Topics in..
.
Chemical lrrstrumentation feature
Edited b y GALEN W. EWING, Seton Holl University, So. O r a n g e , N . J. 07079
A novel featwe (see below) is (lie These articles, most of which are invited contributions by guest authors, are intended to serve the readers of THIS JOURNAL by calling attention to new developments i n the theory, derign, or availability of chemical laboratmy instrumentation, or by presenting useful insights and explanations of topics that are of practical importance to those who use, or teach the use of, modern instrumentation and instrumental techniques.
XXXVI. Recent Instrumentation for UV-Visible Spectrophotometry (Concluded) Part 2: Single-Beam Spectrophotometers (continued) PETER F. LOTT, Chemistry Deportment, University o f Missouriot Kansas City, Kansos City, Missouri 647 70 Coleman Instruments Corporation Division o f t h e Perkin-Elmer Corporation
Four new single beam spectvophotomelers are offered. The least expensive, the Coleman Junior I1 instrnment (Fig. 31)
I
! Figure 3 1 . tometer.
Coleman
Junior
I1
cost of the Coleman Junior I1 is $5!13; bhe Vacnette attachment may also he used with cevtain other Coleman instruments. The Model 101 spectraphnt,mnetel. is a more sophist,ieated instrument, which covers the region from 230 t,o 900 ma. A sit& gmting i h s e d nt 300 mp is employed in the munochromat,or and fixed slils provide a rowtant bsndpass of 10 mr. A tmgst,en lamp covers the region from 34lk!l0tl mp, and a deuteri,m lamp the region fl.am 220 t o 375 mp.
use
of a single phot,odeleetrr~.t,o cover the whole wavelength range and t,a elimi~iate the ineonvonience of shifting phototubes. A four-pusition holder will accept cells from 5 to 20 mm path length. The cost of the UV-visible model 101 is ahout $1675; the instnunent is pictured it) Figure 32. Coleman Instruments also offew the Hitachi Perkiu-Elmer model 139 spectrophotometer. This is a direct reading instrument covering the region from I95 to $1,511 mp with a wavelength acervacy of i O . 5 mp. Tho m o ~ ~ o e h m m a t oemploys r a grating as the dispersiq element and has adjustable slits. Modular constnlct,ion is employed which permit,s great flexibiMy and speed in mount,ing accessory nnits. Accessories olTered include a flame photomet,er, at,omic absorption writ,, fluorophotometer, refleet,anee nuit, spectral elrwgy tworder, Lime drive recorder, and varioru t,ypes of sample cells. The photometer employs the same Ilitaehi dilal-rsrrge phototube as the model 101.
MODEL 101 OPTICAL DIAGRAM
Spectropho-
is an improved ver-iiw of the earlier Junior model. The i1l4l.lrrnent covers the wavelength region fmrn 32.7 ta 825 mr with n chuiee of eithw 20 or 35 m r hartdpass. Tho iwtmment is designed for ~ m ~ t i nnnnlytiral e labm.atary work. Featured is a jack which permits the instrnment t,o be directly connected t,o a recorder, digital mad-out unit, or printer. An accessory which has been recently introduced By C o l e m n ~is their "Vsruette" nssembly. This at,tachment includes a spectrophotomet,er cell which is rdained in the instrument for all samples. T h e sample is pomed int,o the e w e t which has a fnnnel t,op. After the sample has been read, a t,oueh on the vacuum valve removes t,he semple in 2 4 seconds and, according to the manufacturer, flushing and rinsing betweell samples is rarely required. Two erlvet sizes are offered: the 1.75 ml cell has a l em light path and the .5..5 ml cell has a 1.7 em light path. Using either cell and the Vacuette assembly, samples can he analyzed a t the rate of hundreds per Iluur. The basic
@ LIGHT SOURCES
@GRATING
@SOURCE MIRROR @ DICPHRWM @ MRROR @ EXIT SLIT @ SLIT LENS @ FILTERS @ ENTRWCE SLIT ABSORPTIW CELL CONCAVE MIRROR
PHOrOTUEE
Figure 33. Optic01 diogram ond photograph of Hitachi Perkin-Elmer Model 1 3 9 Spectrophdometer.
Figure 3 2 . Optical diogrom and photograph of Colemon M o d e l 1 0 1 UV-Visible Spectrophatometer.
