Adlard E.R. (ed.) Chromatography in the Petroleum Industry
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242
Chapter
9
Removal
of
high baling
mpou&,
fast
reconditioning
of
the
system
Reduchon
of
compkx
samples
onto
onty
a
few
compounds
of
intd
Shortening
of
analysts
hme
r
Protection
of
cdurnns
or
detedm
from
stressing
subdances
Anal-
of
very
dtfferenI
types
of
compounds
in
only
one
system
HeartCut
Analysrs
of
traces
in
the
tailing
of
main
components
Decrease
in
Wecbon
limtts
Vanatm
of
retmllon
timcw
for
compounds
of
interest
Fig.
9.3.
Advantages
of
the use
of
different column switching techniques.
-
The overall temperature limit
of
a chromatographic system is no longer
determined by the phase with the lowest temperature limits; stationary
phases with high and low thermal stability can be operated independently.
Gas-solid chromatography usually requires higher separation temperatures
than gas-liquid chromatography; in a double oven system, the combina-
tion
of
both separation principles is possible in such
a
way that both col-
umns can
be
operated
at
their optimum temperature.
It
is possible to adjust the selectivities
of
both columns by tuning their
temperature (see Section
9.3)
independently, thus having
a
very elegant
way
of
combining the advantages
of
selectivity tuning and column
switching.
-
-
Multi-column
systems
in
gas chromatography
243
/
straieht
on
Fig. 9.4.
Column
switching using valves.
NV
2
Straight
on
cut
Backflush
Fig. 9.5.
Column
switching using a live-switching system.
References
pp.
266-268
244
Chapter
9
-
When combining
two
identical columns, the first operated in a tempera-
ture programmed and the second in
an
isothermal mode, one can exploit
the second column
as
an “identification” column for the determination of
highly precise isothermal retention indices and in this way the advantages
of temperature programmed separation can be combined with those of
isothermal identification of the separated components
[
181.
Finally, the development of the modern microprocessor controlled instru-
ments incorporating a digital adjustment and control
of
temperature and even
pressures and flow rates allows very precise and reproducible results and the
built-in time programmable functions and integration methods make column
switching systems relatively easy to use.
9.3.2
Backflushing
Backflushing is one of the most important switching steps and it is used in
almost all process chromatographic applications. The aim
of
this technique
is
to
shorten analysis time and to obtain
a
definite end to the analysis. Analysis time
can be reduced by eliminating the compounds which are eluted on the first col-
umn after the last compound to be measured. After the compounds
of
interest
have moved from column A to column
B
and the less important components are
still in column A, the direction of the carrier gas stream in column A is reversed
(see Figs.
9.4
and
9.5).
This means that all components that have not left column
A at the time of switching are flushed out from the separation system. A very
typical example of the reduction of analysis time by the use of backflush
is
the
determination of traces
of
acetylene in ethylene. Figure
9.6
shows the chroma-
togram
of
such an analysis without backflush. From the bottom part of the fig-
ure, it can be seen that the use of backflush allows more than twelve analysis
cycles in the same time. Another advantage of the backflush technique is a
definite end to the analysis. In contrast to laboratory gas chromatographs where
late eluting components are usually removed from the system by temperature
programming, the backflush technique removes these components without ther-
mal stress to the columns. Moreover, it is the only method which can guarantee
that all components that can be eluted are really flushed out of the system
so
that
late eluting components are not disturbing further analysis sequences as unex-
pected peaks or
as
a
baseline drift. The column length ratios can, usually without
any problem, be arranged in such a way that the elution time for the last com-
pound of interest in column
B
or
B
and
C
is
at least
as
long
as
the elution time of
that compound in column A. Another common trick when using backflushing
is
to adjust the backflush gas flow rate to be greater than the forward flow rate.
Multi-column systems in gas chromatography
245
Fig.
9.6.
Shortening
of
analysis time by the use
of
backflush, analysis
of
trace amounts
of
acetylene
(1)
in
ethylene
(2).
That means that the time for backflush is long enough to remove all disturbing
compounds from the system and immediately after the elution
of
the last peak of
interest a new analysis sequence can be started.
9.3.3
Cutting
Section
9.3.2
described how the use of backflushing can avoid interferences
or disturbances of the analysis by compounds that would be eluted after the last
component of interest. In addition, the cut technique allows venting out of the
chromatographic system components eluting in front of, or between components
of interest. This can be done by a time-programmed switching of the gas flow
after one or more pre-columns either in the straight-on position (components are
transferred to the main column and to the detector) or in the cut position
(components are vented out
of
the system through a needle valve or a capillary
restrictor with the same flow resistance as the main column). The flow-paths for
both cases are shown in Figs.
9.4
and
9.5,
respectively. There are three important
aims in the application of the cut technique:
References pp.
