Supercritical Fluid Chromatography ( SFC )

  • Supercritical Fluid Chromatography

  • The mobile phase is a supercritical fluid (a fluid above its critical T and critical pressure)

  • Supercritical fluid properties (density, viscosity, and refractive index) vary with T & P

  • Supercritical Fluids

  • At temperatures and pressures above its critical temperature and pressure (critical point), a substance is called a supercritical fluid. The critical temperature is the temperature above which a distinct liquid phase cannot exist. The vapor pressure at its critical temperature is its critical pressure.

  • Where supercritical fluids exist: The forces from the kinetic energy of the molecules exceeds the forces from condensing influence of the intermolecular forces, so no distinct liquid phase exists

  • SFC Mobile Phases

  • Mobile phases should have critical parameters that are easily reached using chromatographic pumps and ovens common to currently used instrumentation.

  • Advantages of supercritical fluids over carrier gasses and liquid mobile phases are in its solubility properties, physical properties, and detector compatibility.

  • SFC Separations

  • SFC is a hybrid of gas and liquid chromatography that combines some of the best features of each

  • As in HPLC, variation of the mobile phase composition affects separation

  • In SFC, mobile phase affinity for the analyte is a function of mobile phase density

  • Density is controlled by controlling system pressure

  • Highly polar samples are not easy to handle (high critical parameters & high reactivity)

  • SFC Advantages vs HPLC

  • Supercritical fluids have low viscosities

   – faster analysis (5 to 10 X faster)

  – less pressure drop across the column   – the use of open tubular columns is      feasible

  • Column lengths from 10 to 20 m are used

  • Can be used with a wide range of sensitive detectors

  • Resolving power is ~5X that of HPLC

  • SFC Advantages vs GC

  • Can analyze non-volatile, polar, or adsorptive solutes without derivatization.

  • Can analyze thermally labile compounds.

  • Can analyze solutes of much higher molecular weight.

  • SFC Instrumentation

  • Solvent delivery system

  • Injector

  • Column/Column Oven

  • Restrictor

  • Detector

  • Data System

  • Solvent Delivery System

  • Maintains precise mobile phase flow (1 to 10 mL/min {OT} or 1 to 10 mL/min {Packed}).

  • Aids in the control of the system pressure (up to 60 Mpa).

  • Moves mobile phase in the liquid state under pressure through the injector & into the column.

  • Injectors

  • Typical HPLC design injectors for packed columns.

  • Split/Splitless valve injector (0.01 to 0.05 mL injections) for open tubular columns.

  • Timed – split injector (0.01 to 0.05 mL injections) for open tubular columns.

  • Detectors

  • Most any detector used in GC or HPLC can be used.

  • FID and UV detectors commonly used.

  • Coupled Detectors

  – MS

  – FTIR

  • SFC Columns

  • Open tubular (derived from GC)

  – Large # theoretical plates (~X500)

  – Easier to control pressure (low P drop)

  • Packed (derived from HPLC)

  – Faster analysis

  – Higher flow rates

  – Higher sample capacity

  • Open Tubular Columns

  • Smaller than GC capillary columns, typically 50 mm i.d., 10 to 20 m in length

  • MP must be more stable due to extreme conditions of supercritical fluids

  • Packed Columns

  • Similar to HPLC columns (10, 5, or 3 mm porous particles)

  • Silica based chemically bonded phases

  • Typically 10 cm long X 4.6 mm i.d

  • SFC and Retention

  • Retention dependent on temperature, pressure, mobile phase density, and composition of the stationary and mobile phase.

  • Complex interactions and not easily predictable.

  • For supercritical fluids

  – solvating properties similar to liquids   – viscosity closer to gases

  • Solvating power a density

  • Temperature/Pressure Effects

  • At lower P, > T, < solubility

  • At higher P, > T, > solubility

  -> T, Pv of solute > solute solubility

  -< fluid density < solubilizing power

  • > T, < solvent r

  • >P, > solvent r

  • Supercritical CO2 Density

  • P (MPa) T (oC) r (g/cm3)

  7.3                    40                    0.22

  7.3                    80                    0.14

  7.3                  120                    0.12

  40                     40                    0.96

  40                     80          0.82

  40      120          0.70

  • Solvent Programming

  • Programming is very useful in controlling solvent strength.

  • Variations in P (density), T, and mobile phase composition.

  • Density programming is most widely used (not simple relationship, T & P).

  -> density, > solubility, < retention

  – Combined T & P programming to control r and thereby solubility and diffusion

  • SFC Mobile Phases

  • Generally non-polar compounds with low to moderate critical properties

  – CO2, N2O, ethane, pentane

  • Normal phase type separations

  – non-polar mp and low polarity sp (substrate + amino, diol, or cyano groups)

  • Elution = function of molecular mass & polarity

  • Carbon Dioxide: SFC Solvent

  • Low Tc

  – operating T as low as 40oC

  • Moderate Pc and rc of 0.448g/cm3

  – reach high r with P < 40 MPa

  • Safe to use

  – nontoxic, nonflammable, noncorrosive, inert

  • Detector compatible

  • Wide r range

  • Other SFC Solvents

  • Nitrous Oxide – Similar in solvating and separations properties to CO2

  • Alkanes – less safe and not as detector compatible than CO2

  – better solvent characteristics for non-polar solutes

  • Halocarbons, xenon, etc. – specialty applications only

  • More polar solvents for highly polar & high molecular weight compounds

  • Solvent Modifiers

  • Add organic modifiers to > solvent strength

  – methanol

  – isopropanol

  – dichloromethane

  – THF

  – acetonitrile

Source: 4414/Chapter 29.ppt

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