AUTOMATED N-15 ANALYSIS OF AMMONIUM IN DIFFUSION DISKS BY THE RITTENBERG TECHNIQUE

A recent development in N-15 analysis of inorganic N in soil extracts involves diffusion of NH3 into acidified glass-fiber disks in disposable specimen containers. A simple and convenient technique was developed for isotope-ratio analysis of the NH4 in these disks using a mass spectrometer equipped with an automated Rittenberg apparatus (ARA-MS). For collection of diffused NH3, the disks were treated with 10 muL of 1 M H2SO4. Following diffusion for 6 d, the disks, containing 50 to 150 mug of N, were transferred to a disposable plastic sample tray (96 wells per tray) and treated with 10 muL of 28.9 M (concentrated) HF, which digested the disks without affecting the tray. The tray was placed in a desiccator containing anhydrous CaSO4, for drying of the digested disks in 1 to 3 d. Good agreement was obtained between N-15 analyses of diffused and nondiffused NH4-N when correction was made for isotopic dilution by NH4 in the reagents used in diffusion. Trays containing the processed disks can be conveniently shipped to a service laboratory for analysis by ARA-MS.