Difficulties in the use of this detector stem from its high sensitivity and low limits of detection. Column bleeding must be avoided. Operation at elevated temperatures accentuates the column bleeding problem. Although not yet demonstrated for operation at temperatures above 50-60” C and on compounds with higher molecular weights (200-300), there would appear to be no reason why such applications could not be made, providing column bleeding is minimized. Suitable means must be taken to prevent passage of no more than approximately 0.1 mg of sample through the detector plasma at any single injection.
ACKNOWLEDGMENTS The authors gratefully acknowledge the assistance of Guy F. Origlio of the U.S.Army Engineering Research and Development Laboratories, Fort Belvoir, Va. RECEIVED for review August 16, 1967. Accepted November 14, 1967. Presented at the 18th Annual Mid-America Spectroscopy Symposium, May 16, 1967. Program carried out under Contract No. DA 44-009-AMC-1576 (T) from the U. S. Army Engineering Research and Development Laboratories.
Specific llnteractions Affecting Gas Chromatographic Retention for Modified Alumina Columns David J. Brookman and Donald T. Sawyer Department of Chemistry, University of California, Riverside, Calif. 92502
The gas chromatographic retention of hydrocarbons has been studied on salt-modified aluminas. The results establish that the retention volume is dependent upon a composite of nonspecific and specific adsorptive interactions. The latter includes effects due to the pi character of the sample molecule and to its planarity. For aromatic hydrocarbons, substituent groups affect the gas-solid interactions, and for olefins the cis isomer is retained more strongly than the trans isomer. A quantitative evaluation of these effects and of different modifying salts permits the design of columns for the selective separation of cistrans isomers and of mixtures containing paraffinic, olefinic, and aromatic hydrocarbons with similar boiling points.
ACTIVATED ALUMINA has been used for many years as an adsorbent in liquid-solid adsorption chromatography. Its use as an adsorbent in gas-solid elution chromatography (GSC) has not been widespread, however, because the resulting chromatographic peaks are asymmetrical due to the nonlinear adsorption isotherms for solutes (1). Much of the asymmetry is eliminated if the adsorbent is partially deactivated with silicone oil or water (2), or with inorganic salts (3, 4). Comparatively weakly interacting adsorbates can be easily and symmetrically eluted from such modified adsorbents (4-6). The sensitivity of ionization detectors permits operation on the linear portion of the adsorption isotherm (7) such that the retention volume is independent of sample size. The study of the adsorption mechanism for various ad(1) A. B. Littlewood, “Gas Chromatography,” Academic Press, New York, 1962, p. 108. (2) C. G. Scott, J. Inst. Petrol., 45, 118 (1959). (3) C. G. Scott, “Gas Chromatography 1962,” M. van Swaay, Ed., Butterworth’s,Washington, 1962, p. 36. (4) C . G. Scott and C. S. G. Phillips, “Gas Chromatography 1964,” A. Goldup, Ed., Institute of Petroleum, London, 1965, p. 266. (5) A. V. Kiselev, “Gas Chromatography 1962,” M. van Swaay, Ed., Butterworth’s, Washington, 1962, p. xxxiv. (6) J. F. K. Huber and A. I. M. Keulemans, “Gas Chromatography 1962,” M. van Swaay, Ed., Butterworth’s, Washington, 1962, p. 26. (7) J. C. Giddings, ANAL.CHEM., 36,1170 (1964).
106
ANALYTICAL CHEMISTRY
sorbents by GSC is of considerable interest. By varying the adsorbent and comparing the adsorption changes resulting for different adsorbates, the adsorptive effect of the different moieties present can be determined. Kiselev (8) has shown that interactions may be divided into nonspecific and specific types from his work with graphitized carbon. A nonspecific interaction is representative of paraffinic hydrocarbons, in which every bond is a sigma bond. In contrast pi-bonded systems can exhibit specific interactions because of the presence of a region of high electron density. The existence of specific interactions presupposes a substrate that couples with the specific center of the adsorbate. Hence, graphitized carbon shows only nonspecific interaction for all compounds while silica or alumino-silicates, for example, can exhibit nonspecific adsorption for aliphatics and a combination of nonspecific and specific adsorption for olefins. Kiselev also notes that the usual conception of polar and nonpolar adsorbates and adsorbents fails to explain the nature of physical adsorption. Other workers have found that a polarity effect exists and that the polarizability of a molecule affects its adsorptione.g., the large retention volumes of olefinic materials on polar adsorbents ( 4 , 9 ) . The present paper summarizes the results of a detailed investigation by gas-solid chromatography of specific adsorptive interactions for several modified aluminas. The effect of increasing polar character while pi-bond and hence polarizability remain constant for a series of adsorbates has been of particular interest. EXPERIMENTAL Analabs Type F-1 activated alumina was used as the adsorbent. In most cases this material was acid washed in 6F HC1 overnight to remove iron contamination. The acid washed material was then washed with distilled water, dried at 120” C, and the 100-110 mesh portion separated by means of ASTM sieves. The salt used for coating was weighed and (8) A. V. Kiselev, “Gas Chromatography 1964,” A. Goldup, Ed., Institute of Petroleum,London, 1965, p. 239. (9) B. T. Guran and L. B. Rogers, ANAL. CHEM., 39,632 (1967).
