U
ANALYTICAL CHEMISTRY ACKNOWLEDGMENT
The authors wish to thank J.
G. Black for his contributions
Figure 6.
during the early stages of dc.velopment of the instrument, 0. S. Duffendack for his continued interest, and Michael Poleshuk, G. W. Anderson, and the staff of the instrument shop, headed by R. P. Johnston, for their excellent work in the fabrication of the instrument.
C h a r t Indicating Stability of I n s t r u m e n t
temperature. For many applications, the drift aould not bo troublesome; however, it is desirable to thermostat the instrument if it is to be subjected t o wide variations in temperature. The chart reproduced in Figure 5 shows a drift of about +O.Ol% aver a &hour period; the run was made in a laboratory room in which the temperature was poorly regulated. The shorbtime variations account for the width and fuzainess of the record shown in Figure 6. These variations amount to about *0.002% or *20 p.p.m. change in flux m one beam. The source of this noise component has not been established; tests have shown, however, that it is not electrical noise in the detecting system. Consequently, the flux may be reduced by strongly absorbant filters in both beams without any reduction in the signal-t+noise ratio. Many industrial uses of the instrument require such filters for sati8frtctory operrttion. The application of instruments of this type to the solution of chemical problems is discussed in another paper ( 6 ) .
LITERATURE CITED
(1) Farren, L. I., Wireless Eng.. 23,330 (1946). ( 2 ) Fastie. , l n ” *W. \ G.. and Pfund, A. H., J . Opfiml Soc. Am., 37, 762 ,AT*,,.
(3) Fowler. R. C.. Reo. Sci. Imlnmzents. 20, 175 (1949). (4) Jamison, N. C.,Kohler, T. R.. and Koppius, 0. G., J . Optical SOC.Am., 38, 1099 (A) (1948). 1.5) . Kivenson.. G.. . Steinback. R. T.. and Rider. M., 163.. 38, lW6 (1948). (6) Koppius. 0.G.. ANAL.CHEM..23,554 (1951). (7) Lev, M.,Elec. Cormnun.. 18. 206-28 (1940). (8) Luft. K. F.. Z. tech. physik, 24. 97 (1943). (9) Martin. G. A,, Instwmmzts. 22, 1102 (1949). (10) Pfund, A. H., Science. 90,236 (1939). (11) Phmd, A. H.,and Gemmill, C. L., Bull. Johns Hc 67,61 (1940). (12) Schmiok, H., U. S. Patent 1,758,088.May 13. I€ (13) Wright, N.. and Herseher, L. W,. J. Optic01 Soc (1946). R E C E W EOotober ~ 2.
1950.
Analysis of Mixtures with Double-Beam Nondispersive Infrared Instrument 0. G . KOPPIUS P h i l i p Laboratories, Inc., Iruington-on-Hudson, N. Y. The general problem of the analysis of mixtures by a nondispersive double-beam infrared instrument has heen investigated because there appears nowhere in the l i t e r a t u r e a n y t h o r o u g h description of metho d s of sensitizing an instrument of t h i s t y p e or conditions u n d e r which it must he used. By utiliri n g the wmpounds occurring in a n y given m i x t u r e as filtering m a t e r i a l i n a suitable a r r a n g e m e n t of cells, it i s shown t h a t a d e q u a t e sensitivity, accuracy, and freedom f r o m interference can he obtained
A .
