Optimized Demineralization Technique for the Measurement of Stable Isotope Ratios of Nonexchangeable H in Soil Organic Matter

Abstract

To make use of the isotope ratio of nonexchangeable hydrogen (δ²H_{n (nonexchangeable)}) of bulk soil organic matter (SOM), the mineral matrix (containing structural water of clay minerals) must be separated from SOM and samples need to be analyzed after H isotope equilibration. We present a novel technique for demineralization of soil samples with HF and dilute HCl and recovery of the SOM fraction solubilized in the HF demineralization solution via solid-phase extraction. Compared with existing techniques, organic C (C_org) and organic N (N_org) recovery of demineralized SOM concentrates was significantly increased (C_org recovery using existing techniques vs new demineralization method: 58% vs 78%; N_org recovery: 60% vs 78%). Chemicals used for the demineralization treatment did not affect δ²H_n values as revealed by spiking with deuterated water. The new demineralization method minimized organic matter losses and thus artificial H isotope fractionation, opening up the opportunity to use δ²H_n analyses of SOM as a new tool in paleoclimatology or geospatial forensics