Chemical Instrumentation
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S. 2. LEWIN, N e w York University, Woshingfon Square, New York 3, N. Y.
T h i s series of articles presents a survey of the badc principles, rlraracteristics, and limitations qf those instruments wh,ich find important applications i n chemical work. The emphasis is on commercially aaailahle equipment, and approximate prices nre quoted to show the order of maqnitnds qf cost of the varioxs types of design andconstruction.
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e k m m t a t other points, and thpir pmergrnt mya are displaced from those of the firskmmtioncd Fa>-. As Figure 3 s h o ~ s , rarh ~ m r r g r n xxvelength t rvill appear as a
9. Spectrophotometers A spectrophatomrter is a photometer ctpipped with a dzsperswe elmenl, such ~ L Sa n m m or a .. eratine. ... so that measureI I . + L ~ .i light i n t w : i t > t x ~ lw. j UL:UW p i n I , v - p < . i ~ t lur v1111tiuw~a~4? cIwr .& 111x6.~ , i I . t . . I , 11tt ),lwtwwtrir measllr~ments are rp~tricted to narrow h a n d of wavelengths, much valuable information can be ohtainrd t,hat is inareessil,le with hroad hand pass filter phot,ometers. A narrow hand pass filter, such R F a n interfereme filter, can give t,hr smne information a t the wavelength it passes as does a good speetrophotometer, hut t h r latter permibs the user to choose :my or all wavelengths over a a i d e range. T h r principal parts of a apectrophotomcter are the light sortwe, monochromat~ar. and photometer, as illortmttd in Fimrrr I .
example, it is often desirable when using a tungsten lamp for measurements in the short wavelength visible t o filt,er out the infrared radiation that is emitted b~ the source in far greater relative intensity (see Figme 2). A high intensity of
Figure 3. Each wavelength in the beom entering the dispersing element emerges separated in space from all other wavelengths. but the width of the entronce slit determines the rpreod in space of each of the emergent wavelengthr.
SOURCE
SUPPLY
FILTER ENTRANCE SLIT
Figure 2. Distribution d relative rodiant energies in the output of o glowing object. The effective temperature of o glowing tungsten filament 1s obavt 3000DC.
I ELEMENT
I
EXIT I
s.1.L
I d ; I
PHOTOMETER
Figure 1. Schematic diagram of the bodc components of a ipectrophomrneter.
Optiml component,^, such as the lenars or mirrors employed for collimation, focusing, etc., a t r e considered aa parts of the major parts jrmt mmtioned.
extraneous radiation entering the monochromator can result in enough scattered light reaching the photometer to impair the validity of the measurements. This is particularly important if the sample- compartment is positioned hetween thp monorhromator and t,he photometer, vide infm.
The Monochromator The nionochromator consists of a, slit system, far fixing the dimensions of the beam of radiation, and a disperning clement, far spreading out the radiation in space in proportion t o wavelength. The prmcnee of the d i t syfitcm is of primary significance in this type of instrummt.
The Entrance Slit The Light Source T h r light source may he a tungstenfilament ineande~cent h d h for measnrement,s in the visible and near infrared rcgionr of the spectrum, a gas discharg~ tuhp for the ultraviolet, or n. Sernst g l o w ~ ror s silicon carhide rod for t,he i f e The total radiation from t,he light source is often rich in undesired wavelengths, and a f i k r may be unrd to :tbsorh most of these without seriously diminishing the desird wavrlcngths. Fur
The role of the enlranee slit is illustrated in Figure 3. If a, primary beam consisting, far example, of four different wavelengths is passed through the dispersing element, each ray of this beam, impinging a t a ppecific paint on thedispersing elrment, will emerge as a spectrum of foor rays. Each of t , h ~ emergent e rays is separated in space from the others which came from the same incident ra,v. However, other incident rays, from other regions of the mtrxnrr slit, i m p i n g ~on thp dispersing
band of light, of m-idth determined by thc width of the entrance slit. If the camponent wavelengths in the primary light are close t o ~ a c hother, or continnot~sin value as is the rape with the light sources generally used in ~pwtrophatometn?, the emergent hand8 of light overlap, and the radiation a t any place is then not a single wavelength, but R m i x t u ~ of ~ wavrlengths. Hence, the narrolr.fr tho entrance slit is, the greater is the npwtml purity of the light nt all places in the dispersed spectrum. H o w ~ w r a, narrow slit means t h a t the total radiant energy entering the monochromator is small, and the intensities a t all places in the dispersed spectrum will be Ion. Therefore, in the design of spectrophotomrters, the width of the e n t r m r r slit is chosen t o provide n, suib able cdmpromise br,twem s p ~ c t r a lpurity and beam intensity. Sine? the intensity of the light emitted by the light source is not the name for all wavdengths, but varies m a r k d l y as indicated for one type of source in Figure 2, i t is generally necesnary t o make the entrance slit continuously adjustable, so that its width can be varied in accordance with the spectral properties of the source in t h r ~mvrlengthregion under study.
The Exit Slit The mil slil serves to select n portion of the dispersed spectrum for vieving by t,he photom~ter, as illustratrd in Figure 4. If the aprctrum is continuous, or consists of rlosply-spaced lines, t,he spread in navelmgths viened by the photometer is proportional to the width of the exit slit. This is true vhether the entrance slit is narrow or wide. The width of the exit slit also cantrols the total light. intensity entering the photometer. Thos, the ~pectralpurity and the intensity of the light emerging from the monochromator depend rrit,ically upon
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Chemical Instrumentation hoth the m t r m e e and exit slits, enell one aff'erting these paramctcrs i n d r p e d ently of the other. I n some instmmnnbs, these two slits arc independently adjustable; in others they are "ganpcd" together and art. varied simultan?ordr.
ENTRANCE SLIT
Figure 4. The exit slit selects e portion of the dispersed spectrum for viewing by the photometer.
The Prism Tho other component that determines bho performance eharactcrist,ics of the monochromator is the dispersing elnrirnt. The two t.yprs of dispersing elpmrnts generally ernployrd are prisnis and diffrnction gratings. A prism disperses a polychromatic bcnm of light into a. spectrum hecsuse its refractive index is not s eonstant, hot is rlifierent for difierrnt wavrIcngths. The deviation in thr dirretion of n l i ~ h heam t produced by pasrage through a prism is illustrated in Figure 5. The variation in refrhctivo indm x i t h wavelength, dn/rlh, is callcd the rlispcrsive power, and is a fundamental characteristic of the prism material. The dispersive power curve for crystalline quartz is shown in Figure 6; other prism materials show similar curve.;.
Figure 5. At the oir/gios interface, o light ray is refracted toward the vertical to the wrface; at the glan/air interface it is refrocted oway from the vertical. The amount of refraction is differentfor each wavelength. The actual separation in space behxen two wavelengths, k and AS, depmds on the dispersive power and on the angle, A , of the prism. I n most prism instruments, the light heam is directed a t the prism so that the internal ray travels parallel to t,he hase of the prism (as in Figure 5); this is called the position of minimum deviation. From the euamctrv
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Journol of Chemicol Educofion
Chemical Instrumentation wavelengt,h, dE/dA, or the ffiWersion, is given hy the expreaaion:
