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recipes [2010/04/30 07:08]
mdaeron
recipes [2012/04/04 09:08] (current)
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 ====== Recipes ====== ====== Recipes ======
  
-Instructions for various experimental protocols.+> ''​Instructions for various experimental protocols.''​
  
 ===== Synthetic calcite precipitation ===== ===== Synthetic calcite precipitation =====
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 ===== CO2-water equilibration ===== ===== CO2-water equilibration =====
 +**Recipe for obtaining equilibrium Δ<​sub>​47</​sub>​ by CO<​sub>​2</​sub>​-H<​sub>​2</​sub>​O isotope exchange:**
 +The way to obtain CO<​sub>​2</​sub>​ with equilibrium Δ<​sub>​47</​sub>​ value at low temperatures is through isotope exchange between CO<​sub>​2</​sub>​ and H<​sub>​2</​sub>​O at the desired temperature. This is commonly done in order to use CO<​sub>​2</​sub>​ δ<​sup>​18</​sup>​O as a measure for H<​sub>​2</​sub>​O δ<​sup>​18</​sup>​O. Clumped isotopes equilibrium is obtained by the same exchange reaction; only Δ<​sub>​47</​sub>​ does not depend on the δ<​sup>​18</​sup>​O of the water used for exchange (beyond linearity correction, of course).
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 +In order to determine the reference frame for Δ<​sub>​47</​sub>​ as it varies among instruments,​ the community had decided to perform CO<​sub>​2</​sub>​-H<​sub>​2</​sub>​O equilibration at 10°C, 25°C and 50°C and use them as a basis for the Δ<​sub>​47</​sub>​ scale, through interlaboratory comparison.
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 +There are many ways to perform these exchange experiments and a lab that is experience in doing that for δ<​sup>​18</​sup>​O,​ should use the same method. For labs that do not perform these experiments regularly, I describe below the way I do it. Again, any way to expose CO<​sub>​2</​sub>​ to water would work, as long as CO<​sub>​2</​sub>​ is then separated from the water before it has a chance to exchange at a different temperature. ​
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 +**Yale CO<​sub>​2</​sub>​-H<​sub>​2</​sub>​O exchange method for Δ<​sub>​47</​sub>​ analysis:**
 +In exchanging CO<​sub>​2</​sub>​ and H<​sub>​2</​sub>​O for Δ<​sub>​47</​sub>​ analysis, the value is not sensitive to the isotopic composition of the water used but is sensitive to temperature deviations, as long as there is trace of liquid water available to exchange. The method is therefore designed to quickly quench any liquid water, instead of focusing on preserving the original δ<​sup>​18</​sup>​O of the water. It is convenient to use water of a range of δ<​sup>​18</​sup>​O values in order to plot Δ<​sub>​47</​sub>​ vs δ47 in the equilibrated gases. For any given temperature this plot should be parallel to the heated gas line, thus reflecting the non-linearity of the mass spectrometer.
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 +We typically use 3 different waters: our lab DIW, water that was enriched by evaporating this water, and mineral water that are <​sup>​18</​sup>​O depleted. This yields a range of δ47 values that is similar to the range of heated gases we typically measure.
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 +Equilibration is performed in a break seal (1/4” OD, ~20 cm long) that is fully immersed in water of the desired temperature (inside a temperature controlled bath), to guarantee that no part of the exchange reaction occurs at a different temperature. ~0.1ml of water is inserted into a break seal and the bottom is frozen in liquid N<​sub>​2</​sub>​. If the lab is humid this will potentially affect slightly the δ<​sup>​18</​sup>​O of the water, but not the Δ<​sub>​47</​sub>​ of the CO<​sub>​2</​sub>​. The break seal is then connected to a vacuum line and the air is pumped away while the water are still frozen (to minimize wetting the vacuum line). The valve to the break seal is closed. The CO<​sub>​2</​sub>​ aliquot is inserted into the line, frozen in a U trap and non-condensable gases are removed (if there are any). The CO<​sub>​2</​sub>​ is transferred into the break seal, which is then fire sealed. Typical CO2 amount: 100-150 μmol (the equivalent of 2-3 sample sizes), though smaller amount is OK as well.
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 +Break seals are immersed in the desired temperature for approximately 3 days (this is likely to be much longer than necessary) to ensure CO<​sub>​2</​sub>​ reached isotopic equilibrium.
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 +In order to extract the CO<​sub>​2</​sub>,​ it has to be first quenched so that it does not continue exchanging with water at lab temperature during sample processing. This can be done by dipping the tip of the break seal in liquid N<​sub>​2</​sub>,​ freezing both the CO<​sub>​2</​sub>​ and the water and preventing further exchange. The upper part of the break seal is scored, inserted carefully into the cracker and mounted on the vacuum line. Then, liquid N<​sub>​2</​sub>​ is replaced by a dry ice-ethanol mix to keep the water frozen but to release the CO<​sub>​2</​sub>​. CO<​sub>​2</​sub>​ is let thaw while headspace is pumped. The break seal is then cracked and processed like a CO<​sub>​2</​sub>​ sample, by passing through a dry ice-ethanol trap into a liquid N<​sub>​2</​sub>​ trap on the vacuum line. 
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 +CO2 that was isolated from water using dry ice-ethanol traps is collected in a dried break seal. Drying can be done by heating break seal with a flame while pumping on the vacuum line. After sealing and disconnecting the break seal the hot tip is dipped into the liquid N<​sub>​2</​sub>​ dewar to quickly cool it, avoiding exposure of CO<​sub>​2</​sub>​ to high temperature. After extraction, vacuum line is likely to contain some water and must be dried thoroughly (I do it with a low oxygen flame) before using it again; extraction should therefore be done at the end of the work day.
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 +CO<​sub>​2</​sub>​ is not likely to be clean enough to measure as is, and should undergo cleaning, just like any other sample.
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-(Not yet written) 
 ===== Heated gases preparation ===== ===== Heated gases preparation =====
  
recipes.1272625701.txt.gz · Last modified: 2012/04/04 09:08 (external edit)