Application of Infrared Spectrophotometry to Quantitative Analysis in the Solid Phase ROBERT S. B R O W N I N G , Sterling-Winthrop Research Institute, Rensselaer, N. Y. STEPHEN E. WIBERLEY, Rensselaer Polytechnic Institute, Troy, N. Y. FREDERICK C. N A C H O D , Sterling-Winthrop Research Institute, Rensselaer, N. Y., Rensselaer Polytechnic Institute, Troy, N. Y.
..I he pressed potassium
and
(3)esamined t,he reproducibility of readings obtained with their instrument nnd esamined some of the sources of error. They concluded that t,he instrument was capable of reproducing absorbance values, over a period of several days, to approximately 4% (2, liniitp). but note that the average absorbance level shifts to a degree necessitating regular running of reference samples. They feel that difficulty in obtaining a 0% transmittance line free of pen drift is a major source of absorbance level variations. I-Tausdorff,Sternglanz, and Williams (9) feel that with the amplifier in a condition of “stable unbalance” transmit’tancereproducibility is within the manufacturers specification of d=0.25% for day to day averages. Seither paper explicitly mentions variations in slit width but BoTvman and Tarpley(3)notethat the use of a wire screen for a sample, instead of a polystyrene film, did not effect precision. Childers and Struthers (6) have observed a long-term standard deviation of 1.85% for the same instrument. Despite these difficulties, infrared spectrophotometry has successfully been applied t.o a variety of analytical problems. While the petroleum industry has undoubtedly been most vigorous in the use of the technique, nevertheless a variety of specific applications in other fields exists. Parke, Ribley, Kennedy, and Hilty ( 1 7 ) describe a method for the analysis of aspirin, phenacetin, and caffeine in tablet,s. A differential method of assay, which has a potential application to the technique here reported, has been described by Hammer and Roe (Y),who claim an accuracy and precision of d=O.l%. Of particwlar interest is the report by Hausdorff (8)on the work of Schiedt. describing the quant,itative results obtained with amino acids by maliing use of pressed potassium bromide pellets. More recently, Jeiisen ( 1 2 ) has demonstrated that this technique is suitable for the quantitative analysis of sodium benzyl penicillin. The potassium bromide pellet technique, which has been described in det,ail (8, 82) 83, 2b), has certain unique advantages, among which is the relative freedom from background absorption. Because of the unique advantages of this method of sample preparation and the specificity of infrared absorption analysis it was decided to attempt to apply the method to the quantitative determination of the alkaloids atropine and scopolamine (hyoscine) as found in commercial antinausea tablet. While these two compounds can be separated and determined by chemical means ( W ) , the quant,ities present in the tablet are small, and the most suitable analytical method, which makes use of the Vitali-&forin reaction, does not dktinguish between the two alkaloids. ThiP reaction has been described a number of times ( 1 , 2, 4,6 ) and its nature is clearly understood (11). Briefly, t’he alkaloid bases are nitrated with fuming nitric acid; an acetone solution of the nitrated material is then treated with strong base to yield a transient purple color. Since it is the tropic acid portion of the iiiolecule which is nitrated, the reaction is clearly not specific. The infrared spectra of some of the alkaloids have been described ( I S ) , \vith recommendations for quant,itative assay ( 19). and Washburn (28) has described a quantitative method for either atropine or scopolamine in ointments. His method, which makes use of a solution of the ointment in carbon tetrachloride. does not apply to the simultaneous determination of both the alkaloids in a mixture. No photometric method of this nature has been found in the literature.
bromide pellet technique has heen utilized as an aid in the quantitative determination, by infrared spectrophotometry, of two closely related alkaloids, atropine and scopolamine. Difficulties encountered in the use of the technique arc discussed, as well as the precision to be expected from the method of application described. Recoveries from standard samples are satisfactory. Reasons for moderate accnracy of the assay in the case of a complex commercial mixture are discussed. Refinements of the method and further work are planned.
