Membrane permeation continuous-flow isotope ratio mass spectrometry for on-line carbon isotope ratio determination

Gaseous membrane permeation (MP) technologies have been combined with continuous-flow isotope ratio mass spectrometry for on-line delta13C measurements. The experimental setup of membrane permeation-gas chromatography/combustion/isotope ratio mass spectrometry (MP-GC/C/IRMS) quantitatively traps gas streams in membrane permeation experiments under steady-state conditions and performs on-line gas transfer into a GC/C/IRMS system. A commercial polydimethylsiloxane (PDMS) membrane sheet was used for the experiments. Laboratory tests using CO2 demonstrate that the whole process does not fractionate the C isotopes of CO2. Moreover, the delta13C values of CO2 permeated on-line give the same isotopic results as off-line static dual-inlet IRMS delta13C measurements. Formaldehyde generated from aqueous formaldehyde solutions has also been used as the feed gas for permeation experiments and on-line delta13C determination. The feed-formaldehyde delta13C value was pre-determined by sampling the headspace of the thermostated aqueous formaldehyde solution. Comparison of the results obtained by headspace with those from direct aqueous formaldehyde injection confirms that the headspace sampling does not generate isotopic fractionation, but the permeated formaldehyde analyzed on-line yields a 13C enrichment relative to the feed delta13C value, the isotopic fractionation being 1.0026 +/- 0.0003. The delta13C values have been normalized using an adapted two-point isotopic calibration for delta13C values ranging from -42 to -10 per thousand. The MP-GC/C/IRMS system allows the delta13C determination of formaldehyde without chemical derivatization or additional analytical imprecision.