Topics in..
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Chemical Instrumentation Edited by GALEN W. EWING, Seton Hall University, So. O r a n g e , N . J. 07079
These articles, most of which are invited contributions
feature
k ouest
instrumentation, or by presentiw useful insights a n d e&lanations of topic; that are of practical importance lo those who use, or teach the use of, modern instrumenintion and instrumental techniques.
XLI. Reflectometers, Calorimeters, and Reflectance Attachments-Part
Dr. Wesley W. Wendlandt was
One
om on Youember 20, 1927, in Gales-
WESLEY W. WENDLANDT, Deportment o f Chemistry University of Houston, Houston, Texas 7 7 0 0 4
INTRODUCTION Perhaps nnfamiliar to the average ehemist is the spectroscopic technique called diffuse reflect,ance spectrosa,py. I n this technique, the radiation reflected from the surface of n sample is debeeted and r e corded a q a function of wavelength or wavenumber. The technique, contrary t,o tl.ansmittance measurements, may he applied to powdered solids, solids, liquids, or pastes. Iliffuse reflectance measurements of a sample are obtained by ure of a reflectometer, spectroreflectometer, or colorimeter. A reflectometer is usually considered to he an instrument which employs a. series of filters to obtain approximately monochromatic radiation; the spectroreflectometer is a reflectometer used in conjunction with a grating or prism manochromator; and lastly, the colorimeter is a filter type reflectometer hut only three filters are employed, those for the C.I.E. color coordinatevalues of X, Y, and Z. The terms are used rather loosely in the literature and hence, there is some confusion as to their exact definitions. Diffuse reflectance measurements can easily be made in the laboratory because reflectance attachments are mail.ilahle for practically all of the well known eommercis1 spectrophotometers. These attachments are basically of three types, as illustrated in Figure 1. I n (a), the integrating sphere type is illustrated. Monochromatic radiation enters thezphere through a side aperture, strikes the sample, the reflected radiation is collected by the sphere; and then detected h y a photo-detector. This is by far the most widely used type of reflectance attachment. I n ( b ) , the r e fleeted radiation from the sample is collected by a n annular, ellipsoidal mirror and then detected by the photo-detector. A
'%ample ~ntegratingsphere
(4 Figure 1 . Types of reflectance onachmentr. fa) Integrating sphere type. (bj Annulor, ellipsoidal mirror type. (cl Reflection type.
simple arrangement is shown in (c) where the reflected radiation is detected by a photo-detector withnut any t,ype of collector device being used. No attempt will be made here to discuss the spectrophotometers which artre employed with the reflectance attachments. These instruments have previously been disousved by Lott (1, 2 ) in this series. Only the att,achments, the specific refle*
ilk, Wisconsin. He attcnded and radnated from high school in LaIrasse, Wisconsin, in 1!145; he re:ived the B.S. degree in chemistry and lathemittics a t Wisconsin State Uniersit,y (River Falls) in 1950; and is M.S. and Ph.1). degrees in inoranic and analytical chemistry from le Stale University of Iowa in 1952 nd 1954, respectively. From 1954 to 1966 he was on the ~culbya t Texas Technological College. n 1966, he became Chairman and 'rofessor of Chemistry a t the Uniersity of Houston. The summers of 954 and 1955 were spent a t the rgonne National Laboratory and he as a Visiting Professor st New Mexico lighlends University during the sumler of 1961. He has published 163 research papers, 0 book chapters, and 5 hooks. His ?search interests are in thermoanlytical techniques, solid-state reacons of courdinntion eampoonds, reectance spect,n,scopy, phutuchemistry, nd the thermal pmpert~esof coordinaon eornpoonds. tometer, or colorimeter, will be described here. More and more interest is heing shown in the technique of diflnse i.eflect,ance spectroscopy. Several books have appeared on the subject (3, 4 ) as well as a recent symposium (5). With the wide availabilit,~of the instrumentation, it is expected t,hnt the teehniqne will find even greater uses in the future.
REFLECTANCE ATTACHMENTS Bausch and 1,ornb. Two reflectance attachments are available for Bausch and I n m h spectrophotometers. Far the Spectvonic 505 spectraphotometer, the sttachment shown in Figure 2 is employed for reflectance measurements. I t consists of a 6 in. in diameter cayt aluminum sphere,
(Continued on page A862)
Volume 45, Number 1 1, November 1968
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Chemical Instrumentation
Figure 2. R e f k c l a n c e ottochmenl for B o u ~ c hond Lomb Spectranic 505 rpectrophotorneter.
runled O H 1,heinlerirw with bariiun sulfnle and eq~~ippcrl with an clid-on winrlow E l l 1 W Y i photomnlliplie~~ittbo, sample and reference holders, and R letis syslem. llot~aalrt,rn.ztie~zllinbionis fucnserl by i.he lens syatcrn onto the sample and refere~rco mat,crinl swfnces. Tho inridenb radinbiou ilh~~niuation is rirculsr in area and ahout, 10 nun i l l diametct.. The specular component of the rcHccted vadiatim may be rejrrted by using light i m p s located nt an allgle af 00' t o the sample and reference malwials. A low s h y - l i g h t level of less t h m O.l'i;, or bet,let., is claimed, with a usnhle wave1engi.h range of 220 t o GTrO mw, or, 220 t o 700 mp wilh s. special phoL~~multiplier lobe and cu-oli' filler.
