Estimation of Types of Nitrogen Compounds in Shale-Oil Gas Oil G. U. DINNEEN,
G.L. COOK, and H. B. JENSEN
Petroleum and Oil-Shale Experiment Station, Bureau of Mines, laramie, Wyo.
F A systematic procedure for the separation and characterization of the types of nitrogen compounds in shaleoil gas oil has been developed. First, a concentrate of the nitrogen compounds was prepared from a gas oil by adsorption. The concentrate was separated into fractions b y molecular distillation and thermal diffusion. The types of compounds present were postulated by interpreting the similarities and differences shown in spectra on the series of fractions. Elemental and functional group analyses, and physical property determinations were used to aid in these interpretations. A quantitative estimate of the composition of the nitrogen-compound concentrate in terms of model compounds was also made.
A
characteristic of shale oil as compared to petroleum is its high content of nitrogen compounds. I n the gas-oil range, which contains compounds having about 15 to 30 carbon atoms, with which this inveqtigation was concerned, about half of the compounds contained nitrogen. Recently, a discussion has been presentrd of the high-molecular-Iyeight hydrocarbons in petroleum ( 3 ) , but no systematic investigation of nitrogen compounds has been reported. This work has been concerned principally with developing techniques to characterize these compounds Information as to the types of nitrogen compounds in the particular gas oil used in the developmtmt has also been obtained. Pyridines, dihgdropyrindines, indoles, and quinolines comprised over half the nitrogen compounds in the gas oil. Rluch of the remainder consisted of compounds having one or more saturated rings condensed with these compounds. I n addition, smaller quantities of compounds having three or more aromatic rings were present. Many of the molecules in these latter two groups also contained an oxygen atom, generally present in a phenolic group. There were several alkyl substituents per molecule for the mono- and dicyclic compounds, and generally only one of the substituents n-as large. The multiring compounds contained a greater number of substituents and no long pi
2026
OUTSTANDING
ANALYTICAL CHEMISTRY
I.
Table
Properties of Nitrogen-Compound Concentrate
Density a t 20' C. 0,994 Refractive index, nz,O 1.5466 Average molecular wt. (ebullioscopic) 317 Elemental Analysis, Wt. % Carbon 83,41 Hydrogen 10.00 Nitrogen 3.79 Sulfur 0 68 Oxygen ( b y difference) 2.09 chains. About twice as many pyridines as dihydropyridines, indoles, and quinolines were found. Only small amounts of pyrroles were found. PROCEDURE
The gas oil was prepared by distillation from a shale oil obtained from Colorado shale by a n N-T-U retort (10). Preparation of a concentrate of nitrogen compounds from the gas oil by Florisil adsorption has been described (2). This nitrogen-compound concentrate comprised 43% of the gas oil and contained 89% of the nitrogen originally present. Properties are shown in Table I. The separation and characterization work are outlined in Figure 1. SEPARATION
Molecular Distillation. The gas oil
from which t h e nitrogen-compound concentrate was separated had a n approximate boiling range of 625" t o l l O O o F. Two separate portions of nitrogen-compound concentrate, each containing about 700 grams, were
charged to a centrifugal, cyclic, batch molecular still. The pressure was maintained at 2 to 10 microns and the rotor temperature mas increased from 120" to 160' C. in 5' increments. Individual distillate cuts were obtained by a single pass of the charge over the rotor at each temperature level. This procedure gave nine cuts and a residue from each charge. To improve separation, cuts from the preliminary distillations \yere redistilled through the same equipment. Cuts 1 and 2 from each of these distillations were combined and charged t o the still. A distillate cut was obtained by cycling the charge several times over the rotor heated to 90" C. The cut was taken when the distillation rate decreased substantially. A second cut was removed from the remaining charge material by cycling it several times over the rotor at 100' C. Cut 3 from each of the preliminary distillations was then added t o the charge and a distillate cut obtained a t a rotor temperature of 110' C. This procedure of periodic addition of material to the charge and increase in rotor temperature was followed until all of the material from the two preliminary distillations had been redistilled, giving eight distillation cuts and a residue. Thermal Diffusion. Each of t h e eight molecular-distillation cuts was charged t o a &foot, concentric tube, steel column (4) having a n annular space of 0.012 inch, a total capacity of 35 to 40 ml., and withdrawal ports to yield 10 fractions of equal size. Average operating temperatures were 97" C. on the hot wall and 64" C. on the cold wall. The temperature of the water a t the inlet was 56' C. and at the outlet 73' C. Each of the batch runs was continued for 170 hours. ANALYTICAL DETERMINATIONS
