Separation of organic and inorganic carbon – a method assessment.

Different methods are described in the literature, detailing the separation of organic and inorganic carbon in soil and sediment samples, so as to get accurate organic carbon concentration (C_org) and isotope (δ¹³C_org) data. Most of these methods use either hydrochloric or sulphurous acid to oxidise carbonates to CO₂, leaving organic carbon behind for analyses. The literature consensus is that acidification methods can cause significant offsets in the C/N ratio, the δ¹³C and δ¹⁵N ratios of treated natural samples. In this study we present data from two acidification methods, and one thermogravimetric method, and assess their impact on isotopic measurements. Rather than using natural samples we produce synthetic samples made of carbonates, mixed with different organic compounds in a matrix of sea sand to better understand the impact of the pre-treatment methods on different types of organic materials. Among the organic compounds are those that can be found in soil samples, such as glucose, lignin, humic acid, but we also use in-house isotope standards such as caffeine.  We show that the thermogravimetric method is not suitable for our synthetic samples as some of the organics seem to be charred, rather than oxidised to CO₂. Nitrogen bearing compounds show the largest isotope offset between acid treated and non-acid treated samples, and surprisingly the acidification treatments have the smallest effect on the carbon isotopic composition of large organic molecules such as lignin and humic acid.