• 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: faculty.atu.edu/abhuiyan/Course/Chem 4414/Chapter 29.ppt