Infrared Analysis of Multicomponent Gas Mixtures Mark M. Rochkind’ Department of Chemistry, University of Rochester, Rochester, N . Y . , 14627, and Bell Telephone Laboratories, Inc., Murray Hill, N . J. 07971 A new method for the infrared analysis of multicomponent gas mixtures which involves a condensed phase sampling technique employing commercially available cryogenic equipment is described. Only a few micromoles of material are required for positive qualitative identification of most chemical species; hence, the method i s sensitiive and suitable for impurity analysis. The technique Is very general in its applicability. Structurally similar molecules are readily differentiated, and molecirles differing only in isotopic content may sometimes be distinguished. Molecules are characterized by a small number of well defined frequencies. The method permits analyses to be executed rapidly and is capable of creditable quantitative results. Herein spectral analyses are demonstrated for a series of 13 CI-C4 hydrocarbons. A NEW SPECTROPH~XOMTRIC TECHNIQUE for the analysis of multicomponent gas mixtures which is capable of high sensitivity and rapid execution has been developed. The method employs a standard double beam infrared spectrophotometer but utilizes an atypical sampling technique involving cryogenic equipment. The sampling technique provides for high sensitivity and gives rise to additional features which recommend this means of analysis. Although this analytical method applies to all infrared absorbing gaseous species, this paper is concerned solely with a random selection of 13 Cl-C4 hydrocarbons. Our aim is to demonstrate that this new sampling teci-nique provides a sound and practicable basis for chemical analysis. Attempts to correlate infrared spectra and structure by means of functional group absorption frequencies began many years ago ( I ) . Today, infrared spectrometry is perhaps the most commonly employed spectrometric technique for the characterization of chemical compounds (2). Techniques have been deve1opt:d for qualitative infrared analysis (3-5); data-handling procedures for the analysis of multicomponent mixtures have been devised (6, 7); and studies relating the contribution of instrumental errors to quantitative determinations have been presented (8). Despite established interest and activity in this area, infrared spectrometry does not yet provide a simple and satisfactory approach to the analysis of gas mixtures, especially multicomponent systems. The reasons for this are clear. Infrared gas phase spectra
1
Present address, Bell Telephone Laboratories, Inc., Murray
are characteristically rich and often exhibit extensive overlap of spectral lines (9). As such is the case for pure compounds, extensive overlap for mixtures of structurally similar molecules is guaranteed. Furthermore, gas phase vibration-rotation band contours are sensitive to pressure and temperature; hot bands when extant serve to complicate the spectra; and for gas cells of conventional design (path length, 10 cm; volume -200 cc), relatively large samples are required to develop sufficient intensity for positive qualitative identifications and to provide suitable photometry for quantitative studies. Despite the fact that structurally similar molecules will have similar group frequencies, the frequencies of normal vibrations are extraordinarily sensitive to overall molecular structure. Hence, band overlap in gas phase spectra is caused primarily by the presence of rotational structure. For example (see reference 9), the 830-cm-l band of ethane has a half-height bandwidth of -100 cm-l; for the 910-cm-l band of 1,3-butadiene the width is -70 cm-l; and for the 950-cm-l band of ethylene or the 1 3 0 0 - ~ m -band ~ of methane the bandwidth appears to be -150 cm-l. These distributions of spectral intensity result not only in overlap; they serve also to dilute the concentration sensitivity of the spectrometric technique. The method of cryogenic sampling we have adopted condenses this broad spectral distribution into a single vibrational line with a characteristic half-height bandwidth of 3-5 cm-’. This results in enhanced (peak) absorption coefficients-Le., high sensitivity-permitting the use of micro samples for routine analyses. The simple condensation of gas mixtures at cryogenic temperatures (4”,20”, or 77” K), however, is not successful for analytical purposes ; the heterogeneous environments which result from sample to sample give rise to spectral shifts which may be large and which obviously cannot be predicted. The existence of such spectral shifts is well known (10-1.3). The alternative is to synthesize a reproducible, quasi-homogeneous inert environment such that characteristic frequencies may be assigned to molecular species and used thereafter for the positive identification of the species so characterized. Our approach then is an adaptation of the matrix isolation technique (14) first suggested some 12 years ago (15, 16),but never developed in a manner suitable for analytical purposes. We adopt this technique because of its band-narrowing effect and because it provides observed fundamental frequencies which in general differ only slightly (less than OSZ) from
Hill, N. J.
