Multimode Separations in a Single Chromatographic

Changing the column temperature and the pressure by using a single system allows different modes of separation to be carried out. Capillary columns, packed or open tubular, facilitate the demonstration

Figure 9 Multi-mode separation of aromatic hydrocarbons and styrene oligomers. Columns: (A) Develosil 100-5 (5 |im silica gel), 150 x0.5 mm i.d.; (B) 13mx53 |im i.d., treated with 1 mol L_1 sodium hydroxide at 55°C for 2 days. Mobile phase: diethyl ether. Initial inlet pressure: programmed as shown in the figure. Pressure drop: 0.49 MPa. Temperature: 220°C. Samples: 1 = benzene; 2 = naphthalene; 3 = anthracene; 4 = pyrene; 5 = polystyrene A-1000. Wavelength of UV detection: 220 nm. (Reproduced with permission from Ishii et al. (1988). Unified capillary chromatography. Journal ofHigh Resolution Chromatography & Chromatographic Communications 11: 801.)

Figure 9 Multi-mode separation of aromatic hydrocarbons and styrene oligomers. Columns: (A) Develosil 100-5 (5 |im silica gel), 150 x0.5 mm i.d.; (B) 13mx53 |im i.d., treated with 1 mol L_1 sodium hydroxide at 55°C for 2 days. Mobile phase: diethyl ether. Initial inlet pressure: programmed as shown in the figure. Pressure drop: 0.49 MPa. Temperature: 220°C. Samples: 1 = benzene; 2 = naphthalene; 3 = anthracene; 4 = pyrene; 5 = polystyrene A-1000. Wavelength of UV detection: 220 nm. (Reproduced with permission from Ishii et al. (1988). Unified capillary chromatography. Journal ofHigh Resolution Chromatography & Chromatographic Communications 11: 801.)

of these different modes of separation with a single chromatographic system. This is because the capillary column can achieve excellent column efficiencies. While universal chromatography allows a number of combinations to be used, it does offer more restrictions, e.g. in terms of permitted column diameters and flow rates, compared with three separate instruments.

Figure 9(A) demonstrates the separation of a prepared mixture of aromatic hydrocarbons and styrene oligomers on a silica gel packed column using the vapour of diethyl ether as the mobile phase. The analytes are isothermally separated at supercritical temperature, and the pressure is programmed so that the aromatic hydrocarbons can be separated in the GC mode prior to the SFC separation of the styrene oligomers. The pressure drop across the separation column is kept at 0.49 MPa by using a two-pump system. The inlet and outlet pressures are controlled by a microcomputer. The hydrocarbons are separated at subcritical pressure; the inlet and outlet pressures are kept at 2.45 and 1.96 MPa, respectively. The styrene oligomers are then separated by pressure-

programmed elution. The profile of the pressure programme is shown in the figure. This type of separation is of practical importance when the sample contains constituents with a wide range of volatility.

Figure 9(B) demonstrates the separation of the same mixture as in Figure 9(A) using an open tubular glass capillary column with 53 |im i.d. x 13 m. The active surface is produced by treating soda-lime glass capillary tubing with 1molL~1 sodium hydroxide aqueous solution at 55°C for 2 days. Nearly the same selectivity is observed between the open tubular and packed silica gel column.

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