Alkylated phenol series in lacustrine black shales from the Noerdlinger Ries, southern Germany

Article abstract of DOI:10.1002/jms.3049

Several series of alkylated phenols were detected for the first time in the extractable bitumens of organic matter-rich sediments from the Noerdlinger Ries (southern Germany). Most abundant and significant constituents comprise those with n-octadecyl, n-eicosanyl, phytanyl, and iso-pentadecyl and anteiso-pentadecyl substituents. The structures of these compounds are suggested from mass spectrometric and retention time data and coinjection with synthetic standards. Diagenetic alteration of phenolic algal lipids is suggested as a possible way to the formation of these compounds in the Noerdlinger Ries sediments. Copyright

Full text of DOI:10.1002/jms.3049

Research Article
Received: 2 February 2012
Revised: 25 May 2012
Accepted: 19 June 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/jms.3049
Alkylated phenol series in lacustrine black shales
from the Nördlinger Ries, southern Germany
Assem O. Barakat,a* Susan Baumgart,b Peter Brocks,b
Barbara M. Scholz-Böttcherb and Jürgen Rullkötterb
Several series of alkylated phenols were detected for the first time in the extractable bitumens of organic matter-rich sediments
from the Nördlinger Ries (southern Germany). Most abundant and significant constituents comprise those with n-octadecyl,
n-eicosanyl, phytanyl, and iso-pentadecyl and anteiso-pentadecyl substituents. The structures of these compounds are suggested
from mass spectrometric and retention time data and coinjection with synthetic standards. Diagenetic alteration of phenolic algal
lipids is suggested as a possible way to the formation of these compounds in the Nördlinger Ries sediments. Copyright © 2012
John Wiley & Sons, Ltd.
Keywords: Nördlinger Ries; n-alkyl phenols, isoprenoidyl phenols; sulfur-rich sediments; NSO fractions
In this communication, we report on the distribution of several
series of alkylated phenols found in small concentrations in two
INTRODUCTION
Short-chain alkyl phenols (C1–C3), along with methoxy-substi-
tuted alkyl phenols and alkylated catechols, are typical pyrolysis
products of lignins and altered lignins, which are widespread
organic compounds in the geosphere.[1,2] They were reported in
the pyrolysates of soils and sediments,[3–6] suspended and
dissolved marine organic matter,[7–10] fossil woods,[11–13] coals[14,15]
and kerogens.[16–18] Because of their high chemical stability and
resistance to microbial degradation, lignin components are consid-
ered specific biomarkers for terrestrial higher plant contribution to
sediments.[11,12] In some of these previous studies,[5,7,8] the origin of
the series of short-chain alkyl phenols was ascribed to tyrosine
moieties protected from degradation by incorporation in melanoidin-
type macromolecules.
fractions of extractable polar lipids of organic matter-rich and
sulfur-rich lacustrine sediments from the Nördlinger Ries in
southern Germany. Structural properties of these compounds
indicate that they are diagenetically unrelated to lignins. Synthesis
of two of the suspected compounds, together with two substitu-
tional isomers, reveals convincing evidence of the molecular
structures of the phenols. The structures of the other related
compounds are inferred from these identifications and supported
by mass spectral interpretation, comparison of the mass spectra
of the individual components with those of the corresponding
trimethylsilyl (TMS) ether derivatives and, wherever possible, by
comparison of the mass spectra with literature data. Extensive use
was also made of mass chromatography MS/MS experiments to aid
recognition of (pseudo)homologous series of phenolic compounds.
There is very little information, however, on the composition of
phenolic components in the extractable fraction (bitumen) of
sedimentary organic matter. One reason is that phenols are not
usually found as monomolecular species in more than small
concentrations due to the ease with which they can autoxidatively
and/or enzymatically polymerize under sedimentary conditions[19]
and thereby be incorporated into the insoluble macromolecular
matrix. However, it should be mentioned that the literature on
polar fossil lipids, which represent a major portion of the total
extractable lipid fraction of sedimentary organic matter that has
experienced little thermal stress, is sparse. Phenols substituted with
long alkyl chains (up to C26) and their methylated homologues
were previously identified in the pyrolysates of the Upper Jurassic
Kashpir oil shales from Russia,[20] of a Cenomanian kerogen from
central Italy,[21] of an Oligocene kerogen from Belgium,[22] of
seabed petroleums and sediment from the Gulf of California[23]
and of Ordovician kukersite kerogen from Estonia.[24] Kukersite is
known to derive from the selective preservation of the outer cell
walls of the fossil alga Gloeocapsomorpha prisca, which were shown
to be the phenol source.[24] In contrast, no precise information on
the source(s) of the long-chain n-alkyl phenols was provided for
the other kerogen samples.