T
HE simplicity and elegance of a recently introduced method
( B 1 23, 26) which makes use of pressed potassium bromide pellets as supporting media for the observation of infrared spectra suggested its potential value as an aid in the quantitative deternilnation of two closely related alkaloids] atropine and scopolamine, in a complex mixture. Present chemical methods for the determination of small quantities of these alkaloids, while eensitive, are not specific. The principles underlying the use of photometric methods for quantitative analysis apply with equal vigor to all spectral regions ( 2 6 ) . Measurements in the infrared, however, are hampered by the introduction of certain practical problems not so evident in the visible or ultraviolet. The nature of energy absorption in the infrared connotes rather narrow spectral bands, g-hile a t the same time, sources of infrared radiation are of relatively low intensity. As a consequence, instruments designed to pass a sufficiently narrow band of frequencies are complex. The effect of the relatively broad band pass in present infrared spectrophotometers on absorption measurements has been studied a t some length. Ramsay ( 2 0 ) has made calculations, utilizing an assumed triangular function of energy over the slit width, which indicate that for a ratio of slit width to band width ( n ) of 0.5, the indicated absorptivities are of the order of 20% below the true values. He suggests a method of integrating absorption over the band to give useful results. Philpotts, Thain, and Smith ( 1 8 ) have examined the same problem, using a transmittance function assumed constant between the limiting wave lengths, and have come to the conclusion that when n is equal to 1, Beer’s law will be obeyed. Observed values will be 15% below true values and a 10% variation in slit width will cause a 3% variation in observed calibration coefficient. While Ramsay ( 2 0 ) feels that slit widths are of the same order of magnitude as band \+idths, Philpotts, Thain, and Smith ( 1 8 ) suggest that n is often greater than 1. Robinson ( 2 1 ) has studied the same problem and come to a similar conclusion; if n is equal to or less than 1, then Beer’s law will apparently hold for observed values. I n addition, he has examined the effects of errors in the 0 and 100% lines (largely owing to noise and stray light) and suggested that errors from the-e sources are minimized if readings are made between 20 and 607, transmittance. Since, as stated, infrared spectrophotometers are relatively complex instruments, attention has been drawn to their ability to yield constant, reproducible values. Fortunately, the instrument used for the work described in this paper, a Perkin-Elmer Model 21, has been discussed in detail. Bowman and Tarpley
7
ANALYTICAL CHEMISTRY
8 INSTRUMENTS AND APPARATUS
Spectrophotometer. A Perkin-Elmer Model 21 recording infrared spectrophotometer, Serial 106, was used throughout the courpe of the work. The following instrument settings were maintained : slit auto; resolution 927; gain 6; response 1; Source amperes 2.8 to 3.0; speed 3; suppression 0. Pellet Die. The die, the design of which was suggested by G. B. Hess of Chas. Pfizer and Co., was made of hardened tool steel. A modification has been described by Merritt and Wiberley (16). The dimensions of the main block are approximately 5 X 5 X 7.5 em. wide. The dimensions of the pellets roduced are 5 X 20 mm. Pellet Holder. The [older is a brass unit designed to fit in the sample beam aperture of the spectrophotometer. HYdraulic Press. A Carver Laboratory press, Serial17733-5, 10-ton capacity, was used to press the pellets. EXPERIMENTAL
o.31
ozt
W
0
U
m r
m 0 m
::I,
,
,
,
,
,
,
,
I
,
,
04 05
4
6
8
10
12
j 14
WAVELENGTH ( M I C R O N S )
Figure 1. Infrared Spectra Top. KBr ellet Bottom. &tract of antinausea components, without the two alkaloids, in KBr pellet
