Adlard E.R. (ed.) Chromatography in the Petroleum Industry
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252
Chapter
9
6
3
2
J
J
i
min.
6
Fig,
9.1
1.
Rapid determination
of
sulphur components
for
Claus-process control.
1,
nitrogen;,
2,
carbon dioxide;
3,
hydrogen sulfide;
4,
carbonoxy sulfide;
5,
sulphur dioxide;
6,
water.
9.4.2
Valveless
flow
switching
A
more complex method of redirecting
flows
in
chromatographic systems op-
erating at elevated temperatures is the use of pneumatic switching. In contrast to
mechanical switching, all moving parts are located outside the heated zones and
the separation path. In such a pneumatic system, the demand for a zero dead
vol-
ume column coupling is obtained by different types of coupling devices mostly
made of glass
or
noble metals. Consequently none of the problems
of
leaks, tem-
perature limitations, adsorption and memory effects or
loss
in efficiency associ-
ated with mechanical valves apply
to
such
a
system.
Multi-column
systems
in
gas chromatography
253
FID
1
Aromatics
FID
2
Saturates
and Olefines
,I
,p
,*.
,I,,
Y
YI
I... I- I"
."
.I-
.
.>I.
-3
I
".I
"I
m
.-
.,.
I
111,"
..
-
.,.
-I
CI
d-
I
CI
CI
u
s--,
Siemens SiCHROMAT
2
Fig.
9.12.
Determination of individual components (paraffins, olefins, naphthenes and aromatics) in
hydrocarbons mixtures
on
capillary columns.
1,
separation
of
aromatics (xylene and higher)
on
a
polar pre-column and distribution to FID
1;
2,
transfer
of
lower to a non-polar second column and
separation up to
C9,
benzene and toluene, detection
of
saturates and olefins by splitting
of
eluent;
3,
detection
of
saturates only
on
FID
3
by means
of
scrubbing
out
the olefins.
PONA
analysis (paraffins, olefins, naphthenes and aromatics), a typical
analysis in the petroleum industry, demonstrates the high performance of a
valveless switching system using capillary columns (Fig.
9.12).
One critical component in pneumatic switching is the carrier gas supply con-
trolled by pressure regulators. The pressure regulation should be very repro-
ducible under all conditions. Two typical valveless flow switching systems are
described below.
9.4.2.
I
Deans-switching
The first pneumatic switching system was developed by Deans
[23]. A
sche-
matic
flow
path of such a system is given in Fig.
9.13.
The principle is based on
References
pp.
266268
254
Chapter
9
solenoid
operation
ON/OFF
~
valve
manual operation
ON/OFF
~
valve
Fig.
9.13.
Flow
diagram
of
a
heart
cutting system
for
two
packed columns due
to
Deans
[33].
the balance of
flows
by in-line restrictors and two pressure controllers (head and
medium pressure) indicated on gauges
(PA, P,).
By opening and closing external
valves
(SVl),
the direction of flow can be changed in this two-column system.
Before starting analysis, the following flow adjustments should be carried out.
Basic
adjustments
-
-
Adjustment
of
head pressure
PA
to the optimum flow with
PR1.
Adjustment
of
medium pressure
P,
by isolating
PR2
with the
odoff
valve
MV,
so
that the naturally occurring pressure
can
be read on
P,
.
For
this
operation
SV
1
is closed.
R1
is chosen such that
a
small fraction of pre-column effluent is passed to
the monitor detector
(Dmn).
NV2
should approximately balance the restriction of the main column.
-
-
Two
phases can be distinguished in the operation modes
of
the illustrated
system:
Sample transfer.
Effluent eluting fiom pre-column
A
enters main column B in
the direction towards the main detector
D~n.
SV1
is
closed during the operation.
A small fraction
of
effluent is split to monitor detector
DmOn.
PR2
supplies the
junction of the columns with a small amount
of
carrier gas.
Multi-column systems in
gas
chromatography
255
Heart cutting.
Effluent eluting from pre-column
A
is vented out of the chroma-
tographic system through needle valve
NV2.
SVl is now open. The gas and the
effluent coming from the pre-column flow out to the atmosphere.
In addition, a small amount of gas coming from
PR2
ensures that a clean cut is
obtained.
By modifling the illustrated system with an additional valve after PRl, the
backflush mode can also be realized.
This system has the following advantages. Transfer times between columns
can be extremely short (milliseconds) and this makes very sharp cuts possible.
A
fraction or fractions can be transferred from the first column to the second with-
out interference from components of no interest, which are vented.
However, the use of Deans-switching is limited by the following:
-
-
Only a few switching modes are possible (backflush and heartcut).
Retention time shifting on the first column is the result of high pressure
differences in the coupling piece when switching between cut and transfer
mode. This affects the cut time period.
The system is mainly applicable to packed columns. The use of capillary
columns requires a focusing trap for the second column.