FLORISIL
A D S O R r T ION
c
MOLECJLAR STILL4*1OU
AND R E S I D U E
THERMAL
DIFFUSION FROM
EACH
SELECTED FRPCTIONS CHARPCTER I Z E D BY SPECTRAL, P H Y S I C A L , A N 1
Figure 1. Outline of separation and characterization of nitrogen compounds in shale-oil gas oil
Spectral. A Consolidated hlodel 21-103C instrument, modified for high mass operation. was used t o obtain mass spectra. The results were plotted as per cent of total ionization in groups according to the %-number in the empirical formula C,H2,+.h'. Ultraviolet spectra were obtained on a Cary ?\lode1 11 spectrophotometer. Appropriate average absorptivities for pyridines a t 2620 A., indoles a t 2900 A., quinolines a t 3000 A., carbazoles a t 3400 A,, and acridines at 3800 A., were determined in methplcyclohexane solution. It was assumed that only the aromatic rings contributed to the absorptivities. The results were then
expressed as per cent of each of the aromatic ring types. Assuming that the remainder of the molecules consisted of alkyl groups, the difference between total rings and 100% was the per cent of alkyl groups. The number of methyl groups in a n average molecule in any selected fraction was calculated from data obtained on a Perkin-Elmer Model 21 infrared ,111 absorption spectrophotometer. measurements n-ere made by integration between 7.14 and 7.45 microns on ITeighed samples dissolved in carbon tetrachloride. Calibration for the top fraction from the thermal diffusion column n-as based on averaged data obtained for 14 alkl-1 pyridines. To compensate for background absorption in fraction 1, a quantity of pure pyridine equivalent to the ring content of the sample was placed in the reference beam of the instrument. Background in each succeeding thermal-diffusion fraction was largely compensated for by placing a n equivalent amount of the preceding fraction in the reference beam. Physical. Molecular weights were determined ebullioscopically by a modified Menzies-ST'right method (6). Benzene was used as the solvent and nbutyl phthalate or n-butyl sebacate as used as a reference standard. Many of the fractions were too dark for an accurate direct reading of refractive index, so each of these was diluted with a transparent solvent that had approximately the same refractive index as the fraction. A series of solvent mixturc s mas prepared which had refractive indices covering the expected range of values of the fractions. The series consisted of binary blends of Decalin (%ao = 1.479) and dimethylnaphthalene (technical, nao = 1.602). The refractive index of the diluted saniple was obtained and the refractive index of the sample was calculated. The sample was diluted with only enough of the solvent to permit a n accurate reading and the darkest of the fractions required about a fourfold dilution. Chemical. K t r o g e n was determined by t h e Kjeldahl method (7). On the molecular-distillation cuts, carbon and hydrogen were determined by the combustion method and sulfur by the oxygen bomb method (1). On the thermal-diffusion fractions, carbon, hydrogen, and sulfur values were obtained from Huffman illicroanalytical Laboratories, Denver, Colo. For all samples oxygen was obtained by difference. Basic nitrogen was determined bv titration with perchloric acid (8). Carboxylic and phenolic oxygen were determined by potentiometric titration with sodium aminoethoxide in anhydrous ethylenediamine using the method of Rloss, Elliot, and Hall (9). Carbonyl oxygen was determined, utilizing the reaction of hydroxylamine with a carbonyl group to form an oxime. This method has been described by Knotnerus (6). RESULTS AND INTERPRETATIONS
Yield and properties of the cuts from
Table II. Yields and Properties of Cuts from Molecular Distillation of NitrogenCompound Concentrate
Rotor Yield,a AAv. T:mp., Weight, Mol. c. 70 Wt. 252 1 90 3.8 2 100 5.6 261 3 ii0 7.4 269 4 115 278 16.6 5 120 17.5 301 6 125 21.3 319 7 130 8.3 318 8 135 6.4 334 Residue 13.1 504 On a no-loss basis. * By difference. Cut No.