Reagents. Reagents used were atropine alkaloid, Mallinckrodt U.S.P. X I I I ; scopolamine hydrobromide, Merck U.S.P. powder; chloroform, I M a l l i n c k r o d t C.P. r e a g e n t ; and potassium bromide, Baker and Adamson reagent, ACS (except as noted). Procedure. P R EP A R A T I O N o F POTA S S I U M 02BROMIDEPELLETS.Investigation was first directed toward the production of satisfactory 03pellets of potassium bromide alone. Baker’s reagent potassium bromide was used for this phase 04 W u of the work. z 0.5 Reproducible pellets should be of the same m r thickness and weight. Potassium bromide, ground I 0 n fine in a porcelain mortar and dried in a muffle m U a t 500” C., was weighed out in 100-mg. portions. The die was then inverted with the plunger extending upward through the body. A spacer was 03inserted between the plunger base and the die body in order to maintain position. The weighed 04potassium bromide was placed in the cavity above t I I I I the plunger and smoothed with a glass rod. The anvil was then placed in position and the entire assembly was reinverted and placed in the hydraulic press. Pressure was applied with the spacer removed. At the end of the assigned Top. Scopolamine, 0.3% in KBr time period the pressure was released, spacing Atropine, 0.3% i n KBr curve of 50% w./w. mixture of atropine and scopolamine, Bottom. Absorbance blocks were placed under the die body, and the =0.3% i n KBr pellet was gently pressed out. A “fill and smooth” technique was adopted. The die was filled with the potassium bromide containing the compound and the excess was removed with the 0 and 100% transmittance levels had been checked. The spectra were then scanned from 2 to 15 microns. a spatula. Pressing time was standardized a t 10 minutes and the pressure a t 100,000 pounds per square inch for the rest There was no decrease in energy transmittance when the pellet of the work, since these conditions gave a reproducible pellet holder was inserted in the beam. -4 fiducial mark was established with average weight (sixteen observations) of 0.1289 gram, on the holder and instrument. Rotation of the empty holder mean deviation of f0.0105 gram, variance 0.000167 gram, standwithin several degrees of this mark in either direction had no ard deviation =k0.0129 gram, and standard error of the mean observable effect on transmission. f0.0032. Absorbance values a t various wave lengths for each of the three atropine systems were plotted against the thickness of the respecA crucial point in a determination of this nature is the measuretive pellets in a manner similar to that adopted for the graphs. ment of the quantity of material in the sample beam. With the The plot for the 0.302% system indicated that Lambert’s law die used, this is perhaps best accomplished by measuring the was obeyed. The 0.603 and 1.06% systems showed deviations thickness of the pellets. Many of these pellets were tapered, of increasing magnitude. These deviations were ascribed to the however. The effect of the taper on the measured absorbance has been evaluated ( 2 4 ) and the error is small, but difficulty in excessive size of the atropine particles and a resultant loss of making the measurement remains. energy by scattering, which would increase with concentration. For this reason a portion of the 1.06% system was sieved through PREPARATION OF CALIBRATION CURVES.It appeared that the bolting d k (200 mesh) and re-examined. The increase in absorpabsorptivities of the alkaloid bases could most properly be determined by running several different concentrations a t varying tivity was marked. thicknesses. Therefore, an experimental program calling for the Because of the interest in the effect of particle size, a new series of atropine systems was prepared by dilution of a 4.914% conmeasurement of pellets made from three concentrations (approxicentrate and screening through 200-mesh bolting silk after mately 0.3, 0.6, and 0.9%) of each of the alkaloid bases in potassium bromide a t each of three thicknesses, was followed as nearly grinding. The potassium bromide wed in these preparations was ground as possible. in a glass mortar and sieved through a 230-mesh standard sieve. A 10.11% concentration of atropine base in the potassium Subsequently it has been stored a t 105’ C. and has shown a bromide for the pelleting tests was prepared by agitating the alkaloid with the ground salt. This was diluted by adding more gradual decrease in the amount of contained water, judging by the decrease in the intensity of the bands at 3 and 6 microns. salt to form mixtures containing 0.302, 0.603, and 1.06%, respec(A trace taken from this material shortly after its preparation tively. Pellets were prepared from these and were inserted in the pellet holder. The holder was inserted in the light path after may be found in Figure 1.)
