The Mass Spectrometric Analysis of Asphalt A Preliminary Investigation R. J. CLERC and M. J. O'NEAL, Jr. Houston Research laboratory, Shell Oil Co., P. 0.Box 2527, Houston 7, Tex.
F A preliminary investigation has indicated that mass spectrometric techniques can be applied successfully to the analysis of asphalt and other residual petroleum fractions. A hightemperature mass spectrometer was temporarily remodeled so that a sample of asphalt could b e placed within the ionizing region. With such an arrangement mass spectra were obtained from mass 24 to about mass 1900. Although the individual mass peaks were not resolved above mass 600, a molecular weight distribution of the unresolved peaks was obtained. Some insight into the composition of asphalt from West Texas straight-run residue was gained. This particular sample of asphalt had a molecular weight range of 500 to 1900 with a mean value of about 900. Fragment ions indicated that certain heterocyclic and aromatic nuclei were the predominant molecular structural groups.
T
successful application of ma3s spectzometry to the heavier dihtillate fractions of petroleum such as lubricating oils and waxes ( I , 3, 4 ) has naturally posed the question of whether the method could be extended to include the asphaltic and other residual fractions. Sormally gas flows through a small leak into the ionizing region (7) but such a vapor-flow inlet system would not be feasible for residual fractions because of their extremely low vapor pressures. I n fact, it was not even known whether sufficient positive ions could be produced from the electron bombardment of asphaltic material to obtain a usable mass spectrum. I n the event that positive ions could be obtained, a high resolution instrument would be needed to resolve such high molecular weight ions. High resolution mass spectrometers are available (6),but the basic limitation is that of ion production. Therefore, it appeared that a preliminary experiment with a conventional analytical instrument would be worthwhile to evaluate the possibility of further applications in this field. It was realized that any spectral data obtained on a conventional instrument would be of HE
380
ANALYTICAL CHEMISTRY
limited usefulness because of inadequate resolution. Nevertheless, a standard Consolidated 180" analytical mass spectrometer was modified to allow the sample to be placed within the ion chamber. This paper describes these modifications and the interpretation of the mass spectrum of a n asphalt sample. EXPERIMENTAL
Several exploratory experiments were conducted to determine the best nianner in which to mount the samples in the ion source. One experiment used a small mesh wire screen to support the asphalt sample directly in the ionizing electron beam. However, essentially no ions were obtained, and it was difficult to control the source operation because the electron beam was blocked by the sample mount. The above results indicated that additional heat should be supplied to the sample and that the sample mount should be designed to eliminate any interference with the electron beam. Accordingly, the sample screen was replaced by a Kichrome heater mounted in the ionizing region adjacent to the electron beam. The heater was constructed with reverse loops on either end to cancel the electric field caused by the direct current heater voltage which otherwise would have distorted the electron beam. The heater lead entered through the unused leak line and was made common t o one of the repeller electrodes. The asphalt sample was prepared from the residue of a molecular distillation of West Texas crude by propane deasphalting in pilot plant equipment, It was necessary t o dissolve the residue in benzene prior to propane deasphalting in order to approach laboratory pentane-deasphalting yields. The per cent composition of this asphalt sample is C, 84.1; H, 8.9; S, 4.6; K, 0.7; 0, 1.8. I t s molecular weight (ebullioscopic) is 1260; softening point, 225" F.; and solid density (20" C.) 1.048. The asphalt sample was dissolved in CC14 and applied directly t o the heater. Solvent evaporation produced an asphalt coating on the heater. The heater was then mounted in place and the instrument brought to operating conditions. After the normal background of the instrument had subsided, the heater voltage was gradually increased until ions appeared. The total ion produc-
tion was a function of the heater temperature. Therefore, the heater voltage was adjusted to produce ions a t about the same order of magnitude as that obtained under normal vapor-flow conditions-e.g., approximately 50 microns pressure on the leak. This required heating the sample to a n estimated 200" to 300" C. Under these conditions the mass spectrum obtained was fairly reproducible for a period of over 1 hour. During this time the mass spectrum of the sample was recorded at various magnet current settings. After about 1 hour the total ion intensity began decreasing, and after about 2 hours the peak intensities were little more than background magnitude. During this time no significant change was observed in molecular weight, indicating little, if any, fractionation. At the conclusion of the experiment, the ion source was removed for observation. KO asphalt deposit was obserl-ed on the heater, but the surrounding region had become coated with a dark film. It thus appeared that most, if not all, of the asphalt had been vaporized from the heater during the course of this evperiment and most of the vaporized asphaltic material had condensed on the surrounding cooler regions of the ion source. The question naturally arises as to whether decomposition or thermal cracking occurred during the experiment. If cracking had occurred, light hydrocarbon gases (ethane, ethylene, propane, etc.) would be expected in rather significant quantities. Such gas production would have resulted in a pressure rise in the high-vacuum analyzclr system which is normally at about l o + mm. of Hg pressure. This did not occur in so far as could be measured. Also. the light saturated gases can be identified in the most complex mass spectrum by simple computation of isotope-free parent peaks, However, no evidence could be found for these gases in the spectra recorded during the experiment. dlthough the amount of methane formed was not known, it is believed from the above considerations that no significant decomposition of the sample occurred and that the spectra obtained represented essentially all of the material n hich had been placed upon the heater. The mass spectrum of the above