EXPERIMENTAL
Materials
All solvents were either distilled in glass grade or spectroscopic
grade and were examined for purity by GC. Potassium hydroxide
(KOH) used for hydrolysis was Suprapur (Merck), and the water
was pentadistilled in an all-glass apparatus. TMS ethers were
prepared using BSTFA containing 1% TMCS (Silyl 991, Macherey
& Nagel, Düren) in the presence of pyridine.
*
Correspondence to: Assem O. Barakat, Department of Environmental Sciences,
Faculty of Science, Alexandria University, 21511, Moharrem Bey, Alexandria, Egypt.
E-mail: asem.barakat@alex-sci.edu.eg
a Department of Environmental Sciences, Faculty of Science, Alexandria University,
21511, Moharrem Bey, Alexandria, Egypt
b Institute for Chemistry and Biology of the Marine Environment (ICBM), Carl von
Ossietzky University of Oldenburg, PO Box 2503, D-26111 Oldenburg, Germany
J. Mass. Spectrom. 2012, 47, 987–994
Copyright © 2012 John Wiley & Sons, Ltd.

A. O. Barakat et al.
Samples
48 ml of water and 14 ml of ethanol. The para-isomer was
enriched in the aqueous phase, whereas the ortho-isomer due
to its higher resistance to deprotonation mainly accumulated in
the organic phase. After separation, acidification of the aqueous
phase, extraction with dichloromethane and removal of the
solvents, the residues of both fractions were hydrogenated by
Wolff-Kishner reduction to the corresponding alkyl-substituted
phenols. Only 4-n-hexadecyl phenol was obtained pure
Nördlinger Ries sediment samples investigated in this study were
supplied by BEB Erdgas und Erdöl GmbH (Hannover). Three
samples were selected from the most bitumen-rich layers in two
boreholes. The geological background of the sediments was
discussed previously,[25] and a characterization of major biomarker
constituents in the extractable polar lipid fractions isolated from
these samples was already reported.[26,27] Briefly, the Nördlinger
Ries is a circular-shaped Miocene sedimentary basin 20 km in
diameter and located about 100 km northwest of Munich in
southern Germany. It was formed by meteorite impact into the Late
Jurassic carbonate sediments about 15 Ma. Bituminous sediments
mainly occur in a laminite series that is at about 250–110 m depth
in the center of the former crater lake. Deposition took place under
stagnant, periodically evaporitic conditions in a semi-arid climate.
The pulverized sediment (140 mesh) was exhaustively extracted
in a Soxhlet apparatus with CH2Cl2 for a period of 1 week. The
extractable lipids were separated by medium pressure liquid
chromatography (MPLC) over silica gel in three steps into the
following compound classes: non-aromatic hydrocarbons, aromatic
hydrocarbons, free alcohols, free carboxyclic acids, a neutral polar
fraction, bases and a highly polar fraction.[28] The neutral polar
fraction was saponified with KOH/CH3OH (0.15 M KOH in MeOH/H2O,
97/3 v/v) by stirring at reflux temperature under N2 for 1 h. The
resulting extract was further separated by MPLC to afford ketones,
ester-bound acids and ester-bound alcohols.
1
enough for NMR analysis. H-NMR (4-n-hexadecylphenol; Bruker
300 MHz, CDCl3, TMS): 0.89 ppm (3H, triplet, 6.5 Hz: CH3-160),
1.27 ppm (26H, multiplet: CH2-30 to CH2-150), 1.58 ppm (2H, multi-
plet, CH2-20), 2.54 ppm (2H, triplet, 8 ppm, CH2-10), 4.67 ppm (1H,
broad singlet, OH), 6.77 ppm (2H, doublet, 8.5 ppm, CH-3 and
CH-5), 7.07 ppm (2H, doublet, 8.5 ppm, CH-2 and CH-6). The
aromatic hydrocarbon signals and the general pattern of the
spectrum match those of 4-n-octylphenol and are clearly distinct
from those of 2-alkyl and 3-alkyl substituted phenols.[30] MS
(2-n-hexadecyl phenol): 318 (M+, 36), 108 (25), 107 (BP), 79 (15),
77 (14), 57 (10), 55 (14), 43 (52), 41 (41); MS (4-n-hexadecylphenol):
318 (M+, 13), 108 (6), 107 (BP), 77 (8), 57 (9), 55 (12), 43 (42), 41 (38).