(1) R. B. Barnes, R. C. Gore, R. W. Stafford and V. 2. Williams, e ANAL.CHEM., 20, 402 (1948). (2) L. J. Bellamy, “Infrared Spectra of Complex Molecules,” Methuen, London. 1958.
(9) R. H. Pierson, A. N. Fletcher, and E. St. Clair Gantz, Ibid., 28, 1218 (1956). (10) I. C. Hisatsune and P. Miller, Jr., J . Chem. Phys., 38, 49
(3) D. Milsom, W. R . Jacoby, and A. R. Rescorla, ANAL.CHEM., 21,
(1 I j C. B. Moore and G. C. Pimentel, Ihid.:38,2816 (1963). (12) D. F. Ball, M. J. Buttler, and D. C. McKean, Spectrochim. Acta, 21, 451 (1965). (133 M. M. Rochkind and G. C. Pirnentel, J . Chem. Phys., 42,
547 (1949). (4) J. D. Stroupe, Ibid.,22,1125 (1950). (5) C. N. R. Rao, “Chemical Applications of Infrared Spectroscopy,” Academic Press. New York, 1963. (6) J. A. Perry and ‘G. H. Bain, ANAL.CHEM.,29, 1123 (1957). (7) E. Pillion, M. R. Rogers, and A. M. Kaplan, Ibid., 33, 1715 (1961 1. (8) D. Z.Robinson, Ibid.,23, 273 (1951,).
(1963).
1361 (1965). (14) G. C. Pirnentel, Specfrochim. Acta, 12,94 (1958). (15) E. Whittle, D. A. Dows, and G. C. Pirnentei, J ~ Chem. . Phys., 22, 1943 (1954). (16) 1. Norman and G. Porter, Nature. 174. 5C8 (1954).
voi.
39, NO. 6, MAY 1967
567
EXPERIMENTAL
TO MANOMETER
TONITROCEN
~~
SUPPLY
TO
TO INC ON
coo WIN
U
Figure 1. Vacuum line for preparation and deposition of sample mixtures Vacuum of
torr required
gas phase values (17-19). The great virtue of the approach proposed is that the vibrational frequencies are well appointed. Rather than an entire spectrum, only a few frequencies characteristic of each chemical species need to be cataloged, Furthermore, bandwidths and relative intensities, being reproducible, serve as confirmations in identifying mixture components. Standard matrix isolation experiments (20) presume t o isolate molecular species in inert matrices by the very slow deposition (flow: 2-3 mmoles/hour) of a highly dilute solution of solute gas in inert matrix gas onto a cooled substratee.g., an infrared-transmitting halide crystal. The technique has met with broad success but is far too time consuming to provide a practical approach to chemical analysis. Our adaptation replaces this slow deposition with an ultrarapid deposition which is, a fortiori,a more reproducible manner of sample preparation. The gas mixture to be analyzed is diluted 1OO:l with dry nitrogen (perhaps other matrix gases may be employed), prepared as will be described, and deposited in aliquots of -500 pmoles onto a CsI, CsBr, KBr, or NaCl window cooled to 20" K. The deposition of a single aliquot requires less than 5 seconds. Hence, the deposition of 40 pmoles of sample (4 mmoles of sample-matrix mixture) is completed in less than 1 minute. The merits of this technique are manifold. First, the constituents of the gas mixture need not be known in advance of the analysis. Second, structurally similar molecules and molecules differing only in isotopic content are often differentiable (though the latter is not demonstrated herein). Third, the method requires only a few micromoles of material for positive identification of a chemical species. Further, !he analysis is rapid; it may be completely automated; aside from the cryogenic equipment (which is available commercially) it requires no instrumentation not commonly found in chemical laboratories; and, not the least important, it requires neither sophisticated skills nor specialized knowledge. The method is highly reproducible, reliably qualitative, and can be readily developed to give creditable quantitative results.
7,111 (17) G . C . Pimentel and S. W. Charles, Pure Appl. C/7em., (1963).
