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.