The mass spectral patterns match those of ortho-undecyl and para-
undecyl phenol.[31] The two hexadecyl phenol isomers can be
distinguished from each other by their retention times, which are
considerably shorter for the ortho-isomer.[32]
For synthesis of 2-phytanylphenol and 4-phytanylphenol,
phytanoic chloride was prepared from commercially available
phytol by acetylation (Ac2O), hydrogenation of the double bond
(Pd/C, H2, ethanol), hydrolysis to release phytanol, oxidation to
phytanic acid (CrO3, pyridine, CH2Cl2) and treatment of the acid
with thionyl chloride, using conventional methods for all steps.
Phenol acylation, separation of 2-phytanoyl and 4-phytanoyl
isomers and hydrogenation were performed as described before.
In this case, the ortho-isomer was strongly enriched in the organic
phase, whereas the aqueous phase contained an approximately 1 :
5 mixture of ortho-isomers and para-isomers. After reduction, the
raw products (70 mg) were filtered using a 32 ꢀ 1 cm i.d. (internal
standard) column filled with 5 g silica gel 100 (deactivated with
5% water) and 100ml cyclohexane/diethyl ether (4: 1) to remove
very polar components and then chromatographed using a
40 ꢀ 1 cm i.d. column filled with 16g silica gel 100 (deactivated
with 5% water) and a succession of 60ml of n-hexane/dichloro-
methane (9 : 1) and 60 ml of dichloromethane as eluents. The
products (7 mg) were eluted in the third 10 ml fraction of the second
eluent. NMR (2-phytanylphenol): 0.87 ppm (12H, broadened doublet,
CH3-160, -180, -190, -200), 0.96 ppm (3H, doublet, 7 Hz, CH3-170),
1.38 ppm (24H, multiplet, C-20 to C-150 methylene and methine
H), 2.62 ppm (2H, multiplet, CH2-10), 4.70 ppm (1H, broad singlet,
OH), 6.95 ppm (4H, multiplet, CH-3 and CH-6). The aromatic hydro-
carbon pattern resembles that of 2-n-dodecyl phenol.[30] MS (2-
phytanylphenol): 374 (M+, 53), 121 (8), 120 (7), 108 (94), 107 (BP),
93 (13), 71 (14), 69 (6), 57 (25), 55 (11); MS (4-phytanylphenol):
374 (M+, 10), 121 (3), 120 (6), 108 (18), 107 (BP), 94 (5), 71 (9), 69
(6), 57 (19), 55 (9). The mass spectral patterns and retention time be-
havior resemble those of phenols with linear alkyl substituents.
Instrumental methods
The GC analyses were carried out on a Carlo Erba Model 6000 gas
chromatograph equipped with a 30 m ꢀ 0.25 mm fused silica
capillary column coated with SE-54. Samples were injected on
column, and the temperature was programmed from 60 to
80 ꢁC at 30 ꢁC/min and then from 80 to 300 ꢁC at 4 ꢁC/min with
an initial hold time of 1 min and a final hold time of 20 min.
Helium was the carrier gas.
The GC/MS analyses were performed on a Thermo Scientific GC
Ultra gas chromatograph equipped with a 30 m ꢀ 0.25 mm fused
silica capillary column coated with DB5-HT (film thickness 0.1 mm)
and connected directly to a TSQ-Quantum GC mass spectrometer
(Thermo Scientific). Samples were injected splitless via a PTV
injector (Thermo Scientific; 1 min injection time, temperature
program: 50ꢁC [0.05 min], 14.5–350 ꢁC/s, 4 min hold), and the
temperature of the GC oven was programmed from 60 (2min) to
350 ꢁC at 3 ꢁC/min with a final hold time of 12 min. He was the
carrier gas (constant flow at 1.2ml/min). The ionization energy
was 70 eV (50 mA filament current), the temperature of the mass
spectrometer ion source was held at 230ꢁC and that of the transfer
line at 300 ꢁC. The quadrupole was scanned continuously over a
mass range of 50–650 u at a rate of 0.5s/scan. The mass spectro-
metric data were acquired and processed with the Xcalibur 2.1
software. For the MS/MS experiments, Ar was used as collision
gas at a collision energy of 12 V.
