Page 54 - Subyek Encyclopedia - Encyclopedia of Separation Science

Sepsci*21*TSK*Venkatachala=BG
I / CHROMATOGRAPHY 49

Table 1 Characteristic values for column parameters related to zone broadening

Parameter Mobile phase
Gas Supercritical fluid Liquid

Diffusion coefficient (m s ) 10 1 10 }10 3 10 5
4
1
2
Density (g cm ) 10 3 0.3}0.8 1
3
4
Viscosity (P) 10 4 10 }10 3 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
1
Optimum velocity (cm s )
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)
4
3
3
Packed 10 }10 4 10 }8 10 4 5 10 }5 10 4
4
4
Open-tubular 10 }10 6 10 }10 5
Phase ratio
Packed 4}200
Open-tubular 15}500
performance start to approach values similar to those seldom more than 5 m long while columns with
for LC and are not easily attained experimentally. lengths from 10 to 100 m are commonly used
Slow diffusion in liquids means that axial dif- in open-tubular column GC, resulting in a 100-fold
fusion is generally insigniRcant but mass transfer increase in the total number of theoretical plates
in the mobile phase is also reduced, requiring col- available. In general, packed columns are used
umns of very small internal diameter, preferably in GC for those applications that are not easily
(10 m, which are impractical for general laborat- performed by open-tubular columns, for example
ory use. Packed columns dominate the practice of LC separations that require a large amount of stationary
while open-tubular columns are equally dominant in phase for the analysis of very volatile mixtures, or
the practice of GC, with both column types used where stationary phases are incompatible with col-
in SFC. umn fabrication, preparative and process-scale GC,
Packed columns in GC are prepared from compar- etc.
atively coarse particles of a narrow size distribution
and coated with a thin homogeneous Rlm of liquid for Liquid Chromatography
high performance. The relatively large particle size The intrinsic efRciency per unit length of packed
and short column lengths are dictated by the limited columns in LC increases as the particle diameter is
pressure drop employed for column operation. For reduced. It can also be increased by using solvents of
thin-Rlm columns, resistance to mass transfer in the low viscosity, which result in smaller contributions to
mobile and stationary phases is the main cause of the column plate height from resistance to mass trans-
zone broadening with a contribution from Sow ani- fer and Sow anisotropy. Operation at low mobile-
sotropy. For thick-Rlm columns, resistance to mass phase velocities compared to GC further minimizes
transfer in the stationary phase tends to dominate. the contributions from resistance to mass transfer in
The intrinsic efRciencies of open-tubular columns the mobile phase at the expense of longer separation
and packed columns of similar phase ratio are times. The pressure drop required to maintain a con-
comparable, but because the two column types dif- stant mobile-phase velocity is proportional to the
fer greatly in their relative permeability at a Rxed ratio of the column length to the particle diameter
column pressure drop, much longer open-tubular col- squared. Since the available operating pressure is
umns can be used. Thus, packed GC columns are Rnite, the column length must be reduced as the