NALYSIS of complex mixtures for one or more components
by infrared spectroscopy has heen developed during recent years as a highly valuable technique. The method is partiqularly useful because the absorption spectrum is a unique property of a compound and, in the absence of chemical reaction, is retained upon admixture with other substances. A wave length-absorp tion plot therefore serves to identify the substances presont, and from the intensity of absorption at properly selected wave lengths, one may determine, by using the well-known Lambert and Beer relationships, the quantity of substances present in the sample having absorption a t those wave lengths. Conventional application of the technique requires the use of an infrared instrument having means for dispersing the radiation employed. Consideration is given herein to the possibility of
for m a n y cases of practical analytical interest. The investigation wvecs t h r e e ranges of oonoentration of interfering components: (1) a wide range s t a r t i n g f r o m zem; (2) a s m a l l r a n g e s t a r t i n g from zem; (3) a range between t w o fixed values of eonoentration. Examples are presented using m e t h a n e , carbon dioxide, and propane for the interfering wmponents for parts 1a n d 2, a n d propane, propylene, and ethylene f o r part 3. These r e s u l t s clearly indicate t h e potentialities of t h i s t y p e of instrument. using nondispersive instruments for the analysis and control of the composition of mixtures. A number of nondispersive instruments have heen described recently (f-lS). In general, they have advantages over dispersive instruments in simplicity, ruggedness, lower casts, and better suitability for plant or control laboratory application. Some general considerations for the application of tho instrument have been published (I, S, 4, 8, 18). A number of the references describe specific applications. However, there appears nowhere in the literature m y thorough description of methods for sensitizing the instrument, conditions under which it must be usod, or factors influencing precision and actecuraxy. This paper describes a means of visualizing the phrnomena occurring in the nondispersive analyzer and derives B general theory for i k use in the snalysis of mixtures. A non-
555
V O L U M E 23, NO. 4, A P R I L 1 9 5 1 selective detector was used in the esperimeiital work; however, it can be shown that, the general method for c2alibrating an instrument utilizing selective detecators (8) (mivrophone type) is essentially the same as discusscd h ( 5 w i i i . Sondispersive infrared i n s t r u m e n t s SOURCE may readily be employed for the deterniination or control of SENSITIZING a single absorbing eonCELL stituent in admixture 1 1 with n on a b s o r b i n g SAMPLE CELL constituents; no special techniques are recpirctd. Adaptation of noitdispersive instruu ments to quantitative Figure 1. Schematic Arrangeanalysis of mixtures men t d Sondispersive Instrument consisting of two or more absorbing components requires that the absorption of coiiipoiients other than the one for which a determination is desired be eliminated or neutralized in some manner. If the ahsorption bands do not overlap (a rather improbable case), intrbrference may be eliminated by any one of several simple filtering arrangements. In the viist majority of practical caws, niiyturcs of interest will contain components havABSORPTION BY ing one or more parCI IN SENSITIZING tially overlapping abzorption bands. Under these conditions a qualitative picture tof t h e p r o c e d u r e U n e c e s s a r y f o r the xxLEFT BEAM RIGHT BEAM elimination of interFigure 2. Energy Picture of Two ference be Heams Modified by Introduction of readily visualized, if Cmmponent A in Sensitizing Cell simplifying assumptions are made. Although no publications have appeared on this subject, Heigl ( 4 ) iliscussed the use of nondispersive dout)lt4)eam instruments for :tnalysis a t a spectroscopic conference a t Brooklyn Polytechnic Inst,itute in August 1919. Independently, Friel ( 3 )discussed his work with a similar instrument a t the Gordon Research Conference in August 1949, and later at the I’ittshurgh Conference on .-\ndytical and Applied Spectrosropy.
1
I t may no\\ t)e s m i that the addition of Ck to the zainple cell has removed more energy from the right beam than from the left beam, because in the left beam in the region of overlap the intensity incident upon B has already been reduced by absorption by A . This mean? that variations in B within the sample cell will cause changes in the intensity diffwence between the two beams. If now, however, component B is introduced into the compensating cell, the energy diagram is as shown in Figure 4. In the right beam, as the concentration of B , CB, is increased in the compensating cell, the intensity (within the absorption band) incident upon CL in the sample cell IS progressively decreased. Thcwfore, a t some unique value for the concentration of B in tht, compensating cell, the energgrenioved by B in thr \ample cell wiII he Cirntical for the t x o beams. ABSORPTION BY ABSORPTION BY and changes in the concentration of R in the sample c ~ l l will therefore not cauw 5 any changc in the intensity difference A--A het\vvt.cn t h e t w o LEFT REAM RIGHT BEAM beamq, ilathematiFigure 3. Energy l’icture of T w o Beams Modified by Introduction of cally this condition Component B in Sample Cell states that the differComponent 4 i a i n sensitizing cell ence of the intensities between the two beams before and rttter thr addition of R to the sample cell must be the same-that is, = AI?.