Synthesis of reference compounds
The 2-n-hexadecyl and 4-n-hexadecyl phenols were synthesized
by acylation of phenol with n-hexadecanoyl chloride[29] obtained
by treatment of commercially available n-hecadecanoic acid
with thionyl chloride. The resulting mixture of 2-n-hexadecyl
and 4-n-hexadecanoyl phenols was dissolved in 30 ml of dichlor-
omethane and extracted twice with a solution of 0.6 g KOH in
RESULTS
During a structural study of the extractable polar lipids of five
sulfur-rich black shales of Miocene age from the Nördlinger Ries,
several (pseudo)homologous series of alkyl phenols were encoun-
tered in two fractions corresponding in liquid chromatographic
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J. Mass. Spectrom. 2012, 47, 987–994

Alkylated phenol series in lacustrine shales
behavior to ‘ester-bound alcohols’ and to ‘aliphatic ketones’.
Alkylmethyl phenols occurring in the ‘ketone’ fraction were more
abundant than alkyl phenols in the ‘ester-bound alcohol’ fraction
of each sample studied. Analytical results for the shale samples
are given in Table 1.
member of the homologous series. No assignment can be made
to the position of the methyl group based on the available data
nor can it be excluded that both possible methyl isomers are
present and coelute under the gas chromatographic conditions
applied.
Additional evidence to support this interpretation comes from
characterization of compound A (Fig. 1), which in general shows
mass spectral features (Fig. 2) similar to those of n-eicosanylmethyl
phenol (M+ = 388), but elutes from the GC column just before
n-octadecylmethyl phenol. This is analogous to the retention
behavior of C20 isoprenoids, which elute at about the same reten-
tion time as the corresponding straight-chain compounds with
two carbon atoms less (e.g. on a CP-Sil-5 stationary phase column,
phytane elutes just after octadecane, whereas phytanic acid elutes
before n-octadecanoic acid). Furthermore, a closer inspection of the
mass spectrum of compound A (Fig. 2) indicates an enhanced
intensity of the ion resulting from g-hydrogen transfer (m/z 122,
base peak) relative to the intensity of the ion resulting from simple
b-cleavage (m/z 121). This feature is not expressed in the mass
spectra of the n-alkylmethyl phenol series and indicates that the
C20 alkyl side chain of compound A is branched at the g-carbon
atom; this fragmentation behavior has been described before, e.g.
for alkylated thiophenes[33] and alkylbenzenes.[34] The retention
behavior and spectral features of compound A, therefore, suggest
that it most likely is 4-phytanyl methyl phenol, the position of the
phytanyl substituent being deduced from the analogy to the
corresponding desmethyl derivative identified on the basis of a
synthetic standard.
Alkylmethyl phenols
The main homologous series in the ‘ketone’ fraction are n-alkylmethyl
phenols with alkyl chains ranging in length from C2 to C24
(carbon numbers in the following text and in the figures always
refer to the length of an unbranched side chain). The main mass
spectral features common to these homologs are the presence
of an abundant molecular ion and a base peak at m/z 121
corresponding to b-cleavage of the side chain. Figure 1 shows
the m/z 121 and total ion current (TIC) traces of the ‘ketone’
fraction obtained from a sediment at 151.5 m depth of well
NR-10. The mass chromatogram suggests that all the isomers, in
addition to the methyl group, carry an unbranched alkyl group. This
is deduced from the utter regularity of the retention times of series
members with increasing parent peak mass. Para-substitution of
the n-alkyl chain is assumed in analogy to the desmethyl compo-
nents discussed subsequently for which identification is based on
comparison with a synthetic reference compound for the C16
Table 1. Analytical results for the shale samples from the Nördlinger
Ries, southern Germany
Well Depth TOC (%)
Extract
(mg/g Corg
Fractionsa
The m/z 121 mass chromatogram (Fig. 1) also shows the
presence of two distinct peaks eluting between the n-tetradecyl
and n-pentadecyl components. From their mass spectra and
relative retention times in the m/z 121 chromatogram, these com-
pounds have tentatively been identified as iso-pentadecylmethyl
and anteiso-pentadecylmethyl phenols. A minor component
eluting after 4-tetradecylmethyl phenol (compound B, Fig. 1),
based on its relative retention time (pointing to three methyl
branches in the side chain compared with that of the n-alkylmethyl
)
Ketones Ester-bound alcohols
NR-10 151.5
NR-10 250.0
NR-30 215.1
12.3
14.4
10.3
206
200
185
8.4
16.4
8.3
0.30
0.61
0.39
aPer cent relative to the total extractable bitumen.
