New and Conventional Evaporative Systems in Concentrating Nitrogen Samples Prior to Isotope‐ratio Analysis

Abstract

AbstractStudies were conducted to quantify and compare the efficiencies of various evaporative systems used in evaporating 15N samples prior to mass spectrometric analysis. Two new forced‐air systems were designed and compared with a conventional forced‐air system and with an open‐air drybath technique for effectiveness in preventing atmospheric contamination of evaporating samples. One of the new system designs was compared with the conventional forced‐air evaporative system and with the open‐air drybath technique for levels of 15N cross‐contamination among evaporating samples. The forced‐air evaporative systems significantly reduced the time needed to evaporate samples as compared to the open‐air drybath technique; samples were evaporated to dryness in 2.5 h with the forced‐air systems as compared to 8 to 10 h on the open‐air drybath. The effectiveness of a given forced‐air system to prevent atmospheric contamination of evaporating samples was significantly affected by the flow rate of the airstream flowing over the samples; the optimum flow rate was different for each of three forced‐air systems tested. When operated at optimum air flow rate, the forced‐air evaporative systems reduced atmospheric contaminants to <0.2µg N per sample, even when a contaminant of ∼77 µg NH3‐N per sample vial was added upstream of the scrubbing systems. The average atmospheric contaminant N found in samples evaporated on the open‐air drybath was 0.3 µ N, indicating very low concentrations of atmospheric NH3 during this study. Both these contaminant levels are sufficiently low to be of little or no significance in most tracer N studies. However, in previous studies we have experienced significant contamination of 15N samples evaporated on an open‐air drybath because the level of contaminant N in the laboratory atmosphere varied and could not be adequately controlled, even when NH3‐containing chemicals, cleansers, and paints were not stored or used in or around the working area. Average cross‐contaminant levels of 0.28, 0.20, and 1.01 µ of N were measured between samples evaporated on the open‐air dry‐bath, the newly‐designed forced‐air system, and the conventional forced‐air system, respectively. The cross‐contamination level is significantly higher on the conventional forced‐air system than on the other two systems, and could significantly alter the atom % 15N of high‐enriched, low [N] evaporating samples. The data from this study demonstrate that effectiveness in preventing atmospheric contamination and potential for cross‐contamination vary from system to system; thus, a given evaporative system should be carefully tested before being used in routine 15N sample processing.