(18) J. C . Evans and H. J. Bernstein, Can. J. Chem., 34,1037 (1956).
(19)J. N. Gayles and W . T. King, Speclrochim. Acta, 21, 543 (1965).
(20) K. J. Rosengren and G . C . Pimentel, J . Chem. Phys., 43, 507 (1965).
568
ANALYTICAL CHEMISTRY
Apparatus. A double beam spectrophotometer, a vacuum system for sample preparation and deposition, and a cryostat capable of holding liquid hydrogen temperatures are required. The cryogenic system used in our work was designed around an Air Products and Chemicals Inc. Model AC-2L-110 Joule-Thomson refrigerator, which was housed in a simple, economically-designed, yet highly versatile brass vacuum jacket. (A detailed description of the cryogenic system employed is available from the author.) Any cryostat system which permits an infrared-transmitting optical crystal positioned perpendicular to the analyzing beam t o be cooled to 20" K, and which further provides a port through which a gas beam may impinge with normal incidence upon the infrared window, is suitable. Our cryostat employed a rotatable cooled window; thus, the latter could be brought normal to both the deposition port and the analyzing beam in turn. A double beam instrument capable of a spectral slitwidth of, at most, 2 cm-l throughout the spectral range 45&4000 cm-l (KBr) or 640-4000 cm-' (NaCI) is required. The work was performed using a purged Beckman IR-12 spectrophotometer. A simple vacuum line such as that pictured in Figure 1 is adequate for the preparation and deposition processes. Valves A-H as shown are hollow-cup high-vacuum stopcocks (Corning #7544), although A and B might be replaced to good advantage with remote control solenoid values. .4 tank of prepurified nitrogen should be connected directly t o the vacuum line via copper tubing, Swagelok fittings, and a glass-to-metal seal, as it is essential to exclude environmental contaminants during preparation of the sample mixtures. A mercury manometer built into the line was used to measure pressures. Materials. Except for nitrogen, the gases used in this work were taken directly from tanks without special purification (see Table I). The nitrogen used was Matheson prepurified grade (99.997z minimum purity). Prior t o use, the latter was flowed rapidly (7.5 mmole.'minute) through a 20-foot coil of copper tubing immersed in a bath of liquid nitrogen. Technique. The fewer the number of components in the gas mixture, the smaller is the quantity of sample gas required for the analysis. A convenient sample (though a smaller sample may be employed) for a mixture containing up to 10 components is 200 prnoles (about 4.6 ml, STP)-Le., 7 mm in a 550-ml bulb. (Such a bulb is readily adapted from a 500-ml round-bottom flask.) Other quantities of sample may be prepared conveniently: 100 pmoles in a 275-m1 bulb, 400 pmoles in a 1100-ml bulb, etc. The indicated bulb sizes are specified so that after dilution of the sample with a 100-fold quantity of nitrogen, the ambient bulb pressure will be approximately 700 torr. The deposition of 6-10 16-ml aliquots of the pressure-equilibrated mixture from such bulbs provides aliquots averaging 0.5 mmole. Aliquots of this size yield clean spectra and transparent, low-scattering, crystalline films. One introduces into an evacuated bulb containing a few clean Teflon chips a measured amount of the gas mixture to be analyzed. A 100-fold quantity of nitrogen is added and the bulb is shaken to induce thorough mixing of the gases. After mixing, the bulb is attached to the vacuum line and the line is evacuated down to the bulb stopcock. Valve C is closed; the bulb stopcock and stopcocks D,E or F, and H are opened; the pressure is recorded and stopcock His closed. The prepared (see below) cooled window is brought normal to the deposition port of the cryostat and all valves from A to the cooled window are opened. Valve B is opened and then closed after pressure equilibration. Valve A is opened and the gas in the calibrated volume (16 ml) is deposited onto the window. Valve A is closed, B is again opened and closed, and A is then opened to deposit the second aliquot. This process is repeated untii a suitable amount of sample mixture has been deposited. Stopcock H may be opened a t
Table I. Materials Compound Methane Ethylene Ethane Cyclopropane Propylene Propane 1,3-Butadiene Ethyl acetylene cis-2-Butene trans-2-Butene Isobutylene Butane Isobutane a As determined by supplier.