Figure 1. Total ion current (TIC) and m/z 121 chromatograms for the ‘ke-
tone’ fraction of a Nördlinger Ries sediment from 151.5 m depth of well
NR-10 ]showing the distribution of n-alkylmethyl phenols. Numbers cor-
respond to the number of carbon atoms in the alkyl side-chain. A is
identified as 4-phytanylmethyl phenol and B tentatively as 4-(40,80,120-
trimethyl)-tridecylmethyl phenol. i = iso, a = anteiso.
Figure 2. Mass spectra (from Fig. 1) of peaks A identified as 4-phytanyl-
methyl phenol and B tentatively identified as 4-(40,80,120-trimethyl)-tridecyl-
methyl phenol.
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A. O. Barakat et al.
phenol with the same number of carbon atoms and the A/n-
eicosanylmethyl phenol retention time behavior), and the
mass spectral molecular ion (Fig. 2) indicate it to be a C16-alkyl
substituted compound; thus, it is tentatively identified as
4-(40,80,120-trimethyl)-tridecylmethyl phenol.
comparatively low abundance) as well as the isoprenoid substitu-
ents of compounds A and B discussed above.
There is some evidence for the presence of higher members of
the homologous series of alkyldimethyl and trimethyl phenols,
but this was difficult to ascertain by mass chromatography and
even MS/MS experiments due to the extreme complexity of the
‘ketone’ mixture in the high-molecular-weight region (TIC trace
in Fig. 1). Nevertheless, we have cursory indications that homo-
logs of n-alkyldimethyl and n-alkyltrimethyl phenols are present
ranging in alkyl chain length from C17 to C20. The order of elution
of these homologs relative to the n-alkylmethyl phenol series
aided in their recognition, although complete mass spectra of
good quality could not be obtained. There was no conclusive
evidence for the presence of long-chain iso-alkyl, anteiso-alkyl
or isoprenoid-alkyl substituted members of the more highly
methylated series.
Further evidence to confirm the identification of the
methylated alkyl phenols was obtained from the trimethylsilyl
derivatives and comparing the mass spectra and distribution
patterns of the silylated and unsilylated components. The most
significant ions in the mass spectra of the TMS derivatives are
an abundant molecular ion (80–100% rel. int. (relative intensity)),
a small (M-15)+ fragment, a base peak (or a peak close to 100%)
from b-cleavage of the alkyl chain and a peak at m/z 73 from
the trimethylsilyl group. The distribution pattern of the entire se-
ries mimics that of the underivatized phenols.
Besides the main extended series of n-alkylmethyl phenols,
three series of pseudohomologous alkylated phenols with
predominantly short-chain alkyl substituents were also detected
in the same ‘ketone’ fraction. These compounds apparently carry
up to three additional methyl (or ethyl) substituents on the
aromatic ring, which results in a shift of the m/z 121 key fragment
by multiples of 14 mass units to m/z 135, 149 and 163, respec-
tively. Partial m/z 135, 149 and 163 mass chromatograms are
shown in Fig. 3 and compared with the m/z 121 trace (diagnostic
ion of alkylmethyl phenols) in the same retention time window.
Closer examination of the single ion chromatograms of m/z 135
and 149, supported by the results of multiple reaction monitoring
(MS/MS) analyses, shows the presence of several structural
isomers for each carbon number in the m/z 135 and 149 series;
the presence of a second isomer of the C3-phenol in the m/z
163 series can be explained by the presence of an ethyl and
two methyl substituents instead of four methyl substituents in
addition to the propyl group. The dominant homologs in the
m/z 135, 149 and 163 traces have total carbon numbers of 10,
11 and 12, respectively, suggesting the presence of an ethyl
substituent besides the methyl groups.
