Oxidized silver cups can skew oxygen isotope results of small samples

Abstract

One 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.

Introduction

Solid samples for oxygen isotope δ¹⁸O measurements are commonly stored in silver cups. However, 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 also form on the cup (Franey et al., 1985; Sanders et al., 2015), particularly 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.

Figure 1. A new silver cup (left) and an old silver cup that has held samples for over a year in room conditions (right). Notice the old cup has turned slightly yellow, especially at its pressed opening.

Objective

Silver oxides or sulfate on silver cups can potentially affect the δ¹⁸O measurements. In this study, we explore this problem and the circumstance that the oxidized silver cups become a concern. 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. We use measurements of oxygen isotope ratios from BaSO₄ standards to illustrate the effects of the oxidized silver cups.

Methods

We put two silver cups into an 80°C oven to speed up the process of oxidation. The cups turn to pale yellow after a month. We also have a silver cup that has sat in room conditions for over a year and turned pale yellow. We then measure the δ¹⁸O of these three silver cups with a Hekatech high-temperature pyrolysis furnace and a continuous flow isotope ratio mass spectrometer system (CF-IRMS, Finnigan MAT 253 in continuous flow mode using a Conflo III open split interface). Before the measurements, the system passed the linearity test, background test and zero test. In the same run, we measure the δ¹⁸O of IAEA-SO-5 BaSO₄ standard to compare the oxygen peak sizes from standard sulfate and the silver cups. δ¹⁸O values (V-SMOW) are calibrated with international standards for oxygen isotopes: IAEA-SO-5, USGS-32, NBS-127 and IAEA-SO-6. The standard deviation of the measurements is 0.18‰.

Results

The three yellow silver cups give small peaks of oxygen in the mass spectrum results (Fig. 2a). The cups stored in an 80°C oven show amplitude-28 of only 141 mV and 112 mV. The amplitude-28 of the cup stored in room conditions for over a year is larger, at 219 mV. δ¹⁸O values (9.4‰, 11.5‰, −4.5‰) from such small amounts of oxygen are not accurate and are only for reference here.

Figure 2. Mass spectrum results from the mass spectrometer. In each mass spectrum, the first three peaks at the left are from the reference gas (tuned for measuring 200 μg of BaSO₄) and the fourth peak is the peak of the oxygen from the sample. (a) An old, pale yellow silver cup gives a small oxygen peak. (b) 43 μg of BaSO₄ gives an oxygen peak larger than that in (a). (c) In comparison, a new silver cup gives no oxygen peaks in the mass spectrum.

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). This means when 200 μg BaSO₄ is used for BaSO4-δ¹⁸O measurements, the oxides from the silver cup contribute less than ~3.5% of the oxygen being measured. However, when only 43 μg BaSO₄ is measured, the oxides from the silver cup can contribute roughly 13% of the oxygen.

Discussions

Our experiment demonstrates that when using conventional sample sizes (e.g., 200 μg BaSO₄), the oxidized silver cups are not a concern for the accurate measurements of δ¹⁸O. However, when only a small sample is available, or the sample is impure so the oxygen content is lower than expected, the oxides on the silver cups can skew the δ¹⁸O results.

The scale of the skewness can vary, depending on the time of storage, the quantities of oxygen compounds and the isotope compositions of oxidants. For instance, δ¹⁸O of both atmospheric O₂ and H₂O vary according to environmental conditions (e.g., Craig, 1961; Klots & Benson, 1963; Benson & Krause, 1980). These oxidants on silver cups will skew δ¹⁸O results differently.

If prolonged storage of samples in silver cups is unavoidable, simple methods to postpone the oxidation process include vacuum-sealing the sample holders and storing the samples in a desiccator. Storing samples in an argon- or nitrogen-filled environment may be considered for storage over an extensive period. The laboratory condition, e.g., whether natural gas is present, matters as natural gas contains a minor amount of H₂S. An effective and clean heating, ventilation, and air conditioning (HVAC) system of the laboratory should also help lessen the oxidation problem.

Conclusions

Our results demonstrate that if small or impure samples are measured for their oxygen isotope compositions, oxidized silver cups that hold the samples can skew the measurements. Our finding suggests that proper sample storage is also an important step for accurate isotope measurements. When small sample sizes are unavoidable, we recommend shortening the storage period with the silver cups and vacuum-sealing the sample holders to delay the oxidation process.