Chemical Instrumentation S. Z. LEWIN,
N e w York Universiw, Washington Square, New York 3, N.
This series of articles presents a survey of the basic principles, characteristics and limitations of those instruments which find important opplications i n chemical work. The emphasis is on commercially available equipment, and approximate prices are quoted to show the order qf magnitude of cost of the various types of design and consbruction.
9. Spectrophotometers Applied Physics Corp. The Cary spectrophotometers manufactured by the Applied Physics Corporation, Pasadena, California, are among the finest instruments of this class currently available. Two basic spedrophotometer designs are produced: one is a twochannel, two-detector instrument with the light beam simultaneously illuminating both channels; the other is a two-channel, single-detector instrument with the light beam alternating between the channels. The optical plan of the Cary Model 11 spectrophotometer (about $8000) is shomm in Figure 37. Light from the source, DOUBLE
MONOCHROMATOR
LIGHT SOURCE
Figure 37. Optical schematic of the Cary Model 11 rpectrophotometer. The two halves of the monochromator ore side-by-side in the horizonfd plane; the two detecton are one below the dher in the vertical plane. which is either a. tungsten or hydrogen lamp in the standard instrument, is chopped a t GO cps to produce an alternsting output signal, and enters the double monochromatar. This contains two 30' Littroa type quartz prisms, so that the light actually undergoes four dispersionstwo in each prism. A slit (H, in the figure) with a 1.5 mm wide opening separates the two halves of the monochromator, so that only a small portion of the spectrum produced by the first prism is allowed to enter the chamber containing
feature
Y.
,,onclu,ed,
the second prism. This drastically reduces the amount of stray radiation that can reach the detector, and is one of the most important advantages of a double monochromator system. The entrance and exit slits of the double manochrcmator (D and L, in the figure) are curved, and are driven bilaterally through a cam and gear train. The slit height is 2.0 em. Thelight beam emergingfrom thedouble monachromator is split into two approximately equal beams by the beam-splitter (Af, in the figure; cf. Figure 34). These beams are directed by mirrors into the sample and reference chambers respectively, rtnd thence to the sample and reference phototubes. Thus, each phototube produces an aac signal, the amplitude of which is proportional to the light intensity leaving the cell being monitored. The phototubes are 1P28 phatomultipliers, matched over the range 185 to 800 mp; other ranges are available on special order. The photometric system operates on the null balance principle. A fraction of the reference phototube signal, which can be adjusted by the operator by means of a slidewire control, is balanced against the sample phototube signal. Any unbalance then remaining is amplified and causes a pen motor to drive the slidewire oontactor toward the belance point. The movement of the oen is therefore oranor-
The magnitude of the reference signal can be adjusted electrically. I h r i n g a. run, the reference signal is kept constant over the entire wavelength interval by means of a slit servomechanism, which causes the slits to open or close as the reference signal wanes or waxes. This automatic arrangement allows variations in the slit width to compensate far the variations in source emissivity and phatotube sensitivity which occur with change in the wavelength of the light. Since two phototubes are employed in this type of instrument design, it is inevitable that they will differ in response a t same wavelengths, even if they are matched exactly a t others. In addition, there are generally differenecs in the two optical paths; e.g., the mirrors in one channel may not have exactly the same reflectivity st all wavelengths as those in the other channel, some of the optical surVolume
faces may have picked up greater amounts of dust or grease than others, corrosion of the reflecting surfaces may differ from mirror to mirror. Therefore, if the reference and sample phototube signals are exactly balanced a t one navelength with the same (or, no) absorber in both paths, they may become unbalanced a t other wavelengths. In the Cary Model 11, a set of 44 adjustable controls is provided, so that the reference and sample signals can be adjusted to equality a t 44 different wavelengths spread throughout the spectral range. This "multipot" (multiple potentiometer) arrangement permits the operator t o achieve and maintain a flat basdine under all operating conditions. The wavelength mechanism is driven hy a synchronous motor; by means of