The facts that for the monomethyl-substituted n-alkyl phenols
(m/z 121 series) only a single peak was found and that isomers
of the more highly methylated analogs were separated on the
column used suggest that the position of the first methyl group
is specific and only a single isomer of this series is present
throughout the entire carbon number range of the homologous
series. Isomers in the mass chromatograms of Figs 1 and 3 repre-
sent iso-substituents and anteiso-substituents, e.g. for C5 in Fig. 3
and C14, C15 and C16 in Fig. 1 (latter one not labeled but doublet
clearly visible; more present according to MS/MS data but of
Alkyl phenols
GC/MS analysis of the ‘ester-bound alcohol’ fraction of the
extracted polar lipids indicated the presence of additional, less
substituted and, thus, more polar phenolic components. These
include a minor series of n-alkyl phenols recognized in the m/z
179 mass chromatogram of the corresponding silylated fractions
(partial section shown in Fig. 4). The mass spectral features
common to the marked compounds are the presence of a
Figure 3. Partial m/z 121, 135, 149 and 163 mass chromatograms for the ‘ketone’ fraction of a Nördlinger Ries sediment from 151.5 m depth of well NR-
10 showing the distribution of the tentatively identified n-alkyldimethyl phenols (m/z 135), n-alkyltrimethyl phenols (m/z 149) and n-alkyltetramethyl
phenol (m/z 163) in the lower molecular weight range. The traces are individually normalized to the highest peak, but information on intensities
(top right) allows comparison of relative abundances between traces. var. = various.
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Alkylated phenol series in lacustrine shales
Moreover, the two distinct peaks just preceding n-pentadecyl
phenol are likely to represent iso-pentadecyl and anteiso-pentadecyl
phenols (all as TMS ethers) analogous to the alkylmethyl phenol
compounds. Further iso-pairs and anteiso-pairs were found for the
C14, C16, C17 (low intensity, not labeled) and C18 substituted
members of the series (Fig. 4). A compound of low intensity eluting
after 4-n-tetradecyl phenol (B0, Fig. 4), based on its relative retention
time and the molecular ion indicating it to be a C16-substituted
compound, is tentatively identified as 4-(40,80120-trimethyl)-tridecyl
phenol (Fig. 5).
Coinjection of the synthesized reference compounds with this
‘bound alcohol’ fraction both in the free and derivatized state
shows that coelution of the natural compounds occurs with
synthetic 4-n-hexadecyl phenol and 4-phytanyl phenol. The
retention time difference to the earlier eluting ortho-substituted
isomers under the gas chromatographic conditions used is more
than 2 min corresponding to about one carbon number. In the
mass spectra of the natural compounds, the low intensities of the
McLafferty rearrangement ions at m/z 108 in the underivatized
sample and at m/z 180 in the trimethylsilyl derivatives support their
identification as 4-alkyl phenols. Analogous to the retention time
behavior of fatty acids and their methyl esters, 4-phytanyl phenol
elutes before 4-n-octadecyl phenol (Fig. 4). Although, according
to Derenne et al.,[32] meta-substituted and para-substituted alkyl
phenols may coelute the presence of the meta-isomers in the
Nördlinger Ries sediments to a significant extent can be excluded
because these compounds are characterized by having a base peak
at m/z 108 in their mass spectra;[32] the same type of rearrangement
products dominates the mass spectra of meta-di-n-alkylbenzenes,
whereas the b-cleavage products are base peaks in the spectra of
the other two isomers.[35]
Figure 4. Partial total ion current (TIC) and m/z 179 chromatograms for
the derivatized (TMS) ‘bound alcohol’ fraction of a Nördlinger Ries
sediment from 151.5 m depth of well NR-10 showing the distribution of
n-alkyl phenols. Numbers correspond to the number of carbon atoms in
the alkyl side-chain. A0 is identified as 4-phytanyl phenol, B0 tentatively
as 4-(40,80,120-trimethyl)-tridecyl phenol.
dominant molecular ion, an intense fragment at m/z 179 resulting
from b-cleavage of the alkyl side-chain and an ion at m/z 73
(TMS). The molecular ions indicate that these compounds have side
chains with 13–21 carbon atoms.
Analogous to the n-alkylmethyl phenol series, inspection of the
m/z 179 chromatogram (Fig. 4) shows a peak (A0) eluting before
n-octadecyl phenol and having a mass spectrum (Fig. 5) with a
base peak at m/z 179 and a molecular ion corresponding to a side
chain with 20 carbon atoms. Again, from its spectrum together
with its relative retention time, this compound is tentatively
identified as the trimethylsilyl ether of phytanyl phenol.
In addition to the long-chain alkyl phenols, GC/MS analysis of
the ‘bound alcohol’ fractions provided evidence for the pres-
ence of methyl phenol (cresol) and dimethyl phenols (xylenols).