source Phillips Petroleum Phillips Petroleum Matheson Matheson Phillips Petroleum Ohio Chemical & Mfg. Matheson Matheson Matheson Matheson Matheson Matheson Matheson
any time to monitcr the pressure in the ballast volume and to determine the quantity of gas deposited. Owing to the sensitivity of the technique, no more than 40 wmoles of sample (4 mmoles of mixture) should be deposited before a spectrum is recorded. In many cases the deposition of even less material will permit qualitative analysis. The infrared-transmitting crystal, which should be large enough IO transmit almost the entire spectrometer beam, is cooled by sandwiching same between two frames of highconductivity copper using about 7 inch-pounds of torque on the brass screws connecting the copper frames and employing indium gaskets. One of the copper frames is then connected to the thermal sink. The cooled window is prepared by depositing onto its surface -1.5 mmoles of pure nitrogen in three equal aliquots. The surface which results is then identical at the start of all analytical experiments. By laying down a layer of nitrogen as described.affer a sample is deposited, it should be possible to perform analyses on a multiple number of distinct gas mixtures employing a single cooled crystal and a single cool-down. When an experiment is completed the infrared window is cleaned by warming the copper blocks to room temperature. Note that the layer of nitrogen which is deposited in preparing the window for deposition protects the window from damage which may result from samples. containing corrosive materials. The manifold pictured in Figure 1 is comprised entirely of dead volume and1 thus should be kept as small as practicable. The size of the calibrated volume can be modified if it is preferred to wcbrk with other ambient pressures. An attempt should be made, however, to retain an aliquot of -500 Mmoles. A higher dilution ratio may be employed. RESULTS
This technique of analysis requires that a standard spectrum be recorded for each chemical species of interest. This has been done for the series of compounds listed in Table I. The results appear in Table 11, where actual observed frequencies, intensities, and bandwidths are listed. Although the frequencies and the bandwidths reported should be reproducible from laboratory to Laboratory, some discrepancies may arise regarding absolute ;intensities. This is discussed more fully below; from the point of view of qualitative analysis, the data reported in Table I1 need not be recorded again. In the region below 2000 c:m-I, the frequencies noted (observed in air) are accurate to within 2 cm-l; in the region 2000-4000 cm-l, the data are in error by fewer than 4 cm-l. For each species, the number of micromoles deposited as determined from differential pressure measurements is recorded. Spectral bands less than 6 % absorbing (absorbances of less than 0.03) are coilected under the heading weaker absorptions. Table I1 lists only the stronger absorptions for each compound. The intensities indicated are not normalized.
Grade Research grade Research grade C. P. grade Research grade C. P. grade Instrument grade C . P. grade C. P. grade C. P. grade C. P. grade Instrument grade
Minimal purity,= 99.98 99.98 99.0 99.0 99.98 99.4 95.0 99.0 99.0 99.0 99.0 99.5
