Moving-Wire Device for Carbon Isotopic Analyses of Nanogram Quantities of Nonvolatile Organic Carbon

We describe a moving-wire analyzer for measuring ^(13)C in dissolved, involatile organic materials. Liquid samples are first deposited and dried on a continuously spooling nickel wire. The residual sample is then combusted as the wire moves through a furnace, and the evolved CO_2 is analyzed by continuous-flow isotope ratio mass spectrometry. A typical analysis requires 1 μL of sample solution and produces a CO_2 peak ~5 s wide. The system can measure "bulk" δ^(13)C values of ~10 nmol of organic carbon with precision better than 0.2‰. For samples containing 1 nmol of C, precision is ~1‰. Precision and sensitivity are limited mainly by background noise derived from carbon within the wire. Instrument conditions minimizing that background are discussed in detail. Accuracy is better than 0.5‰ for nearly all dissolved analytes tested, including lipids, proteins, nucleic acids, sugars, halocarbons, and hydrocarbons. The sensitivity demonstrated here for 13C measurements represents a ~1000-fold improvement relative to existing elemental analyzers and should allow the use of many new preparative techniques for collecting and purifying nonvolatile biochemicals for isotopic analysis.