303
Structural Group Analysis
Despite these limitations, mass spectrometry furnishes useful and adequate information on
the composition of petroleum streams, even if this information is not as exhaustive as might be
required. Structural similarities hindering identification of individual components make, in effect,
identification by type or by homologue only more meaningful, as these similar structures behave
similarly in most processing situations, and knowledge of the individual isomeric distribution would
add little to our understanding of the relationships between composition and processing parameters.
The availability of high-resolution instrumentation extends the scope of mass spectrometry for
both qualitative and quantitative applications. The main advantage of high-resolution techniques is
the separation of fragment and molecular ions of the same nominal molecular weight but different
elemental composition. In addition, most high-resolution instruments are capable of furnishing precise mass measurements, so that the formulae can be calculated from these measurements.
Mass spectrometry should be used cautiously when a maximum amount of information can be
expected. However, heavy oils and residua are for practical purposes beyond the useful range of routine mass spectrometry, but many of the discrete fractions that can now be obtained could be studied
qualitatively by high-resolution mass spectral techniques. Many of the higher molecular weight constituents in heavy oils and residual are nonvolatile. At elevated temperatures, decomposition could
occur in the inlet and any subsequent analysis would be biased to the low-molecular-weight end and
to the lower molecular products produced by thermal decomposition.
Mass spectrometry adds further knowledge to the structural analysis of petroleum by aiding the
researcher in the calculation of ring distribution per molecule ions in a mixture. However, there are
questions about the application of the technique to the nonvolatile fractions of petroleum. The lack of
volatility, which is almost self-explanatory, does not allow accurate identification of the total asphaltene.
Nevertheless, the petroleum industry has long been the leader in the use of mass spectroscopy to
solve its analytical problems. As soon as instruments became capable of producing repeatable spectra in the early 1940s, hydrocarbon mixtures were analyzed by mass spectrometry with speed and
precision not approached by other techniques available at that time. As heat inlet systems evolved,
analysis of gases and low-boiling liquids was followed by type analyses of higher-boiling petroleum
fractions, and advances in mass-spectroscopic analysis of petroleum and its derivatives paralleled
improved instrumentation.
Early work (Lumpkin, 1956) involved development of a procedure for type analysis of the saturate portion of petroleum materials of high molecular weight and employed characteristic fragment
masses of paraffins, noncondensed naphthenes, and condensed naphthenes containing two to six
condensed rings per molecule to delineate these types. This was followed (Lumpkin, 1964) by an
analysis of a trinuclear aromatic portion of a coker gas oil. Six condensed aromatic hydrocarbon
types, five sulfur, two oxygen, and one nitrogen compound were identified and a quantitative analysis based on low-voltage sensitivity showed that derivatives of alkyl dibenzothiophene, phenanthrene, and fluorene were the major types present.
Mass spectrometry has also been employed to investigate the aromatic structural unit of
asphaltenes resins—an analysis of the nitrogen and oxygen compounds from crude oil using highresolution mass spectrometry as the principal analytical tool showed a variety of compound-types
to be present. These included derivatives of indole, carbazole, benzcarbazole, pyridine, quinoline,
pyridone, quinolone, dibenzofuran, naphthobenzofuran, dinaphthofuran, phenol, carboxylic acids,
aliphatic ketones, and esters (Snyder, 1968, 1969).
Further studies using pyrolysis/gas chromatography/mass spectrometry (py/gc/ms) (Speight and
Pancirov, 1984) showed that different sub-fractions of an asphaltene fraction would produce the
same type of polycyclic aromatic systems in the volatile matter but the distribution was not constant.
It was also possible to compute the hydrocarbon distribution from which a noteworthy point here is
preponderance of single ring (cycloparaffin and alkylbenzene) species as well as the domination of
saturated material over aromatic material. The emphasis on low molecular weight material in the
volatile products is to be anticipated on the basis that more complex systems remain as nonvolatile
material and, in fact, are converted to coke.