300
I
1
I
I
(SMOOTHED OUT IN RESOLVABLE PORTION OF SPECTRUM)
W
I 0
z
W
'., 500
Figure 1. asphalt
1000 MOLECULAR WEIGHT
1500
2000
Envelope of peaks in mass spectrum of
asphalt was automatically recorded in the conventional way (7) by a recording oscillograph. The spectrum produced was the conventional display of the abundance of each ion produced in the ion source recorded sequentially with increasing mass of the ions and covered a mass range from 24 to about 1900. The individual mass peaks below 600 were completely resolved while the region between 600 and 800 showed partially resolved peaks. Therefore, specific mass identification could be made in these regions. However, the mass region above 800 represented simply the recorder tracing of the top of the unresolved peaks so that mass identification in this region was based on a previously calibrated magnet current -accelerating voltage relationship. Figure 1 shows the ion abundances observed as a function of specific mass in the 300 to 1900 mass region. This curve represents the envelope formed by the ion peaks in the spwtrum. Peaks mere obtained up to about mass 1900. This probably represents the limit of the instrument rather than the highest molecular weight material in the asphalt sample. However, the small amount of material in the 1500 to 1900 molecular weight region indicates that the amount of material above mass 1900 probably is insignificant in the case of the present sample. INTERPRETATION OF M A S S SPECTRUM
The maximum in Figure 1 is interpreted as being caused by the ionization of the bulk of the parent or molecular ions (3, 4). An &mated base line is shown as that part of the spectrum which is attributed to dissociated and rearranged fragment ions. Figure 2 shows the net difference between the two curves of Figure 1 and represents the molecular weight distribution of the asphalt sample. The ebullioscopic molecular weight is about 30% greater than the average
Figure 2.
molecular weight obtained from the mass spectrum. There are several possible reasons for this difference-e.g., fractionation or cracking in the mass spectrometer or errors in the ebullioscopic method. However, since the above discussions indicate that fractionation and/or thermal decomposition are unlikely, the probable explanation is that of molecular association in solution during the ebullioscopic molecular weight determination. Such a discrepancy in the ebullioscopic method is not unusual for high molecular TTeight residuals. The fragment ions (those ions resulting from molecular dissociation under electron bombardment) are also useful for the identification and analysis of complex mixtures. A type of fragment ion which is particularly useful for the identification of aromatic types is the nucleus ion formed by side chain dissociation and hydrogen rearrangement. It was expected that many of these hydrogen-rearranged fragment peaks, caused by aromatics or heterocyclics which are aromatic in character, vvould be found in a mass spectrum of asphalt. Table I lists the most prominent fragment peaks in the aphalt spectrum in order of decreasing intensity. The ring nuclei assigned to these peaks are also shown. Four of the six prominent peaks are assigned to heterocyclic nuclei. The largest peak (mass 93) is undoubtedly due to a nonhydrocarbon and could be assigned to a phenolic fragment or to a rearranged picolinyl ion (rearranged by hydrogen saturation, which is known to occur in some nitrogen ring structures). The benzene ring derivative (mass 91) was the next most prominent peak. Although the intensities of the next group of peaks (241, 185, 285) were lower than those of masses 93 and 91, they were considerably greater than others in the immediate mass neighborhood. These peaks can also be explained as deriva-
Molecular weight distribution of asphalt
Table 1. Assignments of Fragment Peaks in Mass Spectrum of Asphalt
Mass
Possible Suclei
93
u-
91
241
185
285
202
302
VOL. 33, NO. 3, MARCH 1961
381
tives of thiophenoquinoline as shown in the table. Part or all of the 241 mass could be due to the dithiophenoquinoline structure or to chrysene and its isomers. The last two peaks of still smaller magnitude (202 and 302) can be ascribed to pyrene, benzoperylene, and/or dibenzopyrene types of hydrocarbon structures. Interestingly, there were no significant peaks due to naphthalene, phenanthrene and/or anthracene, or isomers of pentacene. It may be well a t this point to consider the methods used to identify the major A mass spectrum fragment peaks. shows the distribution by mass of the ionized products that result from electron bombardment of the sample. Unfortunately, the spectrum tells us nothing about the identity of an ion except its mass. The problem, then, is to relate mass to molecular structure. The relationships between mass and molecular empirical formula can be expressed as follows: m
=
CnH2rrti
or m = 12n
+ 2n + z = 14%+ z
where m
=
mass
n
=
C
+ N + Q++N25+ 2Q + 45
z = 2-20-2R
C, N, Q, and S
= number of carbon, nitrogen, oxygen, and sulfur atoms, respectively D = number of double bonds (including aromatic) R = number of rings
Thus an ion of a given mass can be definitely associated with certain possibilities of structural formulation within the limits set by the above equations.
