Water-induced errors in continuous-flow carbon isotope ratio mass spectrometry

Formation of HCO2+ from CO2 and background H2O in isotope ratio mass spectrometers has been examined in detail. The process is troublesome because its product is not resolved from (CO2+)-C-13-O-16. Th, resulting, artifactual enhancement of the mass 45 ion current (and analogous enhancement of the mass 46 ion current by transfer of hydrogen to mass 45 species) can cause systematic errors in analyses of C-13 based on measurement of ion current ratios in the mass spectrum of CO2. Such errors are neutralized when isotopic analyses are based on differential comparisons in which ion currents and background water levels are precisely equal during admission and ionization of both sample and standard gases. In continuous-now systems, however, that requirement is generally not met. The resulting systematic error is proportional to the 18/44 ion current ratio. When the widely used MAT 252 mass spectrometer is tuned to yield maximum sensitivity, the constant of proportionality is 26 +/- 2 parts per thousand (i.e., the error will be 0.26 parts per thousand if the mass 18 ion current is 100 times smaller than that at mass 44). Errors can be reduced B-fold when the ion-source residence time of CO2+ is decreased by use of stronger ion-extraction potential gradients. Under those same conditions, sensitivity is decreased by 60 parts per thousand, For operation at highest sensitivity, carrier gas dew points on the order of -70 degrees C are required to obtain errors less than or equal to 0.1 parts per thousand for samples yielding mass 44 ion currents of 10 nA. Carrier gas dew points less than or equal to-80 degrees C are conveniently reached by use of a Nafion dryer operated at approximate to 0 degrees C.