Correlation of Intensity Measurements in Raman Spectra Obtained with Different Instruments D. H. RAh-K, Physics Department, The Pennsylvania State College, State College, P a . graphs is discussed in detail. Equations are derived to enable polarization corrections to be made to the “scattering coefficients” tabulated in the catalog of hydrocarbon spectra by Fenske and his collaborators. By means of curves given in the paper, scattering coefficients given i n the catalog can be calculated approximately as they would be observed i n other spectrographs if the transmission factor for the two kinds of plane polarized light is known.
The chief sources of error in intensity measurements in Raman spectra for analytical purposes are discussed. Three of the major causes of discrepancies between observations of different observers using different instruments are treated and suggestions are made for minimizing these errors. The variation in apparent intensity caused by variable polarization of Raman lines and widely different transmission factors for plane polarized light for different spectro-
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1
A
photographically by means of the density produced on a photoNOTHER paper (1)presents a n outline of the method in use graphic plate, the net effect would be to obtain ‘‘peak” deflections a t The Pennsylvania State College for analysis of hydrouniformly for all lines. The scattering coefficients given in (1)are carbons by means of their Raman spectra and in addition, tabucorrect as far as the variable breadth of the lines is concerned in lates the scattering coefficients and depolarizations of many of the the case of sharp, well-resolved lines, since the standard Av = The aforementioned paper lines of 172 pure hydrocarbons. 459 cm.-l carbon tetrachloride line is sharp and well resolved. points out that the values obtained for the scattering coefficients The difficulties enumerated in the discussion of variable line are somewhat dependent on the instrument used for their deterbreadth above are not so serious as might be expected. I n the mination. I t is well known that in making quantitative analyparticular photoelectric instrument used, no difficulty in quanses with infrared and ultraviolet absorption equipment, it is titative work is experienced from this cause, since the line shape necessary to calibrate the particular instrument employed, stays constant with concentration and thus cancels out of the making use of standard samples. Quantitative analysis by analysis. When other workers use the above data in lieu of means of the Raman effect suffers essentially the same limitations. standards for quantitative work with other instruments, either with respect to standard samples as infrared and ultraviolet photoelectric or photographic, little can be done with respect to spectrophotometry. errors arising from variable line breadth. Fortunately, the The main reasons for the variation of scattering coefficients situation is not very serious in any event, since the broad lines with different instruments are: variable polarization of raman are often weak and frequently, but not universally, are of no lines, variable breadth of lines, and incomplete resolution of interest for quantitative analysis. lines. Variable Polarization of Raman Lines. As for variable D a t a given in the preceding paper (1)represent work on many polarization of Raman lines a considerable degree of conformity hydrocarbons which are rare, and difficult or almost impossible can be achieved with regard to data on scattering coefficients for many investigators to obtain. I t is the purpose of this paper given in the preceding paper and those to be expected using other to acquaint other investigators with the possibilities of using the instruments. above data for crude quantitative work with their own instruI t is well known that a spectrograph does not transmit the two ments in cases where standard samples for calibration are not kinds of plane polarized light with equal facility. Spectrographs available. Incomplete Resolution of Lines. K i t h regard to incomplete resolution of lines, conformity of instruments can be obtained by employing a slit width so that sharp lines 15 cm.-’ apart can just be resolved. Variable Line Breadth. Conformity of variable line breadth cannot be achieved easily for a number of reasons. Because of the necessity of producing a large signal-to-noise ratio in the photoelectric spectrograph, a time constant of some seconds must be employed in the electrical circuit. I t is, of course, necessary to scan the spectrum in a reasonable time. If the Raman lines all had the same breadth, the scattering coefficients T o d d all be relatively correct, regardless of the percentage of full deflection obtained for the lines. However, this is not the case and, as a result, broad lines will have an apparent scattering coefficient which may be as much a s ~ 3 0 7 ~ DEPOLARIZATION FACTOR, P n too great compared t o sharp lines. If Figure 1. Family of Curves Representing Equation ?a Plotted for Various the scattering coefficients were measured Values of T
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OCTOBER 1947
767
of different design and mznufacture, grating instruments as well as prism instruments, differ widely in their ability to transmit t h e two kinds of plane polarized light. These transmission characteristics of spectrographs for polarized light are largely responsible for much of the variation in intensity measurements quoted in the literature. T h e author ( 2 ) has shown how true relative intensities can be measured in Raman spectra. For the convenience of the reader, the derivation of the equation for the true intensity in terms of p n , the depolarization factor for natural unpolarized exciting light, and T , the transmission factor of the spectrograph for the two kinds of plane polarized light, are given beloly.
