Error propagation in normalization of stable isotope data: a Monte Carlo analysis

Abstract

A higher analytical precision of a stable isotope ratio mass spectrometer does not automatically guarantee accurate determination of the true isotope composition (delta-value) of samples, since estimates of true delta-values are obtained from the normalization of raw isotope data. We performed both Monte Carlo simulations and laboratory experiments to investigate aspects of error propagation during the normalization of carbon stable isotope data. We found that increasing both the number of different reference standards and the number of repetitions of each of these standards reduces the normalization error. A 50% reduction in the normalization error can be achieved over the two-point normalization by either analyzing two standards four times each, or four standards two times each. If the true delta-value of a sample is approximately known a priori, the normalization error may then be reduced through a targeted choice of locally optimal standards. However, the difference in improvement is minimal and, therefore, a more practical strategy is to use two or more standards covering the whole stable isotope scale. The selection of different sets of standards by different laboratories or for different batches of samples in the same laboratory may lead to significant differences in the normalized delta-values of the same samples, leading to inconsistent results. Hence, the same set of standards should always be used for a particular element and a particular stable isotope analytical technique.

New organic reference materials for carbon- and nitrogen-stable isotope ratio measurements provided by Center for Ecological Research, Kyoto University, and Institute of Biogeosciences, Japan Agency for Marine-Earth Science and Technology

“…The calculated d 13 C values of other international standards, including IAEA-CH-6, IAEA-CH-3, and USGS40, were well fitted to the literature values within 0.04%, although USGS41 was slightly (0.19%) lower than the accepted value. This is consistent with Skrzypek et al (2010), who have shown that a higher mean error was observed outside the range bracketed by the d values of the anchor standards (L-SVEC and NBS19). Table 2 In the studies of limnology and ecology, stable isotope ratios are often used for investigating nutrient dynamics and food-web structure.…”

Sources of new nitrogen in the Indian Ocean

“…Multipoint normalization was used in order to reduce raw values to the international scale [Paul et al, 2007]. Error propagation for stable isotope data was performed as described by Skrzypek et al [2010]. The external error of analyses (1 standard deviation) was 0.15‰ for δ 13 C and 0.20‰ for δ 15 N. Nitrogen assimilation and C fixation rates ( ρ in nmol N or C L À1 h À1 ) were calculated following Dugdale and Goering [1967] and Knap et al [1996].…”

Changes in latitude and dominant diazotrophic community alter N2 fixation

“…Back in the laboratory on land, filters were acidified and dried overnight at 60°C. Analysis of the 15 N 2 incorporation into the particulate organic matter (POM) was carried out using a continuous flow sys-tem consisting of a Sercon 20-22 mass spectrometer connected with an ANCA (Sercon) at the West Australian Biogeochemistry Centre at the University of Western Australia (Paul et al 2007, Skrzypek et al 2010. Final 15 N atom enrichment was calculated assuming complete dissolution of the gas bubble.…”

“…Multipoint normalization was used in order to reduce raw values to the international scale (Paul et al 2007). Standard selection for δ 13 C was as described by Skrzypek et al (2010).…”