dissolved in 5 ml of distilled water. Next, the appropriate amount of alumina was added, the mixture was stirred with a pointed glass rod, and the water was evaporated on a steam bath. Enough salt was used in every case to give a coating of 10% by weight of the alumina on an anhydrous basis. The coated adsorbent was dried at 110" C before the chromatographic column was packed. The dried adsorbent was packed in a 3-foot length of '/*-inch 0.d. (thin-wall) stainless-steel tubing which previously had been rinsed with both polar and nonpolar solvents. After each increment of packing was poured into the tube, the tube wall was tapped with a metal rod while the end of the column was repeatedly tapped on the floor. Each column was activated for 2 hours at 300" while in contact with reactor grade helium (also used as the carrier gas). Samples of adsorbate were injected by means of a gas-tight syringe. Either the vapor in equilibrium with the liquid sample or liquid injected into an air-filled septum-stoppered vial was used as a source for sample vapor. In general the gram for each adsorbate sample size was about 1 X tested. A modified Barber-Colman Series 5000 gas chromatograph equipped with a flame ionization detector was used for the chromatographic measurements. Relative retention times of the adsorbate were determined and related to the retention time of pentane. At least three injections of pentane were made followed by at least three injections of the adsorbate to be tested; three more injections of pentane were made to ensure that the adsorbent had not changed. By this approach the measurement of flow rates and pressure drops across the column was avoided because these variables affected both sample and standard (pentane) to the same degree. Data for which the average retention times of successive pentane trials differed by more than 1 % were discarded as were those for replicate trials whose range differed from the mean by more than 2%. Surface areas were determined by nitrogen adsorption (10). The relative retention times were converted to apparent retention volumes, VIR,by
Table I.
Retention Volumes Relative to Pentane for Modified Alumina Columns
Adsorbents: 10%salt on acid-washed F-1 alumina Column temp.: 250.0" 2~ 0.3" C
Compound A. Hexane
Cyclohexene Benzene Octane Octene-1 Toluene Chlorobenzene p-Xylene Mesitylene B. Pentane ( V R ,ml)
NaCl
Na2S04NazMo04Na3P04NaOH
1.85 2.66 6.13 6.31 9.60 14.5 16.1 27.2 45.9 5.56
1.78 2.78 6.63 5.41 7.86 13.4 13.9 24.5 44.1 5.10
1.90 2.71 5.91 7.63 10.5 13.4 14.0 28.4 53.0 11.5
1.58 2.38 4.32 4.72 6.75 8.30 9.67 15.3 24.6 6.13
1.38 1.86 2.86 3.62 4.64 4.95 6.17 8.26 13.0 4.20
Table 11. Specific Surface Areas of Modified Alumina Adsorbents
Area, m2/gram F-1 Alumina, acid-washed (unmodified) (A) A 10% NaCl A 10% Na2S04 A 10% NazMoOa A 10% Na3 POc A 10% NaOH
+ + + + +
278 i 7 269 i 7 254 i 7 279 =!= 7 210 zt 5 233 :& 6
order remains the same (with the exception of benzene on Na2S04). Each of the compounds in Table I exhibits good peak symmetry which implies that the most active adsorptive sites on the original alumina are salt-saturated or otherwise blocked, and that each modifying salt affects adsorption in the same manner but to a different degree. Minimum retention volumes are found with the sodium where S is the average apparent retention time ratio of the phosphate and sodium hydroxide modified adsorbents. Both compound in question, compared to pentane, and Y'R(pentane) materials are known to react with alumina and also cause a is the apparent retention volume of pentane. The value of large decrease in surface area relative to the other salts tested the latter was determined by the expression (Table 11). Sodium molybdate produces essentially no decrease in surface area and gives the highest retention volumes V'R(pentane) = f ' R (Tc/TA)* f * Fa (2) of the five modified aluminas. However, the order of eluwhere f l R is the apparent retention time of pentane measured tion is the same as for the other adsorbents (Table I). from the injection point to the