T h e phototube has t w o light detectors built into t,he same envelope and coveys the region from 105 to !15O mp. As l h r wavelength crosses the region from the visible to the neat. iufralwl, WE delwtor gradmlly takes over fuxn the other. For grenler detectio~t sensitivity, as is needcd fol. atomic nhsolptioll and f l ~ m e phntomct~irwwk fat. example, a photomultiplier accessory can be added to the
(Continued on page A8741
Volume 45, Number 4, April 1968
/ A273
Chemical instrumentation instrument to replace the co~iventiold p h o t o t ~ h e . A p h ~ t ~ g m p and h opliral dingram of the model 1 3 ) is shown in Figt11.e 33; the cost of the basic i ~ ~ s t , r u ment is $3100. Durrum Instrument Corporation Unique among single beam spectrophotometers is the Uurrum StoppedFlow spectrophotometer. This instrument is designed t o rapidly mix t.wo solotians (99.6% complete mixing in 0.002 second) and to measure the chsnge in the absorption of light by their reaction ~ r o d n c as t a furretion of time. Reactions having a half-time of O.0OR secvnd cat1 he followed. Because of the speed a t which measurement,^ are made, the output of the inst,rument is displayed on an oscilloscope (wibh storage t,ube) for visl~al observatiou or photographic recording. The stopped-flaw mixing chamber in the instrument eorrespunds to the cell eompartment in s, canvent~iondspeetmphot,ometer and requires only 0.2 ml of each component per determinst.ion. The system is pictured in Figwes 34 and 36. 111 operating t,he instn~rnent,the investigator first sets the monochromator t,o the wavelength corresponding to t,he maximum absorption for the desired compound and adjusts the bandwidth accordingly. He fills two reservoir syringes with the twu reaction components. These fill, in turn, two drive syringes. With the flow
A274 / Journal of Chemical Education
Chemical Instrumentation energy while relainitig x immow bandwidllr, since a. pl.irm has bette~.dispersion tlmn a grabing for shwt wavelength radiation. The grating is employed in the visible and near-IR region where narrow bandwidths are reqoi~.edfar precise measrwemet~ts. A typical tmcit~gisnhow~t ill Figure 36. The cost of the initnunent is about ~10,80lI.
Figure 35. The Durrum Stopped-Flow trophotometei.
Spec-
Figure 36. Typical t r o c e with Durrurn StoppedFlow Spectrophotorneter. Verlicol axis ir 0.1 voltldivinion with 100% ttranrrnittonce ot top of scale and 0% at bottom. Horizontal rcole ir 0.2 second per divi5ion.
Golden Instruments, Inc.
The Spektrokem sperbsophotometer (Figtwe 4 i ) c w e w the regiw fwm 40l1 to 7.711 mp with s baudwidth of 15 mp. Thir t,ra~csmissimgmtitlg i$!slt.nment is \miqr~e iu that rather that! haviug the sample i u x fixed posilio~land rotatiug the gmting to isolate different wavelengths of light, the cuvet, exit slit, and photoeouduetive delectov are moved lateldly to iutereept. the desil.ed prwtiorr of the spertrnm, n patented feat\~re. In wder to stabilize the inst~wmenlagaiwst liue voltage flnctrlntims, a prwtion of the e l w r c ~ ~ist fed both to the light srxwce and the photocondnctive cell i u wr:h a manner that flltot,~mtiorw i u the voltage 10 thc light s o w e e al.eeompensated by the detect"?. Thephotoean-
Figure 37. Golden Inrtrummtr Spe~trophotomet~r.
A276
/
Journol of Chemical Educafion
Speitrokem
Chemical Instrumentation dltclive eell hns a li~~enrrespouse a,that the ( : L I I . I ~Rowing ~ t o the meter is diwetly propa~.tianalto the light impinging on the cell. The c n ~ w n is t snffiriently largo t,o pet.mil, t,he use of a. milliammeter rather than a galvanometer. A flow cell, which employs a peristaltic pump, is ollered as an optional accessory. The instrtme~tt costs about W30 arid with a flow eell system about
53M. Gilford Instrument Laboratories, Inc. The Gilford model 20110 and hadel 240 spectmphotometers Figwes :3R and 39 are similar in that they incotpomle the Gilford rhotometer w i t fos both "rerise absorbance measurements and aulomatic seqrtential sampling. The electronic com-
.~
~~
Figure
~~
38.
Gilford
Model
240
Spectro-
photometer.