Table 111 provides a machine compilation of all bands listed in Table I1 sequenced according to frequency. For this table the intensity data have been normalized to reflect a deposition of 20 pnoles. Where intensity entries are omitted, the respective bands are expected to have an absorbance less than 0.03. An asterisk indicates that the corresponding band is a multiplet with easily resolvable (spectral slitwidth, 1 an-') structure. The relevant component frequencies are listed in Table 11. Table IV records information regarding minimum detectable quantities of material which will yield a positive qualitative identification among the compounds listed in Table I. This table will need to be modified as more compounds are standardized for this means of analysis. Table IV indicates, for example, that 3.5 pmoles are required for a positive identification of trans-2-butene. Such a quantity corresponds to -0.07 ml STP, which is essentially a micro sample. Other listed compounds require even less material, and molecules more structurally distinct will prove still more favorable. Note in Table IV that certain accidental frequency coincidences must be provided for. Figures 2-6 provide several examples of recorded spectra illustrating the specificity of this technique. Figures 2 and 3 show the spectral region 500-1200 cm-* for two samples, each containing four distinct hydrocarbons. In this region group frequencies are widely separated; thus, tentative identification and semi-quantitative measures of each hydrocarbon component may be achieved from this spectral region alone. Figure 2 corresponds to a superposition of 1,3butadiene, cis-2-butene, ethyl acetylene, and isobutane. For this type of condensed phase spectrum of cis-2-butene, the strongest band occurs at 687 f 2 cm-l and has a bandwidth at half-height of about 8 cm-l. Similarly, strong bands in the spectrum of ethyl acetylene are expected to occur at 644 =t 2 cm-I and 763 2 cm-', with weaker absorptions at 510 2 cm-l and 1074 f 2 cm-'. The presence of even the first two of these bands at the appointed frequencies establishes the presence of ethyl acetylene. The spectrum of isobutane is characterized by a 4-cm-'-wide absorption at 1174 f 2 cm-1, although bands considerably more intense appear elsewhere in the spectrum (see Table 11). For the case of 1,3-butadiene, the strongest spectral component occurs a t 910 f 2 cm-I. A similar discussion applies to Figure 3, which corresponds to a superposition of butane, cyclopropane, isobutylene, and propylene. Figures 4, 5, and 6 show detailed spectra of three regions within the available spectral range for a mixture composed of 12 parts isobutane, 10 parts cis-2-butene, 4 parts ethylene, 3 parts propylene, 2 parts isobutylene, 1.5 parts cyclopropane,
*
VOL 39, NO. 6. MAY 1967
569
Table II. Characteristic Spectra for Some CI-C4 Hydrocarbons Moleculea Methane (-43 pmoles)
Absorbance< 0.22 1306 0.08 3025
, cm-lb
Y.
Ad
947 1439 3106
0.42 0.24 0.05
6 2 8
Ethane
829 1377 1467 2894 2924 2953 2989
0.24 0.05 0.11 0.29 0.04 0.18 0.63
1 1 2 2 3 3 3
870 1032/ 3024 3101
0.30 0.24 0.20 0.08
4 4*
Cyclopropane (-21 pmoles)
-1889,
3021, 3078
trans-2-Butene (-21 pmoles)
-1603 Shoulder at 1028 cm-l
5 5
Isobutylene (-20 pmoles)