3 . Spectronic color ~ n ~ i y z for e r Baurch and Lornb Specfronic 20 calorimeter-rpectsoFiguie
photometer.
A romewl~ailower accuracy syslern, Illc Specllnnic 211 Color Analyzer, is availnhle for rwc with the Spectrolcic 20 eo1ot.i~neter-spectraphotometer. The attachment, ns shah-11in Figure 3, eorrsisls of a s m d integrating sphere, n m i ~ m and r lens system, shutter, and s phoiomoltiplie~~ inbe. The sample andlor reference material is rnomilod on the top, athe her t,hnn a t the side of blre sphere. A ~.octnngulnr spui. of incident mdii~tion,2 X X mm in ares, strikes the sample or reference SIIF iacc. Fired slits provide n banbpass width of 20 rnp from 400 t,o 700 mp. Conversion of percent reHeetance vnlues to the C.I.E. Irichromatie S,Y, and Z values is accomplished by n chart snpplied with the at,tnchment. Mcck,nan. Perhaps the mosl widely used attachments for reHcctancc studies are thosefor Beckmall speclrophotorneters. A t the present time, faur sbtschments are zvailnble.; they are for the Models UU, B, IIB, nud IIK-1A and IIK-2A spcctrophat,omet,ers, rrrpectively. 11)Ihc Alodel B reHect,anee nlinchmenl, rnonaeh~~omatic mdiatiun euiers the inte(Conlmmrd on pa!,? A8651
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Figure 4. Reflectonce attachment for the Beckman DB rpectrophotometer (shown mounted on the r p e c t r ~ ~ h o t o m e t e r l .
the area t o X V8in. if so desirod, for larger, less homogeneons samples. The reflcctanee attachment may bo used for solid, powder, or liqnid snmples in the wavelength range of 323 t o LOO0 mp. The most widely used reflectance att,achmen1 is lhnt designed for t h e Aludel 1)U manual spect~~aphotomeler.The attachment has beeu emnlovod for a wide varietv
perhaps not as widely wed a t the present time. The attachmelit consists of a single metal castine containing an elli~soidal mil.r,,r uplicd syst,em a n d a sample~.eferencedrawer. IL is attached t o the sncet,ronhotomete~~ bv asimnlo interchanec
Figure 5. Reflectonce attachment for Beckman Models DK-I A ond DK-?A spectrophotometers. Schematic diogrmm. ib) Attached to the Model DK-2A rpectrophotometer.
grating sphere thmngh a conical tubo alld is reflected t o the surface of the s'ample or refeseuce material by n plane mil.t.or set a! a 4,5" angle. Reflected radiation from the fiample or reference material, between 0" and 23" to the normal, ~.et.url,st o the mirmr, t,hm excl~tdirut h e snecular comoo-
(01
angles between 35' and ST,", strikes t h e ellipsoidal mirror, xnd passes through an opal glass difrwing screen t,o tho detector. Iladiation striking l.he det,edor (either rcdor blue-sensitive phototubes or on later models, a photomulliplier tnbe) produces a voltage &crossn load resistor which is t.hen amplified and registered or the mdl meter. The sample dl.zner seromodates either a solid or a liquid sample and n. standard ref-
(Conlinserl on pagc A S G G )
Chemical instrumentation erenee mstel%d. The sample and the reference material should be approximately the same size, 2.5 in. long X 1.5 in. wide X 1 h ~thick; , a small cup or beaker (nut sup-
Figure 6.
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plied) is used t o retain aliquid or paste-like
The attachment for the Model DB spectrophotameter is illustrated in Figure 4. This unit corrsists of a magnesium oxide coated integrating sphere with two side apertures for the mounting of sample and
Cory Model 141 1 diffuse reflectance qttochment with intagrating sphere in position.
lournol of Chemicol Educofion
reference materials. A dnal plane mirror system directs the monochromatic radiation from t,he spectrophotometer cell compartment into the inbegrating sphere. Using the red-sensitive photomultiplier tube, the wavelength mnge is 380 to '720 mw. The reflectance attachment for the Models 1)K-1A m ~ dUK-2.4 speetrophotometers is shown in Figure 5. This a(.tachment also ut,ilises a magnesium oxide coated integrating sphere. Tho nsmple and reference m a b o d s are illuminated by monochromatic radiation from the monochromator by means of an oscillating plane mirror heam split,ter. Either the total or diffusereflectance of a sample may be mea*wed; the farmer is obtained by mounting the sample a t an angle of 5" to the incident radiation beam. Using a. 1P28 photomultiplier tube and/or a. lead sulfide cell as detectors, the range of the attachment is 210 t o 2700 mp. C a ~ y . Three diffuse reflectance attachments are rtvailable far the Cary Models 14 and 15 spectrophotometen. All three of the attachments are high resolution inst,ruments capable of performing measurements of diffusereflectance, diffuse plus specular refleetmce, and diffuse transmission of transparent or semi-opaque liquid, powdered, or solid samples. The Model 1411 diffuse reflectance attachment, as illustrated in Figure 6, uses either sn integrating sphere or an annular type mirror collect,or. Wit,h the integrating sphere, two beams of chopped monochromatic radiation enter the sphere, via a (Conlinued on page A868)
Chemical Instrumentation lens-and-mirror system, a t a. 90' angle to each orher, and strike the sample and reference materials a t a 0" angle of incidence. The reflect,ed radiabion is collected by the sphere and detected by a photomultiplier tube moimted over an aperture at the top of the sphere. In another mode of measurement, the sample and reference msterials are ill~minatedby an external lamp source, and the reflected radiation deteet,ed by the photomultiplier tube after being passed through the monochromator. With the ring collector accessory, the sample is illuminsted a t a 0" angle of incidence with chopped monochromatic radiation. The reflected radiation is collected by an annular mirror mounted above the sample and is detected by a photomultiplier tube. A screen sttenustor located in the reference beam permits the reference beam to he standardized to a reference samnle.
