Fluid Characteristics

Other pure fluids used A few common fluids are listed in Table 2. However, carbon dioxide remains the fluid of choice for SFC in spite of its inherently low polarity. It has a modest critical pressure and temperature, is inexpensive, readily available, safe, and compatible with the FID.

Some early work used supercritical pentane mixed with alcohols at >235°C. The flammability of such mixtures means that today they are rarely used. Nitrous oxide has characteristics similar to carbon dioxide but is also an extreme oxidizer. It should not be mixed with fuels such as organic modifiers. Fluids like ammonia are also obviously potentially dangerous and corrosive.

Table 2 Critical parameters of some of the pure fluids used for SFC

Critical temp. (°C) Critical pressure (atm)

Table 2 Critical parameters of some of the pure fluids used for SFC

Critical temp. (°C) Critical pressure (atm)

Carbon dioxide

31.1

72.8

Nitrous oxide

36.4

71.5

Sulfur hexafluoride

45.5

37.0

Trifluoromethane

25.9

47.7

Ammonia

132.3

111.3

n-Pentane

196.6

41.7

Methanol

239.4

79.9

Water

374.1

217.6

Some chlorofluorocarbons have interesting properties but have been banned due to their ozone depletion potential in the environment. Several replacement fluorocarbons, especially F-134a, have recently shown potential for extending the polarity range available with a pure fluid. Sulfur hexafluoride has been used as a mobile phase, particularly in the study of hydrocarbons, but is also an ozone depleter. Its main utility is its compatibility with the FID, although a combustion product is hydrofluoric acid. Fluorocarbons exhibit the same problem.

Density At constant temperature, the log of retention in SFC is a nearly linear function of density. Changing the density of the mobile phase can result in a change in retention of 3 or 4 orders of magnitude. Using carbon dioxide as the mobile phase, the usable density range is from gas-like densities (i.e. 0.002 g cm"3) to a density similar to water (>0.95 gcm"3). The typical pressure range is 60 to 400 or 600 bar. The typical temperature range is from 40 to 250°C, but temperatures above 150°C are seldom used.

Modifiers None of the pure fluids used in SFC are polar, and polar solutes are typically insoluble in them. Solvent polarity can be increased by adding polar modifiers, such as alcohols to the nonpolar main fluid. The increase in solvent strength is nonlinear. The addition of 2% methanol to carbon dioxide yields a fluid with the polarity one might expect from 10% methanol. This enhanced solvent strength is due to a phenomenon, sometimes called clustering, where the polar modifier molecules tend to cluster together, creating small pockets of greater than expected polarity. Polar solutes tend to be solvated within, and by, these clusters. The clusters are too small to be considered micelles or a different phase. Individual modifier molecules freely exchange in and out of the clusters.

Once a polar organic modifier is added, the composition of the mobile phase dominates over its density in determining the chromatographic retention of a solute. Gradient elution through composition programming becomes the primary means of retention control. Pressure control becomes a secondary variable. The densities of most binary fluids are unknown, and equations of state of binary pairs are inaccurate, making density programming problematic.

In SFC the composition of the mobile phase can be programmed from 0 to 100% modifier. Methanol is among the most polar liquids that is completely miscible with carbon dioxide. On Snyder's solvent strength scale widely used in HPLC, hexane has a strength of 0, water has a strength of 10.4, and methanol has a strength of 5.1. Thus, the solvent strength can be programmed over approximately half the solvent strength scale available in chromatogra-phy, using a single binary pair of common solvents. Changing the identity of the modifier can change selectivity.

Additives The polarity range amenable to SFC has been extended significantly by the use of ternary mobile phases consisting of carbon dioxide, methanol, and a low concentration of a much more polar substance called an additive. Without an additive most primary aliphatic amines and most polyfunctional acids will not elute or elute with severe tailing. The inclusion of additives often results in elution of symmetrical peaks with high efficiency. Additives appear to function through multiple mechanisms, although much more work is still required to understand their role better. The most effective additives usually contain stronger members of the same functional group. For example, trifluoroacetic acid is usually effective in improving the elution of other acids, while isop-ropylamine is effective in eluting amines. Ion pairing, where an acid is added to a base or a base to an acid is seldom effective.

Viscosity One of the more interesting aspects of the fluids used as mobile phases in SFC is their low viscosity, which is more gas-like than liquid-like, even at high densities. For example, the viscosity of carbon dioxide is typically an order of magnitude lower than water. At 20°C and 200 bar the density of liquid carbon dioxide is over 0.9 g cm"3, yet its viscosity is 10.3 x 10"4 poise. At 20°C and atmospheric pressure, the viscosity of water is 100.2 x 10"4 poise. Low viscosity clearly produces the practical advantage of minimizing pressure drops across columns. Very long, high efficiency columns can be used. Compared to HPLC, an SFC column can be run at 2.5 times the flow with 0.25 times the pressure drop. As the modifier concentration in binary fluids increases, the viscosity of the fluid approaches that of normal liquids.

Diffusion coefficients and speed of analysis Any one column can theoretically be used for GC, SFC or HPLC. Columns can be characterized by the number of 'plates' they exhibit. A column generating many plates can resolve more complex mixtures and is said to have higher efficiency than one generating only a few plates. Another way of describing column efficiency is plate height, H, which is the distance along the column equivalent to one plate. The total number of plates, N, a column exhibits corresponds to the total length, L, divided by the plate height, H (N = L/H).

The van Deemter equation, which can be written as: H = A + 2[D/u] + k[u/D], relates the plate height, H, to the solute binary diffusion coefficient in the mobile phase, D, and the linear velocity of the mobile phase through the column, u. Plots of H vs. u show that the minimum value of H (yielding maximum resolving power for a given column) is the same, regardless of the technique, but the optimum speed for pushing the mobile phase through the column depends on the diffusion coefficient of the solute in the mobile phase.

In GC, values for D are of the order of 0.1 to 1 cm2 s_1. In HPLC, a typical D is of the order of 10~5cm2s~\ Diffusion coefficients in SFC range from 0.01 down to 10~4cm2s~1. As usually practised, SFC is at least 100 times slower than GC but up to 10 times faster than HPLC. As the modifier concentration is increased in binary fluids, diffusion coefficients approach those in HPLC.

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