~
Table 111.
Average JIolecular No. Vt. 1 354 2 346 3 344 4 332 5 317 324 Ij 305 8 302 9 307 10 302 By difference.
I
50-
z
1.5597 1.5651 ...~ 1.5638 1.5607 1.5555 1.5516 1.5482 1.5491 ...
~~
~
Refractive Index, n a$ 1.4793 1.4892 1.5052 1 5256 1 5464 1 5658 1 5799 1 5900 1.5981 1,6047
Elemental Analysis, Wt. N S C H 83.00 9.72 3.97 1.18 83.82 9.63 3.71 0.75 .. -~ 84.39 i.8i 3.60 0.69 84.34 10.05 3.65 0.64 84.27 10.24 3.59 0.56 84.12 10.40 3.62 0.52 83.60 10.52 3.66 0.52 83.12 10.40 3.80 0.55 81.67 9.87 4.46 0.83
70 Ob 2.13 2.09 1.46 1.32 1.34 1.34 1.70 2.13 3.17
C/H 8.54 8.70 8.56 8.39 8.23 8.09 7.95 7.99 8.27
\
Elemental Analysis, Wt. 70
C
H
N
S
05
83.24 83.54 83.67 83 60 84 18 84 54 84 67 84.26 84.07 83.53
12.69 12.16
2.90 3.30 3.40 3 37 3 42 3 55 3 68 3.83 4.10 4.36
0.12 0.12 0.39 0 67
1.05
YD I
-
YD4
-
11.94 11 07
10 51 9 93 9 56 9.28 9.07 8.82
0 50 0 68 0 79
0.69 0.62 0.77
0.88
0.60 1 29 1 39 1 30 1 30 1.94 2.14 2.52
C/H 6.56 6.87 7.01 7.55 8.01 8.51 8.86 9.08 9.27 9.47
25.-
z 50I-
e
n2,0
Properties of Thermal-Diffusion Fractions from Molecular-Distillation Cut 6
Fraction
0
Refractive Index,
25-
a w o 0 .
25 48
118
I88
258
328
398
468
m /e
Figure 2. Peak height vs. m / e for ions
ml e
diffusion fractions from molecular-distillation cuts
Figure 3. Peak height vs. m/e for ions in CnH*n-gN group of number 9 thermaldiffusion fractions from molecular-distillation cuts
the molecular distillation are shown in Table 11. Nitrogen is distributed about evenly among the cuts, but the content of nitrogen compounds, calculated assuming one atom of nitrogen per molecule, increases from about 70 to 90%. As would be expected, the average molecular weight increases with rotor temperature. The decrease in the carbonhydrogen ratio is such t h a t the relation-
ship of these elements in the average molecule for each of the cuts is about C,H2,-11. The increase in molecular weight of 82 or about 6 carbon numbers seems to be due principally to the addition of methylene groups to the aromatic heterocyclic nucleus. Additional methylene groups are also suggested by the decreasing trend shown by the refractive-index values. The residue,
in CnHz,-sN group of number 2 thermal-
VOL. 30, NO. 12, DECEMBER 1958
0
2027
which is 17 carbon numbers higher than the first distillate cut, shows additional unsaturation equivalent to only one aromatic ring. Because of the similarity of the compositions of the molecular-distillation cuts, thermal diffusion should achieve similar separations on each of them. Therefore, characterization of all 80 thermal-diffusion fractions was not attempted. Selected fractions were examined extensively and correlations used to extend the data to other fractions. Most intensive examinations were made on the thermal-diffusion fractions from