Dear Harry, we recently ran Black Shale samples (Toarcian) for isotopic C and N and C/N ratios using the automated GC-MS Carlo Erba-SymbolDS. Our samples ranged from 0.1 to 10% for TOC and from 10 to 90% for Total Carbon. We tried two different ways to isolate kerogen from our sediment: HCl demineralization HF-BF3 maceration (Robl et ali 1993-Org.Geochem. Vol 20, n°2, pp249-255). The HCl demineralized samples gave insufficient concentrated organic matter for C/N and N isotopic analyses. The HF-BF3 tecnique resulted in a final de-ashed product which is sufficiently concentrated in OM for all of our purposes. Elemental analysis of delta 13C-org did not indicate any significant difference in organic alteration using the HCl or HF-BF3 technique. good luck, Antonio Antonio Bacheca Institute de Mineralogie et Petrographie Université de Lausanne BFSH-2 1015 Dorigny Suisse