5
Experiinentally the condition AI, = II, was tcsted using ethane and ethylene for the two components A and B , rc’spectively .4 quartz filter was inserted in the instrument to absorb all radiation longer than about 3.5 microns. In the near infrared region transmitted by quartz, ethane and ethylene have two strong :tborption hands between 3.0 and 3 2 microns which overhp ahout 50%. In addition both have another absorption band bet\$fwi2.0 and 2.2 microns which are about 10% as strong as the former and nhich also overlap. Coniequrntlr , if the weah bands can be neglected, the condition LIZ, = AZ2 ,ihould be fulfilled for these two components to a good approximation. Experimentally, n hen the instrument was wt to be unresponsive to component H (Pthylene), a change in B from zero to 100% in the sample cell caused a shift in unbalance of the tu o 1)eams equivalent to only 2y0 A (ethane) in the sample cell. Therefore the contlition A I , = 112 is fulfilled to a good :ipproximation. ABSORPTION BY C: IN SAMPLE CELL
c,
CA IN SENSITIZING
IN COMPENSATING
EXPERIMENTAL PROCEDURE FOR IDEAL CASE
Consider two compounds, A and B , each having a single overlapping absorption band in the infrared rrgion. For simplicity, assume t,hat the intensity of radiation from the source does not vary with wave length, that the scnsitivity of the detector is constant throughout the wave length rang: undrr consideration, arid that, the absorption bands are rect:tngular-that is, absorp tion coefficients do not vary with wave length within the bands. A scehematic arrangement of the inatrunirnt and the absorption c,ells adapted to this case is shown in Figure I . The essential features are that two identical beams of radiation may be passed through any desired combination of absorption cells and into a detector system capable of detecting and nirasuring differences in intensities of the two transmitted beams. The instrument employed in this stud)- is described in the preceding paper (6). Assume first that a concentration CA of the wmponent to be determined, A , is placed in t,he sensitizing cell. The energy picture of the two beams will then be as shown in Figure 2 . If any concentration, CA, of the interfering component is placed in the sample cell, the energy diagixm will be as shown in Figure 3.
I
I
kL E F T 0EAM
-X RIGHT BEAM
Figure 4. Energy Picture of TWO Beams 3Iodified by Introduction of Component B in Compensating Cell
Furthermore, by the same reasoning it ran be shown that the addition of componcnt A to the sample cell results in the abs o r p t i o n of m o r e energy from the right beam than from the left anti the instrumellt therefore responds to changes in
concentration Of component iZ in the sample cell. Horvever, it rcmains insensitive to changes in concentration of component R. Furthermore, it is readily apparent from this simple model that thc instrument, for small values of A , is practically indepcindvtmt of B when both A and B are present in the sample cell. If large values of A are to be measured, it can be shown that it is only necessary to consider the variation of the concentration of A and not its absolute magnitude. If three absorbing components A , B, and C are considered which have more than one absorbing band in the infrared region, the procedure for making the instrument responsive for one component only remains essentially the same. In this case, from the Conioonrnts A and B a r e i n s e n s i t i z i i i c and *ample cells, respectively
I
ANALYTICAL CHEMISTRY
556 considerations of the previous method, it is necessary to place some unique concentrations of both B and C in the compensating cell, for a given concentration of A in the sensitizing cell. The proper concentrations of B and C must again be determined from the condition that the intensity difference between the two beams shall remain the same when first B and then C is placed in the sample cell. Thus, it is required that the intensity difference remain the same after B is added to the sample cell as it was before, and again after B is replaced by C. There are now three conditions to be satisfied instead of the one in the previous example for two components. Depending on the type of 10 Io mixture, this procedure may not reduce sufficiently the unwanted response caused by CELL 3 L CELL 3R the variations of B and C in the sample cell. From Figure 4 it is evident that the amount of interference can CELL 2L CELL 2 R be drastically reduced by decreasing the intensity of the CELL IL CELL IR radiation f o r a b s o r p t i o n within the region of overlap of the absorption spectra of components A , B , and C. I, I* This can be accomplished by Figure 5 , Experiments, introducing B and C in a Cell Arrangement filter cell, similar to the sample cell, placed across both beams. The optimum ratio of the concentration of components B and C for the filter cell is difficult to establish. Intuitively one feels that the optimum ratio of B to C found for the compensating cell should be used also in the filter cell and experiments demonstrate that this is a satisfactory ratio. The introduction of B and C into the filter cell, however, alters the optical balance and consequently the optimum ratio of B to C for the compensating cell must be redetermined but the ratio of B to C in the filter cell may remain as before. As anticipated, the introduction of the filter cell reduces somewhat the sensitivity of the instrument to changes in concentration of component A (curves a1 and Q Z in Figures 8 and 9). A more detailed discussion of this procedure is planned for future publication in a Philips technical journal.