---
V O L U M E 27, NO. 1, J A N U A R Y 1 9 5 5 A second set of curves was prepared from these new dilutions. The results appeared to be in reasonable conformity with Lambert's law. Meanwhile a scopolamine concentrate had been prepared in,the following way: Scopolamine hydrobromide (0.1444 gram, equivalent to 0.1000 gram of base) was weighed out, and transferred to a separatory funnel. Water (10 ml.) and 10% sodium carbonate solution (2 ml.) were added and the suspension was swirled. This was extracted with 4,3-, and 2-ml. portions of chloroform. T h e pooled chloroform was diluted to 10.0 ml. and dried with anhydrous sodium sulfate. In a slightly warmed mortar, 1.5077 grams of potassium bromide were flattened out and 8.0 ml. of t h e chloroform solution were added. The chloroform was partially removed in a vacuum desiccator and the mixture was ground until dry and warmed a t 60" C. under 50-nini. vacuum for several
9 hours It xas then stored in a dry bottle. The mixture was not dry, as scopolamine base is a liquid, but it was not difficult to handle. This 5.039% concentrate was diluted with the powdered potassium bromide to 0.314, 0.648, and 0.912% scopolamine by weight. Pellets and charts were prepared from this material according to the procedure described.
SuperpoPed absorbance curves for atropine and scopolamine may be found in Figure 2, which also illustrates the absorbance curve of a 50% mixture. It was decided to confine attention to the 5.80-, 11.66- and 13.00-micron bands. The latter two are reasonably discrete and should serve as appropriate points of measurement for the quantitative determination of the alkaloids in a mixture. The 5.80micron band is a strong band, common to both alkaloids, and m e a s u r e m e n t s at this Fave I ' 5 00,u length might help to serve as 81166 checks on the validity of results. 1 /" AI3CQp Washburn (88) made use of a 7e e band a t 11.21 microns for measurement of scopolamine concen6tration and of a band a t 8.56 microns for the determination of the atropine No reason for his choice is given, other than that these were convenient wave ,/ lengths. I t is clear that he did b not attempt to assay mixtures. / / As it had become apparent that the transfer of the alkaloid from the sample under examinaA tion to the potassium bromide / was going to involve solvent evaporation, it was decided to regrind some of the 0.6% atro5 IO 15 20 0 5 IO I5 20 pine system with a few milliliters THICKNESS ( 0 0 0 1 CNCHES) of chloroform and retest it Figure 3. Absorbance of 0.3, 0.6, and 0.9% Atropine in Potassium Bromide Pellets again the absorptivities changed at Indicated Wave Lengths markedly, although to a lesser degree. For this reason the remaining 4.914% atropine conI centrate was ground with chloroI form in a glass mortar and the e mixture, after drying a t 100' C. e 580 u for a few hours, was diluted with / the pon-dered salt to form 0.308, /I 0.624, a n d 0 . 9 1 4 % s y s t e m s . .? These were run again. Carefully obtained calibration data for scopolamine yielded / indicated absorptivities of 3.69 f 0.16 (5.80 microns), 2.21 f. , 0.09 (11.66 microns), and 0.58 * , f 0.06 (13.00 microns), and for L atropine, 3.46 =I= 0.25 (5.80 , microns), 0.241 f 0.036 (11.66 microns), 1 14 0.06 (13 00 microns). To check the reproducibility of this method of preparing the atropine dilutions, portions of atropine were weighed on a m i c r o b a l a n c e i n t o warmed 0 5 IO 15 20 i: 5 IO I5 20 5 IO 15 20 25 mortars. The appropriate quanTHICKNESS (0001 I h C H C S l tities of potassium bromide and chloroform were added and the Figure 4. Absorbance of 0.3, 0.6, and 0.9% Scopolamine in Potassium Bromide mi.;turPs were ground until dry Pellets at Indicated Wave Lengths
.gt 1
i
-
*3t
ANALYTICAL CHEMISTRY
10 and then dried in the oven a t 100" C. for about halt a i l hour. Four samples, of 0.610, 0.621, 0.565, and 0.708y0 concentration, were prepared in this ~ a y .Two pellets were prepared from each of the four systems. Reproducibility was not unreasonable. The corrections applied to the observed absorbanccs were based on the observation that the "absorbance" of the blank potassium bromide pellets was a relative constant, independent of the thickness of the pellet. This phenomenon, which has been noted elsewhere (22, 23), malxes correction a relative]) simple matter, if it is absumed that pellets made of the pot:iiciuni bromide-alkaloid qyqtenis behave i n the same vay.
