Temperature Effect

Figure 1 shows the effect of column temperature on retention behaviour using a packed column and carbon dioxide or methanol as the mobile phase. The relationships between the logarithm of the retention factor of aromatic hydrocarbons, log k, and the recip-

Table 1 Comparison of physical properties of the mobile phase in three chromatographic modes

Chromatography Density(gcm 3) Viscosity(g cm 1s 1) Diffusioncoefficient(cm2s 1)

Supercritical fluid chromatography 0.2-0.9 (0.2-1.0) x 10~3 (0.1-3.3) x 10~4

Liquid chromatography 0.8-1.0 (0.3-2.4) x 10~2 (0.5-2.0) x 10~5

Figure 1 Relationships between the logarithm of the retention factor of aromatic hydrocarbons (log k) and the reciprocal of absolute column temperature (T~1). Columns: Develosil ODS-5 (5 ^m ODS), 150 x0.5 mm i.d. (1-6) or 145 x0.3 mm i.d. (7). Mobile phase: carbon dixoide (1-6) or methanol (7). Inlet pressure: 12 MPa (1-6) or 9.0 MPa (7). CT, Critical temperature. Samples: 1 = biphenyl; 2 = fluorene; 3 = o-terphenyl; 4 = pyrene; 5 = 9-phenylanthracene; 6 = triphenylene; 7 = pyrene. Detector: UV.

Figure 1 Relationships between the logarithm of the retention factor of aromatic hydrocarbons (log k) and the reciprocal of absolute column temperature (T~1). Columns: Develosil ODS-5 (5 ^m ODS), 150 x0.5 mm i.d. (1-6) or 145 x0.3 mm i.d. (7). Mobile phase: carbon dixoide (1-6) or methanol (7). Inlet pressure: 12 MPa (1-6) or 9.0 MPa (7). CT, Critical temperature. Samples: 1 = biphenyl; 2 = fluorene; 3 = o-terphenyl; 4 = pyrene; 5 = 9-phenylanthracene; 6 = triphenylene; 7 = pyrene. Detector: UV.

rocal of absolute column temperature, 1/T, are shown in the figure with an inlet pressure of 12 MPa for carbon dioxide and 9.0 MPa for methanol. It should be noted that the pressure in the column is higher than each critical pressure, e.g. 7.38 MPa for carbon dioxide and 7.95 MPa for methanol. The critical temperatures of carbon dioxide and methanol are denoted as CT in the figure. At each higher critical temperature region, the retention factor increases with decreasing column temperature, while at each lower critical temperature region it decreases with decreasing column temperature. When the mobile phase is liquid, i.e. at a temperature lower than the CT, the retention factor increases with decreasing column temperature, as shown in the case of the methanol mobile phase.

It should be noted that the density of the mobile phase decreases with increasing temperature. The density of carbon dioxide is shown in Figure 2.

It is possible to distinguish a region in which the retention factor decreases with decreasing column temperature (SFC region) from one in which it increases with decreasing column temperature (high pressure GC region). The former region appears at lower supercritical temperatures, in which solvation of the analyte by the mobile phase is dominant, while the latter region appears at higher supercritical temperatures, in which the contribution of volatility is

Figure 2 Density of carbon dioxide.

Figure 3 (A) Negative and (B) positive temperature programming of dialkyl phthalates on an SB-octyl-50 column. Column: SB-octyl-50 (5% n-octylmethylpolysiloxane), 10mx50 ^m i.d. Mobile phase: carbon dioxide. Inlet pressure: 12 MPa. Samples: 1 = dimethyl; 2 = diethyl; 3 = diisobutyl; 4 = di-npropyl; 5 = diiso-butyl; 6 = di-n-butyl; 7 = diheptyl; 8 =di-2-ethylhexyl; 9 = dinonyl phthalate. Initial temperature: 130°C. Temperature programming: (A) -10°C min- for 4 min, and -5°C min-1 for the rest of the analysis; (B) #10°C min-1 for 2 min, # 20°C min-1 for the next 4.5 min and kept at 240°C for the rest of the analysis. Wavelength of UV detection: 225 nm. (Reproduced with permission from Takeuchi et al. (1988). Temperature programming elution in capillary supercritical fluid chromatography. Chromatographia 25: 127.)

250 200

250 200

1 1

1 1 1

x10"'

Figure 3 (A) Negative and (B) positive temperature programming of dialkyl phthalates on an SB-octyl-50 column. Column: SB-octyl-50 (5% n-octylmethylpolysiloxane), 10mx50 ^m i.d. Mobile phase: carbon dioxide. Inlet pressure: 12 MPa. Samples: 1 = dimethyl; 2 = diethyl; 3 = diisobutyl; 4 = di-npropyl; 5 = diiso-butyl; 6 = di-n-butyl; 7 = diheptyl; 8 =di-2-ethylhexyl; 9 = dinonyl phthalate. Initial temperature: 130°C. Temperature programming: (A) -10°C min- for 4 min, and -5°C min-1 for the rest of the analysis; (B) #10°C min-1 for 2 min, # 20°C min-1 for the next 4.5 min and kept at 240°C for the rest of the analysis. Wavelength of UV detection: 225 nm. (Reproduced with permission from Takeuchi et al. (1988). Temperature programming elution in capillary supercritical fluid chromatography. Chromatographia 25: 127.)

dominant. In the intermediate temperature region, both contributions are involved. In addition, nonvolatile analytes cannot be eluted in the high pressure GC region, but they are eluted in the SFC and LC regions. It is clear that both SFC and high pressure GC separations can be demonstrated by changing the column temperature. Negative temperature programming is useful for the former, while positive temperature programming is useful for the latter mode.

Figure 3 demonstrates the separation of dialkyl phthalates using negative and positive temperature programming using an SB-octyl-50 open tubular capillary column and a UV detector. Both temperature programmes are started from the same temperature, i.e. 130°C. The programming rate is changed during the separation so that optimum resolution can be achieved in a reasonable time. The pressure is kept constant at 12 MPa during the separation. In Figure 3(A), SFC-like separation is demonstrated, while GC-like separation is demonstrated in Figure 3(B).

Figure 4 Log kversus 1/Tfor hexane mobile phase. Column: Develosil-60-10 (10 ^m silica gel), 300x0.5 mm i.d. Mobile phase: hexane. Inlet pressure: 0.49 MPa. Outlet pressure: ambient pressure. CT, Critical temperature. Samples: • = benzene; ▲ = naphthalene; ■ = anthracene. Wavelength of UV detection: 254 nm. (Reproduced with permission from Takeuchi et al. (1988). Micropacked column GC with vapor of organic substance as the mobile phase. Chromatographia 25: 994.)

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