A Comparison of Cross‐Contamination in Distillation Units Used in Total Nitrogen and Nitrogen‐15 Analyses

A laboratory study was conducted to investigate 15N cross‐contamination errors between distilled standards [1 mg N as (NH4)2SO4] with various 15N excess contents (0, 3, or 10% excess 15N). Several stills differing in rate of distillation and construction were tested. We also compared the effectiveness of the ethanol (ETOH) cleansing technique with the (NH4)2SO4cleansing technique in reducing cross‐contamination errors. The ETOH wash was less effective than the (NH4)2SO4wash in reducing cross‐contamination errors for samples with normal abundance 15N but was equally effective in reducing cross‐contamination errors for samples with high‐enrichment 15N. The (NH4)2SO4wash sometimes significantly diluted high‐enrichment 15N samples. Ammonia seemed to be absorbed and sporadically released from the nonglass components of the partial‐glass still. Replacing the rubber and teflon components with glass and decreasing the glass surface area of the distillation unit decreased cross‐contamination errors and increased the percent recovery of atom % 15N from the distillation unit.