The transmission of a n instrument, light is
T
=
T ,for natural unpolarized
So’Po [unpolarized]
(1)
where So is the intensity measured for the perpendicular ( I ) component, the electric vector vibrating in the vertical plane, and P o the intensity for the parallel (11) component, or electric vector vibrating in the horizontal plane. Then p,,, the depolarization factor for a polarized line, will be given by
If I t represents the true intensity of the line in question and IO represents the observed natural intensity of the spectrum line, the following relationships result:
+ Io = Po (1 + It = Po (1
Pn)
(3)
PnT)
( 4)
from which is obtained
The scattering coefficient K Oreferred to in the previous paper is defined as
where Io is the observed intensity of the line in question and IO. is the A V = 459 cm.-’ line of carbon tetrachloride-Le., the standard line. The true scattering coefficient, as would be observed by means of a n instrument which did not preferentially transmit one kind
of plane polarized light, we shall call K1. means of the following equation.
K Ois related’to K , by
where superscripts I and s on the pn’s refer to the line in question and the standard line, respectively. The part of the fraction involving only p: and T is a constant for all lines observed under a given set of experimental conditions. Then Equation 7a becomes, for the purposes of transforming the data of the preceding paper ( 1 ):
Let us call the right-hand side of Equation 7af(p,, T ) . I n Figure 1f(p,, T ) !s plotted against pn, using a value of T = 0.90 as was observed in ( I ) .I n order to find K t it is only necessarv to multiply thej(p,, T ) value for a giyen p a by the K Oobserved by Fenske and his collaborators. I n Figure l f ( p n , T ) is also plotted against p n for various values of T ranging from T = 0.3 t o T = 1.0. If a Kt value is known from the data of Fenske et al. the K Ovalue can be obtained for a given instrument b v dividing the value of f(p,, T ) (for a given T appropriate to the instrument in question) into K,. The treatment given above is applicable only to experimental conditions where true t’heoretical depolarization factors are experiment,ally achieved. The method of excitation used by the author and his collaborators (3, 4)has been shown to fulfill these conditions. A recent paper of the author’s has shown that the cylindrical lens method of excitation yields. quantitatively theoretically correct values of p., pe, and p p . Two additional observations concerning the excitation of Raman spectra might be pertinent. The cylindrical lens method of excit,ation, properly used, is the most powerful method of excitation with Tvhich the author is acquainted. The original Wood’s light furnace method of escitation does not yield theoretically correct depolarization values when used in the ordinary manner. LITERATURE CITED
(1) Fenske, M.R., Braun, W.G., Wiegand, R. V.,Quiggle, D., >ICCormick, R. H., and Rank, D. H., .%SAL. CHEM.,19, 700 (1947). ( 2 ) Rank, D. H., J . Optical SOC.S m . ( t o be published October 1947). (3) Rank, D . H., Pfister, R. J., and Grimm, H. H., I b i d . , 33, 31 (1943). (4) Rank, D. H., and Wiegand, R . V., I b i d . , 36,32.5 (1946). RECEIVED ,June 1, 1917
System for Rapid Evaluation of Catalysts for Production of Butadiene from Ethanol SI. H. WHITLOCK’, G. J. H.kDD.ID, ‘ Mellon
AND E. E. STAHLY Znstitute of Industrial Research, Pittsburgh, Pa.
T
HE great number of chemical combinations that must be investigated in a catalyst development program make the use of some type of screening test essential. This program was based on the testing of five hundred catalysts per year, with each material evaluated a t five different condit,ions. I n order to meet t,hese requirements, it, was necessary to be equipped to handle a t least ten catalysts per week, plus a control; it was also desirable t,o be able to do special work on a t least one mat,erial. Laboratory production of large batches of cat,alysts is a slow time-consuming procedure; it is very much easier and quicker to prepare a small quantity of most catalysts. These factors were the 1 Present address, Reeonstruction Finance Corp., Office of Rubber Re‘serve, Washington 25, D. C.
guides which led to the design of a n apparatus to evaluate simultaneously twelve materials using only 20 cc. of each catalyst per test. S\IhLL-SCALE TESTIXG UNIT
h small-scale testing unit was designed, which consists essentially of the feed system, reactor, separation and collection system for C, and lighter hydrocarbons, and sampling system. The main requirements of such a unit are: (1) constant, feed rate ~ the alcohol feed mixtures, ( 2 ) accufor very small rates of f l o for rate control of reactor temperature, (3) continuous separation and collection of a C, and lighter fraction. free from traces of acetaldehyde (and other osygen-containing compounds which might