Figure 39. Gilford Model 2000 Automotk Spectrophotometer.
ponent,s can also he adapted t o other manachromators such as certain Beckman, Zeiss, or Unicam instruments to convert them into direct reading spectrophatometers for fixed wavelength nbsorbance measnrements. The photometer circuit covers the range fmm 0 t,o 3 absorbance units and differs from other circuits by having its output in a direct logarithmic relationship to the radiant energy striking the photoinbe. Since Beer's Law is also x logarit,hmir relationship, the output of the photodetector circuit is thus pmport,ianal to the concent,ration of the absorbing m e t e d . The photometer system incurpot~atesa 1P2R photomultiplier tube in a special feedback circuit which adjusts the voltage applied to the dymdes of t,he tnbe inve~.sely to t,he amount of light falling on the cathode. Th118, for high light levels the sensitivity is decreased while with low light levels the sensitivity is increased. The system is designed so that these changes occur notomatieally and instantaneously which permits operation of the phototube a t extremely low anode ontpot currents regardless of the amount of light impinging on the photocat,hode. (Canlinued on page A882)
A280 / Journal o f Chemical Education
Chemical Instrumentation
Log
A
swhnnre units is reached without entering ~ n the t ~ region wI1el.e the dark c u t ~ e n tis significant. Fot~thot~more,the photo-
(ABSORBANCE)
I0 Figure 40. circuit.
Output voltage renus obwrbonce for a I P 2 8 phototube in the Gilford photometer
This makes i t unnecessary to protect the pho(ocel1 with the rttstomary shutter when the ctwet compartment is opened. Similarly, hecsuse the photometel. circuit has an inherently constant gain throughout its t.ange, it is unnecessary to employ a separate gain or serrsitivity control. Nor is a dark n w t w ~ t balance required since the system does not use the complete ahsetire of light as one of its refel.ence point.;, and the full range uf three ab-
A282
/
Journal of Chemicol Education
metric accuracy of the circuit employed does not depend on the use of high vnllle load resisturs, avoiding the instabilities and attendant need for humidity control with desiccants. Since the feedhack signal which contyds the p h o t o t h e sensitivity originntes from the anode, a n essentia,lly consl,ant anode cmren! is maintained that is immune to power fluctoatious and fatiglle (Conlinialon page A8861
Chemical Instrumentation in the phototube and other components. The output of this constant anode current system is derived from the changing dynode voltage throng11 suitable voltage dividers. Utilization of the dynode voltage as the source of the outprrt signal presents two advantages: (1) Large signals are available for recording without the need for additional amplificntion. (2) The relationship between the radiant light energy entering the phototuhe and the voltage is essentially logarithmic over a. wide range, ss shown in Figure 40, so that in addition to spanuing a range of 3 absorbance units, each 0.1 portion of the absorbance range can be expanded t o full scale on the recordel.. T h e photometer system mhy be w e d for conve~ltiond m a n u d measurements; i t may also be employed in conjunction with the automatic sampling positioner. This unit will automatically position in torn up t o four standard speetrophotometer cells or four flow cells into the light path in a contimring sequence and recofd the changes in absorbance which occur over a period of time, a feature of great interest in studying reaction kinetics. T h e cost of the illustrated n~adel20110 system is about $7300. The model 240 monochmmstor covers the wavelength region irom 185 to 1000 mw and is also designed to function with ihe sequential positionine svatem. Additional accessories " such ss thermostatted cell compartments, flow cells, etc. are available. ~
~~~~~~
-
Figure 41. Gilford Model Spedr~~hotometer.
300 Micro-Svmple
T h e Model 300 spectrophuborneter is desigued for rapid analysis of a series of samples of similar composition. The inatlxment covers the regiou from 340 to 700 mp and features a solid state photometer, x rapid sampling system, and automatic digilal absorhnnce ].end-out. The rapid sampling system, which is offered as an accessory, permits tho analysis of over 10It samples per hu\lr. The cell (which reqni1.e~ a half-ml sample) is filled, {.he absorbance is measured, and the cell is flushed hy 8, v n c w m system operated by a single control level. This instrumeut costs ahout 81700 and is pictwed in Figure 41.
The Heath Co. The Heath Co. is introdociug a series of optical instruments employing a number of modular m i t s built about their recently introduced model EU-700 mono(Catinzed on page Ad86)
~~~
. . ~..~ ~ ~ ~ . . ~ . . .
Volume 45, Number 4, April 1968
/
A285
Chemical Instrumentation chmmator. The monochromator employs 8. C s e ~ . q - T r u t ~ emunvting r with diagonal mil.nm 10 pmvide enll.auce and exit beams on a common c~pt,ieal&xis. Using a 3 mm slit, the remlving power of this grating monochromator is stated to be betiel. than 0.1 mp aud the line pt.ofile half-width less thau (1.03 mp. The wavele~lgthrauge of the mwruchromat~rcan be extended to bekw 180 mp by nitrogen flushingand the upper limit is 1 p. However, with the standard 1P2XA photomultipliel. detector Rwnished by Ileath, the upper limit is restl.ieted to 700 mp. The monochromator employa a ganged bilateral exit and e l ~ t r m c eslit which is symmetric to the optical axis. The slit width is continnously vaviable f w m 5 to 2O(lfl microns and can he read directly in microns h y means of a 4 digit cowter. Provisbms are alw made for font. different slit heights which vmge fwm 0.: t o 12 mm. The wavelength is read dil.ect1.y on a 5
Figure 42. Photograph of Heoth Co. beom rpectrophotometer.
