Topics in..
. Chemical Instrumentation
Edited by
5. Z. LEWIN, N e w
York Universify,
New
York 3, N . Y.
These arlicles, most of which are to be contributed by guest authors, are intended to serue the readers of this JOURNALby calling attention to new developments i n the theory, design, or auailability of chemical laboratory instrumenlation, o i by presenting useful insights and explanations of topics that are of practical importance to those who use, o r teach the use of, modern instrumenlation and instrumenfal techniques.
Commercia.l ggrting spectrographs are available wit,h z variety of mountings. The Rowland mounting shown in Figure 10 is mainly of historical interest,. The t,lrree components, the slit, plate, and grating, lie on t,he radius of a common circle, the Iiowl-lnnd circle, m.hich has a radius of curvat,ure one-half that of the concave .erat.ine. The d a t e and eratine are mounted a t rieht zneles to each ot,her ~ and must he mechanically moved t o scan the spectrum. This disadvantage coupled with high astigmatitim far incident angles greater than 40" resulted in the development of other types of mountings such i s theEsgle, Wadsworth, andSeya-Namioka. The Eagle moont,ing shown in Figure 10
.,
X. Emission Spectroscopy.
Part TWO
Stephen E. Wiberley and Herbert H. Richtol, Department o f Chemistry Rensselaer Polytechnic Institute, Troy, New York
~
~~~~
Monochromators I n emission spectrographs light is dispersed into its component wnvelengths with either a prism or s, grating. Prism materials are primarily fabricated from quartz :!nd glass. Quartz transmitsradiat i m in the region of 2000-3000 A with high dispersion. Glass prisms have a higher dispersion than quartz in tire visible region, but uhsorpt,ion lir@ their range to approximately 3300 A in the ultraviolet. In the fixed position or simple prism spertrogmph, light passes through the prism onw and is focused on the photographic plate. Glass has no polarization properties hut double refraction and circular pvlsriaatian are two fartors which most he cunsidered with quartz. 1)ouble refraction can be eliminated hy having the crystal cut so bhat its optical axis is in s. prinripnl plane and parallel to the prism base. The Cornu desien (shown in Fie. 9 )
and the second component levorotntory, pnl>~riant.ion efreets are eliminated. Compensating right and left polarized lenses are also nserl to eliminate polsrisetiun. The Littrnn mounting employs a simple prism with the back surface of the prism being silvered or nlnminised. This nrrangement shown in Figure 9 ia mare compart and uses less optical material than the Cornu design. Since the light. passes back and forth through the same prism and lens, polarization eRerts are eliminnted. The reflections produced at, the front optical faces of the collimating lens and prism sppozr as st,rey light and darken the plate. This scattered light may be reduced by masking out a portion of the collimator, or by tilting the rollimstor Lens s slight amount vertically. As contrasted with prisms whirh suffer from nunlinear dispersions, gratings produce linear dispersion. Spectrographic reflection gmtings are made by ruling a numher of
REFLECTING COATlHB
\,
COLLIUATINO AND CIYERI LENS
SLIT REFLECTIN@
/
PWOTDDRIPHlE PLATE
REFRACTING PRISM
Figure 9. Schematic diagrams of the Cornu (top) and Litsow (bottom) p r i m rpedrogrophs.
equally spnmd wedge shaped grooves ur lines directly in s p e d alloys sub ns speculum or on an sluminum coating on glass. The aluminum coating prevents deterioration and gives more ultraviolet light. Iteplica gratings cnn be made from a superior grating master and are in common use b d s y . By employing special ruling methods, s large percentage of the incident light may be blazed into n desired order. The dispersion and resolving power are both raised in t,he higher orders \vhirh are vnluable for high resoltxt,iun work and for accurate wavelength measurement,s. The intensity of dilTracted light,, however, decreases with incressing order. The two types of gratings are the concave and plane gratings. The concave grnbing requires no special collimators since i t is bath 3. dispersing and focusing element. In recent years plane grating spectrographs have come into prominenre because of simpler wavelength adjustment,, attainment of higher orders, highly stigmatic designs, and oomp:u:t instroment,ation.
is the most compact of the concave grsting mountings. Sinqe the slit and plateholder are close together, the astigmatism is less than in other forms of Itowland circle mounting. A low power cylindrical lens located betm.een the slit and grating corrects the astigmatism to a large degree. In the Wadswarth mounting shown in Figure 10, the focal length is still half the radius of curvature of the concave grating, but the individual elements are not on the Rowland circle. A concave collimating mirror is introduced, and s stigmatic imagc and linear dispersion are produced. The Seya-Namioka mounting also shown in Figure 10 has an angle of 70' 15' between the entrance and exit sMs. With the grsting rotating there is very slight defoeusing. This mounting is very suitable for vacuum ultraviolet work and is a n excellent scanning monochromator. The Ebert mounting s h o w in Figure I I regenerated interest in plme ggrting spectrographs. This ingenious and compact,
(Conlinfrrd on page A 6 )
Volume 41, Number I , January 1964
/
A5
~
~
-
Chemical Instrumentation
a separation of the image-forming and dispersing elements The Ehert mountme possesses such advantages as ease of
Figure 10. Schematic representations of four mountings with concave gratings. Eagle (lower left), Wadsworth (upper right), and Seya-Namioka (lower right).
inounting was first proposed by Ebcrt ( 8 ) in 1x89 and neglected until 1952 when Fast.ie (!)) independently rediscovered its value. The Ehert mount has a side by side arrangement with the entrant rays on one side of the grating, and the emergent rays on the other side. The Ebert mountis, is whromatic (when a concave mirror is used as the focusing element), resulting in
A6
/
Journal of Chemical Education
Rowland (upper left)*
wavelength adjustment, stigmatic imagc, aresaahility of higher orders, and cornpactness, Scvcrd nudifications of the Ehwt mounting have appeared, pastie has shown an ''under-over" design for the en~ kant emergent, rays and the c Continued on page A8)
~
~
Chemical Instrumentation
MIRROR
sur $R&T, "B
O-=
I[
-
- ~-
--x : E--.:------~::----- ------~~
z-~-
Figure 11. Diagram of the original Ebert Mount lupper), and the Fortie design ilower).
Turner (10) mount,ing has two smsller ronmve mirrors in place of s single largn mirrcr
Typical Commercial Instruments A large t:vnriet,y of spectrographs nrr avnilahle commercially ranging from inexpensive small units for routine rneaslmment,s tu larger more expensive inst,rm ments for fine structure determinatims. Spe':t,rogrrpt~ can hc classified into t,wo hmnd groups; instruments employing photographic measurements and direct reading units. Scanning spoctrornetrrs will not be included in this 8umrnitr.v. Photographic instruments arc inherently more versntile than direct renders, :+Ithough the latter are much faster and mow prerise for routine qunntitwtive nndysis.
(a) Photographic Instruments Fhe 13aosch and i.onrlr 1.5 n l Stiglnatir G r ; ~ t i nSpet:tn,grwplr ~ is a rclativcly inexpensive replira grating spcr.tn>gr:qh The instrument has fixed slits of Ill, 211: and 50 mirrons nnd exposes 10 in. of 35 m n film and 11:~sn fishtail slide hit11 s w ~ 2n mm step apertures. The spectrogr:tph ernplnys x mncave grating with I
