SLONIP

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Sampling and analysis

The most rational approach for monitoring isotopes in precipitation is to perform monthly composite sampling at meteorological stations (Vreča & Malenšek, 2016, Vreča et al., 2022) that are part of a national network and provide reliable meteorological data such as the Slovenian National Meteorological Network maintained by the Slovenian Environmental Agency (ARSO). Therefore, sampling of precipitation for isotope monitoring is performed nowadays at the ARSO synoptic, climatological or precipitation meteorological stations. Precipitation samples are collected by the staff of ARSO from the classical rain gauge collector either three times (synoptic stations) or once per day (climatological and precipitation stations). In few cases is sampling conduced at stations maintained by JSI or volunteers and the samples are collected as soon as possible after the precipitation event. The volume of collected precipitation is recorded and the sample is poured into a plastic bottle with a tight fitting cap. In the laboratory, we remove impurities (e.g. dust, particles) from the composite monthly sample by filtration through 12–25 μm pore size ashless filter papers before taking aliquots for different isotope analyses. Samples for the analysis of stable isotopes of hydrogen and oxygen are stored in glass bottles (minimum 30 mL) and for the activity concentration of tritium in high-density polyethylene bottles (minimum 300 mL).

Stable oxygen (δ¹⁸O) and hydrogen (δ²H isotope composition is determined using isotope ratio mass spectrometry (IRMS). The results are reported as δ values in per mill (‰) relative to the international standards. Activity concentration of tritium (A_s ) is determined by the liquid scintillation counting technique (LSC) following electrolytic enrichment (EE). Results are reported in Bqkg^-1 but for the purpose of SLONIP are expressed as ³H in Tritium Units (1 TU = 0.118 BqL ^-1 ) assuming that 1L = 1 kg of water.

Sampling and analysis were changing in the past and were uniformed for SLONIP stations in 2016. Details about sampling and analysis performed in the frame of SLONIP can be found in References.

More guidelines related to global isotope monitoring can be found at the IAEA web page of the Global Network of Isotopes in Precipitation (GNIP).