molecular-distillation cut 6. Properties of these fractions, which are given in Table 111, are representative of those obtained on other molecular-disti!lation cuts. Significant separation of types of compounds is indicated by the decrease in molecular weights and the increase in refractive indices and carbonhydrogen ratios. Increase in total nitrogen content is approximately inverse to the molecular weight decrease so the content of nitrogen compounds, calculated assuming one atom of nitrogen to the molecule, is about 80% for most of the fractions. The content of sulfur compounds is less than 8% for all f the fractions and is very low, about %, in the first two fractions. The sum of nitrogen, oxygen, and sulfur compounds is close to 100% for the first five fractions. However, it increases rapidly in subsequent fractions, reaching a maximum of about 150% for fraction 10, indicating the presence of more than one hetero-atom in many of the molecules, Values for basic nitrogen, and carbonyl, phenolic, and carboxylic oxygen are shown in Table IV. The size of the individual thermal-diffusion fractions prohibited making all analyses on a single series, but, as indicated, broad trends shown should be similar for fractions from other moleculardistillation cuts, The nitrogen in fraction 1 is nearly all basic whereas only about twofifths of that in fraction 10 is basic. The decrease in carbonyl oxygen from fractions 1 to 10 represents a decrease ~~
Table IV.
1
TD I
c-
w 0-
47
Total
2 3 4 5 6 7 8 9 10
e
2.90 3.30 3.40 3.37 3.42 3.55 3.68 3.83
2.75 3.23 2.96 2.47 2.12 1.77 1.95 1.84 1.97 1.69
ANALYTICAL CHEMISTRY
187
25'
!27
197
46.
Figure 5. Peak height vs. m/e for ions in CnH2,-9N group of thermal-diffusion fractions from molecular-distillation cut 6
'T
n
50-
1
m le
Figure 4. Peak height vs. m/e for ions in CnHzn-~Ngroup of thermaldiffusion fractions from moleculardistillation cut 6
from about four fifths of the total oxygen in fraction 1 to practically nothing in fraction 10. Conversely, the phenolic oxygen increases from a minor proportion of the total oxygen in fraction 1 to about four fifths of the oxygen in fraction 10. The values for carboxylic oxygen show no regularity and generally represent only a small proportion of the total oxygen. Mass spectral data were used to corroborate and extend the ideas formulated as to the types of compounds in the nitrogen concentrate. The spectrum of any one fraction was complex
~
Basic
1 7
m/e
75:
Carbonylb
Phenolice
Carboxylic8
0.02 0.15 0.07 0.09 0.10 0.06 0.00 0.04 0.00 4.10 0.02 4.36 5 Determined on thermal-.diffusion fractions from molecular..distillation cut 6. b c u t 5. 0 c u t 7. 1
L_
Y
Functional-Group Analyses of Selected Thermal-Diffusion Fractions Nitrogen," Wt. % Oxygen-Type Analyses, Wt. yo
No.
