Adlard E.R. (ed.) Chromatography in the Petroleum Industry
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262
Chapter
9
overall analysis time, if the separations are performed simultaneously, is no
longer than 40 min with a greatly increased peak capacity.
(c)
Coupling
of
identical polar columns
for
heart cutting.
The determination
of low concentrations of components
in
the tail of the major component is the
aim of the analysis (see Fig.
9.7).
More details about this technique of sampling
by “heart-cutting” are given in Section 9.3.3.
9.4.3.2
Coupling
of
columns
of
digerent type with intermediate trapping
A
switching system consisting of capillaries has disadvantages; because of
their low sample capacity, the sample
is
often split in the injector and, therefore,
the amount of sample may be too small
to
get good results in trace analysis.
To
achieve an improvement in the lower limit of detection, it
is
desirable to transfer
the total amount of trace components accumulated within the pre-column, with-
out splitting, into the main column. However, interference by the large amount
of solvents and other undesired by-products has to be avoided by means of flow
switching, e.g. by backflushing or cutting, after the pre-column.
By
coupling a packed pre-column with a large sample capacity with
a
high
resolution capillary main column, these problems do not arise [41,42].
Because the carrier
gas
flow through a packed column is much higher than
through a capillary column, only a part of the sample can be transferred. There-
fore, the split point is brought from the injector to the union tee between the two
columns. Discrimination effects typical for split injection are thus avoided.
For optimum performance, broad peaks eluting
from
the pre-column must be
refocused by a cold trap before starting the capillary separation. This trap de-
signed as a connecting tube with low mass for rapid heating can be empty, wall-
coated,
or
filled with packing material. After one
or
more pre-separations, the
enriched trace components, condensed in the cold trap, are thermally displaced.
To
retain full column efficiency for the separation of the species of interest, the
trapped fraction
is
ejected into the hot capillary column with a high heating rate
and broad
or
distorted peaks are eliminated.
If the main separation is performed isothermally, retention index identifica-
tion with high reproducibility
is
possible.
A
modification of the conventional intermediate trap the so-called “total
transfer system” [43] has the advantages of intermediate traps and also elimi-
nates the problems
of
splitting after the packed pre-column. Therefore, the “total
transfer” technique is ideal for sub-trace analysis.
Figure
9.20
shows
a
schematic diagram of this system. The coupling of the
packed and the capillary column is by a glass tube which
is
also the trap. Around
the tube, there
is
heating wire for heating the trap rapidly and transferring the
components to the capillary column. The trap can be cooled by a second glass
tube which
is
designed
as
a mantle around the trap. The negative temperature
Multi-column systems in gas chromatography
263
Denar
Fig.
9.20.
Intermediate trapping
by
total eluate transfer, coupling
of
a packed with
a
capillary col-
umn.
gradient when cooling with nitrogen
is
provided by the inlet of the cooling me-
dium on the capillary side which guarantees a good focusing
of the peak.
9.5
STRATEGIES
FOR
THE
APPLICATION
OF
MULTI-COLUMN
SYSTEMS
9.5.1
General remarks
The full analytical potential of a chromatographic separation can only be used
after the development of a sample specific solution for a particular problem. Be-
cause
of the large number of parameters (e.g. injection procedure, type, length
and diameter
of
columns, types and amounts
of
stationary phases, column tem-
perature and temperature program, type and flow rate
of
the carrier gas and type
and working conditions of the detectors), it is unlikely that there will be only one
clearly defined optimal solution for a given problem. There are rather several
possible solutions for every problem to be solved by chromatography, and the
References pp.
266-268
264
Chapter
9
choice depends on the experience of the user, the equipment available and the
effort required for the maintenance of the equipment.
Therefore, it is
of special importance to establish a consensus between the
applications engineer and the user about the analytical aims to be attained before
identical
/
similar
analytical problems
single-column
system
Fig.
9.2
1.
Guidelines
for
the decision
to
use
a multi-column system.
--;~x;x;x;x~x;x;x;
.I.
,.
,
.x
I
:x.x:1 :x:x.x:
.......
.......
....
.x.x.
.......
y
.......
.......
i:x:
:
:
:
: :
:x:
1:x:
,
:
:
:
:
:x:
.......
.......
.......
.......
'0
,.
....
....
D
..
