302
The Chemistry and Technology of Petroleum
In this context, it would be of interest to fractionate asphaltenes (from different crude oils) by a
standard method, such as ion exchange chromatography (e.g., see McKay et al., 1978; Francisco and
Speight, 1984). From the data, it would be possible to determine the distribution of the amphoteric
constituents, basic constituents, acidic constituents, and less neutral polar species in each asphaltene
and whether or not the difference in aromatic carbon content was due to variations in the relative
proportions of the subfractions.
Carbon-type analysis does not allow the identification of individual constituents but presents an
overall picture of the carbon framework and (if the term can be used without too literal a translation)
actually presents an average molecular structure of these higher boiling materials.
Regardless of the method for separating the hydrogen types, the most derived structural parameters take the form of ratios of the various hydrogen types. The HI/CA (H aru /C ar ) ratio indicates extent
of condensation of the unsubstituted aromatic system and the C su HI (F) ratio gives the degree of
substitution. The C s /C su (n) ratio represents the average chain length, the HN/CN ratio is the degree
of condensation of naphthenic clusters, and fa factor is the fraction of carbon that is aromatic. C A is
the total number of aromatic carbon atoms in the sample, R S is the number of substituted aromatic
ring carbons, and R A is the number of aromatic rings.
The structural details of the carbon backbone obtained from proton spectra are derived by inference, but it must be recognized that protons at peripheral positions can be obscured by intermolecular interactions. This, of course, can cause errors in the ratios that can have a substantial influence
on the outcome of the calculations (Ebert et al., 1987). It is in this regard that carbon-13 magnetic
resonance (CMR) can play a useful role.
Since carbon magnetic resonance deals with analyzing the carbon distribution types, the obvious structural parameter to be determined is the aromaticity, fa. A direct determination from the
various carbon environments is one of the better methods for the determination of aromaticity.
Certain regions are often designated for certain types of carbon atoms (Snape et al., 1979):
heterocyclic + quinone, 155–160; pyrrole-like, 130–150; mono- to polyaromatics, 120–130; εC,
295; δC, 29; γC, 32; βC, 23; and αC, 14.
Through a combination of proton and carbon magnetic resonance techniques, refinements can be
made on the structural parameters. However, with the solid-state high-resolution carbon magnetic
resonance technique, a number of additional structural parameters can be obtained. For example,
quaternary aromatic carbons, tertiary aromatic cartons, ratio of primary to quaternary aliphatic
carbons, and the ratio of secondary to tertiary aliphatic can be estimated (Weinberg et al., 1981).
11.2.2.3 Mass Spectrometry
Mass spectrometry plays a key role in the identification of the constituents of petroleum and of
petroleum products (Aczel, 1979). Mass spectrometric methods are particularly well suited to the
analyses of complex mixtures of relatively simple hydrocarbons that comprise the bulk of petroleum
and its products.
The principal advantages of mass spectrometric methods are as follows: (1) high reproducibility of quantitative analyses; (2) the potential for obtaining detailed data on the individual
components and/or carbon number homologues in complex mixtures; and (3) a minimal sample
size required for analysis. The ability of mass spectrometry to identify individual components in
complex mixtures is unmatched by any modern analytic technique, perhaps the exception is gas
chromatography.
In general, most petroleum samples are very complex mixtures containing literally hundreds
and even thousands of individual components. These components tend to form several homologous
series of the same general formulae and extend from near gaseous materials to nonvolatile components. Thus there are disadvantages arising from the use of mass spectrometry: (1) the limitation of
the method to organic materials that are volatile and stable at temperatures up to 300°C (570°F) and
(2) the difficulty of separating isomers for absolute identification. The sample is usually destroyed,
but this is seldom a disadvantage.
The Chemistry and Technology of Petroleum
In this context, it would be of interest to fractionate asphaltenes (from different crude oils) by a
standard method, such as ion exchange chromatography (e.g., see McKay et al., 1978; Francisco and
Speight, 1984). From the data, it would be possible to determine the distribution of the amphoteric
constituents, basic constituents, acidic constituents, and less neutral polar species in each asphaltene
and whether or not the difference in aromatic carbon content was due to variations in the relative
proportions of the subfractions.
Carbon-type analysis does not allow the identification of individual constituents but presents an
overall picture of the carbon framework and (if the term can be used without too literal a translation)
actually presents an average molecular structure of these higher boiling materials.
Regardless of the method for separating the hydrogen types, the most derived structural parameters take the form of ratios of the various hydrogen types. The HI/CA (H aru /C ar ) ratio indicates extent
of condensation of the unsubstituted aromatic system and the C su HI (F) ratio gives the degree of
substitution. The C s /C su (n) ratio represents the average chain length, the HN/CN ratio is the degree
of condensation of naphthenic clusters, and fa factor is the fraction of carbon that is aromatic. C A is
the total number of aromatic carbon atoms in the sample, R S is the number of substituted aromatic
ring carbons, and R A is the number of aromatic rings.
The structural details of the carbon backbone obtained from proton spectra are derived by inference, but it must be recognized that protons at peripheral positions can be obscured by intermolecular interactions. This, of course, can cause errors in the ratios that can have a substantial influence
on the outcome of the calculations (Ebert et al., 1987). It is in this regard that carbon-13 magnetic
resonance (CMR) can play a useful role.
Since carbon magnetic resonance deals with analyzing the carbon distribution types, the obvious structural parameter to be determined is the aromaticity, fa. A direct determination from the
various carbon environments is one of the better methods for the determination of aromaticity.
Certain regions are often designated for certain types of carbon atoms (Snape et al., 1979):
heterocyclic + quinone, 155–160; pyrrole-like, 130–150; mono- to polyaromatics, 120–130; εC,
295; δC, 29; γC, 32; βC, 23; and αC, 14.
Through a combination of proton and carbon magnetic resonance techniques, refinements can be
made on the structural parameters. However, with the solid-state high-resolution carbon magnetic
resonance technique, a number of additional structural parameters can be obtained. For example,
quaternary aromatic carbons, tertiary aromatic cartons, ratio of primary to quaternary aliphatic
carbons, and the ratio of secondary to tertiary aliphatic can be estimated (Weinberg et al., 1981).
11.2.2.3 Mass Spectrometry
Mass spectrometry plays a key role in the identification of the constituents of petroleum and of
petroleum products (Aczel, 1979). Mass spectrometric methods are particularly well suited to the
analyses of complex mixtures of relatively simple hydrocarbons that comprise the bulk of petroleum
and its products.
The principal advantages of mass spectrometric methods are as follows: (1) high reproducibility of quantitative analyses; (2) the potential for obtaining detailed data on the individual
components and/or carbon number homologues in complex mixtures; and (3) a minimal sample
size required for analysis. The ability of mass spectrometry to identify individual components in
complex mixtures is unmatched by any modern analytic technique, perhaps the exception is gas
chromatography.
In general, most petroleum samples are very complex mixtures containing literally hundreds
and even thousands of individual components. These components tend to form several homologous
series of the same general formulae and extend from near gaseous materials to nonvolatile components. Thus there are disadvantages arising from the use of mass spectrometry: (1) the limitation of
the method to organic materials that are volatile and stable at temperatures up to 300°C (570°F) and
(2) the difficulty of separating isomers for absolute identification. The sample is usually destroyed,
but this is seldom a disadvantage.