582 914 933 998 1376 1440 1455 1650 2926 2945 2978 2990 3089
0.05 0.39 0.04 0.09 0.04 0.07 0.19 0.04
0.07 0.04 0.05 0.05
8 5 4
Propane (-40 pmoles)
1375 1389 1468 1475 2883 2943 2970
0.05 0.05 0.12 0.09 0.16 0.13 0.33
6* 3* 3 2 7 6 Bd
-749, 1058, 1159; 1461, 2906
1,3-Butadiene (-35 pmoles)
535 910 10220 1380 1595 1825 3058 3100
0.11 0.88 0.28 0.18 0.35 0.07 0.05 0.08
3 2 6* 2 4
-990, 1283, 3414
Propylene (-33 pmoles)
Ethyl acetylene (-36 pmoles)
cis-2-Butene (-34 pmoles)
570
e
0.05
0.04 510 644h 0.14 763d 0.14 0.04 1074 2993 (0.07) 0.12 2150 0.03 3283 0.34 3318 567 687 972 1039 1385 1409 1446 1457 1461 1668 2866
0.07 0.20 0.12 0.05
0.07 0.09 0.18 0.06 0.09 0.05 0.05
ANALYTICAL CHEMISTRY
Moleculea
6 Bd
Ethylene (-37 pmoles)
(-40 pmoles)
Weaker absorptions, crn-l8
8 3 3 5
2* 3 3 3 4 5
0
Components: 1020, 1023 cm-1
5
Butane (-35 pmoles)
Isobutane (-34pmoles)
6 8 5
-1274,
1325, 2126
5*
Bd* 3 8 Bd* 4 4
h
3 8 5 3 3
-918, 1010, 1425, 2896
3
3 2 2 3 5
i
Components: 643, 646 cm-l Shoulder at 749 cm-'
AbsorbanceC 2872 0.06 2904 0.07 2931 0.15 2946 0.15 2991 0.09 3028 0.12
Y, cm-l*
975f 1066 1444 1457 2865 2894 2930 2945 2975 3032 3040
0.35 0.09 0.11 0.25
433 893 1060 1379' 1444 1464 1657 2863 2916 2944 2983 2997 3085
0.15 0.36 0.05 0.17 0.10 0.18 0.08 0.04 0.06 0.14 0.13 0.04 0.06
143 968 1382 1464 1469 1475 2871 2885 2930 2937 2971
0.04 0.04 0.11 0.11 0.04 0.14 0.09 0.18 0.06 0.16 0.38
1174 1369 1394 1469 1476 2878 2897 2916 2948 2966
0.05
0.09
0.09 0.13 0.14 0.18 0.14 0.09
0.23 0.07 0.08 0.22 0.22 0.06 0.12 (0.12) '0.63
Weaker absorptions, cm-le
Ad
4 5
6 7 Bd* Bd 9* 1 3
2 7 3 8 9 Bd
~ 1 3 0 5 1380, , 1465, 2916 Components: 972, 975, 979 cm-1
f
5 4
4 6 2 3* 3 4 6
-1280, 1429, 1452, 2736, 2875, 2898, 2925 k
Shoulder at 1382 cm-l
5
4 7 Bd*
4 6 -1294
Bd Bd* 4 3 3 3 3 6 6 6 Bd* 8
Components: 2969, 2974 cm-1 -800, 914, 1330, 1608, 2631, 2727
Number of micromoles deposited is indicated. absorbances of 0.03 or greater are recorded. Frequency uncertainty is less than f 2 cm-' for the region 400-2000 cm-l and less than 1 4 cm-l for the region 20004ooo cm-1. Spectral slitwidth: 1 cm-l over entire spectral range. Parentheses denote uncertainty. c Observed absorbance. d Half-height bandwidth in cm-1. An asterisk indicates a multiplet. Multiplets which are easily resolved (spectral slitwidth: 1 cm-1) are footnoted. Bandwidths in excess of 10 cm-l are recorded as broad (Bd). Footnotes refer e Bands with absorbances of less than 0.03. to resolved multiplets. a
* Only absorptions with
Table 111. Spectral Features Sequenced According to Frequency for the Hydrocarbons Listed in Table I. 1;requencies noted in wavenurnbers. Decimal numbers correspond to absorbances normalized to reflect 20 pmoles of sample. An asterisk denotes a multiplet structure which is easily resolved (see Table 11). Bands for which no absorbances are noted exhibit normalized intensities of less than 0.03. 433 510 535 567 582 644 687 743 749 763 800 829 870 893 910 914 914 918 933 947 968 972 975 990 998 1010 1022 1032 1039 1045 1058 1060 1066 1074 1159 1174 1274 1280 1283 1294 1305 1306 1325 1330 1369 1375 1376 1377 1379 1380 1380 1382 1385 1389 1394 1409 1425 1429 1439 1440 1444 1444 1446
1452 1455 1457 1451 1461 1461 1464
0.15 0.06 0.04 0.03 0.08 0.12 0.08 0.30 0.36 0.50
