This document is a copy and paste from the original Word document created by Joseph Erbland (c 2010).
In order to guarantee a good repeatability of the isotopic measurements with the IRMS, a minimum of 100 nmol of nitrate is needed. Since nitrate ice core concentrations fluctuate between 10 to 800 ppbw, we need to process between 7 to 620 ml of water sample which overpass in most case the maximum volume allowed in the denitrifying bacteria vials (10 ml of sample max). It is therefore necessary to preconcentrate most of ice core samples for nitrate isotope analysis. This is achieved by passing the necessary water sample through an anion exchange resin which traps the nitrate and all other anion species. The nitrate and others species are then eluted with a 1M NaCl solution. Trapping and extraction are quantitative (Figure 1), limiting any possible isotopic fractionations associated with the extraction technique.
- Nitrate concentration in water sample need to be determined accurately to fix the volume of sample necessary to achieve 200-300 nmol of nitrate (absolutely necessary for at least duplicates).
- Prepare a small vial (accuvettes) by mixing roughly 50% v/v of resin (AG I-X8, 200-400 mesh, Chloride form, BIO-RAD) and 50% v/v of NaCl 1M
- Shake vigorously the vial to homogenise the resin solution
- Prepare the resin column (polyprep column, BIO-RAD) by pipetting 0.6 ml of this solution and transferring to the column, verify that you have effectively ~ 0.3 ml of resin
- Rinse the resin with 3 x 3ml of NaCl (1M) and after 3 x 3ml of Millipore water
- Pass your water sample through the resin. The excess water can flow to a sink
- Place your clan vial (accuvettes) below your column
- Elute with 5 x 2 ml 1M NaCl, close the vials, label
- Rinse with 3 ml of Millipore water the resin. The same columns are ready to be used for another sample.
- If the resin turns grey or black, or if you suspect any contamination, change the resin as described before. Depending on the purity of the sample, the same resin can be used many times.
- Note: Record the sample volume (or mass) processed for each sample and its respective concentration, this will avoid us to re measure the nitrate concentration in a NaCl matrix and will allow us to calculate the volume needed to be injected in the bacteria solution (again the target is 100 nmol for one measure)