Fig. 5 : Normalization error of multipoint normalizations based
on the isotope range of the standards. A: matrix matched and bounded
three-point normalizations do not exhibit significant differences with
isotope range; B: matrix matched and bounded two-point normalizations do
not exhibit significant differences with isotope range; C: matrix mixed
and extrapolated three-point normalizations have significantly higher
normalization errors when the isotope range is less than 15‰; D: matrix
mixed and extrapolated two-point normalizations have significantly
higher normalization errors when the isotope range is less than 15‰.
Significance is shown in the compact letter display, where any two
isotope ranges with the same letter are not significantly different as
per Dunn’s post-hoc testing. Nitrogen (uppercase) and carbon (lowercase)
letters are not compared with each other.
Interlaboratory comparison of instrument
linearity
To assess the linearity effect across laboratories and instrument
manufacturers, five different working standards were each analyzed
across a range of sample weights (n=10) while the sample dilution was
held constant, thus producing a range of peak amplitudes. Both
instruments exhibited a non-linear deviation in reportedδ 15N as beam amplitude decreased (Fig. 6) – at
U.S. EPA this deviation occurred at amplitudes below 6nA, and at UNM CSI
it occurred below 2V (peak amplitude is measured in units of current
(nA) on Elementar instruments and voltage (V) on ThermoFinnigan
instruments). The linearity effect of reportedδ 15N occurred regardless of sample matrix and
resulted in substantial (>1.5‰) decrease from the medianδ 15N at both facilities. Instrument linearity
effects for δ 13C were not evident at UNM CSI
across the observed range of peak amplitudes. At U.S. EPA, instrument
linearity effects for δ 13C were not observed
below 20nA, but linearity effects were observed for beam amplitudes
above 20nA (Fig. 6).
Within 48 hours of analyzing the solid samples, diagnostic reference gas
linearity tests were performed for N and C at U.S. EPA and for N at UNM
CSI with the same tuning and configuration. Both facilities displayed a
small reference gas linearity effect for N across the tested range of
peak amplitudes (Fig. 6), with a total isotope range of 0.20‰ and 0.27‰
at UNM CSI and U.S. EPA, respectively. At the U.S. EPA, the reference
gas C linearity diagnostics suggested a consistent inverse relationship
between reported δ 13C and beam height across
the tested range of peak amplitudes (2-12nA), with a total isotope range
of 0.33‰.
To investigate whether the amount of combustion in the elemental
analyzer varies as a function of sample weight, and thus influences the
linearity results, we assessed how the ratio between peak amplitude and
sample weight varied as a function of sample weight (Figure S1).
Combustion effects appeared to be matrix-dependent, with high-organic
matrixes exhibiting a higher variation in the ratio between peak
amplitude and sample weight than other matrixes.