How to fumigate

As a paleoclimatologist and geochemist who studies fossil soils, one thing we often look at is the stable carbon isotopic composition ( ${\delta }^{13}C$) of soil organic matter. However, because these are old soils, most of the organic matter have been either oxidated or comsumed by soil microorganisms, and the remaining organic carbon content (TOC) can be so low (<0.1%）that introduces signifcant uncertainty to the measured ${\delta }^{13}C$.

In terms of carbonate-containing soils, we need to remove the carbonate before measurement, because the ${\delta }^{13}C$ of soil carbonates are much higher than those of soil organic matter. This is especially important for low TOC paleosols, since a tiny bit of carbonate could lead to a hugh bias. The most common way of removing carbonate is called “acid wash”, which refers to treat the samples with diluted acid such as HCl. This method requires the samples to be washed by deionized water multiple times until pH reaches neturality, such as no HCl is left in the samples, which could harm the mass spectrometer. One caveat of this “acid wash” method is that, some of the SOM absorbed onto the soil mineral surfaces or even reside in the crystal lattice of carbonate minerals can be released during the wash and carried away by the solution.

To tackle this issue, another method to remove carbonate called “acid fumigation"is invented. Specifically, the soil samples are directly loaded into silver capsules, wetted, and then put into a desiccator filled with HCl gas. The acid gas will then be diffused in soil water and dissolve carbonates. A key procedure for this method is to ensure that samples are completely dried so that no HCl resides in the soil samples and harms the mass spec.

During my previous fumigation attempt back in China, I usually dry the samples in the oven, which left the oven with potential corrosion and failed to completely remove the HCl (anomalies in the MS curves). So when Jeff the lab manager showed me his way of drying the samples the other day, and I was totally blown away. The trick is to use a continuous gas flow to dry the samples (Jeff uses N2 since it’s inert and cheaper). The gas flow creates a low pressure for the air above the sample capsule, allowing soil water to be fully evaporated. The dried samples were then wrapped with another layer of tin capsules before combustion. Apparently this method is valid since the mass spec is performing perfectly.

The next step for is to compare the ${\delta }^{13}C$ difference between the two carbonate-removing methods, since “acid wash” is still the mainstream method in the realm of paleosol studies. Stay tuned!