law, as in the investigat,ion of complex-format,ion eqnilib~.ia,(C) it is desired to identify species hy the location and shape of the absorption hands, ss in the study of molerulal. struelure of ehromophore-contailring molecules. There are ourrently available rommorcia1 npectrophotometers of many kinds, ranging from moderately-priced, low resolution, limited range instruments intended for routine service in analytical work to expensive, high resolution, versatile, research instruments.
Universal (Modcl 14, with battery and charger, %74:3). Hoth have the mme optical design, which r o n ~ i ~ of t s R tongst,en filament light Roorce, n diffrartion grating for prodwing bhu npwtnm, a lens system, and a fixed slit. A harrier layer photorell is the detector, and its photocurrent in displayed by a galvanometer. A diagram of the optiexl circuit of the Coleman Junior Spertrophotomoter is 8honn in Figure 28. A diffraction grat,ing is mounted in n permanent position hrtween a. pah. of lenses. The monoehromator exit slit i~ located on one side of thifi lens-grating ~yst,em,and the light source is on t,he other aide. Any given wavelength of light from t,he lamp is hrought to s focus a t the exit slit only for s i:ert,ain lamp-to-lens distanrr, determined by the focal lengths of the lenses for that wavelength. Since the refractive index of glass varier wit,h wavelength (rf. prrvioua dismssion of dispe~.sive powcr) the focal length must also vary ait,h wavelength.
Coleman
Also, tho grating spreads out the various wavelengths in the incident beam (see Figure 9 ) ; hence, the angle hetween the light source and the grating most he varied if different wxvclongths ale to he bmoght to t,hr fixed position of the exit slit. Thn6, in order to scan a spertral range, i.e., t,o muse different w:-;lvelengthsto he
The Coleman apectrophotometors are designed for routine analytiral w o ~ k ,and are distinguished by a considered simplicity sndeeanomy of construction, Two models are currently being manufactured: the Junior (Model fiA, $GO), and the
focused sorcessively on the exit slit, it is neccssnry to vary the lamp-to-lens d i p tancc and angle in x manner determined by the dispersion and diffraet,ian ehai.aeteristics of the lens-grabing romhination. A speeinlly-rut ram is a t t , a r h ~ dto t,he wavclmgt,h dial, whirh upon bring rotated tilts thrt,ahle on whirh thelampismoantrd, and rauws th? lamp to travel dong the arr shown hy t.he d a ~ h e dline in Figwe 28. The lamp filament is a thin, straight coil ahout 0.3 in. Long. This light source serves as its own entrance slit,, for the light, is emitted from it in a narrow, sharply-defined area. The exart positioning of this filamcnt on fhr locos shown in the Figure is critical if proper wavelength calibration and spectral resolution are to be oht,ained. The orientation of the filament is fixed by the special slots in the lamp base, which infiure that the lamp ran he inserted in its socket in only one way. Fine adjustment of the filament position i~ made by mems of the adjwting wrem on the shaft of t,he n-ayelength ram. The lamp is an automobile headlight type, with a rated life of 2500 ho~wn,and it draws -1.5 amp. at 6 v.
Figure 29. Electric01 circuit of the Junior Spectrophotorneter.
Figure 28. The optic01 design of the Coleman Junior and Universal Spectrophotorneten.
The stahility of the photometer mcaswements is very dependant upon the constancy of light output of the lamp, sinrr this is s. single-channel, cell-in-rd-out type of instrument. The inevitahle fluctuations that occur in most power line supplies are too great t o permit this type of instrument to be powered directly fmm the line. A good, high-capacity storage battery (fi v, and a t least 100 amp-hours rapacity) is fully satisfactory. Alternntivcly, a n electronically regulated power supply (No. 6-054, 5130), or leas dosirahly, imply a constant voltage trannforme~. applied to the power line (No. 6.056, S"3) may be substituted for the hat,tery.