point of 'maximum eluate Whether, for salt-coated aluminas, the adsorptive surface is (adsorbate) concentration, Tc and T, are the column and merely an attenuated alumina or the salts (as surfaces) adsorb flowmeter temperatures ( " K), respectively, F , is the volume in a similar fashion is unclear. Both acid-washed and nonflow rate at ambient temperature and pressure, and f i s the acid-washed aluminas coated with varying amounts of salts James-Martin gas compressibility correction factor. The give the same order of elution listed in Table I. However, apparent retention volumes were converted to corrected retention volumes, V R ,by the retention of olefins compared to that of paraffins is greatly enhanced by acid-washing the alumina and, hence, by the V R = V'R - v d (3) attendant surface area increase. where V d is the system dead volume (void space) as determined The retention volumes of several closely boiling groups of by injection of methane onto an initial base line of hydrogen compounds with different degrees of polarity are listed in and methane. For these conditions, methane was not reTable 111. If dipole-dipole interaction between the adsorbent tained by the adsorbent. and the adsorbate is the main contribution to retention of polar compounds, then the more polar compound within RESULTS AND DISCUSSION the groups should be eluted last. However, chlorobenzene The relative retention volumes for a series of adsorbates is eluted before ethylbenzene and bromobenzene is eluted on five different salt-coated columns are summarized in Table before cumene, both cases being in opposition to polarity I. Although the absolute retention volume of a given comconsiderations. A possible explanation for the longer repound varies considerably from column to column, the elution tention of the alkyl-substituted aromatic hydrocarbons over the halobenzenes is the electron withdrawing propensity shown by the halogen and the concomitant loss in electron density (10) F. M. Nelson and F. T. Eggertsen, ANAL.CHEM.,30, 1387 (1958). in the aryl ring. This would cause a decreased interaction VOL 40, NO. 1, JANUARY 1968
107
between the adsorbate and adsorbent and be reflected by decreased retention volumes. Another factor may be the polarizability of the molecules which is indicated by their molar refraction. Values of the latter are tabulated in Table I11 and, within the first two groups, give a fair correlation with retention volume. In contrast, the retention volumes of the chlorotoluenes follow closely the order of their polarity (Table 111). The difference between the aryl halides and the chlorotoluenes can be rationalized on the basis that each isomer of the latter has approximately the same ring electron density (neglecting, as a first approximation, the effect of differing position of ring substitution), because each has identical substituents.
Table 111. Effect of Dipole Moment upon Retention Volumes for Compounds of Similar Boiling Point Column: F-1 alumina coated with 10% NaCI, 250° C Boiling Dipole Molar refracpoint, moment, Compound C Debye VR, ml tion, cm3 52.8 36.2 A. Ethylbenzene 132 0.6 38.8 31.2 Chlorobenzene 136 1.7 72.1 40.5 B. Cumene 152 0.8 Bromobenzene 155 1.7 59.4 34.1 79.5 35.8 1.9 C. o-Chlorotoluene 159 72.4 36.2 1.8 rn-Chlorotoluene 161 79.5 36.2 1.9 p-Chlorotoluene 159
Table IV. Effect of Pi Electrons and Structure upon Retention Volumes for Compounds of Similar Boiling Point Column: F-1 alumina coated with 10% NaCI, 250" C Boiling Compound point, O C VR, ml 69 7.89 A. Hexane 63.5 10.2 Hexene-1 cis-Hexene-2 69 10.4 trans-Hexene-2 68 9.20 1,CHexadiene 65 11.5 B. Cyclohexane 81 7.78 Cyclohexene 83 10.5 1,3-Cyclohexadiene 80.5 14.0 1,CCyclohexadiene 86.5 15.4 80 17.0 Benzene
Table V. Effect of Column Temperature on Retention Column : 20 % Na2S04 on F-1 alumina, 80-90 mesh A. Relative Retention of Cis- to Trans-Zpentene Temperature, C VR(ois)/VR(trans) 100 1.33 150 1.22 1.18 200 250 1.07
B. Retention Volume Ratios for Cyclic Hydrocarbons Temperature, "C Compound VR(cpd)/VR(pentane) 150 Cyclohexane 1.76 3.35 Cyclohexene Benzene 8.50 250 Cyclohexane 1.33 Cyclohexene 1.66 2.53 Benzene