1
1
[
1
0
0
[I I
0 1
EXPERIMENTAL EXAMPLES
The procedures described for an ideal case were applied to three practical problems; they gave satisfactory results. The problems were chosen to illustrate the reduction of interference from extraneous components by the use of the compensating and filtering cells. I t seemed desirable to do this for three ranges of concentration of interfering components: (1) a wide range starting from zero; ( 2 ) a small range starting from zero; (3) a range between two fixed values of concentration. All experimental work reported herein was conducted with gases. However, liquid samples would not present any serious differences in technique. Experimental Arrangement. In order to avoid pressure-broadening effects, all data were taken at constant preasure. Accordingly, dry-tank nitrogen, which does not absorb infrared radiation, was used as a diluent. Six absorption cells were used in the tests inasmuch as this arrangement permitted a wide choice of cell combinations. The actual cell arrangement employed is shown in Figure 5. Cells 3L and 3 R were identical in all respects; similarly cells 2 L and 2R were identical, as were cells 1L and 1R. The path length of the radiation through the cells was 20, 5 , and 10 cm. for cells 3L and 3R, 2L and 2R, and 1L and l R , respectively. For all experiments, the cell windows were of rock salt; however, the experimental procedure described herein is satisfactory with other materials.
To test the salient features of the calibrating procedure, t,he following gases whose absorption bands overlap were chosen: methane, carbon dioxide, and propane. The author chose to make the instrument responsive to changes in methane and unresponsive to changes in concentration of propane and carbon dioxide. In order to place all data on a common basis for comparison, it was first necessary to calibrate the amplifier and detector in terms of a known energy difference between the two beams. To accomplish this, all cells of the instrument were iilled with dry nitrogen and the two beams were balanced by means of the trimmer controls. The balanced position was observed as a null indication of the output from the amplifier. A known energy difference between the two beams was introduced by trimming the aperture of one beam 1%. Then the attenuator of the amplifier was adjusted so that the null-indicating meter read full scale for this energy difference. Therefore, any other deflection in per cent off-balance
(yA) X
could be determined from the known
attenuator setting and the off-balance reading on the indicating meter. After this adjustment, the instrument was returned to the balanced position and the trimmer controls were locked into position.
E
0
t
2 2
2-
- 0
8
4
i
-2-
B
*-4-
Pf
-6-
-8 /'
IO
20
30
40
% ICI/CO**BWI
50
60
70
80
90
I(
IN Ne) IN CELL 2R
Figure 6 . Calibration Curves Step 1. S2 Step 2. COS 76 cm. Stel, 3. C3 76 cm.
The calibrating procedure requires that the intensity and spectral distribution of the two beams before entering the cells be the same. A t the balanced position, a null reading on the indicating meter, the two infrared beams were equal in intensity and equal in their spectral distribution. The latter condition was closely approximated because the radiant energy in both beams emanated from the same area of the source. The instrument was now ready for the calibration procedure. Wide Range of Concentrations of Interfering Components. To calibrate the instrument it is necessary to satisfy three different conditions or steps, AI1 = h l z = AZ3, in order to make the instrument insensitive to variations in concentrations of propane and carbon dioxide. (The notation AIII = A12 = A I 3 means that the intensity difference is the same between the two beams of the instrument before and after the addition of component B , and before and after the addition of component C to the sample cell.) Accordingly, methane (component A ) a t atmospheric pressure was introduced into the sensitizing cell, 3L. A ratio of propane to carbon dioxide of 8.2 (CB/CC = 8.2/1) was chosen as a first guess for the proper ratio to introduce into the compensating cell, 2R. Ordinarily cell 3R would be used for this purpose; however, cell 2R was chosen because the path length of this cell enabled the concentrations of propane and carbon dioxide to fall a t convenient points on their respective flow meters. Keeping the ratio of propane to carbon dioxide fixed a t 8.2, the concentration of propane to carbon dioxide in nitrogen (Ca/C02 in Nz) was changed in cell 2R. [The notation (C~/COZin Nz) means % (C~/COZ) % NZ = 100%. Thus, as the per cent of a given ratio of (Ca/C02)was increased, the per cent of NZwas correspondingly decreased. ] For the immediate series of experiments the instrument was calibrated a t the two limits 0 and 100% for propane and carbon dioxide, separately (at atmospheric pressure). A plot was made of the qer cent off-balance as a function of per
