Some instrumental effects in the determination of stable carbon isotope ratios by gas chromatography-isotope ratio mass spectrometry

Some sources of instrumental errors in the determination of¹³C/¹²C in organic compounds by gas chromatography-isotope ratio mass spectrometry (GC-IRMS) have been investigated. For mass 44 ion beam intensities in the range 1·10⁻¹⁰ to 1·10⁻⁸ A, mass-spectrometric pressure effects do not significantly affect data accuracy, thus obviating the necessity of matching sample and reference ion beam intensities for each compound in a complex sample. Data quality is influenced by: (a) the quality of the furnace-tube packing; and (b) the performance of the cryogenic trap. On analysis of large (70 ng) samples, precisions (1σ) of⩽ ± 0.23‰ were obtained using 0.35–0.5-mm-grade CuO. However, there was a marked deterioration in precision (1σ⪯ ± 0.62) when coarse CuO (0.35–1 mm grade) was tested. Use of fine CuO also permits analysis of smaller samples. Inefficient trapping, and release of water during overnight defrosting leads to erroneously lowδ¹³C-values, accuracies of −1.1 to −0.25‰ resulting from analysis of 70-ng samples. Accuracy error increases to⩽1.86‰ on analysis of 10-ng samples. Efficient water trapping combined with the improvements in furnace performance results in precisions generally much better than±0.4‰ and accuracies better than±0.65‰ for high-molecular-weight compounds (268–338 amu), and very accurate and precise results (better than±0.2‰) for lower-molecular-weight compounds (142–173 amu).