The mass spectra of their TMS derivatives are identical to
published spectra.[36]
DISCUSSION
Histograms of the relative abundance of the side-chain (pseudo)
homologs of alkyl phenols and alkylmethyl phenols based on
peak heights in the m/z 179 and 121 mass chromatograms,
respectively, are shown in Fig. 6a and b for the sediment sample
from 151.5 m of well NR-10. A comparison of the distribution
patterns of both series shows a strikingly close similarity, includ-
ing predominance of the n-octadecyl and phytanyl components
as well as the presence of iso-isomers and anteiso-isomers. Such
a remarkable similarity in distributions suggests either a common
genetic origin or a chemical conversion of alkyl phenols into
alkylmethyl phenols (methylation is a common process under
anoxic condition) or vice versa. Alkyl phenol series only occurs
in the shallower sediment from borehole NR-10. If their occur-
rence is considered in terms of diagenetic relationship, involving
degradative reactions to form these compounds, then it is more
likely that they occur in a less saline (implying less reducing or
microbiologically more diverse) environment. On the other hand,
comparison of the distribution patterns of alkylmethyl phenols
for three different black shale samples exhibits major variations
among the samples (Fig. 6b–d). The most obvious feature is the
strong dominance of 4-phytanylmethyl phenol in the sample
from well NR-30, which may be related to lateral facies changes
Figure 5. Mass spectra (from Figure 4) of peaks A0 identified as 4-phyta-
nyl phenol and B0 tentatively identified as 4-(40,80,120-trimethyl)-tridecyl
phenol.
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A. O. Barakat et al.
iso-alkyl
100
80
60
40
20
0
homologs are formed by alkylation of the aromatic nuclei with
primary alcohols[38] or by ring closure of n-paraffins or their precur-
sors.[45] Several authors[40–42] proposed that thermal alteration of
kerogen did not result in generation of alkylbenzenes and that they
originate from a specific biological source or are diagenetically
transformed lipids. On the other hand, the occurrence of a series
of methylated phytanylbenzenes[46] and arylisoprenoids[34] was
reported in a variety of sediment extracts and oils. The formation
of these compounds is proposed to be via diagenetic conversion
of isoprenoid quinones or from direct biosynthesis by specific
archaebacteria[46] or photosynthetic sulfur bacteria.[34]
Fossil evidence of a specific marine biological origin of alkyl-
substituted phenols as well as proof of their presence in extant
freshwater algae, however, has meanwhile been provided. Meta-
substituted phenols with long, linear aliphatic chains up to C19
and without or with one to three additional methyl groups on
the aromatic ring were found, together with a dominant series of
5-n-alkyl-1,3-benzenediols, in the pyrolysate of Ordovician kukersite
kerogen from Estonia.[24] These alkylated phenols apparently were
bound to the macromolecular structure by terminal or mid-chain
bonds. It is assumed that they are derived from highly aliphatic
biopolymers in the outer cell walls of the fossil alga G. prisca. In
contrast to this, no such phenols were found in the pyrolysates of
the Guttenberg Oil Rock (Ordovician), which contains remains of
the same type of fossil alga. This led to the differentiation of two
‘morpho/chemical’ kinds of G. prisca-rich kerogens, i.e. the ‘closed/
phenol-rich’ (Baltic) and the ‘open/phenol-poor’ (North American)
type related to deposition in an environment of higher salinity
in the case of the phenol-rich type.[32] Together with the main
series of meta-alkyl phenols, Derenne et al.[24] detected several
corresponding homologous series of mono-methyl, and some-
times, di-methyl or trimethyl derivatives. In addition, a minor series
of n-alkyl phenols with base peaks at m/z 107 and 121 was
described, which were later identified as 2-alkyl isomers.[32]
Growth of the extant green microalga Botryococcus braunii
(race A) on media with increasing salinity not only has a morpho-
logical effect but also leads to a significant increase of alkyl
phenol content in the polar fractions of the pyrolysates with
increase in salinity.[32,47] Among the alkyl phenols, the desmethyl
compounds were more abundant than the alkylmethyl phenols,
and the ortho-isomers predominated over the meta-isomers
and possibly present para-isomers.[47] No phenols with branched
alkyl substituents were reported. In the hexane-soluble lipids of
an alkadiene-producing strain of race A B. braunii, very long-chain
alkenyl phenols were found in 3.6% abundance.[48] The mixture
consisted of three compounds identified as 6-alkenyl-2,
4-dimethoxy phenol with chain carbon numbers of the alkenyl
substituent of 27, 29 and 31 in a ratio of approximately 3 : 12 : 5.