75-
TO IO
Fraction
2028
i 4
0.88 0.45 0.34 0.30 0.26 0.26 0.21 0.21 0.02 0.01
0.18 0.16 0.20 0.34 0.65 0.87 1.05 1.16 1.26 1.74
- ~ E R M & L - O I F F U S I C N F R A C T I C N NUMBER
Figure 6. Curves illustrating groups into which maxima in mass spectra of thermal-diffusion fractions may be classified
and difficult to interpret. However, by grouping mass spectral peaks according to the empirical formula CnHzn+=Nand considering similarities or differences within these groups for a series of fractions, interpretations could be made. Mass spectra of thermal-diffusion fractions of the same number from each of the moleculardistillation cuts were compared. Illustrative of this comparison are the results for the CnH*%-sNgroup for the thermal-diffusion fractions 2 and 9 that are shown in Figures 2 and 3. The separation achieved by thermal diffusion is indicated by the differences between the sets of curves in the two figures even though they are due to ions having the same empirical formula. For example, in Figure 2 the ions are
Figure 7. Distribution of model nitrogen cornpounds in moleculardistillation cut 6
THERMAL-DIFFUSION F R A C T I O N N U M G E R
concentrated in two regions, about m / e 160 and 328, whereas in Figure 3 the ions are concentrated in the region of ni/e 258. The curves in each figure are generally similar except for an increase in molecular weight with cut number. This may be seen in Figure 2 as the major concentration of ions is a t m/e 146 and 160, whereas the secondary maximum occurs at a n increasingly high m/e. The peak height a t m/e 328, which increases from 0.05 in MD1 to 0.35 in MD8 also illustrates the latter point. This trend is strong support for the concept, mentioned earlier, that the molecular-distillation cuts are similar except for the addition of methylene groups to heterocyclic nuclei. The curves for two groups, C,,HZ,,-~N and C,H2,,-9N, from a series of thermaldiffusion fractions from molecular-distillation cut 6 are given in Figures 4 and 5. Fractions were selected from each series to illustrate the trends that may be observed. The intensities of the maxima in the curves vary systematically throughout the series of thermaldiffusion fractions and so must represent the presence of different types of compounds. Although there are variations among the fractions, the maxima could be classified in three major types which are illustrated by the curves in Figure 6. The type represented by curve A had the greatest intensity in fraction 1 and decreased throughout the remaining fractions. The maximum a t m,'e 134 in Figure 4 is of this type. Curve B represents maxima that reached their highest value in fractions about one third to one half the way down the column, similar to those a t m/e 159 and 313 in Figure 5. The maxima represented by curve C, such as the one a t m/e 243 in Figure 5, increased throughout the fractions.
Maxima of type A were principally in the plots of 2-numbers -5, -6, -7, and -8. Alkyl pyridines have an 2number of - 5 , and cycloalkano pyridines (such as dihydropyrindines or tetrahydroquinolines) have an s-number of - 7 . Thus, the type A maxima correspond to the parent and parent-minusone ions of these types of compounds. Cltraviolet spectra and the high basic nitrogen content of the upper fractions, as well as the fact that thermal diffusion should concentrate this type of compound a t the top of the column, support this assignment. The major concentration of fragment peaks n as in the Cs to CIo range, as illustrated by the maxima a t m / e 134 in Figure 4,which is typical of aromatic compounds with several substituents when all but one of the substituents are small. Infrared spectra showed the presence of chains containing a t least four methylene groups and of about five methyl groups per mole n hich might indicate some branching in the alkyl substituents. Peaks corresponding to a small amount of pyrroles and to the carbonyl-containing compounds discussed previously, were also represented by curve A , Figure 6. Maxima of type B occurred in the plots of 2-numbers -9 to -14. Indoles have an %-numberof -9, quinolines and tetrahydrocarbaxoles have an s-number of - 11, and tetrahydroacridines have an s-number of - 13. While these types of compounds are the preferred assignments, other types of compounds with the proper r-number, such as cycloalkylindoles (z= -11), are possible constituents. Ultraviolet spectra, the decrease in basic nitrogen, and the increase in carbon-hydrogen ratio confirm these assignments. The s-number plots contained maxima in the parent ion region and a t the appropriate ring plus three or four carbon atoms. As
