275
Petroleum Analysis
Group type analysis by means of chromatography has been applied to a wide variety of petroleum
types and products (Chapter 9). These types of analysis are often abbreviated by the names PONA
(paraffins, olefins, naphthenes, and aromatics), PIONA (paraffins, iso-paraffins, olefins, naphthenes, and aromatics), polynuclear aromatic hydrocarbon (paraffins, naphthenes, and aromatics),
PINA (paraffins, iso-paraffins, naphthenes, and aromatics), or SARA (saturates, aromatics, resins,
and asphaltenes).
The USBM-API (US Bureau of Mines-American Petroleum Institute) method allows fractionation of petroleum samples into acids, bases, neutral nitrogen compounds, saturates, and mono-,
di-, and polyaromatic compounds (Chapter 9). Multidimensional techniques, that is, the combination of two or more chromatographic techniques, can be very useful to gain further information
about the individual components of chemical groups. Compounds can be isolated and identified
from complex matrices, and detailed fingerprinting of petroleum constituents is feasible (Altgelt
and Gouw, 1979).
10.8.4 gel PermeAtIon CHromAtogrAPHy
There are two additional techniques that have evolved from the more recent development of chromatographic methods.
The first technique, gel filtration chromatography (GFC), has been successfully employed for
application to aqueous systems by biochemists for more than three decades. This technique was
developed using soft, cross-linked dextran beads. The second technique, gel permeation chromatography (GPC), employs semi-rigid, cross-linked polystyrene beads. In either technique, the packing particles swell in the chromatographic solvent and form a porous gel structure.
The distinction between the methods is based on the degree of swelling of the packing; the
dextran swells to a much greater extent than the polystyrene. Subsequent developments of rigid
porous packings of glass, silica, and silica gel have led to their use and classification as packings
for GPC.
GPC, also called size exclusion chromatography (SEC), which in its simplest representation
consists of employing column(s) packed with gels of varying pore sizes in a liquid chromatograph
(Carbognani, 1997). Under conditions of constant flow, the solutes are injected onto the top of the
column, whereupon they appear at the detector in order of decreasing molecular weight. The separation is based on the fact that the larger solute molecules cannot be accommodated within the pore
systems of the gel beads and thus are eluted first. On the other hand, the smaller solute molecules
have increasing volume within the beads, depending upon their relative size, and require more time
to elute.
Thus it is possible, with careful flow control, calibration, injection, and detection (usually by
RI or UV absorption), to obtain an accurate chromatographic representation of the molecular
weight distribution of the solute (Carbognani, 1997). This must of course assume that there
is no chemical or physical interaction between the solute and the gel that negates the concept
of solute size and pore size. For example, highly polar, small molecules that could associate
in solution and are difficult to dissociate could conceivably appear in the incorrect molecular
weight range.
In theory, GPC is an attractive technique for the determination of the number of average molecular weight (M n ) distribution of petroleum fractions. However, it is imperative to recognize that
petroleum contains constituents of widely differing polarity, including nonpolar paraffins and naphthenes (alicyclic compounds), moderately polar aromatics (mononuclear and condensed), and polar
nitrogen, oxygen, and sulfur species. Each particular compound type interacts with the gel surface to a different degree. The strength of the interaction increases with increasing polarity of the
constituents and with decreasing polarity of the solvent. It must therefore be anticipated that the
ideal linear relationship of log M n against elution volume V e that may be operative for nonpolar
Petroleum Analysis
Group type analysis by means of chromatography has been applied to a wide variety of petroleum
types and products (Chapter 9). These types of analysis are often abbreviated by the names PONA
(paraffins, olefins, naphthenes, and aromatics), PIONA (paraffins, iso-paraffins, olefins, naphthenes, and aromatics), polynuclear aromatic hydrocarbon (paraffins, naphthenes, and aromatics),
PINA (paraffins, iso-paraffins, naphthenes, and aromatics), or SARA (saturates, aromatics, resins,
and asphaltenes).
The USBM-API (US Bureau of Mines-American Petroleum Institute) method allows fractionation of petroleum samples into acids, bases, neutral nitrogen compounds, saturates, and mono-,
di-, and polyaromatic compounds (Chapter 9). Multidimensional techniques, that is, the combination of two or more chromatographic techniques, can be very useful to gain further information
about the individual components of chemical groups. Compounds can be isolated and identified
from complex matrices, and detailed fingerprinting of petroleum constituents is feasible (Altgelt
and Gouw, 1979).
10.8.4 gel PermeAtIon CHromAtogrAPHy
There are two additional techniques that have evolved from the more recent development of chromatographic methods.
The first technique, gel filtration chromatography (GFC), has been successfully employed for
application to aqueous systems by biochemists for more than three decades. This technique was
developed using soft, cross-linked dextran beads. The second technique, gel permeation chromatography (GPC), employs semi-rigid, cross-linked polystyrene beads. In either technique, the packing particles swell in the chromatographic solvent and form a porous gel structure.
The distinction between the methods is based on the degree of swelling of the packing; the
dextran swells to a much greater extent than the polystyrene. Subsequent developments of rigid
porous packings of glass, silica, and silica gel have led to their use and classification as packings
for GPC.
GPC, also called size exclusion chromatography (SEC), which in its simplest representation
consists of employing column(s) packed with gels of varying pore sizes in a liquid chromatograph
(Carbognani, 1997). Under conditions of constant flow, the solutes are injected onto the top of the
column, whereupon they appear at the detector in order of decreasing molecular weight. The separation is based on the fact that the larger solute molecules cannot be accommodated within the pore
systems of the gel beads and thus are eluted first. On the other hand, the smaller solute molecules
have increasing volume within the beads, depending upon their relative size, and require more time
to elute.
Thus it is possible, with careful flow control, calibration, injection, and detection (usually by
RI or UV absorption), to obtain an accurate chromatographic representation of the molecular
weight distribution of the solute (Carbognani, 1997). This must of course assume that there
is no chemical or physical interaction between the solute and the gel that negates the concept
of solute size and pore size. For example, highly polar, small molecules that could associate
in solution and are difficult to dissociate could conceivably appear in the incorrect molecular
weight range.
In theory, GPC is an attractive technique for the determination of the number of average molecular weight (M n ) distribution of petroleum fractions. However, it is imperative to recognize that
petroleum contains constituents of widely differing polarity, including nonpolar paraffins and naphthenes (alicyclic compounds), moderately polar aromatics (mononuclear and condensed), and polar
nitrogen, oxygen, and sulfur species. Each particular compound type interacts with the gel surface to a different degree. The strength of the interaction increases with increasing polarity of the
constituents and with decreasing polarity of the solvent. It must therefore be anticipated that the
ideal linear relationship of log M n against elution volume V e that may be operative for nonpolar