EVALUATION OF A DIFFUSION METHOD FOR PREPARING LOW-NITROGEN SAMPLES FOR N-15 ANALYSIS

Ammonia diffusion techniques have been advocated to eliminate crosscon tamination problems associated with distillation methods for concentra ting low-N-content samples for N-15 analysis. Two potential problems w ith diffusion are nonquantitative N recovery and isotopic fractionatio n. These are most likely to have significant effects for low-N-content samples. We evaluated the ability of a diffusion method to accurately prepare low-N (<1000 mu g N), high-volume (55 mL) simulated acidic di gest samples and quantified sinks for N in the diffusion system. This diffusion system trapped N on an acid-impregnated glass fiber disk sus pended above the sample solution. The N-15/N-14 isotopic ratio of N re covered on the disk declined linearly as the quantity of N in the orig inal sample solution decreased below 500 mu g N. Nitrogen-15 values ad justed for contamination detected in reagent blanks still differed sig nificantly from nondiffused reagent blanks. Significant sinks of N oth er than the acidified disk existed despite efforts to minimize them. T he method produced unexplained changes in isotopic composition of the sample. These results emphasize the importance of thoroughly evaluatin g any NH3 diffusion technique with representative sample types in each laboratory before adoption of the technique for routine use. In addit ion, we recommend standard curves as a more robust method for adjustin g N-15 values for errors associated with diffusion.