+
557
V O L U M E 23, NO. 4, A P R I L 1 9 5 1 cent ropane to carbon dioxide in nitrogen. The resulting data are &own as the curve marked step 1, Figure 6. For step 2, Figure 6, carbon dioxide a t atmospheric pressure was introduced into cell l L l R and the same measurements as in Step 1 were repeated. Finally for step 3, Figure 6, propane a t atmospheric pressure was introduced into cell l L l R instead of carbon dioxide. The immediate experimental objective was to find the proper ratio of propane to carbon dioxide, and the correct per cent of this ratio in the compensating cell, 2R. For these conditions all three curves should have a common point of intersection if AI, = AI2 = A I 2 is satisfied rigorously. However, experimentally, conditions are required in which the differences in the AI'S are small; from the standpoint of the graphs, this means a situation in which the closest bunching of all the curves occurs. The resultant curves for different ratios indicated that the closest bunching occurred between the values of the ratio of propane to carbon dioxide of 58 to 1 and m . For the purposes of the tests in this paper a ratio of propane to carbon dioxide of m (no carbon dioxide) in cell 2R was considered satisfactory (Figure 7). This choice introduced some errors. The results in Figure 7, from the previous consideration, established the operating conditions of the instrument. The common intersection point of the three curves occurred a t an off-balance of -4.15 and 43.5% propane. The fixed signal, -4.15%, was now balanced out to give a new null reading on the meter ( a false zero) by injecting an electrical signal into the amplifier which was of'the same frequency but of opposite phase. Therefore the instrument was now set a t its original optically balanced position, inasmuch as the trimmers were never moved during the calibrating procedure, and the electrical zero was shifted to a new position. This does not imply optical balance between the two beams a t all points. The beams incident on the cell assembly arp optically balanced, but between the cell assembly and the detector they are unbalanced. According to the procedure outlined previously, no deviation is expected from the new balanced position if 100% of either propane or carbon dioxide was introduced into the sample cell, 1 G l R . In order to test this an 1 also to determine the sensitivity of the instrument to methane, each gas was introduced in turn into sample cell 1L-1R and thp per cent off-balance from the new zero was measured as a function of the concentration. These results are plotted in Figure 8 ; curves ai, bl, and CI represent methane, propane, and carbon dioxide, respectively.
-a.
u-4-
5
.
i-5-
E; g-6Y
B -7
I
Figure 7. Step 1. Kz.
Calibration Curves
Step 2. COz 76 cm.
Step 3. Ca 76 cm.
These curves indicate that the instrument was independent of propane or carbon dioxide a t roughly the two concentration points at which it was calibrated. The propane curve was zero a t 81% instead of loo%, because of the difficulty (not inherent in the instrument) of setting the composition a t exactly 43.5% propane a t an off-balance of -4.15%. The curve for carbon dioxide was not zero a t 100% carbon dioxide, nor was it expected to be because some carbon dioxide, instead of none, should have been placed in cell 2R to satisfy the conditions of a common cross point of the three curves.
Concerning the sensitivity of the injtrument, a large defiection resulted when only methane (component A ) was present in the sample cell, 1L-1R. However, this does not indicate good selectivity because the curves show that methane and propane produce an equal but opposite deflection for about the same per cent present in the sample cell, as shown by curves a1 and bl in Figure 8. Hence for this set of Conditions, no off-balance would be observed and the erroneous conclusion would be made that no methane was present. Consequently, a filter cell must be used to eliminate this serious objection.
& y
-1.0
I'. I
I
/
/
/ /
1 1 '
IO
Figure 8.
PO
30
40 50 60 70 % SAMPLE IN C E L L IL-IP
80
90
I
MO
Deviations f r o m New Balanced Position Calibration range 0 to 100% With filter cell az. CH4 in Nr CHI in ?a Propane in NZ bn. Propane in Nt COz in Nn ct. COi in Ni
KO filter cell ai. bl.
CI.