gears, several different scanning speeds are available, ranging from 0.1 to 12.5 ma per see. A mechanical coupling between the motor rtnd the prism tables programs the rate of rotation in accordance with the varying dispersive power of the quartz in order to yield s. linear wavelength scale. Wavelength reproducibility ranges from &0.05 mp a t 210 mp to rt0.3 a t 800 mp. Wavelength accuracy is +0.5 to 1.0 mp over the samo range. Photometric r e producibility is 0.01 or better for ahsorbances between 0 and 2 (+2%T a t lWY0T to &0.02%T a t 1Y0T) in routine use. Because of the low level of stray light, and the good signal-to-noise ratio, useftd results can he obtained a t absorhances as great 8 8 3 (%T as small as 0.1 %). Resolution ranges from 2.0 to 0.1 mp with the hydrogen arc in the ultraviolet region, and from 1.0 to 0.1 mrr with the tungsten lamp in the visible region. The other basic optical design of Cary spectrophotometers is exemplified by the optical plan of the Cary Model 14 spectrophotometer (about $12,000) shown in Figure 38. Here, the double monochromator consists of a Littrow type quitrtz prism in series with a 15,000 lines/inch echelette grating. The entrance, intermediate and exit. slits are simultaneousbadjustable in width over a range of 0 to 3.0 mm. Slit height is 2.0 cm, but a. control is ~rovidedto mask off the upper and lower portions of tho slits, reducing the height to 0.7 rm. -~~~ This is sometimes desirable when the ultimate in resolution is sought, for i t minimiees sn,v mismatch between the slit curvature and the image curvature. The light emerging from the double monachromator is chopped a t 30 cps by a sectored disc ( N , in tho figure) driven by a synchronous motor. Mounted on the same motor shaft is a semicircular mirror (0,in the figure) which causes the chopped light beam to he reflected alternately into the sample chamber (from 0 to R ) and the reference chamber (through 0, then from P to R'). The light from either path is directed by mirrors onto the same photomultiplier detector. Hence, the detector ~
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the figurc) is slid iuto thc position i,,rmcrly occupied hy tho photomultiplier dctector (X), and n mirror (d) is moved into position t o intercept the light from tho monorhromntor and direct i t t,o a lead sulfide photocondnctivc detertor ( j ) . Thus, the light passcs through the mor~ochromntor in the opposite rlircrtion to that cmployerl in tho case oi visihlc or ultrnviolct r:~rli:~-
Chemical lnstrumentlrtion sltcrnately sees the light whirh r m l c thmugh the sample and referenw chmmirls, and its output contain8 an alternation (modulation) that is proportionsl in amplit,ude to th* dilfererm in intt-nit>- in these hesrns.
-
PHOTOMULTIPLIER
1 LAMP
SAY.
I
I
I
Figure 38. Optical schemdic of the Cary Model 14 rpedrophotometer. This imtrumont is also specidly adapted for work in the near infrared (SIR) region, up to 2.6 microns. For this rengo, the positions of light source and detector are interchanged. A tungsten lamp (J-, in
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tion, i ~ the t optical relationships are othcrwiso idontical. This rovcrsd of the posit,ions of light source and detcctar in 91R work is employed to rrdocr tho cffeet (Conlinued on page Afi40)
PHOTOMETER
Chemical Instrumentation of stray radiation. Any infrared radistion emitted by the ~urfacesin the chopper compartment would be measured along with the dcsired monochromatic light if the detector were placed behind the cell compartment. However, in the inver~e arrmgement, this extraneous radiation is fieparated from the wavelength of interest in the course of passing through the monochromator. This reveraibility of the double monochromator is useful in modifying t h r instrument for special applioations. The use of a fiingle detector eliminates the prohlem of matching the eharartvristics of two individual phototubes. However, the possibility of small miamat,ches in the optics of the two channels remains, and the same "multipot" system descrihed above is emplo,ved to permit the baseline to he flattened out as desired. The snbatitution of s. g r a h g for one of t,hc prisms of the Model 11 gives the Model 14 .z higher resolving power and lower temperature coefficient. Other optical and merhsniesl charaeteristies are similar. Resolving powor is hetter than 0.1 mp in thc visible and ultraviolet, and 0.3 mp in the near infrared. Wavelengths are reproducible to 10.05 mp and accurate to &0.5 mp. Photometric accuracy is within 10.002in the 0 to 1 ahnorhanee range, and +0.005 a t an absorbance of 2. A variety of accessories is available for the Cary speetrophotometors, including
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d., 2"Figure 39.
