Supercritical Fluid Chromatography

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Domenic Fields
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1 Supercritical Fluid Chromatography What is a supercritical fluid? Supercritical fluid is a state of matter that is intermediate between a gas and liquid in its properties. This state formed when a gas or liquid solvent is subjected to temperature and pressure condition exceeding a particular critical point. The temperature and pressure at which this point occurs are known as the critical temperature and critical pressure and are characteristic of the solvent. Beyond this point, the solvent will be neither a gas or liquid, but will possess properties of both phases. Whether this supercritical fluid acts more like a gas or liquid will depend on the pressure and temperature. Supercritical fluid properties (density, viscosity, and refractive index) vary with T & P Supercritical Fluid Chromatography Proposed by Lovelock in 1958, in 1962 Klesperwere the first to report supercritical fluids as eluentsin chromatographic separations. A supercritical fluid is obtained by elevating the temperature and pressure above the characteristic critical values of the substance concerned. This reversible physical state possesses unique and interesting properties that can be applied in chromatography. Because the viscosity and diffusivity of this phase are comparable to that of gaseous mobile phases, lower pressures are generated over the system and column. The density and solvating power, on the other hand, approach that of liquid mobile phases, creating a broad application range.however, having to compete with other popular techniques, such as high performance liquid chromatography (HPLC) and gas chromatography (GC), the interest in supercritical fluid chromatography (SFC) was rather limited in the past and applications were relatively scarce. 1

2 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. Paragone delle proprietà dei fluidi supercritici con i liquidi e i gas I dati sono espressi come ordine di grandezza 2

3 Proprietà di alcuni fluidi supercritici Densità della CO2 in functione della pressione a 4 differenti temperature. 3

4 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 analyteis a function of mobile phase density Density is controlled by controlling system pressure and temperature Highly polar analytesare not easy to handle (very high retention and high reactivity) SFC Advantages vs HPLC Supercritical fluids have low viscosities faster analysis (5 to 10 x faster) less pressure drop across the column (however the column end is not at ambient pressure) 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 4

5 SFC Advantages vs GC Can analyze non-volatile, polar, or strongly adsorbed solutes without derivatization. Can analyze thermally labile compounds. Can analyze solutes of much higher molecular weight. 5

6 La prima strumentazione per SFC Solvent Delivery System Maintains precise mobile phase flow (1 to 10 µl/min (OT) or 1 to 10 ml/min (Packed). Aids in the control of the system pressure(upto60mpa). Moves mobile phase in the liquid state under pressure through the injector & into the column. 6

7 Injectors Split/Splitlessvalve injector (0.01 to 0.05 µl injections) for open tubular columns. Timed -split injector (0.01 to 0.05 µl injections) for open tubular columns. Typical HPLC design injectors, both manual or in an autosampler, however precautions must be implemented. Detectors Any detector used in GC or HPLC could be used. Some modification are sometimes necessary and there may be restrictions on use. As an example UV detectors neenspecial flow cell, due to the change in refractive index; FID detector can be used only whit CO 2 and water. Coupled Detectors -MS, sometime need post column addition of suitable solvent -FTIR - Circular dichroism (to establish the elution order of enantiomers 7

8 SFC Columns Open tubular(derived from GC) - Smaller than GC capillary columns, typically 50 µmi.d.,10to20minlengt Advantage: Easier to control pressure(low P drop) Packed(derived from HPLC) SFC is usually a normal phase technique because composition is programmed from low to high polarity. Advantage: - Faster analysis - Higher flow rates - Higher sample capacity For polar solutes, polar stationary phases are used. Classical polar phases included bare silica, cyano, dioland amino. Recently,a number of new stationary phases that include several ethylpyridines and other polar aromatic molecules have been developed specifically for SFC For low polarity solutes, reversed phase columns such as C18, C8, C4, and methyl are sometimes used The use of sub-2-μm particles has become fairly common. The dominant use of SFC in the past several decades has been in chiral separations SFC is also useful for the separation of fat soluble vitamins, carotenoids, and lipids. With such samples the stationary phase is usually C18 8