Valve Column Switching
Pneumatical Column Switching
Gaseous Sample
Liquid Sample
Liquified Gas
Sample
with
Disturbhg
High Boiling Components
or
Sample
with
High
Concentralion Differences
Measuring
of
Trace Components
Multi Component Analysis
Measuring
of
Key
Components
Multi Component Analysis
Measuring
of
All
Components
Sample
with
a Small Difference
of
Boiling Point
-.
..
but
a High Difference tn Polarity
Sample
with
a Small Differences
in Polanty but a
High Difference
of Bolllng Pcint
Sample wth
a
Small hfference
of
Boiling Point and in Pdanty
Sample
wth
a High Difference
of
Bollrng
Pant
and
in
Polany
Sample Consisting
of
Permanent Gases and
Ltght Hydrocarbons
Short Analysis Times
Chemically Sensitive Substancas
Ultra Trace Analysis
266
Chapter
9
development is started. The demand for only a single additional component to be
measured, an improvement in the detection limit or a shorter analysis time can
make an existing solution useless.
For this reason, the following guidelines for an applicational development are
kept very general and they cannot show the appropriate solution in every case.
However, the authors hope that for the majority of analytical problems to be
solved by gas-chromatography, a convenient solution can be found with their
help.
9.5.2
Guidelines
for
the
use
of
single-
or
multi-column
systems
The basic question for the selection
of
a
GC
system (single- or multi-column)
is,
what kind of analytical problems are to be solved. This is important if the
system is mainly used to analyse identical
or
nearly identical samples, typically
for process applications. On the other hand, for constantly changing analytical
problems, an adaptation of the system
is
required, typically for
GCs
in
research
laboratories. Figure 9.21 should help the decision as to whether a multi-column
approach seems to be useful for an individual application.
After the decision
in
favour of a multi-column solution, the separation and
switching systems should be selected; Fig. 9.22 can be helpful in this.
As
a rule,
a current analytical problem will comply with some of the characteristics listed.
The proposed technique depends on the frequency
of
the marked sections. Nev-
ertheless, some individual demands on the system are not taken into considera-
tion, e.g. cost. Also the ease of installation and operation plays an important part
in
the selection of the analytical hardware.
As
a matter of principle, the simplest
possible system should be selected that fulfills the requirements demanded.
9.6
REFERENCES
1
H.-G.
Struppe, in: Handbuch der Gaschromatographie, Akademische Verlagsgesellschaft
Geest
&
Portig
(1984),
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T.
Maurer,
W.
Engewald and
A.
Steinbom,
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I.L.
Davies,
K.E.
Markides, M.L. Lee,
M.W.
Raynor
and
K.D.
Bartle,
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High Resolut. Chro-
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12 (1989) 193.
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W.
Bertsch, Multidimensional Gas Chromatography, in: Multidimensional Chromatography
(H.J. Cortes, Ed.), Marcel
Dekker,
New
York
(1990),
p.
74.
5
J.V.
Hinshaw and L.S.
Ettre,
Chrornatographia
21
(1986) 561,669.
6
J.C.
Giddings,
J.
High Resolut. Chrornatogr. Chromatogr. Commun.,
10
(1987)
3
19.
7
G.
Guiochon and C.L. Guillemin, in: Quantitative Gas Chromatography
for
Laboratory Analy-
ses and On-Line Process Control, Elsevier,
Amsterdam
(1988).
8
J.F.K.
Huber, E. Kenndler,
W.
Nyiry
and
M.
Oreans,
J.
Chromatogr.,
247 (1982) 21
1,
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16
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19
20
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22
23
24
25
26
27
28
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39
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David, M. Proot, G. Dirricks, M. Verstappe and M. Verzele, J. High Resolut.
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8
(1985) 782.
G. Schomburg, LC-GC Mag.,
4 (1987) 304.
U.K.
Goekeler and
F.
Mueller, Process multidimensional gas chromatography, in: Multidi-
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New York
(1990),
p.
191.
W. Bertsch, J. High Resolut. Chromatogr. Chromatogr. Commun.,
1
(1978)
85.
J.C. Giddings, Anal. Chem.,
56 (1984)
1258A.
G. Schomburg, Multidimensional gas chromatography as sampling technique, in: Sample
Introduction in Capillary Gas Chromatography, Vol.
1,
Chromatographic Methods
(P.