-
9-
580,d
8.
't
. a
m1166,u A
1300p
/ /' /
c
I
,/
starch, lactose, magnesium stearate, and oil of peppermint. To prepare the blank the oil of peppermint was ground in a mortar with a portion of the lactose, adding more lactose while grinding until t,he mistule was dry enough to sieve. To this was adtied the remaining lactose and other ingredient.s, n-ith grinding. The entire mixture, after thorough grinding, was traiisfcrred to a bottle and well shaken. hfter several at,tempts to devise a procedure which u o u l t l extract the alkaloids from the powder but, eliminate intcriwing suhatances, the following method was adopted. \Veigh accurately ahout 0.7 gram of ground tablet mixture and transfer to a stoppered centrifuge tube. ddd 10 nil. of water and agitate vigorously for 5 minutes, then centrifuge. Decant. through a small S o . 1 Whatman filter paper into a wparatoi,y funnel. (A vitamin B, separatory reaction vessel is ideal.) W:ish the filter paper with a few milliliters of water. (The solution should a t this time be slightly acid.) Wash twice with 2 1111.of chloroform, discarding each wash. Add 1 nil. of 0.1S sodium hydroxide and extract the solution with 3.0 ml., then 1 .O nil. of chloroform. Pool the chloroform and wash with 10 nil. of water. After washing the chlorofmn, transfer it to a centriiugt? tulie, dry it by adding anhydrous sodium sulfatr, sild cv~ntrifuge. Take 3.0 ml. for the assay.
.,
I I ,
,/' /
Composite Beer's Law Plot for Atropine
Plots of the corrected absorbance values for each alhxloid at three concentration levela are shown in Figures 3 and 4. From these, composite Beer's law plots for the two alkaloids were obtained, as shown in Figures 5 and 6. For convenience, the data were taken at an arbitrary thickness of 0.015 inch. The relative distribution of the points on the Beer's law plots is not affected by this selection. Simultaneous equations for the absorptivity of the system a t 13.00 and 11.66 microns were set up and solved in the usual may (16), using the average values for indicated absorptivity (to distinguish it from true absorptivity) obtained from the collected data. The solutions to these equations follow: C~op0lami.e
=
4.75 A i 1 . 6 6 ~9.9 h
A13.m~
I
cc
CONCENTRATION %
Figure 5.
I
/'
/
(1)
where C = concentration of alkaloid, milligrams per gram; b = pellet thickness, centimeters: and A = corrected :il)sorhance at, the indicated wave length. For notation see (IO). ANALYSISOF KNOWN MIXTURESA N D ANTINAUSEA TABLETS. Two known mixtures of the two alkaloids were prepared from the concentrates used to prepare the standards. Two pellets were pressed from each system and the results were calculated on the basis of Equations 1 and 2. The data show a mean recovery of 104% atropine and 98.2% scopolamine. A mixture of the ingredients of the antinausea tablet with the exception of the alkaloids was prepared to assist in testing estraction procedures. I n addition to scopolamine hydrobromide and atropine sulfate the tablets contain vitamin Bs, vitamin Bg. niacinamide, Benzocaine, Luminal, and excipients, including
/ /-*
/-
I
0
1
2
Figure 6.
