Determination of nitrate at the ng/g level in Antarctic snow samples with ion chromatography and isotope dilution mass spectrometry

Abstract

Nitrate traces in Antarctic snow and hoar-frost samples, which were collected on the Ekstrom ice shelf during January and February, 1987, were determined with ion chromatography at the German Antarctic station “Georg-von-Neumayer”. For comparison, sample preparations for isotope dilution mass spectrometry were also carried out at the German station whereas the mass spectrometric measurements took place in Regensburg. Comparable reproducibilities of 1%–5% were found for both methods of analysing nitrate concentrations in the range of 40–190 ng/g. An excellent agreement of the results of both methods with an average deviation of 0.1% was found in three sets of firn core samples. The average relative deviation of the ion chromatographic values from those of isotope dilution mass spectrometry was −10% for eleven hoar-frost and surface snow samples. The highest nitrate concentrations were analysed in hoar-frost samples with an average value of 548 ng/g. The following order, which corresponds to the age of nitrate depositions, was: new snow (206 ng/g), old surface snow (146 ng/g), and snow from firn cores (61 ng/g). The decreasing concentration with the age of samples is the result of re-emission of nitrate into the atmosphere after its deposition by snow. A seasonal variation of the nitrate concentration with a summer maximum and a winter minimum was observed for firn core samples. Accurate results of analytical data, which are published from all parts of Antarctica today, can only be guaranteed by the application of independent methods, e.g., by ion chromatography and isotope dilution mass spectrometry as it is shown in this work.