Optical schematic of the Generol Electric Recording Spectrophotomeler.
attachments for reflectance or fluorescmco, continuous-flow analysis, automatir program rontrol, and heam condensing.
General Electric A s~ectro~hotometcr, tho photometer section of which is baaed upon a quite different optical scheme from any discussed so far, is the General Electric Recording Spectrophotometer (about $11,000), manufactured hy the General Electric Co., Lynn, Massaehnsetts. The
optical plan of this instrument is shown in Figuro39. T h i ~instrument is intended for the visible region (380 to 760 mp) and can be extended to the near infrared, hut not to the ultraviolet. A projection type lamp, 6 ", serves as the light drawing 18 amp radiation i3 dispersed tryice source. in double monochromator; the spectral handpass emerging through the exit slit is 10 ,ide, and this ,,.idti, is mailltained
,,#
(Continued on page A648)
Chemical Instrumentation consb;mt throughout the spectral rnrgc I,y suita1,lc programming of the entritnco, intrrmcdiats and exit slit nidthr (slits Sm.2 , 2, and d. in the figure). \Tamlcngth sci~nningis arcomplished by displacement of tha intrrmediste slit tahlc through bhr motim of a speeislly curved vavclcngth cam. T h r photomrtw ir hrtsed upon the p n q crties of doubly rcintrting crystals, which :are nt,ilized in the, form of Iloehon aml CVollnston prisms. T h r prinriples involved cnn I P most simply diseunseri i u terms of the diagrams of Figure 40. Wht% ordinary light is incident perpendiculitrly on a clcsvngr f m r of s calcitc crystal, it, I r c i ~ k sup into two mys inside the rryrtill. 0nrpnsst:s throngh thr rrwtnl ond~vinterl, : ~ n di~ pliu~epolarixd (in the plane p w pcndivrllar t,o tha pq,t!r, Figure 10): it is rallrxl thr d i m ray. The othrr, r.rb.rrodinaru r:ty is also plane palnrirpaPrr,iFigurr 40)
40. Iilurtrating the of polarized light by doubly refracting crystals. Figure
A. Ordinary light, incident on o calcite crystal, ond extrai i broken up inlo on ordinary (0) ordinary (El ray. 8. The Rochon prim, in which the 0 ray poser through undevioted,, and the E ray is deviated. C. The Wolloston p~irm,in which both rays suffer deviations.
