Determination of nitrogen-15 by combined system of sealed tube pyrolysis and mass spectrometer

Abstract

A combined system of sealed tube combustion of nitrogen-containing sample and a simple mass spectrometer for detecting 14N-14N and 15N-14N has been established for the ultramicro sampling scale.A sample was placed in a heat-resistant glass tube having a capillary tip at one end as illustrated in Fig. 1, and highpurity oxygen from a stainless steel tube was passed through the glass tube for 8 min to purge the air, then the glass tube was sealed by a small hydrogen flame. The sealed tube was heated in a cylindrical electric furnace at 650°C for 90 min to ensure complete decomposition. The tube was introduced into the bottom of an inverted L-shaped tube as shwon in Fig. 2, while an iron piece was placed at the horizontal part. The inlet system was then evacuated and the iron piece was dropped onto the capillary end of the sealed tube to extract the combustion gas through the broken tip.A simple mass spectrometer, Yanaco Isotope Analyzer Model MSI-10, with the maximum M/e=200, the scan was between M/e = 28 and 29, where 14N-14N+, 14N-14N+ ions were determined. It has been found that an excessive signal response was obtained when the combustion time of the sealed tube was less than 50 min at the heating temperature of 650°C, probably because by incomplete combustion. The analytical results for approximately 10 μg of nitrogen with the proposed method was accurately coincided with those of the Rittenberg method using wet combustion, the latter being carried out by using at least 200 μg of nitrogen.