Lecture Notes on Supercritical Fluid Chromatography

Pages: 14 (2627 words) Published: June 23, 2013
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TOPIC 4

Course Learning Outcomes
Able to :
1. Explain the fundamental concepts & theories of separation techniques in SFC & SFE. 2. Sketch, label the schematic diagrams & discuss the function of each component in SFC & SFE. 3. Identify the strength & limitations of SFC & SFE technique. 4. Suggest and justify the most suitable & efficient separation technique to be employed for an analysis. 2

What is supercritical fluid?
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Critical temperature (Tc) for any substance is a temperature above which it can no longer exist as a liquid, no matter how much pressure is applied. Critical pressure (Pc) is a pressure above which the substance can no longer exist as a gas no matter how high the temperature is raised.

A supercritical fluid is any substance that is above its Tc & Pc [critical point].

CRITICAL POINT

In other words, when a substance is above Tc & Pc, it exists in a single phase which is neither liquid nor gas; this is a supercritical fluid. 4

Example :
CO2 usually behaves as a gas in air at STP or as a solid called dry ice when frozen. If the temperature & pressure are both increased from STP to be at or above the critical point for CO2, it can adopt properties midway between a gas & a liquid.

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More specifically, it behaves as a supercritical fluid above its Tc (31.1 °C) & Pc (72.9 atm/7.39 MPa), expanding to fill its container like a gas but with a density like that of a liquid. Above 304.2 K (31.2 oC) & 72.8 atm, CO2 behaves as a supercritical fluid & shows properties of both a liquid & a gas.

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Supercritical fluid has the unique ability : Diffuse through solids like a gas. Dissolve materials like a liquid. It can readily change in density upon minor changes in temperature or pressure.

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Characteristics of supercritical fluid : They have a combination of vapor & liquid properties. They have densities higher than gas & lower than liquid & viscosities less than liquids. Their diffusivities are gas-like. They transfer mass very rapidly. They are compressible and homogeneous. 8

Supercritical Fluid Chromatography
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The sample is carried through a separating column by a supercritical fluid where the mixture is divided into unique bands based on the amount of interaction between the individual analytes & the stationary phase in the column. As these bands leave the column their identities and quantities are determined by a detector.

What differentiates SFC from other chromatographic techniques (GC & HPLC) is the use of a supercritical fluid as the mobile phase.

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SFC is a hybrid of GC & LC because when the mobile phase is below its Tc & above its Pc, it acts as a liquid, so the technique is LC & when the mobile phase is above its Tc & below its Pc, it acts as a gas, so the technique is GC. Thus, SFC combines some of the best features of LC as well as GC.

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Theory of separation in SFC is based on the density of the supercritical fluid which corresponds to solvating power. As the pressure in the system is increased, the supercritical fluid density increases & correspondingly its solvating power increases.

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A small increase in pressure causes a large increase in the density of the supercritical phase.

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Solvation (or dissolution), is the process of attraction and association of molecules of a solvent with molecules or ions of a solute. As ions dissolve in a solvent they spread out and become surrounded by solvent molecules.

A sodium ion solvated by water molecules

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Therefore, as the density of the supercritical fluid mobile phase is increased, Components most retained in the column can be eluted faster. This will shorten the elution time. This is similar to temperature programming in GC or using a solvent gradient in HPLC. 16

E.g., the density varied during the chromatography is shown by the dashed curve.

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SFC advantages over GC & HPLC
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Compared with gas, supercritical fluid has high density...
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