Sandra,
Ed.), Dr. Alfrd Huethig
(1985),
p.
235.
R. Consden, A.H. Gordon and A.J.P. Martin, Biochem. J.,
38
(1944) 244.
W.D. Snyder, Multidimensional gas chromatography and hyphenated techniques, in: High
Resolution
Gas
Chromatography (K.J. Myrer, Ed.), Hewlett Packard
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W. Engewald and T. Maurer, J. Chromatogr.,
520 (1990)
3.
A.
Zlatkis, J.W. Anderson and
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Holzer, J. Chromatogr.,
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E. Kugler, W. Halany, R. Schlenkermann,
H.
Webel and R. Langlais, Chromatographia,
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(1977) 438.
J.F.K. Huber and H.C. Smit, Z. Anal. Chem.,
245 (1969) 84.
Y.P. Chizhkov and
O.A.
Yushina, J. Anal. Chem. USSR
31
(1976)
1,
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D.R. Deans, Chromatographia.,
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R. W. Slack and A. C. Heim, Int. Lab.,
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88.
G. Schomburg, H. Husmann and
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Hubinger, J. High Resolut. Chromatogr. Chromatogr.
Commun.,
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J.H. Pumell and P.S. Williams, J. Chromatogr.,
325 (1985)
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T.S. Buys and T.W. Smuts, J. High Resolut. Chromatogr. Chromatogr. Commun.,
3
(1980)
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and N. Brenner, Eds.), Academic Press, New York
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L.A.
Sinitzina and B.A. Rudenko,
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Chromatogr.,
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Bastian,
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Behlau, H. Husmann, F. Weeke,
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Mueller, J.
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Rennhard, J. High Resolut. Chromatogr. Chromatogr. Commun.,
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4
1
D.C.
Fenimore,
R. R.
Freeman and
P.R.
Loy, Anal. Chem.,
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Schomburg,
H. Husmann and F. Weeke,
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Chromatogr., 99 (1974)
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M. Oreans,
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Muller,
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Leonhard,
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Heim, Analysis
of
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(1984),
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171.
E.R.
Adlard
(Ed.),
Chromatography
in
the
Petroleum
Industty
Journal of
Chromatography
Library
Series,
Vol.
56
0
1995
Elsevier Science
B.V. All
rights reserved
269
CHAPTER
10
Supercritical jluid extraction
T.P.
Lynch
Anabtical and Applied Science Division,
BP
Group Research and Engineering Centre, Chertsey Road,
Sunbury-on
Thames,
Middlesex,
UK
10.1
INTRODUCTION
When a substance is maintained at a temperature above its critical point, the
vapour and liquid have the same density and it becomes impossible to liquefy it
by increasing pressure
[l].
Thus, a single phase is maintained regardless of the
applied pressure. This single phase is called a supercritical fluid and is character-
ized by a critical temperature
(T,)
and a critical pressure
(PJ.
The critical para-
meters for some fluids which have been employed for analytical studies are
listed in Table
10.1.
The supercritical phase has unique properties which can be employed to great
advantage in many processes requiring selective solvation. Basically, a super-
critical fluid has the beneficial properties of both liquids and gases. It has a
density approaching that of the liquid and therefore a high solubilizing power.
The diffusivity is intermediate between a liquid and a gas and the viscosity is
similar to
a
gas. Furthermore, the density
of
the fluid, and hence its solvating
power, can be varied by changing the applied pressure. Thus, a supercritical
fluid can be considered as a solvent with continuously adjustable solvent power
and this can be utilized to great advantage in separation processes both on an
industrial and analytical scale. The theory, development and application of su-
percritical fluid technology has beenhe subject
of
much research in recent years
and several texts
[2-6]
have appeared which cover these subjects in detail. All
of
these texts are recommended to anyone considering working with supercritical
fluids. In this text, we are specifically concerned with analysis in the petroleum
industry and it is interesting to briefly consider the contribution made by this
industry to much
of
the early research in supercritical fluids both in analytical
and process applications.
References
pp.
