Untitled Document
Contamination-free biomarker analysis of shales using
oxidative microwave digestion
Janet Hope and Jochen J. Brocks
Research School of Earth Sciences, Australian National University, Canberra,
ACT 0200, Australia
Figure 1. Mass chromatograms
of the hydrocarbons of a 1.6 billion year old sample from the McArthur
Basin in northern Australia. (A) Untreated sample showing a mixture
of contaminants and indigenous biomarkers. (B) Predominantly indigenous
hydrocarbons after treatment of the rock sample with hot nitric acid.
BAQC = 5,5-diethylalkanes (branched alkanes with quaternary carbon);
BHT-CHO = 3,5-di-tert-butyl-4-hydroxybenzaldehyde; Fl = fluorene;
DBT = dibenzothiophene; DMP = dimethylphenanthrenes; MeFl = methylfluorenes;
Me2Fl = dimethylfluorenes; MP = methylphenanthrenes; MPyr = methylpyrenes;
P = phenanthrene; Phth = phthalates; Pyr = pyrene; Std = standard;
¨ = alkyl diadamantanes; Á = alkylcyclopentanes; • = n-alkanes.
Surficial contamination of drill core and outcrop samples with anthropogenic
hydrocarbons is a common phenomenon and can compromise the analyses of
molecular fossils, particularly of lean and very ancient samples. A survey
studying the molecular content of the exterior and interior portions
of 26 rock samples from a wide range of drill cores and outcrops, found
that all samples were surficially contaminated with petroleum products
[1]. For compact and impermeable rock samples, surficial contaminants
can be removed by trimming of surfaces [1]. However, we demonstrated
that this is not possible for fissile and fractured samples where contaminants
may have entered fissures and cracks. We tested whether contaminant hydrocarbons
can be removed with solvents by extracting intact pieces of diesel stained
shale with dichloromethane using an Automated Solvent Extractor (ASE).
After 3 extraction cycles only ~40% of the diesel was removed, demonstrating
that solvent rinsing does not efficiently eliminate surficial petroleum
products. In a second experiment, we subjected diesel stained shale with
hot concentrated nitric acid in a microwave digestion oven. This treatment
successfully removed 98.5 to 100% of the contaminant hydrocarbons from
the shale.
We further tested the microwave digestion technique on a Precambrian
shale. Figure 1 shows the hydrocarbons of this sample after heating of
the rock in concentrated nitric acid at 180°C for 30 minutes. The treatment
successfully removed nearly 100% of all contaminants while the indigenous
diamondoid and polyaromatic hydrocarbons were retained. The experiments
demonstrate that our oxidative microwave digestion technique is highly
efficient for the removal of surficial hydrocarbons and other contaminants.
Brocks J. J., Grosjean E., and Logan G. A. (2008) Assessing biomarker
syngeneity using branched alkanes with quaternary carbon (BAQCs) and
other plastic contaminants. Geochim. Cosmochim. Acta 72, 871-888.