Gas Chromatography

Gases of low viscosity with favourable solute dif-fusivity, such as hydrogen and helium, are commonly used as mobile phases in GC. For these gases the minimum in the plate height occurs at a high optimum mobile-phase velocity, resulting in efficient and fast separations. At these high mobile-phase velocities the contribution from axial diffusion to the column plate height is minimized. For thin-film columns, resistance to mass transfer in the mobile phase is the main cause of zone broadening, while for thick-film columns resistance to mass transfer in the stationary phase is equally important. Since diffusion in gases is relatively favourable, the column internal diameters required to maintain an acceptable contribution from resistance to mass transfer in the mobile phase offer little difficulty in practice. For supercritical fluids, solute diffusivity is not as favourable as for gases and in the case of liquids must be considered unfavourable. The unfavourable slow optimum mobile-phase velocity in SFC (in practice open-tubular columns are operated at 10 or more times the optimum velocity to obtain an acceptable separation time) requires significantly smaller internal diameter capillary columns than those needed for GC to minimize resistance to mass transfer in the mobile phase. At mobile-phase velocities used in practice the contribution of axial diffusion to the column plate height is negligible compared with the contribution of resistance to mass transfer in the mobile and stationary phases. For fast, high efficiency separations, column internal diameters < 100 |im are required and much smaller diameters are preferred. As densities and solute diffusivity become more liquid-like, column dimensions for reasonable

Table 1 Characteristic values for column parameters related to zone broadening

Parameter

Mobile phase

Gas

Supercritical fluid

Liquid

Diffusion coefficient (m2s_1)

10-1

104-103

10-5

Density (g cm"3)

10-3

0.3-0.8

1

Viscosity (P)

10-4

104-103

10-2

Column length (m)

Packed

1 -5

0.1-1

0.05-1

Open-tubular

10-100

5-25

Column internal diameter (mm)

Packed

2-4

0.3-5

0.3-5

Open-tubular

0.1-0.7

0.02-0.1

<0.01

Average particle diameter (^m)

100-200

3-20

3-10

Column inlet pressure (atm)

< 10

<600

< 400

Optimum velocity (cm s"1)

Packed

5-15

0.4-0.8

0.1-0.3

Open-tubular

10-100

0.1-0.5

Minimum plate height (mm)

Packed

0.5-2

0.1-0.6

0.06-0.30

Open-tubular

0.03-0.8

0.01-0.05

>0.02

Typical system efficiency (N)

Packed

103-104

104-8 x 104

5 x 103-5 x 104

Open-tubular

104-106

104-105

Phase ratio

Packed

4-200

15-500

Open-tubular

15-500

performance start to approach values similar to those for LC and are not easily attained experimentally. Slow diffusion in liquids means that axial diffusion is generally insignificant but mass transfer in the mobile phase is also reduced, requiring columns of very small internal diameter, preferably < 10 |im, which are impractical for general laboratory use. Packed columns dominate the practice of LC while open-tubular columns are equally dominant in the practice of GC, with both column types used in SFC.

Packed columns in GC are prepared from comparatively coarse particles of a narrow size distribution and coated with a thin homogeneous film of liquid for high performance. The relatively large particle size and short column lengths are dictated by the limited pressure drop employed for column operation. For thin-film columns, resistance to mass transfer in the mobile and stationary phases is the main cause of zone broadening with a contribution from flow ani-sotropy. For thick-film columns, resistance to mass transfer in the stationary phase tends to dominate. The intrinsic efficiencies of open-tubular columns and packed columns of similar phase ratio are comparable, but because the two column types differ greatly in their relative permeability at a fixed column pressure drop, much longer open-tubular columns can be used. Thus, packed GC columns are seldom more than 5 m long while columns with lengths from 10 to 100 m are commonly used in open-tubular column GC, resulting in a 100-fold increase in the total number of theoretical plates available. In general, packed columns are used in GC for those applications that are not easily performed by open-tubular columns, for example separations that require a large amount of stationary phase for the analysis of very volatile mixtures, or where stationary phases are incompatible with column fabrication, preparative and process-scale GC, etc.

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