The introduction of a common filter cell containing the interfering components permits the conditions A l l = AIz = AZ, to be more nearly satisfied. To test experimentally the effect of the filter cell, the same problem was investigated. The equivalent of a 5-cm. filter cell filled with propane a t atmospheric pressure was placed across both beams. Cells 2L and 3R were used for this purpose. Since the cells were of two different path lengths, cell 2L was filled with propane and a mixture of propane and nitrogen was introduced in cell 3R so that they were equivalent. Again the proper ratio of propane to carbon dioxide and the correct per cent of this ratio in the compensating cell, 2R, was redetermined. The conditions AI1 = AIz = Ala were satisfactorily approximated when the ratio of propane to carbon dioxide was m (no carbon dioxide in cell 2R) and the concentration of propane was 78.7%. The off-balance for this condition was - 1.470. From the considerations discussed in the experimental procedure, one would nom expect the deviations corresponding t o Figure 8 (bl and CI) to be much smaller in this case. To test this point, propane and carbon dioxide were separately introduced into the sample cell, lL-IR, and the deviations from the new balanced position, - 1.49;b and 78.7% propane, were recorded. As shown in Figure 8 ( b and ~ c p ) ,the complete variation of either propane or carbon dioxide from 0 to 100% corresponded to, a t most, lY0 of methane present in cell 1L-1R; in Figure 8 (bl and c l ) the maximum deviation corresponded t o about 9%. The sensitivity to methane, on the other hand, was reduced only a small amount by comparison (curves UI and U Z , Figure 8). The effect of the filter cell on carbon dioxide was small and consequently some carbon dioxide should have been introduced into the filter cell as shown by curves c1 and cz, Figure 8. Small Range of Concentrations of Interfering Components. By a more detailed consideration of the argument prasented in
ANALYTICAL CHEMISTRY
558 the experimental procedure it can be shown that the conditions = d 2= AI3 are more nearly approximated if the concentrations of the calibrating components become small. To test this feature experimentally, data were taken for 0 to 10% calibrating range for both propane and carbon dioxide. Results were obtained for two conditions: with no filter cell, and with a 5-cm. filter erll of a 2 to 1 ratio of carbon dioxide to propane a t atmospheric pressure. Again the proper ratio of propane to carbon diouide and the correct per cent of this ratio in the compensating w l l , 2R, were redetermined for the two sets of eonditions, with a n d without the filter cells. These data arc shown in Figure 9.
cell 1G1R. Sets I and 2 in Figure 10 show the corresponding curves for two different ratios of ethylene to propane, 2 to 1 and 1 to 2 ( Y = 2/1 and 1/2). The optimum ratio is determined by the graphs exhibiting the closest bunching of all four calibrating curves. Of the various ratios tried, the value of 1 to 2 waq the bmt approximation as shown by set 2, Figure 10. With the ratio of ethylene to propane of 1 to 2 in nitrogen, the optimum concentration for operation occurred a t the value of 75% which was present in cell 3R. For this concentration, the instrument was -7.75% off-balance. Thus, these results established the operating Conditions of the instrument for the calibrating mi\tures, 1 , 2 , 3 , and 4. Under the operating condition described, the instrument w a* now ready for the important tests of selectivity and sensitivity. These data are given by Figure 11. For the tests on selectivity the following mixtures were introduced into the sample cell, 1G 1R*
7,methane, 50% ethylene, (50 - z)% nitrogen propane, (50 - z)'% nitrogen where t varied between 15 and 50% 111. xyo ethylene, 25% propane, (75 - z)% nitrogen where I. 11.
2%
z varied between 15 and 60%
-
s I,-.
J
IV.
IO
Figure 9.
