Processing samples for nitrate analysis
After we filter out the absorbing impurities, we use the filtered water (melted snow) to measure the concentration of dissolved ions. We focus on nitrate concentrations, but we also get the concentration of other species as well (both anions and cations). We are able to measure the ion concentrations right here in the lab in Utah using a collaborator's ion chromatograph (I.C.), which is pictured below. This separates the ions from each other in a column, and detects their individual abundance based on their conductivity.
We only take a small aliquot of our sample for ion concentration analysis. The rest of the sample is used for measuring the isotopic composition of nitrate. This we will do back in the lab in Seattle, but we do as much of the sample prep as we can here in Utah. At this point we have a fairly large volume of water for each sample (about 100 mL). We pour this water through a resin. Below is an image of a funnel (yellow) on top of a column filled with resin.
The image below shows several columns filled with 1 mL of resin.
It takes awhile for the entire sample to pass through the resin, so we speed it up by processing more than one sample at a time, and by using a pump to evacuate the chamber shown in the image below (chamber is labeled "supelco").
Once all of the nitrate from each sample is "stuck" to the resin, we elute (or kick off) the nitrate with 10 mL of a solution of NaCl (table salt). We then have our sample in a much more compact volume. This 10 mL of solution containing NaCl plus the nitrate from the snow will be shipped back to Seattle where we can measure the isotopic composition of the nitrate. More on that later...