Oxygen Plasma Ashing for successful preparation of samples - Stable isotope ratio measurements, performed by mass spectrometry, can provide accurate information in a number of geoscience applications, including the reconstruction of past glacial/interglacial temperature variations for example.
One of the most common problems in stable isotopic measurements is the presence of hydrocarbon (e.g. bitumen) and adhering organic material. Effective removal of this hydrocarbon material needs to be accomplished by a method which in turn does not itself also alter the initial isotopic composition.
Oxygen plasma ashing can be employed for this purpose but care is needed to ensure that the initial isotopic composition is retained.
Recently published work from Imperial College London using the Henniker HPT-100 demonstrated the routine preparation of geological samples without altering the initial isotopic composition and provided the conditions and methods by which this should be undertaken.
Please find the abstract below;
Effects of Oxygen Plasma Ashing treatment on Carbonate Clumped Isotopes
All information courtesy of Online Wiley Library (2020), Qi Adlan, Amelia J. Davies, Cédric M. John, First published: 04 April 2020,https://doi.org/10.1002/rcm.8802
Figure above. Oxygen plasma ashing pre-treatment rate of success on carbonate reservoir samples contaminated by hydrocarbon (bitumen) (30), based on 57 carbonate clumped isotope measurement of MacDonald et al, 2015. The clumped isotope measurement rate of success based on acceptance of Δ48 offset and 49 parameter contaminant level
Rationale
For clumped isotopes analysis (Δ47), hydrocarbon and organic molecules present an important contaminant that cannot always be removed by CO2 purification via a poropak trap. Treatment by low‐temperature oxygen plasma ashing (OPA) is a quick and easy approach; however, the impact of this treatment on the original carbonate clumped isotopes values has never been fully assessed.
Methods
We tested the isotopic impact of OPA using three natural samples selected for their large range in initial Δ47 values. Crushed and sieved (125 μm mesh) samples were placed into a Henniker Plasma HPT‐100 plasma system and treated at a flow rate of 46mL/min and a power of 100W at a vacuum of 0.2 mbar for 10, 20, 30 and 60 minutes prior to clumped isotope analysis using two MAT 253 isotope ratio mass spectrometers modified to measure masses 44‐49.
Results
OPA treatment for 30 minutes or more on calcite powder samples has the potential to alter the clumped isotopic composition of the samples beyond analytical error. A systematic positive offset is observed in all samples. The magnitude of this alteration translates to a temperature offset from known values ranging from 4°C to 13°C. We postulate that the observed positive offset in Δ47 occurs because the bonds within lighter isotopologues are preferentially broken by plasma treatment, leading to an artificial increase in the ‘clumping’ value of the sample.
Conclusions
We recommend that any laboratory performing OPA should shorten the runs to 10 to 20 minutes, or to do successive runs of 10 minutes followed by sample stirring, as this procedure showed no alteration of the initial Δ47 value. Our results thus validate the use of OPA for clumped isotope applications and will allow future research using clumped isotopes in challenging samples, such as oil‐stained carbonates, bituminous shales, or host‐rocks with very high organic carbon content.
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