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Chemical hstrumentafion The diffractiongrating is a l,lxstia repliea of a master grating, and rontuinr; 7500 lines per linear inch. The grating is I > l a r ~po d that a large part of the light in-
Junior Spectraphotometer is cxtwmely simplo, consisting of a i,arrier layer photocell and a galvanometer. The zero of the galvanometer can be adjusted mechanically, by mean8 of a lever bar scaeaaihle from tho top of the instrument case. The magnitude oi the d~flprtionproduced hy a
dry cell is employed to oppose p u t of the photoeell current, leaving a difference current to deflect the galvanometer. The galvanometer ~ c d eon which the light spot from t,hr gdvanometer mirror is focused has a mmovable translucent panel, so that various panels calibrated in YJ, optical density, or concentration of some constit,uent, ran he subntituted by the operator. A filter i~ mounted in front of the photocell (t,he "integral filter" shown in Figure 28) which is designed to compensate for bhe variation in spectral sensitivity of the photoeell aver the range used in the instrument 1400 to 700 millimicrons). Thnt is. a t t,ho?irnavrlrnethr for which the nhotocell has low sensitivity, the filter passea a large fraction of the light; where photocell sensitivity is high, the filter passes a smaller fraction of the light. Test tube-shaped euvettes st.e used, and cell holders are available for micro (ca. 0.007 ml) to macro (10 ml) sized samples. For higher precision work, square euvettes with optically polished surfaces are avnilable. The dispersion of the instrument in lineal, and the hand width is 35 millimicrons uniformly over the entire range. Photometric aecursry is flYoTand reprodneihility ia 0.2C1,T a t any %?' value. Modifications of the Coleman Junior Spcrtrophotometer are available wit,h an exte~naljack for plugging the galvanometer circuit into a flame photometer or Huorimeter (Model GC, $460),and with a highrrre~olutiondiffraction grating having ~
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Figure 30. Schematic of the Coleman Universal Spectrophotometer.
tensit,y ia concentrated in the fi~.st.ord~l. sper:tvum. The illuminated nl.r!tb in "/, X in. The photometer circuit of t h t Coleman
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given light intensity ran he adjusted electrically, by means of the fine and coarse controls shown in Figure 29. In the eketrieal adjustment, n. current from a small
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(Continued on page A610)
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Chemical instrumentation 14,400 liner per inch, giving n Imnd gnsa width of 20 mp (Modcl 6D, 'R465). The Coleman Univrt.snl Sp~ct,lophotometw, hfodel 14, embodies the same optical circuit as the .Junior model, and also employs s barrier layer photocell and galvanometer read-out. However, thc clcrt,rieal circuit is more elahovate and vcrsutile, as shown in the sehrmatir diagram 1.cproduced in Figure 30. Light int,ensities can he measured by direct deflection of t,he galvanometer, as in the Junio~.modd,or hy a nnll-balance arrangement, i n which the galvanometer serve8 only to detect urlhalance, and the reading that is mad? is of t,he position of the eontactor on s d k k wire n:sistol.. The variable resistors marked y a h ,line and coamr are employed as in Figllrp 29, t,o
Figure 31.
Optic01 design d the Bow& and Lomb Spectronic 20 Calorimeter.
oppose n rwrmt from a dry rcll against tho phat,ocd out,pttt, thus adjusting full-scale d~flwtionof thtr gnlvnnomrte~.to eorre-
pond to some reference condition, such as the light. tritnsmi~~ion through a cuvette filled with the pula solvent. The eontld lalded hlk is used fol. adjusting the galvanomrtcl. zcro when the ohotocoll is dark: by thc puw solvent is taken as zero for the purpoms of eslrulation, and in this instntment this low light intensity is electrically Ix~akad-outhy the blk control. The vari~ h l rcsistol. e muked bal is used when nulll)rtlanre mcwsurementa are desired, and the fluor control adjusts the span of the bal slidrwirr, thus controlling the sensitivity of tho 1,alnnring dial. Thr galvanometer gains in sensitivity hy tho long aptirnl lever arm between the coil and tlw scale, and this is fitted into the compnct space of the instrument hy means of sweral mirrors, producing a so-called "foldrd" optical path. The nensitivity in O.IX15 mirlmnmpwe per male division, and the galvanom~tmcan he used for any cln; rent measuring application independently of the spect,rophot,ometor hy attaching thc external r i l w i t through the nephelomeler socket shown in Figure 30. Tho instrument is designed to take, as accessories, Asme photometer, nophelometer, and Roor i m ~ t e rattarhments. Photometric nreoracy is stmnt *0.5%T.
