STABLE CARBON ISOTOPIC ANALYSIS OF LOW-LEVEL METHANE IN WATER AND GAS

Sample introduction systems have been developed to significantly exten d existing isotope ratio monitoring gas chromatography/mass spectromet ry techniques to allow the simultaneous determination of the concentra tion and stable carbon isotopic ratio of methane from low-concentratio n aqueous and gaseous samples, These systems, along with improvements to the basic analytical system, are used with 1-50 nM water samples an d 45 ppb-250 ppm (v/v) gas samples. The detection limit of the techniq ue is 200 pmol of CH4 in either aqueous or gaseous samples, The precis ion of isotopic measurements of replicate samples is less than or equa l to 0.8 parts per thousand for water rind 0.9 parts per thousand when 200-pmol gas samples are analyzed, the latter improving to less than 0.1 parts per thousand with 4-nmol or larger gas samples.