The cut valve is still in the sample path. This means a limited lifetime for
this valve.
The flow through the cut exit depends on the switching position and thus
a constant
flow
is not available at this point. Therefore monitor detection
is only possible when using an additional restriction outlet between cou-
pling piece and cut valve. With this modification, the effluent is continu-
ously split to the monitor detector.
-
-
-
Deans demonstrated that, in some cases, two packed columns in series with
different stationary phases and heart cutting between them can have a much
greater separation power than a single capillary column
[33].
In such a packed
column system, the source of separation power for complex mixtures, in spite of
lower number of plates, lies in the difference in selectivity between the two sta-
tionary phases and in the elimination of interfering components of no interest.
However, packed columns were mainly used in coupled systems
for
simple ana-
lytical problems having
a
less complex sample composition.
9.4.2.2
Live-switching
The live column switching system
[34]
is based on the principle of pneumatic
switching developed by Deans. The differences to Deans-switching are the fol-
lowing:
References
pp.
266268
Chapter
9
n
Fig.
9.14.
Scheme
of
the small pressure differences in the coupling piece.
-
In contrast to the Deans system which operates with significant flow dif-
ferences between the pre-column and main column, the live system
is
based on small pressure differences (see Fig. 9.14) in the coupling piece,
which results in very small flow differences. Therefore, the live system
guarantees a constant flow through all the columns and restrictors, so that
the cut exit can also be connected to the detector.
The live switch consists of a special coupling T-piece which is a part of a
"pneumatic bridge" configuration (see Fig.
9.5).
It operates
in
a very
similar manner to the Wheatstone bridge, consisting of four (flow) resis-
tors (needle valves
NV2,
NV3,
and restrictors
R1,
R2).
The differential
pressure
(6P)
in the bridge diagonal is adjustable in the negative direction
with
NV2
and instantly switchable with solenoid valve
SV
CUT.
There is an additional
flow
channel with only a small resistance, which is
located in one diagonal. Either the total flow from the first column is di-
rected into the second column or it is diverted from the first column via
the restrictor
R1
into the cut exit, due to a reverse flow depending on the
differential pressure applied.
The specially designed T-piece, shown in Fig. 9.15, avoids unswept vol-
umes by small gaps
- especially between the column and the coupling
tube. Back diffusion of the sample into the gaps is thereby eliminated and
-
-
-
Multi-column
systems
in
gas chromatography
257
1-
Fig.
9.15.
Dead-volume free live-T-piece
for
coupling
two
capillary
columns.
peak distortion is also prevented in the cut path. Capillary columns can be
used as well as packed columns without loss of efficiency.
More principal column configurations are possible (backflush, heartcut,
distribution and backflush sum).
-
In this system, high precise pressure controllers are used,
so
that the relation
between head and medium pressure is constant over the complete analysis time.
Only under correct pressure regulation are the switching times of, e.g. heartcut,
precise and reproducible. Regulators for this system based on the principle of a
nozzle/baffle plate arrangement are built up completely without a diaphragm,
so
that the carrier gas supply cannot be contaminated by degassing softening agents
and also the influence of ambient temperature change is eliminated.
From the above characteristics, the advantages of live-switching are clear. In
Fig.
9.16,
this system is compared with a mechanical column switching unit
where the switching valves are located in the heated zone.
For the first time, this technique made it possible to use capillary columns in
process chromatography
[35],
where it is essential to switch columns (the mini-
mum requirement is the possibility of backflushing the column). Figure
9.17
References pp.
266-268
258
Prevention
of
band broadening
and memory effects
Inertness
Speed
of
switching
cost
Ease
of
installation
Ease
of
operation
Avai
I
a bi
I1
t
y
Suitability for automation,
Capillary GC
and High Temp. GC
I
-valves valveless
1
Chapter
9
unsuitable moderate good excellent
refers
to
poctcd
EOIUEIRS
dy;
for
CtpilLry
colrtcnos
valves
arc
not
likely
to
bc
asable
Fig.
9.16.
Comparison
of
a
valveless live system
with
a mechanical column switching system.
shows a typical process GC application (by packed and capillary techniques) in a
steam cracker plant for process control. The higher performance of capillary col-
umns in contrast to packed columns is demonstrated. The capillaries are coupled
without valves, the packed column system is switched with valves.
9.4.3
Column-switching as
a
sampling technique
If
trace components are to be determined in complex matrices, special prob-
lems arise using single column GC systems.
The signal-to-noise ratio of trace components is normally too low for reliable
detection. The significant components must be enriched and transferred from an
unsuitable matrix into a new solvent matrix. After sample pre-treatment by, for
example, extraction or LC separation, a large sampling volume must be intro-
duced into the chromatographic system to increase the signal-to-noise ratio of
the trace components. The consequent overloading results in poorer resolution
and the broad solvent peak can overlap neighbouring minor components.