These possibilities can be further limited by other considerations. For example, mass 302 cited above could be caused by CnHB (2 = -6), a hexadecylbenzene. This type of molecule can be eliminated because of the kind of sample under consideratmion. Another possibility would be CsH26 (2 = -20), which is equivalent to a molecule containing a dihydropyrene nucleus with a 7-carbon atom side chain. This would be a light oil component and also not a likely possibility in an asphalt sample. A more likely pos(2 = -34), which is sibility is dibenzopyrene and/or its isomers. However, the molecular weight distribution of this asphalt sample would indicate that the peak a t mass 302 is not caused by dibenzopyrene itself, but rather by a dibenzopyrene nucleus linked to other groups within the asphalt molecule. I n this case the dibenzopyrene nucleus (mass 302) is formed by rearrangement (hydrogen saturation) of the dissociated nucleus. Such rearrangements are prevalent for highly condensed aromatics. Since little is known about the molecular composition of asphalt, Table I can be considered only as an indication of possible structures within the asphalt molecule. It appears, for example, that asphalt does not consist of very highly condensed polyaromatic structures. Rather, these mass spectrometric studies have indicated that the degree of condensation is unlikely to be very great although the number of different ring systems may be very large. Similar conclusions based on the physical properties of asphalts have been reported (2, 6). This preliminary investigation has
indicated that mass spectrometric techniques might be useful by providing information about the molecular structure of asphalts that is not obtainable a t the present time by any other technique. The full possibilities of this technique can be realized only after further study with very high molecular weight pure compounds and the simplification of asphalt samples by various separation methods. ACKNOWLEDGMENT
The authors acknowledge the help of A. Hood, C. K. Hines, C. E. Davis, and R. Y. Seaber in connection with this investigation. LITERATURE CITED
(1) Brown, R. A., Skahan, D. J., Cirillo,
V. A., Melpolder, F. W., ANAL.CHEM. 31, 1531 (1959). (2) Hillman, E. S., Barnett, B., Proc. Am. Sac. Testing Materials 37, 11, 558 (1937). (3) O’Neal, M.,, J., in “Applied Mase Spectrometry, p. 27, Institute of Petroleum, London, 1954. (4) O’Neal, M. J., Hood, A., Clerc, R. J., Andre, M. L., Hines, C. K., Fourth World Petroleum Con ress, Section V/C. Remint 3. Carlo 8olombo Pub.. Rome, 19’55. ’ (5) Traxler, R. N., Romberg, J. W., Petroleum Engr. 30, No. 11, G37 (1958). (6) Voorhies, H. G., et a$,, in “Advances in Mass Spectrometry, p. 44, J. D. Maldron, ed., Pergamon Press, London, 1959. -_._
(7) Washburn, H. W., in “Pfiysical Methods in Chemical Analysis, Vol. I, p. 630. Academic Press, New York, 1950. RECEIVEDfor review July 18, 1960. Accepted November 7, 1960. Division of Petroleum Chemistry, 138th Meeting, ACS, New York, N. Y., September 1960.
Flame Spectrophotometric Study of Silver JOHN A. DEAN and CHARLES B. STUBBLEFIELD‘ Department of Chemisfry, Universify o f Tennessee, Knoxville, Tenn.
b The flame emission characteristics of the two ultraviolet emission lines of silver at 328.0 and 338.3 mp from aqueous solution and from solutions of several organic solvents have been thoroughly studied in oxygen-acetylene and oxygen-hydrogen flames. Parameters investigated include oxygen and fuel-gas flows; optimum ratio of flows, oxygen/fuel gas; effect of different burner heights; and background emission. The spectral and radiation interferences of metals and anions that are commonly associated with silver have been examined. 382
ANALYTICAL CHEMISTRY
F
methods for silver have been reported by Rivkina (Q),and Pungor and KonkolyThege ( 7 ) have studied briefly the behavior of the 338.3-mp silver line. Rathje (8)applied the flame photometric method to zinc-cadmium phosphors and Galloway (6) determined silver in blister copper. Nevertheless, there is a distinct lack of information regarding the optimal conditions for the determination of silver, as well as information about interferences arising from anions and cations commonly encountered during silver determinations. This inLAME SPECTROPHOTOMETRIC
vestigation describes a thorough study of the flame emission characteristics of silver in oxygen-hydrogen flames, in oxygen-acetylene flames, and in flames into which an organic solvent is aspirated. REAGENTS AND APPARATUS
Reagents. Silver, standard solution, 10,000 pg. per ml. Dissolve 15.74 grams of ACS grade silver nitrate, 1 Present address, Columbia-Southern Chemical Corp., Corpus Christi, Tex.
