GC Using an LC System

In GC permanent gases are usually employed as the carrier gas. The vapour of an organic substance can also be used as the mobile phase for GC when the pressure in the column is lower than the vapour pressure at the operating temperature. In contrast, when the pressure in the column is higher than the vapour pressure, analytes are subjected to LC-mode separation. By controlling the column temperature and pressure, it is possible to demonstrate both GC and LC using a single LC system. It is convenient to use a micro-LC system because the heat capacity of micropacked columns of 0.2-0.5 mm i.d. is small.

Figure 4 shows the relationships between the logarithm of the retention factor and the reciprocal of the absolute column temperature when hexane and silica gel are used as the mobile and stationary phases. In Figure 4 the inlet pressure is 0.49 MPa, while the outlet pressure is atmospheric. The mobile phase is liquid when passing through the flow cell of the UV detector. The critical temperature and pressure of hexane are 234°C and 3.0 MPa, respectively. The critical temperature of hexane is denoted as CT in the figure. It should be noted that the pressure in the column is lower than the critical pressure. At higher

Figure 5 GC separation of alkylbenzenes at an inlet pressure of 0.49 MPa using hexane as a carrier gas. Column: Develosil-60-10 (10 |im silica gel), 300x0.5 mm i.d. Mobile phase: hexane. Inlet pressure: 0.49 MPa. Outlet pressure: ambient pressure. Column temperature: 205°C. Peaks: 1 = benzene; 2 "toluene; 3 = naphthalene; 4 = o-xylene; 5 = n-propylbenzene; 6 = mesity-lene; 7 = sec.-butylbenzene; 8 = n-butylbenzene; 9 = n-amyl-benzene. Wavelength of UV detection: 254 nm. (Reproduced with permission from Takeuchi etal. (1988). Micropacked column GC with vapor of organic substance as the mobile phase. Chromato-graphia 25: 995.)

Figure 5 GC separation of alkylbenzenes at an inlet pressure of 0.49 MPa using hexane as a carrier gas. Column: Develosil-60-10 (10 |im silica gel), 300x0.5 mm i.d. Mobile phase: hexane. Inlet pressure: 0.49 MPa. Outlet pressure: ambient pressure. Column temperature: 205°C. Peaks: 1 = benzene; 2 "toluene; 3 = naphthalene; 4 = o-xylene; 5 = n-propylbenzene; 6 = mesity-lene; 7 = sec.-butylbenzene; 8 = n-butylbenzene; 9 = n-amyl-benzene. Wavelength of UV detection: 254 nm. (Reproduced with permission from Takeuchi etal. (1988). Micropacked column GC with vapor of organic substance as the mobile phase. Chromato-graphia 25: 995.)

temperatures linear relationships between log k and 1/T are observed, where the GC separation mode is involved. Since the applied pressure is lower than the critical pressure, the hexane vaporizes at a temperature lower than the critical temperature. When the outlet pressure is higher than the vapour pressure at the operated temperature, the hexane is liquid in the column and analytes are separated in the normalphase LC mode; almost linear relationships between two parameters are observed. At some intermediate temperature the retention factor drastically changes with column temperature, where the state of the hexane in the column changes from liquid to gas, and both liquid and gaseous hexane exist at the boundary region in the column.

Figure 5 demonstrates GC separation of alkylben-zenes using hexane vapour as the mobile phase. The analytes are monitored by a UV detector at 254 nm. The mobile phase is liquefied while passing through a 60 cm x 70 |im i.d. capillary tube connecting the separation column and the flow cell of the detector. The capillary tubing is kept at ambient temperature.

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