Another limitation is the possible disturbing influence which a major compo-
nent may have on the performance of the column (e.g. water using molecular
sieves) and detector (e.g. chlorinated solvents using
ECD).
Multi-column systems in
gas
chromatography
259
0.0
16.0
J-
8.0
Fig. 9.17. Separation
of
a C1<4 hydrocarbon mixture.
I,
i-butane;
2,
n-butane;
3,
1-butene;
4,
i-
butene;
5,
trans-butene; 6, cis-butene.
(a)
With packed columns in
16
min; (b) with capillary col-
umns in
8
min.
“Integrated” pre-separations, like those performed on-line within the closed
GC
system, are preferable to off-line sample pre-treatments which carry the risk
of undesirable changes in the original relative concentrations of a significant
component.
In the following sampling techniques, the first using a two-dimensional capil-
lary column system and the second, using a system with different
type
of col-
umns (packed-capillary) and intermediate trapping, are discussed.
References
pp.
266-268
260
Chapter
9
9.4.3.
I
Samplingfr.om
a
capillary pre-column
to
a main capillary column
Grob Jr.
[36],
Schomburg
et
al.
[37]
and others pointed out that by column
switching in a typical
MDGC
system containing
two
or more columns coupled in
series, the problems mentioned above can be avoided. In this system, in which
the first column serves as a sampling unit for the main column, only selected
parts of the original sample are, in most cases, transferred into the second col-
umn. This operation can be considered
a
“selective” sampling technique, if the
analytical data (response, retention) are taken from the separation in the second
column.
Other “selective” sampling methods like pneumatic sampling switching tech-
niques (Deans-
or
live-sampling
[3
S]),
temperature programmed sample transfer
(e.g. ”moving needle”
[39])
or
head-space analysis
[40])
may also be useful for
overcoming the negative influence of column overloading by the matrix solvent,
but are not discussed here.
Three examples of multi-dimensional sampling onto capillary columns are
discussed
in
more detail:
(a)
Polarhon-polar column-coupling.
The aim of the analysis
is
a simulta-
neous determination
of
polar substances by monitor detection and non-polar hy-
drocarbons transferred to the main column and the main detector. The determi-
I
1
1’
1
C-Heranr
10
Bulanol
2
/I
2
N-Hexane
1
1
Isobutanol
12
Bulanol
13
Benzene
5
Acelone
J
14
Toluene
15
Ethylbenzene
16
P-Xylene
,
17
M-Xylene
,
18
0-Xylene
I
1
I!
’I
R
I
o“;?
Light
benzene
..
.
I
I
5
5
-
lime. minutes
Stenens
SiCHROMAT
2
Fig.
9.18.
Multi-dimensional separation
of
oxygenated components in gasoline using
a
system
of
coupled columns with different polarities.
(A)
Chromatogram
of
gasoline obtained with polar pre-
column
(TCEP);
(B)
separation
of
cuts on
a
non-polar main column (OV-I).
Multi-column systems
in
gas chromatography
26
1
nation of oxygenated components in gasoline (Fig.
9.18)
demonstrates the im-
provement in resolution for significant groups of peaks by changing polarity
from the first to the second column. The first separation is carried out by using a
very polar
tris-(cyanoeth0xy)propane
(TCEP)
column. All aliphatic components
up to
Ctt
are eluted before the aromatics. The unresolved components are sepa-
rated on the main column after the first column. Overall analysis time
is
35
min.
(b)
Coupling
of
columns with identical polarity but difSerent
film
thickness.
The separation of a crude oil fraction in the
C
number range
C4-C30
seen in Fig.
9.19
is a remarkable example of the separation of mixtures ranging widely in
volatility. The first separation is carried out by using a non-polar thin film col-
umn. All components, covering the entire volatility range, are eluted within a
reasonable time. The second separation is to resolve the volatile hydrocarbon
isomers of the
C
number range up to
CI2,
now using a non-polar thick film col-
umn. Optimal operation of the
two
columns is only achieved with different tem-
peratures or with temperature programs maintained within
two
ovens. The
Fig.
9.19.
Separation
of
crude oil using a system of two coupled alkylpolysiloxane columns with
different phase ratio.
(A)
Chromatogram of crude oil.
(B)
Crude oil pre-separation. Cut after elu-
tion
of
dodecane. (C) Separation of cut in carbon number range C2-C12. Sample, 0.3
pl
of crude
oil; columns
(A,B),
30
m
SE
52,
1
pm
film
thickness; 50
m
OV-1;
temperatures
(A,B),
5
min 50°C
iso, 50-300°C
(8
"Chin), (C) 5 min iso, 0-50°C (lO"C/min), 50-250°C (CC/min); carrier gas
(A,B),
1.1
bar
H2,
(c)
0.8
bar
H2;
analysis time,
40
min.
References
pp.
266268