266268
246
Chapter
9
-
Extended column life and prevention of detector contamination by selec-
tive transfer of only the analytically significant components into the main
column. We have pointed out the advantages earlier.
Reduction of the complexity
of
multi-component samples and thereby re-
duction of the probability of peak interferences. This is
a
very helpful tool
when only a few components of a complex sample are to be determined.
In cases where different stationary phases are used the well known effect
of interferences between components which are separated on the individ-
ual columns but have the same overall retention time on the combined
columns can be avoided.
A third and very important application of cutting techniques is the
so-
called “heart-cutting”. In trace analysis, the column usually has to be
overloaded in order to obtain a sufficient signal from the trace component.
This causes difficulties when the trace
is
eluted on the tail of the major
component. Using a cutting device
so that only the trace and the underly-
ing small amount of the major components are transferred to the second
column gives a substantial improvement in separation and a decrease in
the detection limit for the trace component. The second column only has
to separate the trace from the small amount of the tail
of
the major com-
ponent. The typical peak shape of the tail can be seen in the simple ex-
ample of a “heart cut” given in Fig.
9.7.
-
-
9.3.4
Distribution cutting
The column switching configuration called distribution cutting is used either
to
shorten analysis time or in cases where very different components are to be
separated on
two
different columns simultaneously. It is realized by the same
switching configuration as for cutting but in distribution mode, the “cut” compo-
nents which are vented to waste in cut-mode are by-passed to the detector
through a needle valve or a capillary restrictor only or to a combination of an
additional column and a restrictor which together have the same flow resistance
as the main column.
A
typical example of the shortening of analysis time is
given in Fig.
9.8.
The second column is only necessary for the separation of
components 11-13. Components 1-10 will be by-passed directly to the detector
after the first column.
Figure
9.9
shows that the elution
of
all the components through the second
column would lengthen the analysis time. This
is
significantly reduced by the
use of the distribution technique.
A
typical example of the other strategy of dis-
tribution, the simultaneous separation of very different components within only
one analysis is the application of permanent gases and hydrocarbons.
A
pre-
Multi-column
systems
in
gas chromatography
247
column has to separate the sample into a hydrocarbon fraction and a permanent
gas fraction. The permanent gases are separated
on
molecular sieve whereas the
hydrocarbon fraction
is
distributed to another column which would not be able
to
separate the permanent gases.
An
example
of
the resulting chromatograms
is
given in Fig.
9.10.
3
Fig.
9.7.
Improved detectability
of
traces in the tail
of
the major component by heart-cut.
1,
Main
component;
2,
3,
trace components to be determined;
4,
typical peak shape
of
the cut. Top, without
heart-cut; bottom, with heart-cut.
References
pp.
266268
248
Chapter
9
Fig. 9.8. Shortening of analysis time by distribution (chromatogram), analysis
of
a
gasoline frac-
tion,
GC
conditions: column
A,
17
m
X
0.20 mm
HP-PONA;
column
B,
33 m
X
0.20
mm
HP-
PONA,
carrier hydrogen, temperature 30°C.
1,
butane;
2,
i-pentane; 3, n-pentane;
4,
2,2-
dimethylbutane;
5,
3-methylpentane;
6,
hexane;
7,
methylcyclopentane;
8,
2,4-dimethylpentane; 9,
benzene;
10,
cyclohexane;
1
1, cyclopentane; 12, 2,3-dimethylbutane; 13, 2-methylpentane. Sepa-
ration of components
1-10
on column
A
only and distribution directly
to
the detector, separation of
the
components 11-13 on columns
A
and B.
Retention Imin.1
25
20
-
15
10
5
0
~
----
-~
i
~-
7
---
_-
-
12345678910111213
Measuring
components
1
t(R) precolumn t(R) main column
'/
minimal cycle
Comp.11-13 not separated after precolumn
Fig. 9.9. Comparison of retention times with and without distribution (components and
GC
condi-
tions see Fig. 9.8).
Multi-column systems
in
gas chromatography
249
5
9
3
A
2
1
Fig. 9.10. Distribution onto
two
different types
of
columns, separation
of
permanent gases on
a
molecular sieve (A) and
of
lower hydrocarbons on
a
modified Spherosil
(B).
1, oxygen; 2, meth-
ane;
3,
ethane;
4,
ethene;
5,
propane;
6,
ethine;
7,
iso-butane;
8,
n-butane;
9,
propene.
9.3.5
Special switching techniques
9.3.5.1
Summation
of
backjlushed compounds
For certain applications, it is required that all components, even very high
boiling ones must be detected. A straight elution of all components often pro-
longs analysis time unnecessarily, and in many cases, higher boiling compounds
do
not elute at all, causing build up on the column with deterioration of effi-
ciency or selectivity. In these cases and when only the determination of late
eluting compounds as a sum parameter is
of
interest, a backflush sum
is
used.
