Vacuum Apparatus to Concentrate Ammonium Solutions for Nitrogen Isotope‐Ratio Analysis

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Abstract
Concentrating NH4 solutions in preparation for 15N analysis usually is accomplished by evaporating samples to dryness with open‐air or forced‐air systems. This paper describes a newly developed drying apparatus that operates under vacuum to facilitate evaporation, prevent sample contamination, and avoid several problems associated with conventional evaporative systems. Standard (NH4)2SO4 samples containing 0.20, 0.50, 1.00, or 2.00 atom % 15N were acidulated with H2SO4 or H3BO3 plus H2SO4 and evaporated to dryness with the vacuum system, a conventional forced‐air system, or a conventional open‐air system. No significant contamination by atmospheric NH3 was found in samples acidulated with H2SO4 and dried with the vacuum apparatus. In comparison, absorption of atmospheric NH3 during evaporative processing of H2SO4‐acidulated samples with the forced‐air or open‐air system resulted in dilution of 15N concentrations in excess of 0.027 and 0.047 atom % 15N in standards containing 1.00 or 2.00 atom % 15N, respectively. The atom % 15N concentrations of evaporated samples containing H3BO3 plus H2SO4 were inconsistent. No significant N loss, isotopic fractionation, or atmospheric contamination was detected in samples acidulated with H2SO4 and heated to 130 °C under vacuum for up to 4 h past dryness. Moreover, no significant cross‐contamination was detected in 4‐mL samples of varying 15N concentration that were acidulated with H2SO4 and dried under vacuum, indicating that the vacuum apparatus can be used to simultaneously process samples differing in 15N concentration. The apparatus can accommodate up to 80 samples, depending on sample volume and container size (< 5–250 mL).

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