0.24 0.23
0.07 0.35
I)
0.05 0.16 0.24 0.03
4
0.05
0.09 0.03
0.11 0.14 0103 0.17 0.10 0.06 0.04 0.03 0.04 0.05
0.13 0.04 0.10 0.11 0.11 0.12 0.04 0.25 0.05 0.06
1464
0.18
0.06 0.06
1475
a
0.12
1465 1467 1468 1469 1469
+
0.05 0.08
a
ISBBUTYLENE ETHYL ACETYLENE BUTA DI €NE C I 5- BUT ENE PRllPYLENE ETHYL ACETYLENE CIS-BUTENE BUTANE PRBPANE ETHYL ACETYLENE IS 0 BUT AN E ETHANE CYCLBPRBPANE ISBBUTYLENE BUTAOIENE I SBBUT AN€ PRBPYLENE C IS-BUTENE PRBPYLENE ETHYLENE BUTANE CI S-BUTENE TRANS-BUTENE BUTAOIENE PRk3PY LENE C I S-BUT EN€ BUTAOIENE CYCLBPRBPANE C IS-BUTENE PRBPYLENE PRBPANE ISBBUTYLENE TRANS-BUT ENE ETHYL ACETYLENE PRBPANE ISBBUTANE ETHYL ACETYLENE I S B BUT YL €NE BUTAOIENE BUTANE TRANS-BUTENE METHANE ETHYL ACETYLENE ISB BUTANE ISllBUTANE PRBPANE PRBPVLENE ETHANE ISBBUTYLENE TRANS-BUT €NE BUTAOIENE BUTANE C I S-BUTENE PRBPANE ISB BUT AN€ CIS-BUTENE CIS-BUTENE ISBBUTYLENE ETHYLENE PRBPYLENE I SBBUTYLENE TRANS-BUTENE CIS-BUTENE ISBBUTYLENE PRBPYLENE CIS-BUTENE TRANS-BUTENE CIS-BUTENE PRBPANE BUTANE ISBBUTYLENE TRANS-BUT EN€ ETHANE PRBPANE BUTANE ISUBUTANE BUTANE
1475 1476 1595 1603 1608 1650 1657 1668 1825 1889 2126 2631 2727 2736 2863 2865 2866 2871 2872 2875 2878 2883 2885 2894 2894 2896 2897 2898 2904 2906 2916 2916 2916 2924 2925 2926 2930 2930 2931 2937 2943 2944 2945 2945 2946 2948 2953 2966 2970 2971 2975 2978 2983 2989 2990 2991 2993 2997 3014 3021 3024 3625 3028 3032 3040 3058 3076 3078 3085 3089 3100 3101 3106 3250 3283 3318
0.05
0.13 0.20
0.08
0.03 0.04
0.04 0.09 0.03 0.05
0.04 0.13 0.08 0.10 0.15 0.09 0.04 0.04 0.06
0.07 0.03 0.03 0.13 0.09 0.09 0.07 0.14 0.04 0.14 0.09 0.07 0.09 0.37 0.17 0.22 0.18 0.13 0.32 0.03 0.05 0.04 0.04 0.20 0.04 0.07 0.14 0.09
0.03 0.06 0.03 0.05 0.08 0.03 0.07
0.19
4
PRBPANE I SBBUTANE BUTADIENE CYCLIPRBPANE I SBBUTANE PRBPYLENE ISBBUTYLENE CIS-BUTENE BUTAOI EN€ €THY LENE ETHYL ACETYLENE ISBBUTANE ISBBUTANE I SBBUTYLENE I SBBUTYLENE TRANS-BUTENE CIS-BUTENE BUTANE C I S-BUTENE ISBBUTYLENE I SBBUTANE PRBPANE BUTANE ETHANE TRANS-BUTENE C I S-BUTENE I SB8UTANE ISBBUTYLENE C 1 S-BUTENE PRBPANE ISBBUTYLENE TRANS-BUTENE ISBBUTANE E THANE ISBBUTYLENE PRBPYLENE BUTANE TRANS-BUTENE CIS-BUTENE BUTANE PRBPANE ISBBUTYLENE PRBPYLENE TRANS- BUT EN€ C I S-BUTENE I SBBUTANE ETHANE I SBBUTANE PRBPANE BUTANE TRANS-BUTENE PRBPYLENE I SBBUTYLENE ETHANE PRBPYLENE C I S-BUTENE ETHYL ACETYLENE ISBBUTYLENE BUTADIENE €THY LE NE CYCLBPRBPANE HETHANE C I S-BUTENE TRANS-BUTENE TRANS-BUTENE BUTADIENE PRBPYLENE ETHYLENE ISBBUTYLENE PR 0 PYL EN€ BUTADIENE C YCL BPRBPAN€ €THY LENE ETHYL ACETYLENE ETHYL ACETYLENE ETHYL ACETYLENE
VOL 39, NO. 6, MAY 1967
571
';E
701
80
70
4-
5011
GO
Va
50
w0
i
40
C
30 30
20
/1IIIIII/1IIJ///j
A
L
1200
c
1200
1100
1000
900
800
700
600
500
Figure 2. Superposition of condensed phase survey spectra, 20" K, 500-1200 cm-I, of 1,3-butadiene ( A ) , cis-2-butene (B), ethyl acetylene (C), and isobutane (D), each deposited as a 1 solution in nitrogen Spectral slit width: 600 cm-l, 2.4 cm-l; 900 cm-1, 1.1 cm-1; 1200 crn-l, 1.25 cm-1 WB = window band
and 1 part trans-2-butene. Note that each component of this 7-part mixture except trans-2-butene is readily identified from the spectra in Figures 4 and 5. Identification of the trans-2-butene was missed in this analysis owing to the deposition of an insufficient quantity of mixture to permit
