At the present time, this method could not compete with neutron activation analysis or vacuum or gas fusion for bulk analyses because of the time required for an analysis. In addition to a lengthy instrument clean-up, approximately one hour per sample is required and a standard should be run every few samples to be sure no vacuum leaks or cross-contaminations have occurred. However, the technique is unsurpassed when the examination of oxygen inhomogeneities is desired. Several modifications are being made to improve the capabilities of the instrument. Preliminary work indicates that the sensitivity of the technique can be improved by using l60produced by positive argon ion bombardment.
The use of a crysosorption pump similar to that of Harrington, Skogerboe, and Morrison ( I ) should reduce the oxygen blank and ion counting will improve the accuracy and precision in the measurement of low intensity secondary ion beams. ACKNOWLEDGMENT The author thanks T. C. Wilder for providing the in-house oxygen in copper standards and D. A. Corrigan for his discussions of the role of oxygen in copper technology.
RECEIVED for review April 20,1970.
Accepted June 18,1970.
Determination of CI2 Alkylnaphthalenes and Methylbiphenyls in Aromatic Fractions by Capillary Gas Chromatography Jifi Mosteck?, Milan Popl, and Josef K i i i Institute of Chemical Technology, Department of Synthetic Fuel and Petroleum, Technicka' 1905, Praha 6, Czechoslovakia A description i s given of the separation of dimethylnaphthalenes, ethylnaphthalenes, and methylbiphenyls using three capillary columns with different packing selectivities. The efficiency of separation was studied on the determination of these hydrocarbons in fractions of pyrolysis (steam cracker) oil and black coal tar. UV spectrometry was applied to the determination of 2,6- and 2,7-dimethylnaphthalenes, where chromatographic separation did not succeed. This combination facilitated a satisfactory qualitative and quantitative determination of all components. The relative retention times of all components related to naphthalene were measured for all three capillary columns. The method of linear regression then served to determine the slopes of lines and correlation coefficients for the relative retention time logarithm values VI. boiling point. Further, a chart was plotted for C,, alkylnaphthalenes showing the retention time of a component of a column with polyethyleneglycol adipate VI. the retention time on a column with Apiezon L. This chart indicates the differing polarity of individual isomers.
WHILETHE DETERMINATION of naphthalene and methylnaphthalenes in aromatic fractions usually is no great problem, the analysis of dimethylnaphthalenes is much more difficult. Papers discussing this problem differ even in such basic data as the boiling points of the dimethylnaphthalenes (1-5). Moreover, in natural materials these compounds are always accompanied by methylbiphenyls and, to a lesser extent, by further components like alkylindenes, alkylindanes, or even heterocyclics. Such complicated mixtures present a problem which cannot be solved by using a single chromatographic column. The analysis of such mixtures has been achieved by using three columns with different stationary phase selectivities. A correct and reproducible determination of Ciz alkylnaphtha(1) Ta-Chuang Lo Chang and Clarence Karr, Jr., Anal. Chim. Acta, 24, 343 (1961). (2) J. Q. Walker and D. L. Ahlberg, ANAL.CHEM., 35, 2028 (1963). (3) F. J. Kabot and L. S. Ettre, ibid., 36, 250 (1964). (4) B. J. Mair and T. J. Mayer, ibid., p 351. ( 5 ) M. I. Gerber, E. M. Terent'eva, V. S. Orlova, and V. P. Kondrat'ev, Nefrekhimiya, 5, 776 (1965). 1132
lenes and methylbiphenyls was achieved by employing further analytical methods, i.e.,UV spectrometry and mass spectrometry. EXPERIMENTAL Pyrolysis oil is a heavy liquid product distilling above 180 "C. This oil is formed as the by-product of naphtha cracking when manufacturing ethylene and propylene and represents a complex mixture of higher aromatics (6). For analysis, the cut boiling between 130 and 160 "C at 20 mm (i.e., 250-280 OC at 760 mm) was used and is designated Fraction 1 in the following. From the black coal tar, a fraction boiling between 130 and 160 "C at 20 mm (Le., 250-280 "C at 760 mm) was used after removing pyridine bases (extraction into 30 % HzS04) and phenols (extraction into 10% NaOH). This fraction is designated Fraction 2 in the following. Apparatus. The standard gas-liquid chromatographic set CHROM 2 with flame-ionization detection was used, with nitrogen as the carrier gas. All analyses were executed isothermally. A Hamilton 5-pl syringe was used for injection. The capillary columns were made of stainless steel and had i.d. of 0.25 mm. Column with Apiezon L (Metropolitan-Vickers Electrical Co., Ltd.) length 100 m; working conditions: 180 "C, nitrogen flow 0.32 ml/min, column efficiency 86,000 theoretical plates. Column with m-bis(m-phenoxiphenoxi)-benzene (BPB) (Consol. Vac. Co.) length 50 m ; working conditions: 200 "C, nitrogen flow 0.25 mlimin., column efficiency 75,000 theoretical plates. Column with polyethyleneglycol adipate (PEGA) (Carlo Erba Sp.A.Milano) length 50 m ; working conditions: 190 "C, nitrogen flow 0.30 ml/min., column efficiency 52,000 theoretical plates. The columns were coated the usual way using a 1 3 z solution of Apiezon L in benzene, a 10% solution of BPB in benzene, and a 10 solution of polyethyleneglycol adipate in benzene. (6) J. Mosteck9, M. Popl, and M. Kuras, Erdoel Kohle, 22, 388 (1969).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
f
90
6'
TIME, MINUTES
'
0
Figure 3. Typical chromatogram of Fraction 1 PEGA at 190 "C. Peaks as given in Table I 93
6o
TIME, MINUTES
30
Figure 1. Typical chromatogram of Fraction 1 Apiezon L at 180 "C.Peaks as given in Table I
c
90
I1
I
0
T I M E , MINUTES
Figure 2. Typical chromatogram of Fraction 1
1
i Y
90
6
Figure 4.
