External-source contamination during extraction—distillation in isotope-ratio analysis of soil inorganic nitrogen

The sources of contamination introduced during the extraction, distillation and drying phases of isotope-ratio analysis of soil inorganic nitrogen (ammonium and nitrite + nitrate) were identified, and the individual amounts of contaminants were quantified by isotope dilution. The procedure involves addition of internal standard solutions of ¹⁵N-labelled ammonium and nitrite to reagent blanks which are carried through each stage of the analysis at the same time as the test samples. Potassium chloride extractants, filter-papers, distillation reagents and atmospheric ammonia all contributed to dilution of the sample ¹⁵N. Some materials tested contained sufficient contaminants to cause large errors in the determination of sample ¹⁵N abundance. Both the amount and isotopic composition of contaminants can be determined by the isotope-dilution procedure, which permits the measured sample ¹⁵N abundance to be corrected for contamination.