Bausch and Lomb
A simple, economioal, good-quality hrbad hand apectrophotometer utilizing a r e p l i ~ adiffraction grating as the d i e persing element is the Spectronic 20 Colorimeter ($235) of the Bausch and Lomh O ~ t i c a lCo.. Rochest,&. New York. The optical oirruit of this instrument is shown in Fidrre 31. The light source, phototube, and entrance and exit slits are fixed in position, and the grating is turned in order to scan through t h r ~pectralrange. Since a grab ing is wed, the di~persionis constant, and fixed elits ran he employed. The lamp and I P ~ Rsystrm produces a beam of collimatad light on tho grating, and eliminates any critica1it.v of tho location or dimensions of the lamp filament. A vacuum phototube is used as detertor, and its output is fed to a bridge amplifier circuit, where i t is amplified and read out on a rugged mieroammotcr. The entire amplifier circuit is mounted on a. removable card; the manufacturing tech~
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(Crmlinued on. page A6IB)
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Chemical Instrumentation nique of "printed circuitry" is used in producing t h k card, and it, in inexpensive enough t h a t i t may be replaced by an entire new circuit, card whenrvw tmuhlc :LI~S. A ti-v, automobile headlight type of t,ungst,en-filament lamp is used us the light source. A voltage stabilizer is nn integral part of the inst,nrmeat, and .SPI.WS to smooth out power supply fluot,ustions as la1.g~as 1 v. For larger fluctuations, an adrlitioniil voltage stabilizer is required. The power input for lamp and amplifiw is the 1 Ill v ac line. The diffraction grating containn about 15,000 lines per inch, and providnl, with the fixed slit widths employed, u hand pasa nidt,h of 20 millimicrons over the m t i w spectral range of 375 to 650 mp. By rrplacing the standard phot,otube with a ~.edsensitive one, and adding a, filtet. to removt. scattered light of short wavelengths, the ntnge can be extended to 950 mp. A very attractive, high resolution, recoding speetrophotometer based upon it twn-grating monochromator has recently heen placed on the market by Bauarh and Lomb. This is the Speetmnie 505 (complete, for 200-650 mp, $4285), m d is show11 in optical plan in Figure 32, and in nrtnnl appearance in Figure 33. The two replica gratings each contain about 30,500 lines per inch. The fir& pvodwm an initial dispersion of t,he light
(Continued on page A516)
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Figure 32.
Optical circuit of the Bauwh ond Lomb Specfronic 505 recording ~ p e d r o p h o t o r n ~ t ~ r .
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Figu~e33.
Photograph d the Bourch and Lomb Spectronic 505
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Chemical Instrumentation incidcnt on i t ; thesecond grating sevvcs the functions of doubling this initial dispersion, and eliminating the interference of second and higher order spectra with the desired first a&r. The entranco and exit dits m e tired in sizc, since the dispersion of the grating is uniform. With the standsvd slits. the hand pass width is 0.5 mp. A h g e r exit slit is provided for law intensity massuremnrts, such ss are encounkred in ~.eflectnneeor fluorescence work, giving a band pass width of 5 mrr. On special order., narrow slits arp available to givc hand widths of 0.2 mp. This i~ a double-beam, ratio-lwording instlwnont. The light from thc source is focused as x 10 X enlarged, achromatic image of tht, lamp filament on the mtr.anee slit, from whence it diverges to the d l i matine mirror. which directs n nnt.allcl
I m m splitter. The latter is a n arrsngrment of alternately oriented mirrors, as shown in Figure 34, whieh sends half of thebeam to the reference channel, and the other half to the sample channel. The light leaving the beam splitter is chopped by the beam switch, whieh consists of a rotating housing ~urroundingit, with holes 80 plared as to pass the light alternately to the referenre and sample channels reapert,ively, during each cycle of rotation. The light from the two channels in raused to comc to a focus a t the photom~~ltiplie~.
Figure 34.
Detail of the beam rpliner used in 505. showing the amk of alternately-oriented reflectorr.
t h e Spechonic
The latter, themfirre, alternately seea the light that came through the sample and reference channels, and produces a n alternating signal the amplitude of whieh is s. measore of the ratio of the two intensities. The speed with which the instrument scans through its wavelength range is set a t a convenient, high value. However, by x iecdbsck arrangement between the recording pen motion and the scan drive motor, the scanning speed automatically deweases when the rate of change of pen travel increases. Thus, the spectrum is sennned rapidly over regions where there is no atrueture, and slowly whero there are :~bsorption bands; tho sharper tho band is, the more slowly does thc instrument a r m , and the more time does tho pen havri to drew an accurate record.
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Chemical Instrumentation Removable gears permit the total wavclength range in a scan to he 150, 300, or 600 mp on the 11 X 17 in. chart paper provided. The wavelength span is linear. The chart drive can be disoonnected if it is desired to record absorbance as a function of time. Gears are also provided far recording of light intensities in terms of percent transmittance (on ranges of 0-100, 0-10, or 0-300%), or optical density (on ranges 0-1, 1-2, or -0.3 to +0.7). A combination light source is available that contains a tungsten lamp, a mercury lamp (for wavelength calibration), and a hydrogen lamp. An 1P28 photomultip l m tuhe is used as detector over the entire range of 200 to 700 mp. Wavelength accuracy and reproducibility sre f0.5 mp; photometric accuracy and reproducibility 1 0.006 a t a n optical density of 0.4 (i.e., 1 0 . 3 % a t 25%T).