Backflush sum means in practice that the same switching configuration as
for
backflush is used but in this case, the backflushed effluent is not flushed out of
the system but is by-passed to the detector. All components which, at the time
of
References
pp.
266268
250
Chapter
9
switching, are still in the first column, are backflushed. In this manner, the sepa-
ration already achieved is eliminated by the reversal of carrier gas flow; all com-
ponents are eluted simultaneously and detected as one peak.
9.3.5.2
Stoppedflow/stuttering
In
all systems with more than one flow path to the detector, it is possible that
different components are eluted to the detector by both paths simultaneously. If
there is no possibility of avoiding this by a proper arrangement of column
lengths and flow rates, then
two
more or less tricky column switching techniques
can help to solve the problem without the necessity of using a second detector.
The first variant (unusual in capillary systems)
is
to stop the flow in one column
for a short time by by-passing the carrier gas through a needle valve with the
same flow resistance as the “stopped” column. In this way, all components
which are in the “stopped’ column will elute a little later. But in every case, one
has
to take into account that during the time the flow
is
stopped, peak broadening
processes by diffusion continue. Our experience is that immobilization of some
components, especially hydrogen, often results in band broadening or a
loss
of
sensitivity.
The same effect, a selective delay of one or some components, can be
achieved by short periods of flow reversal. Usually the backflush mode can be
used for this technique. Switching of the backflush valve for a short time causes
a delay for all components which are still in the first column. This so-called
“stuttering” technique can also help to avoid the simultaneous detection of
two
separated components from different flow paths, but of course the application of
stuttering is limited by the same disadvantages as stopped flow.
9.3.5.3
Recycle Chromatography
The idea in recycle chromatography is to achieve the required separation, not
by
the use of a very long column, but by a repeated separation of the sample on
one and the same short column. This can also be realized with a column switch-
ing technique. Recycle chromatography offers the chance to get a large number
of theoretical plates with a very small carrier gas pressure and, in the case of
capillary columns, with short and inexpensive columns. The principles of recycle
chromatography especially in preparative mode have been treated by Chizkov
et
al.
[31].
Some of the known applications are listed in
[4]
but it seems that the
technique
has
not found widespread use up to now.
9.4
PRACTICAL ASPECTS
Instrumentation for multi-column gas chromatography ranges from very ele-
mentary arrangements to highly complex and automated systems. In the simplest
Multi-column systems
in
gas chromatography
25
1
case, only a conventional gas chromatograph with single inlet, single detector
and
two
columns is required. These
two
columns can be coupled either by valves
or more elegantly by a valveless pneumatic switching device. Instruments spe-
cially designed for multi-dimensional
GC
may also incorporate such functional
elements as auxiliary inlets/outlets, selective detectors, dual ovens, automatic
timers and various intermediate traps for focussing components of interest.
A
multi-column system using traps is one possibility for the utilization
of
column-
switching as a sampling technique.
9.4.1
Valve
switching
Multi-column systems using valve switching have been in use successfully for
more than
30
years
[32].
Valves are the simplest way of switching effluent from
one column to another and are very similar to gas sampling valves. Others used
for column switching are rotary, sliding or piston valves. Some versions are de-
signed with up to
12
ports, miniaturized or remote controlled. Figure
9.4
(see
Section
9.3.1)
shows a diagram of a complex valve switching system.
Commonly, valve switching systems are used with packed columns. However,
in recent years, packed columns have been more and more displaced by capillary
columns, especially in single but also in multi-column systems, because of their
greater efficiency. These capillary multi-column systems are mostly switched by
pneumatic switching devices. Nevertheless, for some analytical requirements,
the use of mechanical switching in combination with packed columns has some
advantages.
As
many different column configurations as necessary can be built
and therefore many analytical problems can be solved with one system. Their
relatively large internal volume does not affect the analytical separation because
of the high carrier gas flow rates through packed columns. However, if micro-
packed or capillary columns with smaller inside diameter and lower operating
flow rates are used, peak broadening and tailing with a loss of efficiency may
occur. These adverse effects can be overcome by cold trapping the solutes on the
head of the column. The temperature limitation due to the sealing material
within the valve is another disadvantage when using mechanical column
switching by valves. None of the materials inside the valves present an ideal sur-
face for gas chromatography. Adsorption and memory effects can limit the ap-
plication, especially for trace component analysis.
However, because packed systems with valve switching are simple, inexpen-
sive, versatile and easily maintained, such systems are still used, especially in
on-line process control and for gas analysis. A very simple but reliable and rapid
analysis for process control is the determination of sulphur components in the
Claus process shown in Fig.
9.1
1.
Refirences
pp.
266268