One of the primary methods to simultaneously determine carbon and nitrogen isotope compositions (d13C and d 15N) - as well as percent carbon and nitrogen, and carbon:nitrogen ratios of organic material - involves using an elemental analyzer in combination with a continuous flow isotope ratio mass spectrometer.

While effective, this method has the risk that the presence of inorganic carbon may significantly affect these measured values (d13C and %C) and therefore calculated organic C:N values as well. This problem is due to the appreciable differences in d13C between organic and inorganic carbon. As a result, even small amounts of inorganic amounts of inorganic carbonate (less than one percent mass) can increase measured d13C values and overestimate the amount of organic carbon.

In a recent publication in Rapid Communications in Mass Spectrometry, Toti Larson and colleagues of Los Alamos National Laboratory describe the development of a new pretreatment method for the removal of siderite (FeCO3, an inorganic carbonate), from geological samples being subjected to organic carbon isotope and C:N ratio analysis. As siderite is a relatively common diagenetic carbonate created in reducing environments such as bogs, it can complicate the carbon isotope analysis for paleoclimate studies.

To tackle this problem, Larson's team developed a novel acidification strategy (sample acidification is a common approach for the removal of inorganic carbonate). To avoid the loss of organic material associated with standard decanting and rinsing methods, an in situ acidification technique using 6N HCl and silver capsule sample holders was optimized. Tested on both modern and ancient samples, the method was found to avoid potential errors caused by decanting techniques, and to efficiently remove the relatively insoluble siderite.

Article by Larson et al. on pretreatment method for siderite removal, Rapid Communications in Mass Spectrometry, 2008