108
ANALYTICAL CHEMISTRY
Hence, only boiling point differences and polarity differences contribute to their relative retention. The retention volumes of several closely-boiling linear and cyclic six-carbon hydrocarbons are tabulated in Table IV, and indicate the effect of pi electrons and structure on retention. These compounds have been chosen on the assumption that their similarity in boiling point reflects a similarity in degree of nonspecific adsorption. The data in Table IV indicate that retention on NaClmodified alumina is a function of the degree of pi-electron character of the adsorbate plus a steric factor. Presumably, as suggested by Phillips (4), the cis-olefins can more closely approach the adsorptive surface and, hence, interact more strongly than the trans isomers, although they possess the same degree of pi character. The hexene-1 isomer is held more strongly than the trans isomer, presumably because of the diminished amount of carbon chain interference to the double bond's interaction with the adsorptive surface. The presence of a geometrical factor is supported by the data for n-hexane and cyclohexane in Table IV, which establishes that the linear molecule is retained longer than the cyclic compound even though that latter has a 12" C higher boiling point. This probably results from the fact that for n-hexane all six carbons can be made to approach a plane, while only three or four carbons lie on plane in cyclohexane. The longer retention of 1,Ccyclohexadiene relative to the 1,3-isomer may be due to differences in the amount of nonspecific adsorption between the two, or may be because 1,4-~yclohexadiene can bring the most pi-bonded carbons into direct contact with the adsorptive surface. The effect of column temperature on the relative retention of compounds is illustrated by Table V. Clearly lower column temperatures provide greater resolution, and an enhancement of the specific interaction due to pi electrons. Adjustment of column temperature provides an important variable for idealizing the separation of a complex mixture. The retention volumes of the n-paraffin series (pentane through dodecane) increase with increasing chain length as well as with increasing molecular weight or boiling point. Because at temperatures below their boiling points these compounds have heats (enthalpies) of adsorption on alumina of approximately the same magnitude as their vaporization enthalpies, a plot of the logarithm of retention volume us. boiling point should be linear for data determined in this temperature range. This expectation has been realized by Patton, Lewis, and Kaye (11). Above the boiling points of the hydrocarbons, however, such a plot is curved at the low temperature end, but straightens to a limiting slope (Figure 1). However, for benzene and the methylated benzenes a linear relationship is observed. This line is considerably above the paraffin line because of specific interactions of the adsorbent with the aromatic rings. Mono-olefins give a fairly straight line intermediate between the methylated-aromatic line and the paraffin line because the degree of specific interaction (pi-character) is less than for the aromatic molecules. Plots similar to Figure 1 also have been prepared for the NaCl, NazMo04,N a 8 O 4 , and NaOH coated columns. The slopes for the various classes of compounds with each column are summarized in Table VI. Reference to the slope values establishes that the sodium molybdate modified column
(11) H. W. Patton, J. S. Lewis, and W. I. Kaye, ANAL.CHEM. 27, 174 (1955).
3.0
Table VI. Slopes of Log Retention Volume cs. Boiling Point Plots (see Figure 1) Column: 10% salt on acid-washed F-1 alumina, 250" C Compound Type NaCl NazS04 NazMo04Na3P04 NaOH Paraffins" 0.0101 0.00973 0.0112 0.00937 0.00641 Methylbenzenes 0.0133 0.0105 0.0138 0.0106 0.00893 0.0106 0.00841 0.0104 0.00806 0.00667 Mono-olefinsb
2.0
a
>"
Limiting slope. Calculated from mean values of all geometrical isomers used.