.3
4
5 6 7 8 CONCENTRATION %
I
9
I
I
1011
I2
Coniposite Beer's Law Plot for Scopolaniine
Thc abvorbance curve of a pellet prepared in this way from the blirnk mixture is shown in Figure 1. The chloroform solution was handled much as described for the preparation of the scopolamine standards: transferring the aliquot to a warmed glass mortar containing a known weight of powdered potassium bromide (in this case, about 0.12 gram), removing the bulk of the chloroform in a vacuum desiccator, grinding the remaining solvent with the potassium bromide until dry. t,hen drying for a short while in R 100' C. oven. Two commercial lots of tablet,s supplied by W. C:. RlacLennan of Winthrop-Steams, Inc., were examined in the same ~ v r t y . I'cllvt tl':ices appear i n Figure 7 , Results arc, a* t'ollows: ~
I:ound
~~
_ ~ - _f / _T ~ b k Found
~
~
~~~
Claiiiiad
Ili 132
113
188
117
%!I
230
81 2'2''
DISCUSSIO\
An examination of the results e h o \ ~ sthe relative error to he high. Some potential sources of error have been noted, including
V O L U M E 2 7 , NO. 1, J A N U A R Y 1 9 5 5
11
mean of i 5 . S % , in contrast to 324.3% for the same group on the basis of thickness. A potential source of error in this method is the presence of iuterference patterns in t,he trace (see Figure I, top). This phenomenon has been noted and ut'ilized for the measurement of cell thickness. I t remains to choose the W appropriate pellet thickness to eliminate the z u difficulty. m a: Particle distribution should not be Y signifi0 VI m cant, feature i n this case, as Lambert's lan. is ohrycd fairly well for the standards ( 1 4 ) . The effect of partirk size, or, more accuratclly. control of particle size, has not adequately bwii investigated in this work. While this should 2 4 0 ' 8 I2 14 not be a significant factor with the liqui(l scopolamine, it is planned to measure the pi'WAVELENGTH f MICRONS) "sizes Of the group Of atropine Figure 7. Traces of Extracts from Commercial intinausea Tablets monsible for the bad scatter on the coini~o~itc~ (Beer's law) plot, and in addition, measurc I>\ cheniical means, the alkaloid concentrations of the same systrlnl. the folloiving: slit witltli variation, instrument reproducibility, Testing the recovery of known amounts of alkaloid from :I pellet taper, particle distribution, and particle size. Ot.hei-3 blttiik should be deferred until the previous suggestions have hwn arise in connection with the preparation of the s:tniple-for esfs\:t~nined, but will, of course, be undertaken 1% hen other contliample, incomplete e.xtrnction of t,he alkaloid, loss on ghssivare tion* harc h e n phon n to he reproducible. in grinding (t.his applies also t o the standards), erroneous blank corrections, interference pnttei,Iis. : ~ i i dpossible unknown fact.ors. LITER4TURE ClTED The possibility of interaction hctw-een the alkaloid and the potassium bromide is felt to be smsll. as ip the possibility of damage Allport, N. L., and Jones. S . R . . Quart. J . Pharm. a i d P / I U I I / L ~ while pressing. col., 15,238 (1942). Allport, N. L., and Wilson, E. S., Ihid., 12, 399 (1939). Slit width variation wits felt to be insignificaiit in this case. Bowman. H. LI.,and Taruley, W.B., AppE. Spectroscopy, 7, 57 Variation in slit n-idth was checked by observation of the slit counter during operation. \-dues were observed to vary by less than 1 micron, or less than & I % a t 5.80 microns. Even during a short period when a noisy detector amplifier necessitated dightly loKer resolution to compensate for increased noise t,he variatiori did not exceed f1.5% of the average a t 5.80 microns. This is well under the 10% limit, which, it was noted, would cawe an error of 3%.
Table I.
a
Assay of Atropine-Scopolamine Tablets Tablet A
Tablet H
Satiiple wt., g. KBr. g I't,llet thickness, inch
0.7034 0.1474 0.0170
0,7370 0,1600 0,0202
Absorbance 11.66~ Obsvd. Corr.(l 1 3 ,OOfi Obsvd. c0rr.a
0.213 0.163
0.230 0.180
0.198
0.207 0 . 132
0.123
Corrections were applied on the basis of t h e absorbance curve of Figure.