n-ith the preceding group, this suggests multiple substitution with only one of the substituents having more than two carbon atoms. Infrared data showed a slightly higher concentration of methyl groups probably indicating some additional substitution on the larger rings. bIaxima corresponding to curve C nere distributed about evenly throughout all the z-numbers. Infrared spectra on the lower fractions from the thermaldiffusion column indicated about nine methyl groups per molecule and no long chains. This value for methyl groups is about 50% greater than that obtained on the upper thermal-diffusion fractions. The trend indicated is considered reliable, but the method used does not give dependable absolute values on the materials in this boiling range. The absence of long chains mould also be required in these fractions because nearly all of the carbon atoms are used in rings and methyl groups. To indicate the relative importance of the various groups of compounds, the quantities present in each thermaldiffusion fraction from molecular-distillation cut 6 were estimated from mass spectra. The results, which \vere calculated from a summation of selected parent and parent-minus-one peaks, are shown in Figure 7 as relative per cent of nitrogen compounds. Some of the groups shown in the figure contain ions of more than one of the types discussed previously. ils a check of the results, the carbon-hydrogen ratio was calculated from them and was found to agree closely v i t h that shown in Table 111 for each fraction. I n the low-numbered fractions the molecules probably contain only nitrogen in addition to carbon and hydrogen. However, in the highernumbered fractions, oxygen, which was shown to be largely phenolic, is probably also present. Therefore, the names given on the figure are intended only as an aid in picturing the types of compounds that may be present. For example, the area labeled acridines includes any type of compound having an %-number of - 17, such as tetrahydrobenzocarbaxoles, or a molecular vieight corresponding to azaliydrocarbons in the -17 group, such as tetrahydrobenzacridines containing a phenol group. The results on this one cut should be rcprescntative of all the distillation cuts. Another presentation of the data is based on the number of aromatic rings in the molecule. On this basis, single-ring compounds, which are essentially alkyland cycloalkanopyridines, comprise about 35% of the nitrogen compounds in the gas oil. Bicyclo aromatic compounds, indoles and quinolines, make up 25%. The remaining 40% are multiring compounds, many of which contain both oxygen and nitrogen. As stated, only small quantities of pyrroles were found. The reactivity of these VOL. 30, NO. 12, DECEMBER 1958
0
2029
and other compounds may have caused them to be lost during treatment. ACKNOWLEDGMENT
The authors express appreciation to
J. S. Ball, C. S. Allbright, R. A. Van Meter, I. A. Jacobson, C. F. Dieter, and R. A. Neyer who participated in early phases of the problem. They also thank W. S. LlcAuley, R.A. Robb, D. G. Earnshaw, and J. A. Lanum for assistance in obtaining the analytical data presented.
L!TERATURE CITED
.,
(1) Am. SOC.Testing Materials. Standard Method D 129-49r1949 Book of Stand-
ards, part 5, pp. 770-2.
( 2 ) Dinneen, G. U., Smith, J. R., Van hleter, R. A , , Allbright, C. S., Anthoney, W. R., - 4 ~ 4 CHEW ~ . 27, 185 (1955). (3) Hood. 4.. Clerc. R . J.. O'Neal. hl. I
,
J., Preprints, Division of Petroleum Chemistry, ACS 2, No. 1, 221 (1957). (4) . . Jones, A. L., Milberger, E. C., Znd. Eng. Chem. 45; 2689 ( 3 5 3 ) . (5) Kitson, R. E., Oemler, A. S., Mitchell, John, Jr:, AsAI. CHEM:21, 404 (1949): (6) Knotnerus, J., J . Znst. Petrol. 4 2 , 355 (1956). (7) Lake, G. R., RlcCutchan, Philip, '
Van Meter, Robin, Neel, J. C., ANAL. CHEM.2 3 , 1634 (1961). (8) Moore, R. T., McCutchan, Philip, Young, D. A., Zbid., 23, 1639 (1961). (9) hloss, AI. L., Elliot, J. H., Hall, R. T., Zbid., 20, 784 (1948). (10) Ruark, J. R., Berry, K. L., Guthrie, B., U. S. Bur. hlines Rept. Invest. 5279 (1956). RECEIVED for review February 8, 1958. Accepted July 28, 1958. Symposium on Kitrogen Compounds in Petroleum, Division of Petroleum Chemistry, 132nd hleeting, ACS, Sew York, September 1957. Work done under a cooperative agreement between the UniverEity of Wyoming and the Bureau of Mines, IT. S. Department of the Interior.
