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Write in experimental protocols, e.g.:
- CO2-water equilibration
- heated gas preparation
Recipe for heated gases:
Heated gases are CO2 samples whose isotopes are randomly distributed among all isotopologues and therefore have Δ47 value of zero. This is obtained by heating the CO2 to 1000°C.
Seal CO2 in quartz break seals (ampules) that are roughly 20cm long and 6mm OD. Heat break seals in a furnace at 1000°C for about 2 hours. Remove from the furnace while still at 1000°C (furnace will be glowing red). This will allow fast quenching to room temperature and would prevent re-equilibration at an intermediate temperature. Use long tongs to remove break seals from furnace, place them on a heat resistant surface. DO NOT TOUCH (they are hot). The break seals cool down sufficiently to hold them after 5-10 minutes. With the correct type of quartz heated CO2 can be stored at room temperature (I tested up to a year but typically do not store it for more than a month). Measure them like a sample (including GC cleaning).
To make a heated gas line: use a range of δ13C and δ18O of the CO2 such that δ47 (which is about δ45+δ46) would bracket the range of your samples. This can be done either by having 2 types of CO2 and creating mixtures between them (remember that the mixing must be done before heating, as Δ47 does not mix linearly), or by recycling samples that are large enough (namely recollect the CO2 left in the bellows after the run and use it to make a heated gas). In both cases note that δ18O in heated gas tends to be a few ‰ more depleted than the original gas, so in order to bracket your enriched samples (most marine carbonates) you would need CO2 with relatively enriched δ18O (this can be done with enriched δ13C but it is difficult to fine a CO2 cylinder with enriched δ13C). Probably the easiest way is to use CO2 from a cylinder as the depleted end (but try not to use the common CO2 that has δ13C of -30 to -50‰ as the δ47 would be lighter than any likely sample). Then equilibrate a large aliquot (I do it in 500ml batches at atmospheric pressure) of that cylinder CO2 with water that is 18O enriched by evaporation.
Types of quartz: Make sure to test your quartz before using it to store heated gases. I tested 2 types of quartz tubes. ¼” tubes purchased from GM Associates were tested for storing heated gases and gas was found to be stable during 1 year. These tubes tend to produce signal in mass 48, that can be removed using the typical GC cleaning. Tubes from Hereaus do not produce signal in mass 48, however, heated gases can be used only if freshly made. After 2 weeks the Δ47 signal is partially lost and after a month the Δ47 value is closer to room temperature equilibrium than to heated gas. It is not recommended to use these tubes.
- sample purification
- phosphoric acid preparation
- synthetic calcite precipitation====== ====== Headline ======