1, blank tablet mixture.
Instrument reproducibility leaves rooni for improvenient but generally is a minor source of error (3, 5, 9). Pellet taper, already noted, is not highly significant in itself (ad)?except that it impairs the accuracy of thickness measurement. Measurements on a number of pellets pressed under the same conditions were examined to test the possibility that pellet weight might be a better measure of sample in the beam than is the thickness. The average deviation of the mean of the ratio waF nearly &3%, a value judged t o be somewhat higher than that which could be accounted for hy error in measurement of thickness. To further test this point, absorptivities have been calculated on the basis of pellet weight for a group of scopolamine pellets. Analysis of the data yield an average deviation of the
(1953). Rrunaell, h.,CanbAcL, T.. and Seydlita, H., Farm. Rezu. 46, 729 (1947); reviewed in Qiiart. J . Pharm. and Pharmaeol.. 21, 72 (1948). Childers, E., and Struthers, G LV., A 4 ~CHEM., ~ ~ 25, . 1.711 (1953). Colby, A. B., and Beal, J. L. J . Bm. Pharm. Assoc., 41, 351 (1952). Haminer, C. F., and Roe, H. It.. Ax.4~.CHEW,25, 668 (1953). Hausdorff, H., A p p l . Spectroscopy. 7, 75 (1953). Hausdorff, H., Sternglana, H., and Williams, V. Z., Ibld., 7, 63 (1953). Hughes, H . K., et al., ASAL. CHEM.,24, 1349 (1952). James. W. O., and Roberts, 31.,Quart. J . Pharm. and Plrurniucol., 18, 29 (1945). Jensen, J. B., A c t a Chent. Scand., 8,393 (1954). Marion, I,., Ramsay, L). A , , and Jones, R . N., J . A m . C h m i . Soc., 73,306 (1951). L\lellon, 11. G., ed., "Analytiral .ibsorption Spectroscopj-." Kew York, John Wiley Br Sons, 1950. LIerritt, P. E., Ph.D. thehis. Rensselaer Polytechnic Institute. 1954. Nachod, F. C., Zippin, C., and TIinkel, E. T.. J . .4m. I'hn),m. Assoc., 38, 173 (1949). Parke, T. V., Ribley, A. hf.. Kennedy, E. E., and FIilty, K.K.. ANAL. CHEM.,23, 953 (1951). Philpotts, A. R., Thain, K., and Smith, P. G., Ibid.. 23, XiC; (I 951). Pleat, G.B., ILirley, .J. 1.. and Wiberlcy, S.E., J . A m . Pharur. Assoc., 40, 107 (1951). Rainsay, D. 9., J . A m . ('/!( ( t i . .9oc.. 74, 73 (1952). Robinson, D. 2.. d s a ~C. m : x 23,273 (1951). Schiedt, U., 2. .Yatirr.foi,vli., 8b, ($6 (1953). Schiedt, U., and Reinwein. H., I b i d . , 7b, 270 (1952). Spell, A., and Hector. IT. E., paper presented a t Pittsburgh Conference on Applied Spectroscopy and Analytical Chemistry. Pittsburgh, P a . , March 4, 1954. Stimson, AI. l l . , and O'Donnell, 11. J., J . Am. Chem. Soc., 7 4 , 1805 (1952). Strong, F. C., A x i r . . CHEM.,24, 338 (1952). l'rautner, E. hI.,Neufeld, 0. E., and Rodwell, C. N., Aicstra2;orL Chem. Inst. J . & Proc., 15, 55 (1948); reviewed in Anal!/.-/. 74, 55 (1949). Washburn, W.H., ,J. - 4 1 ~Plia,nz. Assoc., 41, 603 (1952).
RECEIVED for review
July 19, 1954. Accepted October 20, 1954
