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132
Chapter
5
Yamamoto
[38]
used TLC on silica gel to obtain 11 fractions of azaarenes
from
crude oils and rock extracts from Japan and Sumatra.
Reversed phases
(RP),
impregnated-phases (e.g.
Ag+)
and multi-dimensional
TLC methods have not found very widespread use in oil geochemistry, unlike
many other areas
161
and future investigations may prove valuable for charac-
terization of polar fractions. For example, argentation TLC was useful in the
separation of various classes of sulphur-containing components from the malte-
nes
of
an immature bituminous shale
[ls]
and for the separation of unsaturated
components from the “off-line” pyrolysates of algal ultralaminae
[20].
Non-planar TLC-FID (Iatroscan) is an under-utilized method which allows re-
producible (and if
&+
fractions are examined, quantitative) analysis of whole
oils or oil fractions
[39,40].
The method is particularly useful for replicate analy-
ses
from
which
a
meaningful statistical appraisal of results can be obtained.
In-
teresting refinements include the use of Age-TLC-FID chromarods [41] and the
addition
of
a flame thermionic detector for the analysis of nitrogenous com-
pounds
[42].
5.3.2
HPLC
HPLC is one of the most widely used modern chromatographic techniques
and numerous routine applications have been found
in
petroleum geochemistry.
Recent interesting studies include preparative and automated compound class
separations on silica and cyano-bonded silica with good
(1
00
f
1.3%) recoveries
[43];
HPLC coupled with Iatroscan has been used for the quantitative analysis of
coal-derived
oils
[44]
and determination of aromatics in oil distillates by 2-D
HPLC with column switching
[45].
Use of reversed phases allowed Garrigues
and colleagues to study the retention behaviour of all
25
dimethylphenanthrene
isomers and to identify the compounds in crude oil
[46]
whilst a class separation
of nitrogen aromatics was possible with polar aminocyano-silica HPLC columns
[47].
Use of HPLC with refractive index detection facilitated identification of a
new series
of
rearranged hopanes
in
sediments and petroleum
[48].
A
quantitative method using backflush HPLC techniques allowed a determi-
nation of the heavy constituents
of
a crude oil to be made
[49].
HPLC continues
to
be
routinely used for characterization of both free-base and metalloporphyrins
in
oils and sediments
[50]
and new compounds are still being found
[33,34].
In-
deed, HPLC coupled to element-selective detection
is
increasing
in
popularity
for the determination of metal speciation in oils (for review, see
[51]).
A
recent
example is the determination of metalloporphyrins in coal extracts by HPLC-
ICP-MS
[52]
as illustrated in Fig.
5.2,
showing the distribution of gallium por-
phyrins
in
coal.
A
conventional HPLC-UV chromatogram is shown for compari-
Chromatography
in
petroleum geochemistry
133
1
750
600
0
300
150
A
B
I
I
I
I
I
10
30
50
70
90
110
Time (min)
Fig.
5.2.
HPLC-ICP-MS mass chromatogram
(mlz
71)
illustrating the distribution
of
gallium-
containing porphyrins in a coal from the British Coal National Coal Bank (Markham Main). Inset
shows more conventional HPLC-UV chromatogram. Mobile phase
1
mYmin,
1
mM t-butyl-
ammonium dihydrogen phosphate
(150/)
in methanol; column
30
mm
X
3.9
mm octadecyl silane,
4
pm Novapak (Waters). ICP-MS
was
VG
Plasmaquad
2.
For operating conditions, see
[52].
son. Such methods show promise for the future study
of
metalloporphyrins, al-
though further research will be needed to achieve reproducible quantitation
[52].
5.3.3
GC
Capillary GC continues to be used extensively for crude oil characterization
and can be considered a routine technique in the field, particularly for biomark-
ers and other hydrocarbons
[7-91.
A
review
of
up-to-date GC publications has
appeared
[5].
A
recent study reported separation and identification
of
21
light
References
pp.
139-1
41
134
Chapter
5
end,
83
whole oil and
17
heavy end components of Vlaska oil from the Adriatic
Basin
[53].
Routine analyses typically use polydimethylsiloxane or phenylmethylsiloxane
stationary phases (e.g.
OV-1
or
OV-5
type) but even with these phases retention
indices of compounds are unfortunately reported irregularly. More unusual GC
phases are sometimes employed. For example, certain liquid crystal phases have
proved to be unrivalled for separation of isomers of alkylaromatics such as di-
methylphenanthrenes and dibenzothiophenes
[46,54]
although column availabil-
ity and the limited range of operating temperatures may place practical restric-
tions on their extensive use for crude oil analysis. Impressive separations can
also be obtained by
2-D
GC; a good example is the quantitation of
aa
and
&?
isomers of dimethylnaphthalenes for maturity index calculations with the added
advantage that no sample preseparation was needed
[55].
