COLUMN SWITCHING
Column switching
Column switching provides many advantages with
its various operating modes Straight, Backflush,
Cut.
Two columns are usually connected in series. The col-
umns have e.g. different separating characteristic or dif-
ferent temperatures.
More complex switching is possible depending on the
problem. The type of switching used in your chromato-
graph can be found in the application documents.
Straight
The components pass through columns A and B in suc-
cession.
Backflush
Once a number of components have passed through
column A, the flow direction in this column is reversed.
The components which are still present in column A are
collected again at the beginning of this column and
flushed out into the open air via the backflush outlet or
as a backflush total to a detector.
This permits the analysis cycle to be drastically short-
ened: higher boiling components which only pass
through column A slowly need not be removed at a
higher temperature, a process which often requires a
long time.
Cut
Once a number of components have passed through
column A, they are flushed out through the cut outlet.
They never reach column B.
This method is used
to keep components away from column B which
cannot be backflushed because they are followed by
components which are still of interest
if components falsify the result or damage
column B
to directly pass components onto a further detector,
e.g. in order to shorten the duration of the analysis
to divide components further on a third column
(distribution).
Live switching or diaphragm valve?
The columns can be connected using the live T-piece or
diaphragm valves.
Advantages of live switching
The sample only comes into contact with metal
parts in the live T-piece
There are no temperature imitations because the
switching valves are mounted outside the oven
Maintenance-free since there are no moving parts
Minimum dead volume, highly suitable for capillary
columns.
Disadvantages of diaphragm valve switching
The sample comes into contact with plastic compo-
nents in the valve (Viton/Teflon)
Influence of diffusion through the diaphragm
Dead volume too high for capillary columns
The maximum analysis temperature is 150 °C.
Column A
Column B
Column A
Column B
Column A
Column B
Live T-piece
Diaphragm valve
Column switching
6 - 2 schaltg.chp, Issue August 17, 1999
To Next Page
Loading...