given in Table I.
Figure 7. Cory cell rpoce diffuse reflectance attachment, thowing the rornple sphere (top1 and the reference box (bottom).
The cell-space io(nl diffuse reflectance attachment, hlodel 14117.50 (for the Cnry Modcl 14 spectrophat,omete~.)and 1511000 (for the Cary hlodel 15 spectrophotometer), is illmtrated in Figore 7. Thereflect,tmco stt,achment consists of two integration chambers; a spherical chamber which is used for thepmple, and a light integrating box in which is positioned the reference material. Each chamber contains a plane mirror which is used t o focus the mouachromatic radiation onto the samnle and
system is employed to compensate for any difference in absorption by the sample and reference chamber walls. The third reflectance attachment, the Model 50-400-000. is shown in Fieure 8.
(Continued on page A870)
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Chemical hstf~mentation diameter integrating sphere, coated on the inside with magnesium oxide. The sample is mounted in s. horizontal plane to facilitate measurements on uncovered liquids, pastes, and powdered samples, as well as solid samples. Table
I.
RCA 7326, or R C A 6217. Apparently, no ~~~~
~~~~~
Wavelength range Nominal resolution
Detect,ian Sample
Journal of Chemical Edumfion
Ring Mirror
2500-7000 1 2.5 to 7 A
2200-7000 1 . 3 to 7 A from 3000-6000 A: 20 A a t range limits Integration, I)irectianal, 18Oe 45"&7" angle (2r solid angle) cone With either accessory, semi-upaque or transparent liquid, n o d e r e d . 01.solid s a m ~ l e s
Monochromatic radiat,ion from the spectrometer cell compartment. is directed into the attachment aperture by means of a lens and mirror ssrmgement. A flat mirror directs the ra.di.diat,ion downward from the sphere aperture t o Lhe sample aperture a t the bottom of the sphere. The reflected sample radiatia; is collected by the sphere and detected by either a photomultiplier tube or s. l a d sulfide detector. The photomultiplier tubes suggested for the attachment are the Domont 7664,
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Specifications a n d Resolution o f The C a r y Model 141 1 Reflectance Attachment Sphere
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snnele . o~~ t,nhe's soectral resoonse curve is ideal for all types of reflectance measurements. Measurements can be made, using any of t h e above detectors, from 2500 to 20,000 A (can he extended to 25,000 A by use of a. pen-period control accessory). To make measurements of diffuse transmittance, the sample is placed into t h e holder "V" bluek at the entrance
,
aperture of the sphere and the detector a t the top aperture. G e l z e d Electric. Perhaps the oldest and most widely used recording spectrophotometer for the visible range is the General Electric instrument, developed by A. C. Hardy in the 1920's and first sold commercially in the 1930's. More than 300 of the instruments have been sold to dare. Some of the units shipped in the 1030's are still in use today; this is cer; (Continued on page A874)
Chemical instrumentation
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tainly some sort of a record for instrument longevity and use. The inst,rument, as shown in Figure 9, is used t o measure and record the color of samples in the visible range of 380 to 700 mp. An opt,ical sccesw r y extends the range t o 1000 mp. Also avsilable is nn optional sut,omatic tristimulos illlegrator which simdtaneously eomputcs iho S,Y, and Z color coordinates. 111 priltciple, the instrument is a doublobemm speclrophot.o~net,erwibh x double monocbromatrr. hIowchlametia light from a slit is trawmit1,od sueceasively ihrrrodl n balance ilochon. Wollnslull " prism, x,Lat,ing polal.ising filter, separate double lenscs, nnd into t h e 8 in. diameter i~degrsting sphcrc. Tho reflected light from the sample and reference mat,erials is collected by l,he sphere and reaches t,he photolrtbe thlrmgh a plastic m d light pipe. The diameter of [.he sample and reference material aperture is ' 3 / i s i ~ ~Smallcr . areas can be obtained by (we of a beam reducing lens accesswy. 1 I A difruse ~.eflectmre nit,achment is available for me on t h e consists of lwo precisim cast plastic spheres, each eoittcd with magncsiom w i d e