0
s
Table VII. Specific Interaction Free Energies (- AGspecifio), Calories/Mole Column: 10% salt on acid-washed F-1 alumina, 250" C NaCl Na2S04Na2Mo04Na3P04NaOH Single pi-bond (normal olefins) 440 490 410 230 310 Aromatic ring 1050 1230 (planar) 900 850 560 Nonplanarity -180 -180 -60 -60 0
I .o
0.0 0
260
Figure 1. Logarithm of retention volume cs. boiling point for a 10 Na2S04modified alumina column (acid-washed) at 250OC A , normal alkanes; B , cyclic alkanes; C , normal alkanes; D,benzene and methylated aromatics. Cyclohexene indicated by 0and substituted aromatics by appropriate symbols
is the most selective of those tested for paraffins and methylbenzenes. Thus, for this adsorbent the change in retention volume is greatest for a given change in boiling point of the adsorbare. The sodium chloride modified column is as specific as the sodium molybdate column for mono-olefins. If the increase in retention volume with carbon number (and hence with boiling point) for the paraffin series is identified with an increase in the nonspecific adsorption, then plots similar to Figure 1 can be used to elucidate the nature of the specific adsorption process. This assumption seems reasonable in view of the smoothness of the normal alkane curve in Figure 1. This implies that the degree of nonspecific interaction is related to the ease of condensation of the adsorbate or, possibly, to the molecular weight of the adsorbate. Using this approach, the vertical distance between the normal alkane line and any point on the plot is related to the degree of specific interaction shown by the compound representing that point. If there are several sources of specific interaction then the total free energy of interaction, AGa, is given by
where the specific interaction summation includes all possible sources of interaction other than the nonspecific one. Such effects as pi-character and steric effects are included in the specific interaction portion. In the absence of absolute values for the total free energy of adsorption, the specific interaction energies can be evaluated by considering the vertical distance between the nonspecific interaction line (the normal alkane line) and any point. This distance must be due only to specific interaction and can be related to the free energy of specific interaction by the relation (12) (12) E. Cremer and R. Muller, 2. Elecrrochem., 55,66 (1951).
(AG3*)
- (-AGz*)
= 2.3 RTlog
(VRJJ'RJ
=
2.3 RT(D) (4)
which gives the difference in the standard free energies of two adsorbates. The vertical distance, D, corresponds to the logarithm of the retention volumes (see Figure 1). The use of this equation to calculate specific interaction energies is unequivocal when any two compounds are compared; its extention to the extrapolated curve between two paraffins is justified on the basis that this curve would be followed by intermediate linear paraffins if they existed. Equation 4 has been used to calculate the specific adsorptive effect of a single double bond by substituting the D value measured from the normal alkane line to the mono-olefin line; the mean value for each modified alumina is summarized in Table VII. The specific adsorptive effect of an aromatic center has been determined by measuring the distance from the paraffin line to the methylated benzene line; the average value for each modified alumina also is summarized in Table VII. Reference to Figure 1 indicates that aromatic molecules with bulky side groups are eluted before the more planar aromatics such as benzene and methyl substituted benzenes. This behavior appears to be related to the degree of nonplanarity of the adsorbate, because the deviation of ethyl benzene from the planar aromatic line is less than either that of cumene or t-butylbenzene. Similar results are obtained for the other four modified columns. Presumably molecules with bulky substituents offer greater obstruction to the aromatic center attaining a position parallel to the adsorptive surface. The quantitative effect of this nonplanarity effect has been calculated by Equation 4; the mean of the result for cumene and t-butyl benzene for each column is summarized in Table VII. Evidently, the sodium chloride and sulfate adsorbents have similar specific interaction effects as do sodium phosphate and molybdate. In general, there is an approximate correlation between the interaction free energies and the number of pi electrons in the sample molecule. Reference to Figure 1 indicates that aryl halides are eluted earlier than predicted by a simple planar aromatic specific interaction. This earlier elution appears to be related to the V O L 40, NO.
1 , JANUARY 1968
109
Table VIII. Aromatic Substituent Effects on Retention; Decrease of Free Energy of Specific Interaction Column: 10 NazS04on acid-washed F-1 alumina, 250” C Substituent UP - AGsipma,cal/mole -CHI -0.17 0 -F +0.06 0 - CI +0.23 -440 - Br 1-0.23 -730
decreased ring electron density and, hence, diminished interaction between adsorbate and adsorbent. If this is the case, the Hammett sigma constant, u p , which is a measure of the decrease in the electronic ring density (I3), should indicate the magnitude of the decreased interaction. This decrease has been evaluated by Equation 4 for the Na2S04column; the results and the sigma constants for the parasubstituted halogens are tabulated in Table VIII. The positive Hammett sigma constant implies that the ring electron density is decreased, which corresponds to the proposed decrease in interaction. The planar aromatic line undoubtedly includes the effect of increasing the ring electron density by successive addition of methyl groups. Hence, the -CH3 substituent lies on the aromatic line and provides (13) E. S. Gould, “Mechanism and Structure in Organic Chemistry,” Holt, Rinehart and Winston, Inc., New York, 1959, p. 221.
