Packed Column
A chromatographic device must contain a distribution system where the solute can exchange between a moving phase and a stationary phase. This distribution system usually takes the form of a cylindrical body (although not necessarily so, e.g. a TLC plate) in which one phase is held stationary and another, immiscible phase, allowed to pass through, or by it. The cylindrical body is called the column and can take two forms depending on how the stationary phase is contained in it. In one form (the capillary column) the stationary phase is held as a thin film adhering to the column walls. In the other form, the column contains an inert supporting material carrying the stationary phase absorbed on it and is called the packed column. The packing usually consists of a very inert porous material such as Celite (a diatomaceous earth made up of the skeletons of diatoms), calcined Celite (in the form of powdered fire brick) or a synthetically prepared support made from Celite. To minimize surface activity the support is often washed with acid to remove traces of iron and other heavy metals, then with water and acetone and dried. The material is then treated with hexachlorodisilazane to block any hydroxyl groups remaining on the support surface. The stationary phase is usually added to the support as a solution in a suitable solvent and ‘tumble dried’ in a rotating flask. Stationary phase loading can range from about 2% by weight to a maximum of about 15% by weight depending on the nature of the sample, the nature of the stationary phase and the anticipated sample load. In adsorption chromatography the adsorbent itself is packed into the column. The support or adsorbent is normally packed into the column by mechanical vibration. The disadvantage of the packed column over the capillary column is its relatively high flow impedance.
Author: RPW Scott
Book:Preparative Chromatography
Section:Preparative Packing-Preparative-Columns LC-Columns
are other problems
that need addressing, once packed, the practical lifetime of a column is also
uncertain. The changes in performance of a preparative HPLC column that occurs
with time depends upon the stability of the packed bed. Frequently, the bed
settles after operation for even a short time and the top of the column needs
to be repacked. Sometimes channels are formed in the bed, in which case the
entire column has to be repacked. The rate of settling again depends upon the
diameter of the column. This bed instability arises because there is a
significant change in wall support as the column diameter increases. In
analytical columns the walls are relatively close to the center of the column
and 'bridges' of packing particles can be formed across the bed, as shown in
Figure 16. These bridges allow the longitudinal forces acting on the packing
within the column to be dissipated to the walls. When a column is packed, it is
never in its optimal configuration and there are always areas
Preparative Packing-Preparative-Columns LC-Columns
Author: RPW Scott
Book:Preparative Chromatography
Section:Preparative Packing-Preparative-Columns LC-Columns
may need to be
determined by experiment. During the initial pressure adjustment some of the
packing passes into the column and forms a lightly packed bed at the bottom of
the column. The exit valve is hen rapidly opened and the sudden flow of gas
packs and compacts the bed at the same time. After packing, the reservoir is
carefully removed so as not to loosen the top of the packing and connected to
the sampling system.
LC Columns
If particle
sizes in excess of 20 mm are used, then
the column can often be dry packed, with appropriate tapping, or, even better,
with longitudinal and radial sonic vibration. The variance per unit length
obtainable from a preparative LC column should be less than 2 particle
diameters (determined using analytical scale samples). It is worth remembering
that (as already discussed) when designing preparative columns, it is better to
obtain the necessary efficiency using a longer column packed with larger
particles, than the converse. The long column
Preparative Packing-Preparative-Columns LC-Columns
Author: RPW Scott
Book:Gas Chromatography
Section:YES GC-Columns Packed-GC-Column
There are two
types of columns in common use in GC and they are the conventional packed column
and the open tubular column. The former are usually 2 to 4 mm I.D. and 1 to 4
meters long and, packed with a suitable adsorbent, are mostly used for gas
analysis. As a result of the simpler injection procedure and the more precise
sampling method, the packed column tends to give greater quantitative accuracy
and precision. However, despite its problems with sample injection, the open
tubular column is seen as the 'state of the art' column and is by far the most
popular column system in general use. The length of open tubular columns range
from about 10 m to 100 m and can have internal diameters from 100 mm to 500 mm.
The stationary phase is coated on the internal wall of the column as a film 0.2
mm to 1 mm
thick.
The Packed GC Column
Packed columns
are usually constructed from stainless steel or Pyrex glass. Pyrex glass is
favored when thermally labile materials
YES GC-Columns Packed-GC-Column
Author: RPW Scott
Book:Plate Theory and Extensions
Section:Plate-Theory Mobile-Phase-Compressibility
Consider the
column depicted in Figure 3.
Figure 3.
Pressure and Velocity Distribution Along a Packed Column
The column may
be packed or an open tube but in this example, a packed column will be
specifically considered. Let the column have a length (L) and inlet and outlet
pressures and inlet and outlet velocities of (Pi), (Po),
(ui) and (uo), respectively. The pressure and velocity at
a distance (x) from the front of the column will be defined as (Px)
and (ux), respectively. According to D'Arcy's equation for fluid
flow through a packed bed, at any point in the column,
&
Plate-Theory Mobile-Phase-Compressibility
Author: RPW Scott
Book:Principles and Practice of Chromatography
Section:Principles Basic-Chromatograph Column
is employed. Nevertheless, a
liquid thermostatting medium introduces difficulties when changing columns and
with column detector connections and is thus, not commonly used. The
temperature program can be controlled by a microprocessor incorporated in the
programmer or can be controlled from a central computer that governs the
operation of the whole instrument.
The GC column
can be a packed or open tubular and thus the oven must be capable of taking
both. The open tubular column is by far the most popular partly because they
are considered state of the art and not because they necessarily provide an
improved performance. Open tubular columns will always provide the highest
efficiencies but, if correct operating procedures are adopted, in general,
analyses carried out on packed columns, are likely to provide greater accuracy
and better precision and repeatability. Packed GC columns are usually made of
stainless steel or glass and open tubular column almost
Principles Basic-Chromatograph Column
Author: RPW Scott
Book:Capillary Chromatography
Section:Capillary Apparatus Connections
Types of Small Bore Columns
The first small bore column to be explored was the packed column. The mass sensitivity of a detector-column combination is a function of the I.D. of the column and so the reduction of the columns diameter is highly desirable for trace analysis (mass sensitivity is the minimum detectable mass, as opposed to the minimum detectable concentration and depends not only on the detector sensitivity but also on the dimensions of the column). The I.D. of small bore packed columns is considered to be 1 mm or less but GC columns having an I.D of less than 0.5 mm are extremely difficult to pack. Small bore packed columns, however, are popular and extensively used in LC. The wall coated open tubular column (WCOTT), the impressive term given to capillary columns, have I.D.s lying between 100 mm and 550 mm and carry a film of stationary phase on the wall surface that ranges between 5 mm to 50 mm thick.
The third type of small bore column is the porous
Capillary Apparatus Connections