20
30
50
€4 70 % SAMPLE IN CELL IL-IR
40
80
90
IO0
propane, 25% ethylene, (76 -
t%
z varied l~ttir-cen15 and 60%
L)%
nitrogen where
Deviations from New Balanced Position
Calibration range 0 to 10% No filter cell With filter cell al. CH4 in ae. CHI in NZ bl. Propane in X I h. Propane in Nz CI. Cot in Xz CE. COa in Ne
Nz
-3-
-
0 -4-
h was anticipated, reducing the range of the concentration of the calibrating components (from 0 to 100Toto 0 to 1 0 ~ omateri) ally diminishes the deviations within the reduced calibration range when propane and carbon dioxide were introduced into the sample cell, 1L-IR, as shown by curves bl and c1, Figures 8 and 9. In addition, a pronounced reduction in the deviations occurred when a filter cell x a s introduced across both beams, curves b2 and cz, Figures 8 and 9. Furthermore, as the range of the calibrating concentrations becomes smaller the sensitivity to methane increases; in each case the introduction of a filter cell reduced somewhat the sensitivity toward methane. The calibration curves for methane are only accurate enough to indicate this general trend. Range of Concentrations of Interfering Components between Fixed Limits. The most useful application of the instrument is the analysis of mixtures in which the concentration of interfering components changes over the limits, one of which is not zero (25 to 50%, etc.). The following gases were used: propylene, C3--; propane, C3; and ethylene, Cz--. These components were rhosen in preference to those considered previously because their absorption bands overlap much more throughout the infrared ipcctrum. Propylene (component A ) was determined in the presence of propane and ethylene when the concentration of each of these components changed a t random between 25 and 50%. To calibrate the instrument, propylene a t atmospheric pressure was first introduced into the sensitizing cell, 3L. Four sets of mixtures of propane and ethylene rrere used to calibrate the instrument. These mixtures were a5 folloivs: 1. 25% propane, 25% ethylene, 50% nitrogen 2. 25% propane, 50% ethylene, 25y0 nitrogen 3. 50% propane, 25% ethylene, 25% nitrogen 1. 50% propane, 50% ethylene
A number of ratios, 1 to 3, 1 to 2, 1 to 1, and 2 to 1, of ethylene to propane (C2--/Ca = Y ) n w e tried in order to find the proper ratio of these gases in the compensating cell, 2R. I n Figure 10 a plot was made of the per cent ethylene to propane in nitrogen (C*--/Cs = Y in K2), in cell 3R as a function of the per cent offbalance with each of the Calibrating mixtures, 1, 2, 3, and 4,in
0
1
:-5D
-
c (
4 W
0 -6-
z a
J
d -7 Y
k + 5 -88w a -9 -
-11
,
k ,
I
7 C3 25% cz-- 50%-Ne; (2) 2 5 5 cs, 50% b 1)e - -2,52$% kz: (3) 50%'C3, Za% Ce--, 23% Nz; (4) CZ--(Sets 1 and 2, no filter cell; Set 3,
,50% Cs, 50%
with filter cell)
To test the sensitivity, the following propylene (component .4 ) mixture was introduced : V. propylene (100 - z)% of a 50% ethylene to 50% propane mixture where x varied between 0 and 8% These data show that the maximum variation of both propane and ethylene within the range 25 to 50% corresponds to a measurement of 1% in the determination of propylene. (Unfortunately there was insufficient time to prove experimentally for this particular set of components that another choice of the ratio of ethylene t o propane other than the one exhibiting the closeRt bunching of all four calibrating curves would result in larger deviations by propane and ethylene than those exhibited by set
V O L U M E 23, NO. 4, A P R I L 1 9 5 1
559
2, Figure 10. However, for another gaseous miature, methane, carbon dioxide, and carbon monoxide, this point has been verified.) In view of the large amount of overlapping of the infrared absorption bands, it is rather remarkable that a nondispersive instrument can be made to exhibit this degree of sensitivity and selectivity to propylene. For many applications it is desirable to further reduce the deviations resulting from propane and ethylene and thereby improve the accuracy of the determination of propylene. This can be accomplished by placing across both bmms :I filter cell containing propant' and ethylene.
I
/x
latter case the optimum point of operation was much more easily ascertained. The error in measuring the concentration of the components would not warrant an expansion of the coordinates of set 3 in order to investigate whether or not a single intersection point of all four curves actually occurred as indicated. A repetition of the experimental conditions of the previous example, Figure 11, with the addition of the &cm. filter cell showed that the deviations produced by the interfering gases were drastically reduced. In fact, in view of the errors made in the composition of the components it cannot be said that observed deviations were real. Furthermore, the sensitivity to propylene s a s reduced only a small amount by comparison. These results clearly indicatr the commercial potentialities of this type of inqtrurncwt. 4CKNOW LEDGM ENT
m
I
IO
Figure 11.
