Nitrate isotope measurements

Lab Note #17
Apr 14, 2014
Hi All,

After a meeting in Australia followed by a well-deserved vacation, Maria is back in Seattle and is working in the lab.  Last week, she started the process of measuring nitrate isotopes in the snow samples.  This is a time-consuming and expensive process, and is where about half of the money you donated goes to support.  The money is for "consumables" (e.g. bacteria media, vials, standards, gases, liquid nitrogen, etc) used during analysis.  Consumables are supplies that cannot be re-used.

The nitrate we will measure is dissolved in water (melted snow).  In order to measure its isotopes (both nitrogen and oxygen), we need to convert the nitrate to oxygen gas (O2) for measuring oxygen isotopes and nitrogen gas (N2) for measuring nitrogen isotopes.  To do this, we first use bacteria to do the job of converting our sample from a liquid to a gas!  We use a specific type of "denitrifying" bacteria called Pseudomonas aereofaciens.  The nitrate in our samples is a nutrient source for the bacteria, but they don't use all of it.  The "waste" from the consumption of nitrate is N2O gas (nitrous oxide, or laughing gas), which is how we first get the nitrate into the gas phase.  The image below shows the bacteria in a nitrate-free media solution prior to use.  The bottles are being shaken which helps the bacteria grow.




Maria pipettes some of this bacteria in solution into sample vials shown below.



She then places the samples in a bubbler to get rid of any N2O gas.



Once all of the N2O gas is gone, she adds a melted snow sample (which has been previously concentrated, see prior lab notes) to each vial, and places them on a shaker overnight where the bacteria go to work converting dissolved nitrate into gaseous N2O.



The N2O gas that the bacteria produce accumulates and remains in the "head space" of the vials, awaiting the next step.  More on that later in the week!
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