the base for reference of the halide substituent effects. The effect of fluoride substitution is too small to measure, and conforms to the small value for the sigma constant. CONCLUSIONS
The data establish that physical adsorption on salt-modified alumina is a composite of nonselective (nonspecific) and specific contributions. The latter is a mixture of selective effects due to the pi-character of the adsorbate, its planarity, its substituents (if aromatic), and its cis-trans geometry (if olefinic). Specific salt effects have not been established, but are the subject of additional studies. Gas-solid chromatography provides a convenient means for the study of adsorption in terms of adsorbent, adsorbate, and inorganic salt-pi electron interactions. The approach, as outlined, should provide a convenient means for the study of molecular structure, the pi-character of organic molecules, and the electronic effects of aromatic ring substituents. From an analytical view, GSC with modified alumina adsorbents provides selective separation of cis and trans isomers and of mixtures containing paraffinic, olefinic, and aromatic hydrocarbons with similar boiling points. RECEIVED for review June 8, 1967. Accepted October 11, 1967. Work supported by the U. S. Atomic Energy Commission under Contract No. AT(l1-1)-34, Project No. 45. We also are grateful to the Public Health Service for an Environmental Sciences Predoctoral Fellowship to D.J.B.
Gas Chromatographic Analysis of Isomeric Diaminotoluenes L. E. Brydia’ and Friso Willeboordse Chemicals and P h t i c s , Union Carbide Corp., South Charleston, W. Vu. 25303 A gas chromatographic method has been developed for the determination of the isomers of diaminotoluene. The method consists of reacting the diaminotoluene isomer mixture with trifluoroacetic anhydride to form the corresponding diamides. The diamides are then analyzed using one of two procedures. Procedure A i s a simple and rapid method for the quantitative determination of the diaminotoluene isomers but does not resolve the 2,4- and 2,5-isomer derivatives. This is not a serious limitation for analysis of diaminotoluenes produced by the direct nitration of toluene since the 2,5-isomer is normally present only in trace amounts. Procedure A thus serves as an excellent method for quality control of plant-grade material. Procedure B resolves all five isomers normally present in crude diaminotoluene. However, the procedure is slightly longer than Procedure A because of an evaporation step and longer retention times.
DIAMINOTOLUENES have established significant importance as chemical intermediates because they are basic ingredients in the manufacture of polyurethane foams and elastomers. Production of these materials usually begins with the direct nitration of toluene to produce a mixture of isomeric dinitrotoluenes. The dinitrotoluenes are hydrogenated to the corresponding diaminotoluenes, also called tolylenediamines Present address, Chemicals and Plastics, Union Carbide Corp., Bound Brook. N. J. 08805 1
110
ANALYTICAL CHEMISTRY
(TDA), which are then phosgenated to toluene diisocyanates (TDI). The toluene diisocyanates are reacted with an active hydrogen-containing polyfunctional molecule, usually a polyol, to foim the urethane. An accurate determination of the positional isomers of the intermediate diamines is important. The ortho isomers (2,3- and 3,4-) are not phosgenated to the corresponding diisocyanates, but form undesirable ureas, and therefore lower the overall efficiency of the TDA-TDI process. It is also desirable to know the relative concentration of the meta isomers (2,4- and 2,6-) since they affect the process exotherm conditions and the ultimate physical properties of the polyurethane. Despite the importance of the isomer content of diaminotoluenes, no suitable, single method for this determination has been reported. The existing methods for this determination have recently been summarized by Mathias, who used NMR for this purpose ( I ) . The NMR method is based on the slight separation of the methyl proton resonances of the TDA isomers in deuterated chloroform solution. This method lacks sensitivity to minor components and gives relatively poor accuracy. An infrared spectrophotometric method (2), specifically developed to determine the 2,4- and 2,6-isomer content of a binary mixture, also lacks sensitivity (1) A. Mathias, ANAL.CHEM., 38, 1931 (1966). (2) A. I. Finkel’shtein and E. N. Boitsov, Zacodsk. Lab., 26, 959
(1960).