T M E , MINUTES
103
Typical chromatogram of Fraction 2
BPB at 200 "C.Peaks as given in Table I
Apiezon L at 180 "C. Peaks as given in Table I
UV-spectrometric analyses were carried out on the Unicam Model SP 800 B spectrophotometer in silicon cuvettes 10 mm thick. Mass spectrometric analyses were carried on the mass spectrometer LKB 9000 (LKB Produkter AB Stockholm) equipped with an attached gas-liquid chromatographic set. Identification of Hydrocarbons. All the hydrocarbons in the investigated fractions were identified by addition of standards, independently of the three columns used. Besides that, the capillary BPB column was coupled to the mass spectrometer, and the mass spectra of the individual peaks were obtained. Quantitative Analysis. Since natural materials of varying composition were subjected to analysis, only the relative concentration was determined. The relative concentration of the individual isomers of Clzalkylnaphthalenes and methylbiphenyls was established by measuring the areas with a planimeter, because the use of an integrator did not give reliable results owing to the imperfect resolution of some of the peaks. When using artificial mixtures of known composition, this method did not cause larger deviations than 2x rel.
to the dead volume of the column, which measured by determining the elution time of methane. Table I further lists the boiling points. Issuing from the established relative retention times, the method of linear regression was used to calculate the slopes of the straight lines for the values of the relative retention time logarithm cs. the boiling point, separately for C12 alkylnaphthalenes and methylbiphenyls. Table I1 shows the slope values of the straight lines and the correlation coefficients for the retention time logarithm values us. boiling point of the respective component for all three columns used. Since, contrary to other authors (2), separation of 2,6and 2,7-dimethylnaphthaleneswas not achieved on any of the columns, careful rectification was applied to yield a narrow fraction containing 42 of these isomers. This fraction was then diluted with 2,2,4-trimethylpentane to a concentration of 0.04 gram per liter. Extinction was determined at wavelength of 323 nm, where 2,6dimethylnaphthalene has a characteristic absorption not shown by the other C12 alkylnaphthalenes contained in this fraction. The calibration curve was plotted from synthetic mixtures of 2,6- and 2,7-dimethylnaphthalenes of known concentration in 2,2,4-trimethylpentane. The comparison cuvette contained also 2,2,4-trimethylpentane. The 2,7-dimethylnaphthalene content was taken as the difference between the 2,6-dimethylnaphthalene plus 2,7-dimethylnaphthalene total determined by chromatography and the 2,6-dimethylnaphthalene content determined by spectrophotometry. On the basis of these measurements, it was then possible to determine the distribution of CL2 alkylnaphthalenes and
RESULTS AND DISCUSSION Figures 1,2, and 3 show typical chromatograms of Fraction 1 on columns with Apiezon L, BPB, and PEGA. The chro-
matograms indicate that Fraction 1 contained mainlyCI2alkylnaphthalenes, biphenyl, acenaphthene, and methylbiphenyls. Figure 4 shows the chromatogram of Fraction 2 on the capillary column with Apiezon L. Table I lists the relative retention times of the individual identified components related to naphthalene and corrected
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
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Peak No. 1 2 3 4 5
6 7 8 9 10 11 12 13 14 15 16 17 18
Compound Naphthalene 2-Methylnaphthalene 1-Methylnaphthalene Biphenyl 2-Ethylnaphthalene 1-Ethylnaphthalene 2,6-Dimethylnaphthalene 2,7-Dimethylnaphthalene 1,7-Dimethylnaphthalene 1,3-Dimethylnaphthalene 1,6-Dimethylnaphthalene 2-Methylbiphenyl 2,3-Dimethylnaphthalene 1,4-Dimethylnaphthalene 1,5Dimethylnaphthalene I ,2-Dimethylnaphthalene 4-Methylbiphenyl 3-Methylbiphenyl 1$-Dimethylnaphthalene Acenaphthene
19 20 a Reference 4. Estimated on the base of relative retention. Others (1).
