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be understood from consideration of the physical arrangement of an EI ion
source. The source includes magnets that collimate the electron flow from
the filament through the ion box. Once formed, ions are accelerated across
magnetic field lines which induce minor but measurable mass selectivity. As
the mean-free path of ions depends on the relatively high pressure in the
closed ion source, the mean position from which ions escape the ion box
depends on pressure, as well as on mass. This translates into a subtle depen-
dence of isotope ratio on pressure, which becomes apparent in the precision
of these measurements. The severity of this effect in any particular source
defines the relative importance of sample/standard pressure matching. Con-
tinuous-flow IRMS instruments have only become available because modern
ion sources have been designed for maximum linearity and because the He
carrier gas flow maintains the ion source pressure nearly constant and inde-
pendent of the level of sample gas flowing into the source.

4.2.3 Continuous-Flow IRMS
4.2.3.1 Bulk Stable Isotope Analysis (BSIA)
In the IRMS inlet system referred to as “continuous flow,” a He carrier gas
passes continuously into the ion source and sweeps bands or peaks of analyte
gas into the source for analysis. This approach overcomes the sample size
requirements for viscous flow because the He carrier gas stream maintains
viscous conditions independent of sample size. Although originally devel-
oped for on-line coupling of a gas chromatograph (GC) to an IRMS, this
approach has also been successfully used to connect IRMS to a variety of
automated sample processing devices such as an elemental analyzer.
With the increasing use of bulk stable isotopes analysis (BSIA) in fields
such as geochemistry, ecology, bio-medicine, and nutritional sciences, a
great demand existed for the rapid analysis of large numbers of solid sam-
21
ples. In 1983, Preston and Owens demonstrated the first CF interface to a
multi- (dual) collector IRMS for the bulk analysis of nitrogen from solid
15
samples. Precisions of about SD (d N) = 0.7‰ were obtained for 3.5 mmol
aliquots of N derived from urea in CF mode after flash combustion and
2
separation in a GC equipped with a gas separation column. These authors
observed degradation in the precision due to ion source nonlinearities when
analyzing samples of varying size, as is now observed in demanding CF
analyses. This system was the first high-precision multicollector CF-IRMS.
Two years later, these authors extended the instrument to the analysis of C
with similar reproducibility. 22
Modern commercially available elemental analyzers provide an auto-
mated means for on-line high-precision isotope ratios for bulk analysis of
solid and nonvolatile liquid samples. Samples are placed in a capsule, typically

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