: ~ n de m r r p l;\tvrally d i q h c o d iron, the ordinary ray. Since t h 0~ and E rays i ~ r o ~ ~ P ~ toI Pe:tch I othw, and t,he lateral displnct.m~:ut is small, the>- cannot he rffrrtivaly scpsratcd from each other. Honover, in the Itochon and Wollaston prisms, two picccs oi crystal are cemented togethrr with their optic nxrs perpendicular, ;tnd t.he wnrrgcnt r:tys divwge from each othr,r, pormitting ready soparittian of the two. I n the G. Is:, spertm~dmbometer, thc light issuing from tho m- mhromator first enters a Iloehon priam. The polarizrd I m m s leaving this prism are separated a t tha slit beforr the \I'ollitston prism, and (Confinired on page A844)
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Chemical Instrumentation only the 0 beam enters. This is split into two heams inside the prism. The relative intensities of these two heams is a. function of the angular position of the Roehon prism, and can be varied hy rotation of that component. The two heams leaning the Wallastan prism constitute the sample and reference beams; the relative intens i t i e ~of these heams are halanced by ratation of the Rochon prism. A rotating polarizing filter mounted hehind the Wollaston prism alternately extinguishes the two polarized beams, serving the same function as the mechanical choppers used in other spectrophotometers. The phototuhc detector is mounted a t the base of a diffusing sphere, the interior surface of which is oasted with magnesium oxide smoke. The diffusoly reflected light from a sample and a reference plaque is collected by moans of a Plexiglam rod mounted an the axis of the sphere, and this light is piped down to the detector. If there is a difference in reflectivity hetween the sample and reference plaques, the detector sees a flickering intensity and generates an sc signal. This is amplified, and serves to drive s, motor r o t a h g the Rochon prism until null balance is reached. The recorder pen is geared to the movement of the Rochon prism. For transmission messurements, the sample plaque is replaced by an MgO block, and the sample is placed a t the entrance port to the sphere. Wavelength accuracy is f1 mfi; photometric accuracy is +0.5%T. Trsnsmit-
adin&, as required for the specification of rolor according to the C.I.E. system.
Unicorn A line of instruments quite similar in fundamental design to the Beckman spectrophotometers, treated in detail earlier, is manufecturod by Unicam Instruments, Ltd., Cambridge, England (available in the U. 8. through WilkensAnderson Co., Chicago 51, Illinois).
The Model SP 500 spectrophotnmetnr ($2800) is z h o m in optical plan in Figure 41. It contains an Ultrasil (i.e., fused silica) 30" Littrow type prism as dispersing clement. The use of fused silica permits somewhat better transmission of ultraviolet than does crystal quartz; the range (Continued o n page A6461
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instrumentation of this instrument is from 18,i ,nil to 1 #.
Thc rest of the optics and &ytronics are w r y similar t o those of the Iirekrnsn Model DU. The Model SP 600 ($1020) is rlrsigned
Figure 42.
Optical schematic of the Unicom Model SP 600 gloss prism spectrophotometer
for the visible region only (360 t o 1000 mrr) and employs a glass prism, its illustrated in Figure 42. This differsfrom the Beckmsn Model I3 in employing a focusing
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7 id collimating systmm that is separatr from the dispersing dement. The spertral band width is lcss than 3 mp over the central part of t,he range, increasing t o 10 mp a t the estrmwa. Stmy light is lem than 1 7 , a t all wavelengt,hs. Other characteristics :are similar t o the Beckmm Model B
The M o d d SI' TOO (311,200) is a. danhle-beam recordingspectrophotometer, covering t h e range from 186 mp in the (Confmued on page A6481