Table I. Relation Retention T i m a Apiezon L Boiling point at 180 "C 217.96 1.OO 241.14 1.60 244.18 1.77 255.0 2.10 257.9 2.38 258.67 2.43 262. On 2.63 2.63 262.0 2.76 262.9 2.87 265.0 2.92 265.5 2.96 260.0 3.16 268.0 3.20 268.5 3.26 270.1 3.32 271.1 3.32 270.0" 3.42 272.7 3.89 276. 5b 3.95 277.2
Table 11. Values of Slopes for Relative Retention Logarithm us. Boiling Point Correlation Column Slope coefficient Alkylnaphthalenes Apiezon L 0,01140 0.99557 BPB 0.01142 0.98 178 PEGA 0.01435 0.98493 Methylbiphenyls Apiezon L 0,00578 0.99815 BPB 0.00968 0.99883 PEGA 0.00960 0,99889 Table 111. Distribution of Alkylnaphthalenes and Methylbiphenyls Relative amount, by wt. Compound Fraction 1 Fraction 2 Alkylnaphthalenes CI2: 2-Ethylnaphthalene 11.3 15.1 I-Ethylnaphthalene 5.7 3.0 2,6-Dimethylnaphthalene 6.7 13.3 2,%Dimethylnaphthalene 7.3 13.9 1,7-Dimethylnaphthalene 10.4 11.9 1,3-Dimethylnaphthalene 17.9 13.1 1,6-Dimethylnaphthalene 16.0 17.2 2,3-DimethyInaphthalene 6.5 4.8 1,4-Dimethylnaphthalene 5.3 2.5 1,S-Dimethylnaphthalene 3.8 2.3 1,2-Dimethylnaphthalene 9.1 2.9 1&Dimethylnaphthalene Present Nonproved 100.0 100.0 Methylbiphenyls: 2-Methylbiphenyl 22.6 14.8 4-Methylbiphenyl 41.1 55.9 3-Methylbiphenyl 36.3 29.3 100.0 100.0
methylbiphenyls in the investigated samples, as is shown in Table 111. When comparing the separating efficiency of the capillary columns, it becomes evident that the column with Apiezon L facilitates good separation of most CI? alkylnaphthalenes. 2-Ethylnaphthalene and I-ethylnaphthalene are separated 1134
BPB at 200 "C 1.00 1.50 1.68 2.15 2.10 2.21 2.23 2.23 2.38 2.50 2.51 2.51 2.73 2.78 2.84 2.99 3.16 3.31 3.57 3.73
PEGA at 190 "C 1.oo 1.41 1.60 2.11 1.84 1.93 1.98 1.98 2.20 2.27 2.27 2.34 2.57 2.57 2.62 2.82 2.94 3.08 3.42 3.55
quite well, 2,6- and 2,7-dimethylnaphthalenes are not separated, partially separated are the l ,3- plus l ,6-dimethylnaphthalenes and 2,3- plus 1,4-dimethylnaphthalenes. The 1,2dimethylnaphthalene peak also contains 4-methylbiphenyl. Conversely, the separation of biphenyls on this phase was very poor, since only biphenyl and 3-methylbiphenyl are fully separated, 2-methylbiphenyl forms a shoulder on the 1,6-dimethylnaphthalene peak, and 4-methylbiphenyl has an identical retention time with 1,2-dimethylnaphthalene. The use of the BPB column presents an advantage in the full separation of 2-ethylnaphthalene, Furthermore, very good separation of 2,3- and 1,4-dimethylnaphthalenesis achieved. The peaks of 1,5-dimethylnaphthaIene and 1&dimethylnaphthalene are fully separated from other components. A drawback is in the small difference of the retention times of 1-ethylnaphthalene and 2,6- and 2,7-dimethylnaphthalenes. As regards biphenyl separation, biphenyl and 3-methylbiphenyl are again completely separated, while 2-methylbiphenyl merges with the peak of the 1,3- and 1,6-dimethylnaphthalene and 4-methylbiphenyl has a n identical retention time with a n as yet unidentified C13alkylnaphthalene, the presence of which has been shown by mass spectrometric analysis. The PEGA column allows a good determination of both ethylnaphthalenes, whereas for the dimethylnaphthalenes separation deteriorates. An advantage of this column is the good separation of 2-methylbiphenyl, which was not the case with the two preceding columns. Also the peak of acenaphthene is in this case free of all other components. The presence of 1,8-dimethylnaphthalene in Fraction 1 was fully proved by means of mass spectrometry coupled with the BPB capillary column. However, since considerable interference of C13alkylnaphthalenes exists in this region, the quantitative determination of the above component is not exact and has not been attempted. On the basis of PEGA analysis, it may be stated that the concentration of l&dimethylnaphthalene is the lowest of all isomers. The results show that chromatographic determination of individual compounds in the course of the analysis of natural materials is difficult. By using columns with different selec-
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