The author wishes to acknowledge the interest and guidance of 0. S. Duffendark, president of Phillips Laboratories, Inc., during the course of this investigation. Also he wishes to thank N. C. Jamison, R. C. Hughes, and T. R. Kohler for many valuable discussions on this subject, and, in particular, Reina Hutner of the theoretical group for her aid in the theoretical interpretation of the experimental results and for her many helpful suggeetions in the preparation of this paper.
20
30
Y X
40 50 IN CELL IL-IR
60
1
LITERATURE CITED
70
Fastie, W.G.. and Pfund, A. H., J. Optical SOC.Am.,37, 762
Deviations from N e w Balanced Position
(1947).
To test this point experimentally the &em. filter cells, 2L-2R, were filled a t atmospheric pressure with the optimum ratio of ethylene t o propane, 1 to 2, previously determined for the compensating cell, 3R. Again the entire calibration procedure was repeated I t was found that the best ratio of ethylene to propane was again 1 to 2 as shown by set 3, Figure 10, and the optimum point for operating the instrument occurred at a concentration of about 75% ethylene to propane (1 to 2), in 2*5% nitrogen in cell 3R. For this concentration the instrument was -10.7y0 off-balanw. .4 comparison of sets 2 and 3 shows that in the
Infrared Analysis of
Fowler, R. C., Rev. Sci. Instruments, 20, 175 (1949). Friel, D. D., . 4 ~ . 4 ~CHEM., . 22, 505 (Abstract) (1950). Heigl, J. J., Skarstron, C. R., and Hinlicky, C. W., private conimunication. Jamison, K.C., Kohler, T. R., and Koppius, 0. G., -4~ar.. CrisM.. 23, 551 (1951). Kivenson, G., J . Opticul SOC.Am., 40, 112 (1950). Kivenson, G.. Steinback, R. T., and Rider, M., Ibid., 38, 1 W i (1948).
Luft, K. F., Z. tech. Phosik, 24, 97 (1943). Martin, G. A . , Instruments, 22, 1102 (1949). Pfund, A. H., Science, 90, 236 (1939). Pfund. A. H., and Gemmill, C. L., Bull. J o h w Hopkins H o s ~ . . 67, 61 (1940).
Schmick, H., U. 8. Patent 1,758,088 (May, 13, 1930). Wright, S . , and Rersrher, L. W., J . Optical SOC.Am.,36, 195 (1946).
RECEIVE hIarcti D
23 19.50
cis- and trens-Decahydronaphthalene
J4Y SEII)\I
\\I,
Sinclair Rejining Co., Hcrrrey, I l l .
T
i a colorless liquid, has t i density of 0.8963 gram per ml., and boils a t 194.6" C. (382" F.).
Throughout this analysis a Model 12-B Perkiu-Elmer recording infrared spect,rometer was used with sodium chloride prism and cell window. All the cells used were calibrated for cell thickness against a "standard" cell by means of relative absorbances at the samc wave length and with the same hydrocarbon solution.
Complete infrared spectra covering the range from 2 to 16 microns were run first for spectroscopically pure samples of cisand trans-Decalin i n order to determine what differences existed between the representative spectra of the two compounds (Figures 1 and 2). The purity bf t,he cis- and trans-Decalin used was greater than 99.9%, as determined by the National Bureau of Standards, from which these samples were purchased. From Figures 1 and 2 it would seem that in a solution of cis- and tiansDecalin, the infrsrrtl absorption bands which occur a t 10.81 anti 12.12 microns are representative of trans-Decalin, R-hile the infrared absorption bands which occur a t 11.42, 11.70, and 12.50 microns are representative of cis-Decalin.
HE possibility of utilizing an infrared spectrometer for dis-
tinguishing isomers has long been known as one of the main features of such an instrument. The following is a discussion of the method that was used for an analysis of cis- and trans-decahydronaphthalene (Decalin). I t is believed that the procedures discussed are general enough to indicate the method that might be followed for m y similar type of analysis. INSTRUM ENTATION
PROCEDURE
cis-Decahvdroiiaphthalene (cis-Decalin, biryclo [4,4,0]-decane) I
Prevent addresh. S o r t h American Aviation. Inc.. Downcy. Calif.
Trans-Decahydronaphthalene (trans-Decalin, bicyclo[4,4,0]decane) is also a colorless liquid, has a density of 0.8699 gram per ml., and boils a t 185.6" C. (366" F.) ( 1 ) .