Perkin-Elmer A douhle monoohromator recording spectrophotometer employing quartz optics is the Model 4000-A spectrophotometer of the I'erkin-Elmer Corp., Norwslk, Connecticut ($8950). Tho optical diagram is shown in Figure 35. A two-position mirror focuses light from either a tungnten or a hydrogen lamp onto the entrance slit of the monochromrttor. The beam is rendered parallel by a eollimating mirror, and dispersed by the first quartz prism. The prism has a. Littrow mirror hacking, so that the light entcring
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Figure 35. O p t i c o l rchemofic of the Perkin-Elmer M o d e l 4000-A recording rpectrophofomefer. 1, light source mirror; 3, 9, collimating mirrors; 4, 8, Littrow-mounted quartz prisms; 6, mirror slit; II, chopper; 14, cuvetter; 1 5 , PbS detector; 16, photomultiplier detector.
the prism is reflected nt the hack surface, undergoing furt,hev dispersion, and omerging from the same prism face through which it entered. A narrow hand of this initial soeetrum onsties throueh s mirror; slit (a T-shaped slit.,with one member oft,hr T a mirraverl surface) into the ~t:lmld half
of the monochromator, whew it is irrrthcr dispersed hp the second Littrow-monntcd quartz prism. The selected savelongths emerge from the exit slit and are chopp~d, t,o create s fluetnatine lieht, aimal for " pnsipr amplification subsequently in tho (Conlinued on page A618)
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Chemical Instrumentation electronic circuitry. This light is thrlr split into two heams, one of which passes t,hrough a reference ehitnncl, whilr t,he other trnverfirs the sample channel. Two detectors s e n e the two heams, prodnring two indepcadent electrical signal^. These two ar signals are compared lcet~onirnlly t,o yield t,he rntio of the light intensities in the two rhannels. Th? eomhination of two pvisms gives high d i s p ~ ~ k mand , the IISC of p ~ . i ~ m s rnthet. t,han gratings leads bo very low row tont of smttwcd light in thp final measurcd intensity. This type of i.onatruetion is, however, ronsiderably more expensivr than the daublr-grating monochromatar discossed pr~viaoaly. The stray radiation is ICSS than 0.005% between 220 and 1500 mr. Rt.~olntion is 0.1 mp ~t 250 mp. in~rcasing to 3 mp a t 1500 mp. This inst,rnment fcatures variahle scnn mtrs, controlled by the rate-of-rhange of pen trxvd, so thnt scanning is rapid owl. regions with little structure, and slow where ahsorption hands a n encountered The 100% line of a recording spertvaphotometer is the line that is dl.swn n h m thr lxference s u h ~ t a n c e(e.g., nil. 01. solvent) is in both channels. Ideally, this should be a perfectly Rst line at 100%,T, hut in practice eonsiderahle deviation from t h a t value may he expertm, d w t o non-uniform differences (optical, clertriml, and mechanical) between the two c.hnnn~lr; t,hese deviations may vary v i t h time, and with manipulations of the envottes. This is the reason thnt a hasp line should b r run a t frequent intervals with evew s w h instrument. The Morlrl 1000-A provides a convenient d w t m n i r eontrol for operator adjustment of the 0 0 I . .ilong the length oi thc tcnt,wn wavelength potentiom~tcrare I8 taps, rnch ronnerted to a msnually ndj~lstnhle varixhle resistor. The movahlr rontact of the heliral potentiometer is driven hy the wavelength motor and Rcmr the ~ i g h k e ntape, spread across the wnvrlength SCBIP. At any point whew the 100% line is too high, one of these ndjnstshle resistors can be varied t o bring it, down: if a t Born? other mvelcngth the 100% line is too low, a diffcrrnt rrsistor ran he varied to raise it. This ~portrophotometercall Ile nard t o cover the range 200 t o 2800 mp, employing a photomultiplier detertor fol. the ,IItr:wiolet and visible regions, and R lend nulfidr photocondurt,ive dctcrtor for the near infrared. Wavelength acrmwy an th? recording paper rangw from +0.08 lnp at 200 mp t o + I 0 mp a t 1500 mp. Photometrir arrurary is +0.5%1'. The recovcler is of the drum typr, taking an I I X li-in. (,hart. The slit is adjuatahlr and is arnlmt? to &0.003 mm below 0.010 mm. A number of accessories is av:~ilxlrlr,indudine a diffuse lpflprtanrr ?inhem
Zeiss
A qw,~.taprism, non-rerotding apwtroplrotomvter similar in d ~ s i g nto thv l3wli(Conlinwd on pngr &SO)
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hand width is 1.5 mu, Chemical h ~ f ~ ~ m e n f f l fonly h,te, if the spectral ~f 0.2% noise is acceptable, man Model DU is mxnofartwed hy the Carl Zei~sargmisntion, nvnilahl~in the
the hand width can be reduced to 0.5mp, and for 0.3% noise, a hand width of 0.2 mp ran he obtained.