Chemical instrumentation ultrilviolet to 3 . 6 ~in the infrared. Two interchangeable dispersing elements are employed; a fused silica prism is used for tho range 186 mp t o 2.3 p, and a grating with 7500 lines/ineh replaces the prism for the range 2.3 to 3 . 6 ~ . n3en the grating is used, unwanted ordrrs of diffraction are removed by filters, a germanium lilt& for 2.3 t,o 3 . 0 ~ ,and a lead sulfide filter for 3.0 to 3 . 6 ~ . The monochromator and cell compartments me gas-tight, and can he flushed with dry nitrogen to eliminate interferences from atmospheric constituents. The light is~uingfrom tho manochromator falls on a beam ~plitter,which sends beams to the sample and reference chambers. A aynchronous chopper rotating a t 25 cps allan.s h ~ a m nto pass dtcrnately to carh compartment. I n addition, each light hrem is chopped a t 300 cps; hence, the detretor output signal has a. frequenev of 300 eps, and oontainsa modulation a t 25 eps that is proportional to the diiTerence in light intenfiitiefi hrtwem the sample and reference beams. A three-stage negative feedback amplifier feed^ R signal proportional to tho ratio of the fiample and rcfcronre intensities to the self-halancing potentiomotcr recorder. A photomultiplier tube is employed as the detector far the ult,mviolet and visihle (to 750 mp), and a. lead sulfide cell servos over the rest of the wavdenpth range. A multipot control (cf. discussion of Cary Model 11) is used topormit flettcning out of the haseline over tho spectral range of interest. Stray light ia less than 0.57" a t 200 mw, and 1888 than 0.17, over m o ~of t the range. Photometric accuracy is 1 0 . 5 % T . Scan ia linear in frequency (wavenumbem), and wa;venumber accuracy is f100 cm-' in the ultraviolet, decreasing to +5 c m r l in the near infrared. Process and Insirurnenis
Before the advent of the nev, relatively moderstelv ~ r i c e drecordine soectrooha-
strument manufactured hy Procoss and Instruments, Brooklyn 33, Kew York. This was their Model R 8 3 recording spectropbotomrter, which eonristed of a unit to he attached to the Beekman Model nu monoehromittor, thereby converbi"g t,hat manual instrument into an automatx recording one (price, $6700, monochromator and light source8 not included). This device contained a 1,eeds and Korthrnp Speedomax G recorder, n motor-driven esm mechanism for producing a linear rate of wavelength scanning, a slit control program far automatically adjusting the slit width as the wavelength varied, and s. cell holder that rapidly alternated the sample and referonce cells before the detector. A photomultiplier detector was substituted for the photatuhes upp plied with the manual instrument.
(Continued on page A660)
Chemical Instrumentation This company is now msnrdnctoring n complete spectrophotometer, in 8. marluill version (Model MOS-5, nhont $3600) and u d h necessoriea to convert it to an nutomatic recording inst,rument (t,otnl cost, ahout $5000). I t is a douhle-henm, single detector design, wit11 no ixnm split,t,ing, hut with n motor-driven mirror system t o send the light l,rilm :~lt.ernnt~rly through ~nmplt:and rr:fcrenee cells.
A high q d i t y grsting spectrophatometer is av:~ibhle from Optien, Inc., U'sshington 18, 1). C. This is produead as eithcr a manual, single-beam instrument (Model CF4, Single-Ream Mnnunl, Range 185-1000 mp, $4Xi5), or s dmhlebeam, recardingunit (.Model CF4, DoulrlcBeam Recording, 185 mp to 4 p, $10,700). The dispersing element is a plane dilbfrsetion grating with Littrow mirror backing, ruled with 15,000 linrajineh, for the rmpe 185-1000 mp, or 7500 lineajinch for thc range 1 to 4 p. The wavelength scnlo is very close t o linear, nfi is the dispersion. Since a grating is used, the spectral image is straight (not curved, ns is the ease with prism in~t,rommts)and straight slits can he used. The spertral range is not limitcrl by the airsorption characteristics of n prism, but by the emisaivity of tho l i m p and the sensitivity of the det,vetor. 8tmy light is 2% a t 190 mp and ixgligil,la abavc 210 mu. Interference of i l i d ~ s r order
Figure 43. Simplified schematic of the meoruring circuit employed in the Opllm Model CF4 single beam mmud spectrophotometer. The measuring circuit i~ a 1,nlmced amplifier bridge, as illusbrated in Figure 43. The detector is a IF28 photomultiplier tube for 185 t o 750 mp. A rcdsensitive photomultiplier is avsilahlr far the range 600 t o 1000 mp, and n lead sulfide cell is used beyond lp. I n the double-beam instrument the light, path is alternated between the sample and (Conlinxed on page A663)
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Chemical Instrumentation reference cells by means of a pair of ratating mirrors. A single detector views the beams, and generates a signal that is amplified and recorded as the ratio of the two intensities. Photometric accuracy is better than &0.15%T with the manual instrument, +0.2%T with the recording unit. Wavelength reproducibility is &0.05 mp over the entire range. The band widths obtainilhle with the single-beam instrument under various conditions are shown in Figure 44.