The long-chain alkyl phenols in the Nördlinger Ries sediments
have a distribution pattern that is different from all of those
found in the pyrolysates of fossil algal-rich kerogens, extant algae
and in the extractable lipids of extant algae. On the other hand,
both the occurrences of fossil remains of thick-walled algae in
the Nördlinger Ries kerogens[25] and of indications of enhanced
salinity in the former crater lake suggest environmental condition
that make it at least conceivable that the alkylated phenols
described in this study are of direct biogenic origin. The major
difference to the other occurrences is the mixture of n-alkyl,
monomethyl-alkyl and isoprenoid-alkyl substitution patterns in
the alkyl phenol series of the Nördlinger Ries samples.
(a) Alkylphenols
NR-10 (151.5 m)
anteiso-alkyl
phytanyl
n-alkyl
13
14
15
16
17
18
19
20
21
Carbon number of alkyl side-chain
100
80
60
40
20
0
(b) Alkyl-methylphenols
NR-10 (151.5 m)
2
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Carbon number of alkyl side-chain
100
80
60
40
20
0
(c) Alkyl-methylphenols
NR-10 (250.0 m)
2
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Carbon number of alkyl side-chain
100
80
60
40
20
0
(d) Alkyl-methylphenols
NR-30 (215.1 m)
2
3
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Carbon number of alkyl side-chain
Figure 6. Comparison of (a) alkyl phenol and (b) alkylmethyl phenol
distributions in a Nördlinger Ries sediment from 151.5 m depth of well
NR-10; (b–d) comparison of alkylmethyl phenol distributions in three
different black shales from the Nördlinger Ries.
because the drill site was closer to the shore of the former crater
lake. The difference between the two samples from well NR-10 is
less pronounced and mainly relates to a lower concentration of
short-chain compounds, a higher relative abundance of the C15
iso-alkyl-substituted methyl phenol and a slight shift in the
maximum of the distribution to lower carbon numbers in the
deeper sample.
Long-chain alkylbenzenes have long been recognized as
widely occurring constituents of many geological samples.
Homologous series of long-chain n-alkylbenzenes (up to C35) were
reported in a variety of crude oils and sediments[37–41] and in pyrol-
ysis products of coals and kerogens.[42–44] The origin of these
straight-chain alkyl aromatics is not yet well understood. There
are, however, indications that alkyl aromatics and alkylcycloparaffin
Particularly the iso-alkyl and anteiso-alkyl moieties suggest the
involvement of microbial lipids in the formation of the alkyl
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J. Mass. Spectrom. 2012, 47, 987–994

Alkylated phenol series in lacustrine shales
phenol series due to the abundance of the corresponding
branched-chain fatty acids in certain bacteria. Also, the dominance
of the C18 and C20 n-alkyl substitutes in the alkyl phenol and
alkylmethyl phenol series bears some resemblance to the chain
lengths of fatty acids in microbial organisms, the same applies to
the relatively high abundance of the C17 and the C19 homologs.
So, alternative to a direct biogenic origin, a diagenetic coupling of
fatty acid-derived lipids (intact or with partly degraded chains) to
phenols may have been involved in the formation of the
compounds described here. Supporting evidence comes from the
fact that besides phytanyl (C20), a C16 isoprenoid appears to be
coupled to the phenols. Had these compounds a biosynthetic
origin, C20 and C15 substituents would rather have been expected.
The C16 isoprenoid moiety may have been derived from partial
degradation of the phytyl chain in chlorophyll a or b or the phytanyl
units in archaeal membrane lipid, because in addition to photosyn-
thetic algae, archaea are likely to have contributed to the organic
matter in the black shales of the Nördlinger Ries formed under
strongly reducing conditions.
At present there is no direct evidence for the diagenetic
reaction pathway tentatively outline previously, which also did
not easily explain the large variety of alkylated phenols detected
and particularly not the formation of the short-chain components
with their increasing abundance toward short alkyl chain lengths.
On the other hand, it is clear that the necessary reaction partners
from algal cell walls and aliphatic lipids would have been
available in the Nördlinger Ries sediments. Given the shallow
burial of the sediments and the accordingly mild temperatures
these sediments have ever encountered, microorganisms may
eventually have mediated the coupling reactions.
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We are grateful to BEB Erdöl und Erdgas GmbH (Hannover) for
providing the Nördlinger Ries samples and permission for
publishing the results. Extraction/lipid chromatography and
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