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

摘要

Gaseous membrane permeation IMP) technologies have been combined with continuous-flow isotope ratio mass spectrometry for on-line delta C-13 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 delta C-13 values of CO2 permeated on-line give the same isotopic results as off-line static dual-inlet IRMS delta C-13 measurements. Formaldehyde generated from aqueous formaldehyde solutions has also been used as the feed gas for permeation experiments and on-line delta C-13 determination. The feed-formaldehyde delta C-13 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 C-13 enrichment relative to the feed delta C-13 value, the isotopic fractionation being 1.0026 +/- 0.0003. The delta C-13 values have been normalized using an adapted two-point isotopic calibration for delta C-13 values ranging from -42 to -10 parts per thousand. The MP-GC/C/IRMS system allows the delta C-13 determination of formaldehyde without chemical derivatization or additional analytical imprecision.