Measurement precision and accuracy of high artificial enrichment ¹⁵ N and ¹³ C tracer samples

Oversaturation of the Faraday cup amplifiers of isotope ratio mass spectrometers when using tracers that are highly enriched in heavier isotopes (up to 99.9%) remains a major bottleneck to obtaining high-precision measurements. The memory effect plays a key role in reducing tracer sample measurement precision and accuracy. Several sample preparation approaches are known to reduce memory effects and to improve tracer sample measurement precision. However, the potential benefits when using very high enrichment tracer samples (> +1000 mUr) have not been tested.

In this study, we test how specific sample positioning for measurements and frequent use of natural isotope abundance reference materials within the sequence affects the precision and accuracy of isotopic ratio analyses when using a Flash elemental analyser coupled to a Delta^plus XP isotope ratio mass spectrometer for very high enrichment (> +22000 mUr) ¹⁵ N tracer sample measurements. Furthermore, we investigate if tracer sample dilution with natural isotope abundance materials reduces memory effects and increases measurement precision and accuracy when measurements of high-enrichment ¹⁵ N and ¹³ C biomass tracer samples are conducted.

Frequent use of natural isotope abundance materials and specific positioning increased ¹⁵ N tracer sample precision, but it had a negative effect on the precision of quality control substances. ¹⁵ N and ¹³ C tracer sample dilution improved measurement precision by a maximum of ±0.9 mUr; however, a strong linear relationship between the original and the calculated φ values was found. Highly enriched ¹⁵ N tracer samples caused a maximum memory effect of 0.11%. High levels of ¹⁵ N abundance within the samples affected measurement accuracy by an average of 6.7%.

We conclude that highly enriched tracer samples do not require dilution before analysis. Tracer sample precision can be improved by using a specific measurement order of expected isotope abundance and by the frequent use of natural abundance reference materials.