Band wldth
mu 2.0
Other Instruments
In conclusion, mention should be made of the availability of several other csmplete speetrophotometers, as well as of the component parts of such instruments, from which the user can build his own equipment. The fact that these instruments are not discussed as fully as some of the preceding units is not to he taken as an implication of poor design or performance. It is rather a oonsequence of space limitations, and the fact that the basic principles of their design have already been fully covered. Leeds and Northrup Co., Philadelphia 44, Pennsylvania, manufacture a singlebeam, recording spectrophotometer utilizing a 30,000 lines/inch diffraction grating s s the dispersing element. I t is specifically designed far the rapid scanning of emission spectral line intensities in flames,
Figure 44. Spectral band width obtainable with the Optico Model CF4 monochrommor as a function of the detector and the wavelength.
ares, etc., over the range 210 to 700 mp with a resolution of 0.5 mp a t 300 mp. A similar instrument, manufactured by Land-Air, Inc., San Leandro, California, covers the range 300 to 1150 mp, and has a sensitivity of 0.5 microwatts far full scale deflection when the slits are set for 10 mp bandpess. Small, limitedresolutionspeetrophatometers for the near ultraviolet and visihle regions (350-750 mp) are made by Jouan, Paris, France (Junior Model, $616 a t port of entry) and by Lange, Berlin, Germany
(availi~blethrough Epic., Inc., New York 38, New York). The Uvicord spectrophotometer, imported from Sweden, is available from LKB Instruments, Ine., Ta~ashington14, D. C. A double-beam microspeotrophotometor for quantitative analyses of solutions as small as 0.015 ml in volume and as low in concentration as fractional parts per million, is made by Jarrell-Ash Co., Iiewtonville, Massachusetts. A 30,000 lines/inch grating is used; bath beams alternate in (Continued a page A6541
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Chemical Instrumentation falling on the same photomultiplier detector. A variety of excellent monochromators is available, and we can do no mare than list the manufacturers here. Vacuum lillravzolet Monoch~omators: Baird-Atomic, Inc., Cambridge 38, Massachusetts; Paul R1. McPherson, Acton, Massachusetts: Jarrcll-Ash Co., Newtanville 60, Mass. Double P ~ i s r nMonoch7omators: Kipp and Zonen, Delit, Holland (available in tho U. S. through James G. Biddle Co., Philndelphin i , Pennsylvxnis); Farrand Optical Co., New Yark 70, N. Y.; Photovolt Carp., Kew York 16, N. Y.; Hilger and Watta, Inc., Chicago 5, Illinois (c/o Engis Equipment Co.). Gratinq A?'onochromalors: Farrand Optical Co.; Hilger and Watts, Inc.; Bauseh and Lomh Optical Co., Rochester, New York. PolwizingiPlale Qzmrtz Monochmmator: Cambridge Thermionic Corp., Cambridge 38, Massachusetts.
Bibliography GIBSON,K. S., and KEEGAN,H. J., "Calibration and Operation oi the General Electric Recording Spectrophotometer of the National Bureau of Standards," J. Opt. Soc, Amer., 28, 372 (1938). GILLAM,A. E., and STERN,E . S., "An Introduction to Electronic Absorption Spectroscopy in Organic Chemistry,"
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E. Arnold. Ltd.. London. 2nd Ed.. ' 1957, chap: 2. GILLIAM, LOTHIAN, MORTOX, imd COOPER, "Spectraphotometer Terms and Symbols," Annlysf, 67, 164 (1912). HARDY, A. C., ' 8 H i ~ t o rand y Design oi thc Recording Spectrophotometer," J . Opt. Soe. Amw., 28, 360 (1938).
