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New and Conventional Evaporative Systems in Concentrating Nitrogen Samples Prior to Isotope-ratio Analysis¹

ABSTRACT

Studies 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.

¹ Contribution from the Hydro-Ecosystem Research Unit, USDA-ARS, Fort Collins, CO 80522.

² Biological Science Technician, and Soil Scientists, USDA-ARS, P. O. Box E, Fort Collins, CO 80522.

Received for publication October 6, 1986.