~i~~~~ 36. s~hemoticdiagram, and .Oval appearance, of the Cod Zeirs Model P M Q I1 Spectro photometer.
U. S. through Carl Zeiss, Inc., N e w York I t is their Model PMQ 17, New York. I1 Spectrophotometer ($4400), and is shown in Figure 36. Either a, tungsten or hydrogen light sourcein thelamp housing has its light outout focused onto the entrencc slit of the knoehromator. The light is chopped at, this point, then collimated, and finally dispersed by pasaitge through a Littrowmounted quartz prism. The selected band of wavelengths leaving the exit slit traverses a cuvette, and impinges on the detector (either a photomultiplier tube for the W and visihle, or a photoconductive cell for the near-IR). The ae signal from the detector is amplified, and presented for read-out on a gslvsnometer. The spectral range is 200 to 2500 m r ; wavelength aocursey is f0.05 m r a t 250 mp, increasing to +8 mp a t 1500 mr. The monochromator is detaehahle fmm the lamp and cell housings, and is completely symmetrical, so that either slit can he used as the entrance slit. This facilitates modification of the instrument for npcrinl applieations or unusual types of xbsorption cells. The chopping of the light beam, and the use of ac amplification, eliminate the pmbIem of drift in the measurements due to change in the lamp intensity or power s u p ply voltage. Hence, high eapacit.~storage batteries. or eleetronicallv.. regulated power .~ supplies are not needed for the lamp eireuit. The band pas8 width of t h i ~type of speetrophotarneter depends upon the value of the amplifier gain heing used which in t u r n depends upon the photomot,ric acemacy dosired. For example, if the monochromator is set a t 320 mr, tho light source is the trrngst,en lamp, and the detect,or is the phot,omultiplier, a noise level of O.lYo can he achieved a t full-scale deflection of the read-out galvanometer
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Other instrumontation is also avsilahle from this manufacturer, including a double monochromator (with either two quartz or two glass prisms), as well as reflectance, fl~ioresrenw,and flame attachments.
Bibliography
A. O., "A CARY,H. H., A N D BECKMAN, Quarte Photoelectric Spectrophotometer," J . Opt. Soe. Amer., 31, 682-51 (1941). CASTER,W. O., "Varishility in the Beckman Spectxophotometer," Anal. C h m . , 23,1229-36 (1951). Em-ING, G. W., AND PARSONS,T., JR., "Intercornparison of Beckman Spectroibid., 20, 423-5 (1948). GIBSON,K. S., AND BALCOM,M. M., "Transmission Measurements with the Bcckman Quartz Spectrophotometer," J . Opt. Soc. Amer., 37, 5 9 3 4 0 8 (1947). GOLDRING, L. S., HAWE%R. C., HARE, G. H., BECKMAN, A. O . , A N D STICKNEY, M . E., "Anomalies in Extinction Coefficient Measurements," Anal. Chem., 25, 869-78 (1953). HARDY,A. C., "Illuminating and Viewing Condition8 for Spectrophotom~tryand Colorimetry," J . Opt. Soc. Amer., 35, 289 (1945). MII,LER,W.C., HARE,G., STRAIN,D. C., GEOROE,K. P., STICKNEY,M. E., AND BECKMAS, A. O., "A New Spoctrophotometcr Employing a Glass Fery Prinm," ibid., 39, 377-86 (1949). ROYER,G. L., LAWRENCE, H. C., KODAMA, S. P., A N D Warren, C. W., "Manual and Continuous Recording Attachments far the Bcckman Model D U Spectrophot o m ~ t e r , ".4nal. Chem., 27,501-6 (1055). Woon, R. W., "Improved DiRraction Grating8 and Rcplicns," J. Opt. Soc. Amer., 34,509 (1944). Nezt: Continuation of the survey of commcrrial speetrophotomelers.