Many of the compo-
nents of crude oil contain chiral centres and this stereochemistry confers high
information content on such molecules
[7,9].
The separation of enantiomers of
various biomarkers on chiral GC phases is therefore of interest
[56]
and the col-
umns suitable for such analyses are readily available as a result of a dramatic
expansion of research into the GC properties of cyclodextrin phases.
An automated method for molecular weight determination of crude oils by GC
has been published
[57]
and a good correlation with the results of cryoscopic
molecular weights found. The usual molecular weight range of GC analyses
(viz.
typically
C1-C40)
has also been considerably extended (e.g. to Cso) since capil-
lary high temperature GC columns have become commercially available (see
reviews
[3,58-60]
and Chapter
3
of this volume). Although SIMDIST GC analy-
ses have in fact allowed some separation of >C, components for some years
[61,62],
present HTGC columns allow analysis of even relatively involatile or-
ganometallic species such as metalloporphyrins whilst retaining reasonable GC
resolution
[63,64].
Indeed, when coupled with element selective detectors (e.g.
MIP, ICP-MS
[22,65])
HTGC offers
a
rapid fingerprinting method for or-
ganometallic compounds
in
oils. The use of element-selective detection with GC
has been reviewed
[66].
Despite the high resolving power and extended working range of modern
capillary GC, a substantial proportion
(3690%)
of fresh and biodegraded crude
oils
is
still unresolved and chromatograms of crude oils often contain “humps”
or
so-called unresolved complex mixtures (UCMs). This is illustrated by the
flow diagram shown
in
Fig.
5.3
which shows that for heavily biodegraded Vene-
zuelan Tia Juana Pesado crude, which is imported into the
UK
for lubricant
manufacture; current methods result in a detailed knowledge of only about
5%
of
the
petroleum.
A
few attempts to characterize hydrocarbon UCMs have been made recently
[67-703.
These have used degradative oxidation followed by GC and GC-MS in
Chromatography in petroleum geochemistry
135
100%
-2
Asphaltene
(20%
of
oil)
(80%
of
011)
Aliphatic
(23.2%
of
oil)
Aromatic
(21.6%
of
oil)
(35.2%
of
oil)
I
Re;ol;d
I
I
Unrgewlved
I
1
Re;;d
I
1
Unrgee3;ved
I
(2.3%
of
oil)
(20.9%
of
oil)
(I
8%
of
oil)
(33.4%
of
oil)
Fig.
5.3.
The limitations of chromatography in petroleum geochemistry? Flow diagram illustrating
the lack of molecular information available for a biodegraded Venezuelan crude oil. Although a
greater proportion of non-biodegraded oils is identifiable, a substantial proportion
of
even these
remains as unresolved complex mixtures.
conjunction with other methods. The method typically produces about 20% of
resolvable and identifiable oxidized products, mainly acids, lactones and ketones
(Fig.
5.4).
The bulk
of
the
UCM,
although oxidized, is still unresolved and re-
mains a challenge for chromatographers.
5.3.4
GC-MS
Capillary GC-MS has perhaps made the single greatest contribution to the
analysis of petroleum hydrocarbons in the last 20years and hundreds, if not
thousands,
of
compounds have been identified. For example, over
700
com-
pounds were identified in a shale oil
[71]
and
450
compounds in a coal
tar
an-
thracene oil
[72]
and coal tar pitch on
two
stationary phases
[73].
However, it
should be emphasized, that the proportion of oil represented by these resolved
components
is
not always high, and many remain unresolved and unidentified.
A
review lists retention indices for
400
terpenoids on methylsilicone and Carbowax
20M
phases
[74]
and many
of
those compounds discovered in petroleum which
Refirences pp.
139-141
136
Chapter
5
44
12
20
20
36
44
52
0
0.
L
0
1
4
20
25
36
52
12
Time (mid
Fig.
5.4.
Gas
chromatograms of (a) a fuel oil unresolved complex mixture
of
hydrocarbons and (b)
enhanced resolution achieved by oxidative degradation to n-acids
(e),
lactones and ketones. Re-
solved components comprise less than
1%
in
the fuel oil and about
20%
in the oxidized
oil
(reproduced from
[70]).
Chromatography
in
petroleum geochemistry
137
provide petroleum geochemists with palaeoenvironment, maturity, biodegrada-
tion and migration information are terpenoids,
so
such data may be important.
