Da. Merritt et al., ISOTOPE-RATIO-MONITORING GAS-CHROMATOGRAPHY MASS-SPECTROMETRY - METHODS FOR ISOTOPIC CALIBRATION, Organic geochemistry, 21(6-7), 1994, pp. 573-583
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).