301-303
270
Chapter
10
ThSLE
10.1
CRITICAL PARAMETERS FOR SOME FLUIDS USED
IN
ANALYTICAL SUPERCRITICAL
FLUID TECHNIOUES. (DATA TAKEN FROM
SF
SOLVERTM PACKAGE, ISCO, USA)
Fluid
Critical temperature Critical pressure
Tc
(“C)
pc
(atm)
Xenon
Carbon dioxide
Nitrous oxide
Sulphur
hexafluoride
Propane
Ammonia
Pentane
16.7
31.1
36.6
45.7
96.8
132.4
196.6
57.6
72.8
71.5
37.1
41.9
112.0
33.3
A
review
of
patents relating to supercritical fluids will show a large number
relating to the oil industry. For example, one
of
the earliest patents was filed in
1927
and referred to a “Process for Treating, Separating, and Purifying Oils”. In
this patent, the authors refer to a separation where
5
kg
of
mineral lubricating oil
with an original viscosity
of
5.2
cps was treated with
15
kg of liquid carbon di-
oxide. The carbon dioxide extract was found to yield
0.5
kg of oil with a viscos-
ity of
2.2
cps while the residual
4.5
kg had a viscosity of
5.9
cps. This patent is
referred to by many of the subsequent petroleum-based patents including that
assigned to Shell Development Company, San Francisco, CA, in
1940
which is
the earliest found by McHugh and Krukonis
[2]
which refers to the use of a su-
percritical fluid.
However, perhaps the best
known
process which relates to the successful ex-
ploitation of the dramatic changes in properties which can occur around the
critical point is the technique of propane deasphalting which is still in use today
for
refining oils. The oil industry
is
also much in evidence when the develop-
ment of analytical supercritical fluid techniques
is
considered. It
is
generally
agreed that the first demonstration of supercritical fluid chromatography (SFC)
was by Klesper
et
al.
[7]
for
the separation of porphyrins, compound types which
are
of
great interest to the petroleum industry. Furthermore, Sie and Rijnders
[S-
113
of
Shell Laboratories in Amsterdam are widely credited with much of the
pioneering development
of
SFC
and in fact with using the term supercritical
fluid
chromatography for the first time
[12].
Following this, Jentoft and Gouw at
Chevron reported the application
of
pressure programming
[13]
and mobile
phase modifiers
[14].
So,
we can see that the oil industry has been a major force
in the development of all aspects
of
the application of supercritical fluids. This
text, however, is directed at the application of chromatography in the petroleum
industry and as the more mature area
of
supercritical fluid chromatography
Supercritical fluid extraction
27
1
(SFC)
is dealt with in another chapter, the remainder of this chapter is devoted to
the more recent technique of supercritical fluid extraction
(SFE)
which is cur-
rently being proposed as the sample preparation method
of
choice for many ap-
plications.
10.2
WHY
USE SUPERCRITICAL
FLUID
EXTRACTION?
Supercritical fluid extraction has become a favoured means of analytical
sample preparation in recent years, particularly for the removal of analytes of
interest from solid matrices prior to chromatography. Although the petroleum
industry deals mainly with liquids, there are many areas where the analysis of
solid matrices is of interest. But why should we choose to employ a supercritical
fluid when traditional methods such as distillation and solvent extraction have
sufficed for many years?
To answer this, we have to again consider the properties of supercritical fluids
and what these mean in terms of the ideal features of an extraction solvent.
10.2.1
Technical advantages
10.2.1.1
Solubility
The first requirement for an extraction solvent is that it must dissolve the
species of interest and the great feature of supercritical fluids is that they have
liquid-like densities and therefore liquid-like solvent powers. The variation of
solubility with density for naphthalene in carbon dioxide at
55°C
is shown in
Fig.
10.1
and we can see a
two
order of magnitude increase in solubility on go-
ing from a density of
0.6
to
2.1.
One problem that has been encountered in the
use of single component supercritical fluids is that those with easily attainable
critical parameters are relatively non-polar and the type of components which
can be solubilized are therefore also limited by their polarity. This problem can
be partially overcome by using “solvent modifiers”. These are polar solvents
such as methanol which are added to supercritical fluids such as carbon dioxide
to alter its polarity and hence enhance the solubility of polar compounds in the
supercritical mixture. The mechanism by which these modifiers work
is
complex
and still the subject of much research but is thought to involve processes such as
hydrogen bonding and molecular clustering
[
151.
10.2.1.2
Selectivity
With conventional solvent extraction, the only variable which can be readily
altered to change solubility significantly is temperature. However, with a super-
References pp. 301-303