Nitrate isotope measurements continued

Lab Note #18
Apr 17, 2014
After the bacteria have had enough time to convert all of the nitrate in solution into N2O gas, we kill the bacteria with sodium hydroxide (NaOH).  The picture below shows Maria doing just that.

After that, the samples are loaded onto the auto sampler, which is attached to the front end of our mass spectrometer.

The autosampler injects the N2O gas in each sample into a flow of helium (He) gas through stainless steel tubing (if you look closely, you can see lots of stainless steel tubing in the image below).  Each sample vial contains N2O gas, but it is not pure.  In order to purify it, the He flow carries the N2O first through two traps, one for carbon dioxide (CO2) and water, and the other for volatile organic compounds (VOCs).  In the image below, the white tube near the foreground of the picture is the CO2 and water trap, and the tube behind it (wrapped in heating tape) is the VOC trap.  The whole apparatus shown below is called the "PreCon" (for preconcentration).

After cleaning up the sample of CO2, water, and VOCs, the N2O gas is then "concentrated" cryogenically by freezing it into a liquid nitrogen trap.  The liquid nitrogen is in the blue dewar on the right of the image above.  The image below shows Maria pouring liquid nitrogen into the trap.

Once the N2O gas has been concentrated in the liquid nitrogen trap, the PreCon lifts the trap out of the liquid nitrogen so that it will continue downstream through the stainless steel tubing with the flow of He gas.  The N2O + He then flows through a heated (800 degrees C) gold tube, where it decomposes into N2 and O2 gas.  The heated gold tube is located in the cage looking thing towards the back of the PreCon picture above.  The He + N2 + O2 gas then flows through a gas chromatograph.  The gas chromatograph separates the N2 + O2 gas mixture, so that the N2 and O2 gases enter the mass spectrometer at separate times so that they can be measured for their nitrogen and oxygen isotopic composition separately.  The gas chromatograph is inside the box labeled "Gasbench II" in the image above.  The picture below shows an image of the mass spectrometer.

In my next post, I will describe how the mass spectrometer works.
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