Comparison of Procedures for Reducing Cross‐contamination During Steam Distillations in Nitrogen‐15 Tracer Research

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Abstract
A study was conducted to compare the effectiveness of various procedures for reduction of cross‐contamination errors when unenriched samples of (NH4)2SO4 containing 0.5 mg of N were steamdistilled in a glass distillation apparatus following samples enriched with 50 atom % 15N. Twelve procedures were compared in this study, including most of those previously proposed in the literature. None of the procedures completely eliminated cross‐contamination; however, some were considerably more effective than others. The results suggest that, when a glass distillation apparatus is used in 15N‐tracer research, the best way to reduce cross‐contamination error is to distill two aliquots of each sample whenever possible, discarding the first distillate and retaining the second for isotope‐ratio analysis. When this is not possible, it is recommended that 1 mL of 1M formic acid be distilled for 1 min and 25 mL of 16.4 M (95%) ethanol be distilled for 3 min between samples. This procedure can be made more effective by increasing the amount of formic acid used and extending distillation beyond 1 min.

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