ISOTOPE-RATIO-MONITORING GAS-CHROMATOGRAPHY MASS-SPECTROMETRY - METHODS FOR ISOTOPIC CALIBRATION

In trial analyses of a series of n-alkanes, precise determinations of C-13 contents were based on isotopic standards introduced by five diff erent techniques and results were compared. Specifically, organic-comp ound standards were coinjected with the analytes and carried through c hromatography and combustion with them; or CO2 was supplied from a con ventional inlet and mixed with the analyte in the ion source, or CO2 w as supplied from an auxiliary mixing volume and transmitted to the sou rce without interruption of the analyte stream. Additionally, two tech niques were investigated in which the analyte stream was diverted and CO2 standards were placed on a near-zero background. All methods provi ded accurate results. Where applicable, methods not involving interrup tion of the analyte stream provided the highest performance (sigma = 0 .00006 at.% C-13 or 0.06 parts per thousand for 250 pmol C as CO2 reac hing the ion source), but great care was required. Techniques involvin g diversion of the analyte stream were immune to interference from coe luting sample components and still provided high precision (0.0001 les s-than-or-equal-to sigma less-than-or-equal-to 0.0002 at. % or 0.1 les s-than-or-equal-to or less-than-or-equal-to 0.2 parts per thousand).