tivity, a mutual shift of the individual types of hydrocarbons takes place (in this case of biphenyls and alkylnaphthalenes) and thus it is possible to establish the individual components such more accurately. Table I i illustrates the changes of the slope and thus also the changes of separation when using different columns. While the BPB column did not bring about practically any change of the slope compared to the Apiezon L column for alkylnaphthalenes, a substantial increase of the slope occurred with methylbiphenyls. On the other hand, the PEGA column changed the slopes for both types of compounds. The use of such a system of columns allows not only the resolution and quantitative determination of most components of complex mixtures, but also helps derive the structure of a compound according to the change of the elution time on various columns. The high values of the correlation coefficients shown in Table I1 prove that the values established are in good agreement with the straight line equation. For quantitative analyses, good use was made of the fact that the separation of 1,7-dimethylnaphthalene was good on all the used columns. When calculating the relative concentration of the individual components all calculations were related to the area of this peak and thus the relative concentration of the individual components was established even in the case of 4-methylbiphenyl, although the peak belonging to this compound could not be separated from other interfering components on any of the columns. The area corresponding to 4-methylbiphenyl was determined from the difference of the area of 1,2-dimethylnaphthalene plus 4-methylbiphenyl peak on Apiezon L and of the area of the 1,2-dimethylnaphthalene peak on BPB. Since the positions of the methyl groups in dimethylnaphthalenes allow us to assume different polarity of the molecules, the values of the relative retention times on PEGA were plotted os. the values on Apiezon L. These columns had different selectivity for CL2alkylnaphthalenes, as can be seen from the slopes of the straight lines in Table 11. The chart, shown in Figure 5, resulted in three parallel straight lines, including all the CI2alkylnaphthalenes except 177-dimethylnaphthalene. Figure 5 indicates that the smallest change of the relative retention time with the change of column selectivity occurs in the case of 2,G-, 2,7-, l,G-, 1,4- and 1,5-dimethylnaphthalenes. These dimethylnaphthalenes therefore have the lowest polarity. A more considerable change of the retention time takes place with the 1,3- and 2,3-dimethylnaphthalenes,and finally the largest change occurs with the 1,2- and 1,%dimethylnaphthalenes and both ethylnaphthalenes. The position of 1,7dimethylnaphthalene is between the medium and large change although the location of this isomer on the medium line could have been expected.
Figure 5. Relative retention of alkylnaphthalenes Clz on PEGA VS. Apiezon L Numbers of compounds as given in Table I When considering the isomers on the individual straight lines, it appears that the lowest change of the relative elution time with the change of column selectivity occurs in the case of such configurations, where the methyl groups are as far distant from each other as possible. Thus it can be explained that 2,G-dimethylnaphthalenes are the highest. A second interesting finding is that an alkyl group in position 1 increases the polarity of the naphthalene molecule much more than an alkyl in position 2. This is supported by the fact that a medium change occurs with 2,3-dimethylnaphthalenes, whereas 1,8-dimethylnaphthalene shows the highest change. Since the chromatograms show some unidentified peaks in the region between 1,2-dimethylnaphthalene and acenaphthene, the BPB capillary column, coupled with the mass spectrometer, was used for the identification of these components. The analysis established that these components are formed by CL3 alkylnaphthalenes, mainly methylethylnaphthalenes. Mass spectrometric analysis of Fraction 1 and 2 was also carried out using the method of molecular ions (6) at an ionization electron energy of 10 eV. It was established that these fractions contain besides the alkylnaphthalenes and biphenyls about 5 mass of alkylindenes Clr-Cl3. RECEIVED for review Feburary 10, 1970. Accepted June 2, 1970.
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