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Comparison of Procedures for Reducing Cross-contamination During Steam Distillations in Nitrogen-15 Tracer Research¹

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.

¹ This study was a part of Project no. ILLU-15-0392 of the Agric. Exp. Stn., College of Agriculture, Univ. of Illinois at Urbana-Champaign. Presented before Div. S-4, Soil Sci. Soc. Am., Las Vegas, NV, 26 Nov. 1984.

² Assistant Professor, Dep. of Agronomy, Univ. of Illinois, Urbana, IL 61801.

Received for publication April 22, 1985. Accepted for publication August 27, 1985.