Abstract
AbstractOne of the commonly used analytical approaches for measuring oxygen isotope ratios δ18O of solids (organic and inorganic) is to pyrolyze the samples to gaseous phases and then send the gas into an isotope ratio mass spectrometer system. Solid samples for δ18O measurements are usually stored in silver cups because of its low reactivity towards oxygen and other oxidants. Samples in silver cups can be dropped directly into the carbon column of the pyrolysis furnace. However, the silver cups can tarnish and then be oxidized over a prolonged storage period. We find that while a small amount of silver oxides does not affect measurements with appreciable sample sizes, it can skew isotope results of small samples. We thus recommend careful storage of samples in silver cups to minimize oxidation, such as under an air-isolated condition, and avoiding prolonged storage for accurate δ18O measurements.
Highlights
Solid samples for oxygen isotope δ18O measurements are commonly stored in silver cups
We measure the δ18O of silver cups that appear pale yellow, in order to identify if oxidation of the cups can skew δ18O measurements of small solid samples
In the same run, ~200 μg of BaSO4 (IAEA-SO-5, about 27% of the mass is from O) produces an amplitude-28 at about 6,500 mV, and 43 μg BaSO4 at about 1,500 mV (Fig. 2b)
Summary
Solid samples for oxygen isotope δ18O measurements are commonly stored in silver cups. Silver cups tarnish (Ag2S) upon exposure to H2S in the air even at parts-per-billion concentrations of H2S (Franey et al, 1985), especially near the opening of the cups where they are pressed to avoid the samples from falling out (Fig. 1). Other than Ag2S, a small amount of AgO, Ag2O, Ag2SO3, Ag2SO4 can form on the cup (Franey et al, 1985; Sanders et al, 2015), in the presence of ozone in the air (Wiesinger et al, 2013). Some laboratories store the silver cups in ovens to avoid moisture, which can speed up the oxidation process. Even if the silver cups are stored in room conditions, they can still turn yellow after storage for about a year
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