digit cowlel. which has a fr.nel imal scale of 0.02 mp per divisirm to pel.mit waveleltgth readabilitv of 0.01 mu. A n electronic
supplies pulses to a sieppirjg motor which drives a preci~ion lead screw to give scanning speeds which va1.y from (1.005 tu 2.0 mp per senmd in niue steps. These pulses also contlal the dl.ive of the chart, recorder motor to pernit exact synrhlmniaation of the wavelength drive with the chart. I n addition, rapid slewing can be done in either directiorc at appro xi mat el,^ 540 mp per minllle. For single beam spect~nphotometric measurements, a light sourre module (EU701-50) is attached to one end of the monochromator. A cell compwtment madnle (EU-70-11) and a photom\dtiplier module (EU-701-30) are attached to the other end of the munochromalor. The outpnt from the photomultipliet. is then presented on a Heath strip chart 1.eeorde1. (EUW-20M). The cost of the monoch~~omatar alone ir $119.5; the cost of the cumpletesiltgle beam speetlophotometer showu i l l Fig~tre 42 would be nppnmimatcly I I . I t is anticipated that an addi(iona1 module will he intmdmed in !he riear. 1ul1rt.ewhirh will permit conventinns1 double beam renrrciing. McKee-Pedersen Instruments M P I also offers a modular npplrmsh for spect~.ophotometlieit~st~.ttme~tlntic,t~ whirh is built alonnd the use of the 1IP system 1000. This system is essellt idly all malng compiltel. collsirting of :I gntllp of vpel.;l(Conlimred on page A2g0)
A286
/
lournol o f Chemical Educafion
Chemical Instrumentation tiw;rl nmplifiers. The s i g d is p m e n t e d in this case I,y the spect~~~,ph,,lo~net~er and the ~.eadmttis presented cm a ~.ecol.del.. The instl.~,mentis pirttwed in Figure 43. The mowrrhram,ztrn employs a. ,500 l i w pet. mm grntirig which is hlaaed nt 400 mp and pwvides n wnvelel+$h r m g e fl.mn 200 mp to 1 3 p with au neciwacy of about + I mp. Tlawever, the icse of the i w t l . ~ m e n t for speain,photometry i.; cwt.ently limited t q the visihle and near-IIL vegirm ~wlessthe ~ l s e rran s~tpplya deulevi~msowce. The slit wicllll i s : ~ d j ~ w t d ~ l ehoth n ~ dslit widths m d wavelenglll~a1.e read fwm t u n indiFigure 43. MP-Sy5tem 1000 Recording Spectrophotometer. rntiug dial.;. Siugle ordwlhle bemn operatime is pcwihlc Ih~.r,~tgh the Itre of s pair Shimadzu of matched phototnhei. In addiliw the 1)U iustl.unen1. The instl.tment employs The Model Q V 50 (Figwe 44) spectmrendrnlt (in trmsmittnnce rn s h s w h a ~ ~ c e a Lit t w w mounted prism mrmochromator units) may nlw be drme h y manna1 1b1111 plratometer 110th in e r l e t x d nppeamnce and a null halnnce potentiumetrie system and intel.~lnloptical design, appears to he hnlnuci~~g rn digitally. By w e of approf w 1,eediug either ahsorbanee or. Il.ansmitpriate ope~.ntiowdimpliliel. ru~>tiertir,~ia, qnite similar to the well-knowti Beckman tawe. An o~ltlet f u r connecting an exderivative or integrated s11et.tt.n r s t i hc t e n d ~wmlrlet.is provided. Although the ~.et.o~.detl nod mensitled. monorh~.omatal.wavelength dial is ealiThe rosl ,,f the simple 111111 l~alanre hmted Rom 180 to 2000 mp, the usable ~ l ~ c t ~ ~ ~ , g l ~ ~is, l nhuul ~ , n ~ cW2150. t c ~ ~ Fw ~mcgef o r the imtrnment ir ftwn 183 to n l ~ a $4400 ~ ~ t tllc 1 1 5 ~ 1 call . ~ I I I . L ~ ~ Ii11611.11~JC 1200 mu. A sernitmnsiatoriaed auwer sunrne~italion wlricll will e~tahlehim lo pel.f < ~ t . mall spet.ti. the ALP-syslern 1000. T h e povatea a mcvctwy lamp for wavelength system is vcyy flexi1,lr :md individual wlits cdibmtion. The hasic instrument cwts h ~ :LS~ nds.znrple ~ rr,mparImrllt
