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The Chemistry and Technology of Petroleum
10.8.7 suPerCrItICAl FluId CHromAtogrAPHy
A supercritical fluid is defined as a substance above its critical temperature that has properties not
usually found at ambient temperatures and pressures. Use of a fluid under supercritical conditions
conveys upon the fluid extraction capabilities that allows the opportunity to improve recovery of a
solute.
In supercritical fluid chromatography, the mobile phase is a substance maintained at a temperature a few degrees above its critical point. The physical properties of this substance are intermediate to those of a liquid and of a gas at ambient conditions. Hence, it is preferable to designate this
condition as the supercritical phase.
In a chromatographic column, the supercritical fluid usually has a density about one-third to
one-fourth of that of the corresponding liquid when used as the mobile phase; the diffusivity is
about 1/100 that of a gas and about 200 times that of the liquid. The viscosity is of the same order of
magnitude as that of the gas. Thus, for chromatographic purposes, such a fluid has more desirable
transport properties than a liquid. In addition, the high density of the fluid results in a 1000-fold
better solvency than that of a gas. This is especially valuable for analyzing high-molecular-weight
compounds.
A primary advantage of chromatography using supercritical mobile phases results from the mass
transfer characteristics of the solute. The increased diffusion coefficients of supercritical fluids compared with liquids can lead to greater speed in separations or greater resolution in complex mixture
analyses. Another advantage of supercritical fluids compared with gases is that they can dissolve
thermally labile and nonvolatile solutes and, upon expansion (decompression) of this solution, introduce the solute into the vapor phase for detection. Although supercritical fluids are sometimes
considered to have superior solvating power, they usually do not provide any advantages in solvating power over liquids given a similar temperature constraint. In fact, many unique capabilities of
supercritical fluids can be attributed to the poor solvent properties obtained at lower fluid densities.
This dissolution phenomenon is increased by the variability of the solvent power of the fluid with
density as the pressure or temperature changes.
The solvent properties that are most relevant for supercritical fluid chromatography are the critical temperature, polarity, and any specific solute-solvent intermolecular interactions (such as hydrogen bonding) that can enhance solubility and selectivity in a separation. Nonpolar or low-polarity
solvents with moderate critical temperatures (e.g., nitrous oxide, carbon dioxide, ethane, propane,
pentane, xenon, sulfur hexafluoride, and various Freons) have been well explored for use in supercritical fluid chromatography. Carbon dioxide has been the fluid of choice in many supercritical
fluid chromatography applications because of its low critical temperature (31°C, 88°F), nontoxic
nature, and lack of interference with most detection methods.
10.9 MOLECULAR WEIGHT
The molecular weight (formula weight) of a compound is the sum of the atomic weights of all the
atoms in a molecule and can be determined by a variety of methods (Cooper, 1989). Petroleum, being
a complex mixture of (at least) several thousand constituents requires qualification of the molecular
weight as either (1) number average molecular weight or (2) weight average molecular weight.
The number average molecular weight is the ordinary arithmetic mean or average of the molecular weights of the individual constituents. It is determined by measuring the molecular weight of n
molecules, summing the weights, and dividing by n.
The weight average molecular weight is a way of describing the molecular weight of a complex
mixture such as petroleum even if the molecular constituents are not of the same type and exist in
different sizes.
Even though refining produces, in general, lower molecular weight species than those originally in the feedstock, there is still the need to determine the molecular weight of the original
The Chemistry and Technology of Petroleum
10.8.7 suPerCrItICAl FluId CHromAtogrAPHy
A supercritical fluid is defined as a substance above its critical temperature that has properties not
usually found at ambient temperatures and pressures. Use of a fluid under supercritical conditions
conveys upon the fluid extraction capabilities that allows the opportunity to improve recovery of a
solute.
In supercritical fluid chromatography, the mobile phase is a substance maintained at a temperature a few degrees above its critical point. The physical properties of this substance are intermediate to those of a liquid and of a gas at ambient conditions. Hence, it is preferable to designate this
condition as the supercritical phase.
In a chromatographic column, the supercritical fluid usually has a density about one-third to
one-fourth of that of the corresponding liquid when used as the mobile phase; the diffusivity is
about 1/100 that of a gas and about 200 times that of the liquid. The viscosity is of the same order of
magnitude as that of the gas. Thus, for chromatographic purposes, such a fluid has more desirable
transport properties than a liquid. In addition, the high density of the fluid results in a 1000-fold
better solvency than that of a gas. This is especially valuable for analyzing high-molecular-weight
compounds.
A primary advantage of chromatography using supercritical mobile phases results from the mass
transfer characteristics of the solute. The increased diffusion coefficients of supercritical fluids compared with liquids can lead to greater speed in separations or greater resolution in complex mixture
analyses. Another advantage of supercritical fluids compared with gases is that they can dissolve
thermally labile and nonvolatile solutes and, upon expansion (decompression) of this solution, introduce the solute into the vapor phase for detection. Although supercritical fluids are sometimes
considered to have superior solvating power, they usually do not provide any advantages in solvating power over liquids given a similar temperature constraint. In fact, many unique capabilities of
supercritical fluids can be attributed to the poor solvent properties obtained at lower fluid densities.
This dissolution phenomenon is increased by the variability of the solvent power of the fluid with
density as the pressure or temperature changes.
The solvent properties that are most relevant for supercritical fluid chromatography are the critical temperature, polarity, and any specific solute-solvent intermolecular interactions (such as hydrogen bonding) that can enhance solubility and selectivity in a separation. Nonpolar or low-polarity
solvents with moderate critical temperatures (e.g., nitrous oxide, carbon dioxide, ethane, propane,
pentane, xenon, sulfur hexafluoride, and various Freons) have been well explored for use in supercritical fluid chromatography. Carbon dioxide has been the fluid of choice in many supercritical
fluid chromatography applications because of its low critical temperature (31°C, 88°F), nontoxic
nature, and lack of interference with most detection methods.
10.9 MOLECULAR WEIGHT
The molecular weight (formula weight) of a compound is the sum of the atomic weights of all the
atoms in a molecule and can be determined by a variety of methods (Cooper, 1989). Petroleum, being
a complex mixture of (at least) several thousand constituents requires qualification of the molecular
weight as either (1) number average molecular weight or (2) weight average molecular weight.
The number average molecular weight is the ordinary arithmetic mean or average of the molecular weights of the individual constituents. It is determined by measuring the molecular weight of n
molecules, summing the weights, and dividing by n.
The weight average molecular weight is a way of describing the molecular weight of a complex
mixture such as petroleum even if the molecular constituents are not of the same type and exist in
different sizes.
Even though refining produces, in general, lower molecular weight species than those originally in the feedstock, there is still the need to determine the molecular weight of the original