9 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 Supercritical CO 2 Density P (MPa) T ( o C) ρ(g/cm 3 )

10 Modifiers or cosolvents Most SFC applications are performed on relatively polar stationary phases with CO2 modified with an organic solvent and sometimes other highly polar components, such as acids and bases, called additives. Methanol is by far the most widely used modifier and among the most polar modifiers completely miscible with CO2. SEGUE Advantages of methanol as co-solvent include: availability, inexpensiveness, complete miscibility with CO2, low UV cut-off (about 205 nm), relatively low toxicity. 10

11 In recent years, SFC has seldom been performed using pure CO2. The addition of a polar modifier, for example, an alcohol such as methanol, ethanol or isopropanol, decreases diffusion coefficients significantly. The addition of only 5.5 % methanol decreases diffusion coefficients by nearly half. Higher concentrations have an even larger effect. Since SFC is usually performed with between 5 and 50 % modifier, it is typically stated that SFC, with modified mobile phases, is three to five-timesfaster than HPLC, with the same chromatographic efficiency, on the same sized particles. This is still true when using columns packed with sub-2-μm particles. This means that run times are one-third to one-fifth as long and throughput is three to five-times greater than in HPLC (or UHPLC), on the same sized column. Re-equilibration is fast, resulting in short cycle times for gradient analysis..re-equilibration is much slower in presence of basic or acidic additives. New SFC instrumentation 11

12 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 ρ and thereby solubility and diffusion La densità della CO2 pura in funzione della pressione a 4 differenti temperature 12

13 Density of MeOH/CO2 mixtures as a function of pressure at 50 C. Si possono utilizzare altri modificanti a minore polarità Modifier P Hildebrand elution strength (on silica) Water 9 Methanol Acetonitrile Methoxyethanol 5.7 Acetone to 0.53 Ethanol Isopropanol Ethyl acetate to 0.48 Tetrahydrofuran Dichloromethane

14 % corrisponde alla percentuale di metanolo nella CO2 1 = Caffeine2 = Theophylline3 = Theobromine The effect of modifier concentration on the separation of 1-caffeine, 2-theophylline 3-theobromine, 4-uracil at 4 ml/min, 50 C, 150-bar outlet pressure., RX-SIL column, 4.6 by 150 mm, 3.5 μm. Note the unusual multiple peak reversals. 14

15 EFFETTO DELLA TEMPERATURA E DELLA PRESSIONE 4 ml/min of 7.5 % MeOH, 150-bar outlet pressure. 4 ml/min of 7.5 % MeOHat 50 C. Generally, changing temperature tends to have only modest effect on retention, but can often cause significant changes in selectivity Increasing the temperature from 50 to 60 C resulted in improved resolution, but only at significantly longer retention times. At 10-degrees lower temperature (40 C), the baseline resolution of both the first and last pairs was lost. Also pressure is considered to be a secondary control variable, but can have modest effects on retention and selectivity. Returning to 50 C and 150 bar, the column outlet pressure was varied to observe the effect of column outlet pressure on retention and selectivity. At progressively lower outlet pressures, resolution degraded. At 120 bar, the last two peaks coeluted, but at 100 bar they reversed elution order. 15

16 2 ml/min of 20 % MeOH and 100- bar outlet Fastest separation with resolution near 3 for all pairs. The numbers under the peaks are the resolutions between the peaks. Conditions: 3.5 ml/min of 20 % MeOH, 30 C and 100 bar. temperature had little effect on either retention orselectivity under theseconditions Effetto sulla separazione di composti chirali 16

17 3-idrossifenil, 3.5µm Chinolina 17

Chromatography. Gas Chromatography

Chromatography. Gas Chromatography Chromatography Chromatography is essentially the separation of a mixture into its component parts for qualitative and quantitative analysis. The basis of separation is the partitioning of the analyte mixture