1100
1000
900
800
700
600
500
Figure 3. Superposition of condensed phase survey spectra, 20' K, 500-1200 cm-I, of butane ( A ) , cyclopropane (B), isobutylene (0,and propylene (D),each deposited as a 1 solution in nitrogen Spectralslit width: 600c1n-~,2.4cm-l; 900 cm-l, 1.1cm-l; 1200 Ern-', 1.25 cm-l WB = window band
observation of its isolated 1066-cm-' band. The strong 975cm-l feature of trans-2-butene is effectively masked by the great excess of cis-2-butene in the sample. It should be clear from these data and Tables I1 and 111 that the region of the spectrum below 1700 cm-l is far more useful for a pending analysis than is the region 1700-4000 cm-l. For most cases, absorptions beyond 1700 cm-l provide only confirmation of an analysis based on observed bands far to the red. DISCUSSION
Table IV. Detection Sensitivity Minimum detectable Prominent Compound Y , cm-1 quantitya bands, cm-lb hi ethanec 1306 4.0 Ethylene 947 1.8 Ethane 829 2.6 2989 Cyclopropane 870 1.4 Propylened 914 1.7 Propane" 1475 6.5 2970 1,3-Butadiene 1595 1.5 910 Ethyl acetylene 763 5.0 3318 cis-2-Butene 687 3.3 trans-2-Butene 1066 3.5 975, 2975 Isobutylene 893 1.1 Butane' 1475 5.0 2971 lsobutane 1369 2.9 2966 a Micromoles, based on 0.02 absorbance or, where a more intense band is present to confirm the assignment, 0.015 absorbance. * A prominent band is at least twice as intense as the corresponding band noted in column 2. c trans-2-Butene at 1305 c n r l is 0.25 as intense as methane at 1306 cm-1; 10 pmoles are required for positive identification of methane at 3025 cm-l. d In the presence of butadiene (1595 cm-l) which has a strong absorption at 910 cm-:, propylene is detected at 998 ern-'; 5.8 pmoles is then the minimum detectable quantity. c In the presence of butane, propane should be confirmed by observing its weaker absorption at 1389 cm-I. f In the presence of propane and (or) isobutane, butane must be detected at 743 cm-l, in which case 17 pmoles are required for positive identification. ~~
572
ANALYTICAL CHEMISTR"
Whereas frequencies, bandwidths, and even relative intensities are expected to be reproducible from laboratory to laboratory provided the experimental procedure described is adhered to, some discrepancies regarding absolute intensities are bound to arise. This results from our assumption of a sticking coefficient of unity for all gases and our further assumption that all gas emanating from the deposition port is
r-
" 2
1
LI
[I:
1000
380
960
542
520
900
880
869
Figure 4. Condensed phase spectrum, 20" K, 850-1010 cm-I, of mixture containing 4 parts ethylene ( A ) , 10 parts cis-2butene (B), 12 parts isobutane (C), 2 parts isobutylene (D),3 parts propylene ( E ) , 1.5 parts cyclopropane (F), and 1 part trans-2-butene, deposited as a 1 solution in nitrogen Approximately 100 pmoles of mixture were deposited. Spectral slit width: 0.75 cm --i WB = window band; CON = contaminant
Table V. Selected Characteristic Frequencies for Some C,-C4 Hydrocarbons
-
Molecule Methaned
70 -
60
Ethylene
Ethane
50 -
Cyclopropane i c
Propylene
m
v, cm-Ia
1306 3025 w7 1439 829
2989 870 1032 914 998 1455
Propane
L - - l -1440 'I
1420
I
1
1400
8
1
1380
I
1
1360
1
1
1,3-Butadiene
Figure 5. Condemed phase spectrum, 20' I