Structural Group Analysis
Despite these limitations, mass spectrometry furnishes useful and adequate information on
the composition of petroleum streams, even if this information is not as exhaustive as might be
required. Structural similarities hindering identification of individual components make, in effect,
identification by type or by homologue only more meaningful, as these similar structures behave
similarly in most processing situations, and knowledge of the individual isomeric distribution would
add little to our understanding of the relationships between composition and processing parameters.
The availability of high-resolution instrumentation extends the scope of mass spectrometry for
both qualitative and quantitative applications. The main advantage of high-resolution techniques is
the separation of fragment and molecular ions of the same nominal molecular weight but different
elemental composition. In addition, most high-resolution instruments are capable of furnishing precise mass measurements, so that the formulae can be calculated from these measurements.
Mass spectrometry should be used cautiously when a maximum amount of information can be
expected. However, heavy oils and residua are for practical purposes beyond the useful range of routine mass spectrometry, but many of the discrete fractions that can now be obtained could be studied
qualitatively by high-resolution mass spectral techniques. Many of the higher molecular weight constituents in heavy oils and residual are nonvolatile. At elevated temperatures, decomposition could
occur in the inlet and any subsequent analysis would be biased to the low-molecular-weight end and
to the lower molecular products produced by thermal decomposition.
Mass spectrometry adds further knowledge to the structural analysis of petroleum by aiding the
researcher in the calculation of ring distribution per molecule ions in a mixture. However, there are
questions about the application of the technique to the nonvolatile fractions of petroleum. The lack of
volatility, which is almost self-explanatory, does not allow accurate identification of the total asphaltene.
Nevertheless, the petroleum industry has long been the leader in the use of mass spectroscopy to
solve its analytical problems. As soon as instruments became capable of producing repeatable spectra in the early 1940s, hydrocarbon mixtures were analyzed by mass spectrometry with speed and
precision not approached by other techniques available at that time. As heat inlet systems evolved,
analysis of gases and low-boiling liquids was followed by type analyses of higher-boiling petroleum
fractions, and advances in mass-spectroscopic analysis of petroleum and its derivatives paralleled
improved instrumentation.
Early work (Lumpkin, 1956) involved development of a procedure for type analysis of the saturate portion of petroleum materials of high molecular weight and employed characteristic fragment
masses of paraffins, noncondensed naphthenes, and condensed naphthenes containing two to six
condensed rings per molecule to delineate these types. This was followed (Lumpkin, 1964) by an
analysis of a trinuclear aromatic portion of a coker gas oil. Six condensed aromatic hydrocarbon
types, five sulfur, two oxygen, and one nitrogen compound were identified and a quantitative analysis based on low-voltage sensitivity showed that derivatives of alkyl dibenzothiophene, phenanthrene, and fluorene were the major types present.
Mass spectrometry has also been employed to investigate the aromatic structural unit of
asphaltenes resins—an analysis of the nitrogen and oxygen compounds from crude oil using highresolution mass spectrometry as the principal analytical tool showed a variety of compound-types
to be present. These included derivatives of indole, carbazole, benzcarbazole, pyridine, quinoline,
pyridone, quinolone, dibenzofuran, naphthobenzofuran, dinaphthofuran, phenol, carboxylic acids,
aliphatic ketones, and esters (Snyder, 1968, 1969).
Further studies using pyrolysis/gas chromatography/mass spectrometry (py/gc/ms) (Speight and
Pancirov, 1984) showed that different sub-fractions of an asphaltene fraction would produce the
same type of polycyclic aromatic systems in the volatile matter but the distribution was not constant.
It was also possible to compute the hydrocarbon distribution from which a noteworthy point here is
preponderance of single ring (cycloparaffin and alkylbenzene) species as well as the domination of
saturated material over aromatic material. The emphasis on low molecular weight material in the
volatile products is to be anticipated on the basis that more complex systems remain as nonvolatile
material and, in fact, are converted to coke.