The term “biomarker” has been used to describe those compounds whose struc-
tures suggest an unambiguous link with a precursor biochemical and such com-
pounds are used for oil-source rock and oil-oil correlations [7-91. Despite the
usefulness of MS and tandem MS in identifying many of these compounds, the
occurrence of many structural and stereoisomers of hydrocarbons in oils means
that GC retention time is still an extremely valuable guide to compound identity.
For example, an extended injection temperature hold time GC method was
needed to separate 14a(H), 17a(H) and 14&H), 17P(H) isomers of C21-norchol-
estanes in mature sediments [76].
A
review of the compound classes discovered
is
outside the scope
of
this chapter and readers are referred to other sources for a
fuller discussion [7-9,79,80] but recent advances have been made in the identifi-
cation of non-hydrocarbons such as thiophenes and sulphides
[
18,s
13,
phenols
[82] and naphthenic acids [83].
A
rapid method of biomarker analysis in oils and
source rocks is thermal extraction GC-MS
[
841.
5.3.5
GC-isotope
ratio-MS
GC-isotope ratio-mass spectrometry is a potentially valuable addition to the
petroleum geochemists armoury of instrumental techniques and commercial in-
struments are now available. The method allows the rapid determination of the
13CP2C ratio of individual organic compounds in complex mixtures, which is
useful for correlation and possibly for assignment of the biological origins
of
the
compounds [SS-881. However, GC co-elution of certain components may mean
that further developments in chromatography are necessary before accurate 613C
values can be obtained. For example, y-Cerane from eukaryotic heterotrophs was
found to co-elute with 3p
(CH,)
hopane from methanotrophs in extracts of the
Green River oil shale. Peak deconvolution was needed to obtain adequate 6I3C
data [89].
5.3.6
LC-MS
LC-MS has still not found wide application in petroleum geochemistry, even
though the combination of HPLC technology with mass spectrometry would ap-
pear to be a powerful combination for the characterization of petroleum frac-
tions. An efficient and innovative data reduction process has been developed for
LC-MS analysis of hydrocarbon mixtures [90] and LC-MS has been evaluated
for the analysis of lubricating oils [91] the hydrocarbons of which are predomi-
References
pp.
139-141
138
Chapter
5
nately UCMs [69,70]. LC-MS has proved to be extremely valuable for the
analysis of chlorophyll-derived pigments in sediments [92] and may therefore be
valuable for the examination
of
geological analogues
of
these pigments
(viz.
geoporphyrins) in the future.
5.3.7
SFC,
GPC
and
SEC
Some of the fractions of petroleum which are less accessible to the geochem-
ist (e.g. “Heavy Ends”) and which are not amenable to examination by GC and
GC-MS, can nonetheless be examined by SFC-GPC andor SEC methods. These
chromatographic techniques can be coupled with a variety
of
detectors including
FID
and
MS
[93,94,97,98].
For
instance, saturates, aromatics, resins and asphal-
tenes in petroleum were separated in 15 min by
SFC-FID
[94] and carbon num-
ber ranges up
to
C, can be eluted [95]. SFC-MS is well suited to PAH analysis
in fossil fuels [96] and has good selectivity for porphyrins [93].
Applications of GPC to petroleum and coal analysis have been reviewed [6].
GPC has been used to study the behaviour of
C3G52
petroporphyrins from Ath-
abasca asphaltenes [99] and maltenes from Utah bitumens were found to have
average molecular weights of 300
to
nearly 12
000
[IOO]
by GPC. Porphyrins,
aromatics and alkenes can be determined by GPC in petroleum without sample
preparation [loll.
SEC on pure silicon carbide columns has been used to determine the molecu-
lar weight of
V
and Ni complexes in heavy ends of petroleum (see [6]) and the
use
of
SEC
with element-specific detectors has been reviewed [102]. The mo-
lecular weight distributions of aromatics and alkanes has been determined by
SEC
and used to differentiate natural gas condensates from petroleum
[
1031.
5.4
SUMMARY
A
wide variety of chromatographic techniques has been used in petroleum
geochemical studies, but GC and GC-MS remain overwhelmingly most popular,
and these can be considered as routine methods. Whilst this has led to the ac-
quisition of a great deal
of
knowledge about a small proportion of the compo-
nents
of
petroleum, much remains to be learned about the geochemistry
of
even
the hydrocarbons,
a
substantial proportion of which are
left
unresolved by the
best
chromatographic methods. Methods such as LC-MS and SFC may prove
increasingly useful for fractions such as resins, asphaltenes and polars, which
have previously proven difficult
to
characterize.
Chromatography
in
petroleum geochemistry
5.5
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