A multi-scale approach to determine accurate elemental and isotopic ratios by nano-scale secondary ion mass spectrometry imaging

RATIONALE Nano-scale secondary ion mass spectrometry (NanoSIMS) is still hampered by a lack of appropriate calibration method for the quantification of elemental and isotopic ratios in heterogeneous materials such as soil samples. The potential of 13C-15N-labeled density fractions of soil to calibrate the C/N, 13C/12C and 15N/14N ratios provided by NanoSIMS was evaluated. METHODS The spatial organization of soil particles found at the macro- and micro-scales were compared. The C/N, 13C/12C and 15N/14N ratios measured at the macroscopic scale from different density fractions using an elemental analyzer coupled to an isotope ratio mass spectrometer (EA/IRMS) were compared with the corresponding micro-scale NanoSIMS measurements. When the macro- and micro-scales patterns were similar, macroscopic scale measurements obtained by EA/IRMS and the corresponding NanoSIMS C/N and 15N/14N ratios averaged per fraction were used to obtain correction equations. The correction method using the internal calibration procedure was compared with the traditional one using a single organic standard. RESULTS It was demonstrated that the correction method using an internal calibration procedure was applicable for NanoSIMS images acquired on more than 500 mu m2 per fraction and provided more accurate C/N and 15N/14N ratios than the traditional correction method. CONCLUSIONS As long as the NanoSIMS sampling was representative of the macroscopic properties, the correction method using an internal calibration procedure allowed better quantification of the isotope tracers and characterization of the C/N ratios. This method not only produced qualitative images, but also accurate quantitative parameters from which ecological interpretations can be derived. Copyright (C) 2012 John Wiley & Sons, Ltd.