Gas-Liquid Chromatography of Hydroxyl and Amino Compounds Production of Symmetrical Peaks H. S. KNIGHT Shell Development Co., Emeryville, Calif. Tailing is caused by nonideal adsorption of sample components on the surface of the support. It is reduced b y adding a strongly adsorbed material continuously with the helium carrier gas. For example, added water reduces tailing of water and alcohol peaks. Leading, in which the forward edge of the peak is distorted, is caused by nonideal solution effects, It is controlled by raising the temperature.
G
as-LIQUID chromatography has become a popular analytical tool because of its versatility and simplicity. One factor tending to decrease its effectiveness has been the occasional appearance of unsymmetrical peaks. I n extreme cases such peaks may obscure adjacent small peaks, but more generally, they simply complicate the area measurement. The dissymmetry may take two forms. I n one the forward edge of the peak is distorted, and this is here called leading. I n the other the rear edge of the peak may be elongated, which is called tailing. Leading can occur with hydrocarbon samples. These are generally free of tailing in gas-liquid chromatograph17 ( 3 ) . Both leading and tailing are encountered with more polar samples. For example, tailing virtually prevents the analysis of a mixture of lower alcohols and water on a dialkyl phthalate or silicone column. Tailing is less pronounced on more polar solvents such as triethylene glycol, but this type of solvent may not be otherwise suitable for the sample.
2030
ANALYTICAL CHEMISTRY
I n this paper the more fundamental causes of distorted peaks are discussed, and some simple remedies are suggested. Apparatus factors that may cause tailing, such as slow sample vaporization and dead space leading to diffusional effects, are neglected as being relatively unimportant in well-designed equipment.
compound 2 in a binary. Note that in an ideal system a2 and al are both unity and the emergence times of compounds 1 and 2 are inversely proportional to their vapor pressures. An increase in a value has the same effect as increasing the vapor pressure.
APPARATUS
Two kindsof peak distortions are prevalent (tailing and leading). These may be interpreted in terms of a values as follows. The symmetrical peak is Gaussian, or bell-shaped, and a t the base of the bell, the extreme tips represent regions of lowest solute concentration. If the a value a t low concentration is higher than in the bulk of the peak, these traces of material tend to remain in the carrier gas. Consequently the tips move ahead. The rear tip merges with the peak and the leading tip enlarges. This is leading. Conversely, if the n value a t lowest solute concentration is a minimum, the tips are retarded and tailing results. If only a trace of solute is present, its emergence time nil1 be governed by the applicable a valuethat is. relative to a large peak. a small peak n ill be early in the leading case or late n.here tailing is involved. There are t x o mechanisms by n hicli the a value might be expected to chaiigc. with concentration. These involve solute-solvent ( 7 ) and adsorption effects. Solute-solvent effects can lead to either higher or lower values of n a t lon concentration, depending on the system. Lower values result from chemical interactions or from molecular size differenrw. The former are uncommon, and
The unit first used for this work was described earlier (1, 4 ) . The temperature was controlled by a bimetallic regulator. Recently a Beckman GC-2 gas chromatograph has been employed. This model has a controller t h a t providesacontinuously variable heat input. The IO-foot by */4-inch outside diameter coiled columns were packed with Johns-llanville Co. crushed firebrick of 20 to 30 or 30 to 60 mesh. The solvent concentrations given later are percentages of the dry support. Neither particle size nor the solvent concentration has any major effect on peak shape. THEORY
The theoretical basis on which the data are interpreted is given by Pierotti et al. (7') and was employed earlier for gas-liquid chromatography of hydrocarbons ( 3 , 6 ):
where a is an activity coefficient and is a measure of the over-all nonideality of the system, t is a n emergence time of the peak maximum measured from the air peak, and Po is a vapor pressure. Subscripts 1 and 2 refer to compound 1 and
CAUSES OF PEAK DISSYMMETRY
