Molecular Decay of Plant Biopolymers - Biopolymers: A Molecular Paleontology Approach - page 4

et al. 2007b , c ). Compounds generated were identifi ed by comparing their spectra
with those reported in the literature (Gupta et al. 2006 ; Gupta and Pancost 2004 ;
Ralph and Hatfi eld 1991 ; van Bergen et al. 1997 ) as well as those obtained from
model reference compounds lignin and cellulose commercially obtained from
Sigma-Aldrich. Modern and fossil samples were pyrolysed using a CDS 5150
Pyroprobe by heating at 650 °C for 20 s. Compound detection and identifi cation
were performed with on line GC-MS in full scan mode using a Hewlett Packard
HP6890 gas chromatograph interfaced to a Micromass AutoSpec Ultima magnetic
sector mass spectrometer. GC was performed with a J&W Scientifi c DB-1MS
column (60 m × 0.25 mm I.D., 0.25

m fi lm thickness) using He as carrier gas. The
oven was programmed from 50 °C (held 1 min) to 300 °C (held 28 min) at 8 °C min −1 .
The source was operated in the electron ionization (EI) mode at 70 eV ionization
energy at 250 °C. The AutoSpec full scan rate was 0.80 s/decade over a mass range
of 50-700 Da with an inter-scan delay of 0.20 s.
For analysis of experimentally matured samples, samples were subjected to
thermodesorption of weakly bound or non-covalently bound components
(at 310 °C, Gupta et al. 2007a ) and then subjected to pyrolysis at 650 °C in order
to analyze the macromolecular component. Fresh, decayed, and fossil leaves
were extracted 15 min × 3 times with 2:1 Dichloromethane: Methanol to remove
soluble lipids.
Leaves of decay series samples were air dried. They were softened in water
before they were prepared for embedding and thin sectioning. Conventional embed-
ding and thin sectioning (2

μ

m) methods were applied with Toluidine Blue as the
stain. Mounted thin slides were observed with a Nikon light microscope and photo-
graphed using an integral Nikon digital camera. For scanning electron microscopy
(SEM), the leaves were softened and cut transversely in their middle portion. One
piece of each leaf of about 1 mm long was mounted on a SEM stub with one of its
cut sides facing upwards. Leaf pieces on stubs were air dried and sputter coated with
approximately 20 nm of gold and examined with a JSM-6400 Scanning Microscope
at 15 kV. The fossil leaf of Metasequoia from Miocene Clarkia was macerated in a
30 % HF solution overnight to dissolve the adhering matrix before cutting, and then
treated similarly for SEM observation.

μ

Chemistry of Modern Plant Tissue and Decay Series Samples

Figure 1.1 shows the Py-GC-MS total ion trace of fresh leaves (sample 1) of
Metasequoia glyptostroboides from which the lipids have been extracted together
with traces of leaves subjected to environmental decay (samples 4-6). Lignin
(guaiacyl units), polysaccharides (e.g., levoglucosan from cellulose), C16 saturated
and unsaturated fatty acyl moieties (derived from internal lipids and cutin), ben-
zenes, vinyl phenol and alkyl phenols are abundant, as expected for a modern gym-
nosperm leaf (van Bergen et al. 1997 ), whereas n -alkanes and n -alk-1-enes were
detected in trace amounts (as also noted in Metasequoia by Yang et al. 2005 , Fig. 3).