Raman CO₂ calibration methods

So when I get emails about how to do this I won't have to write a response from scratch!

1) High pressure optical cell (HPOC)

The HPOC is just a box with a transparent glass window and some plumbing which delivers ultra-high-purity CO2 from a gas cylinder. Andras Fall and Hector Lamadrid both used this method at Virginia Tech to build calibration curves for the Raman “densimeter,” and there are more details about this method in Hector's paper. More recently, Charlotte DeVitre at Cornell University wrote a paper documenting this method even more thoroughly. Similarly, some people use a stage with a capillary tube instead of the HPOC chamber (I think the USGS lab in Reston VA does this; not to be confused with the sealed capillary tube method described below). The capillary tube can withstand higher pressures and is significantly more user-friendly.

References:

Advantages: probably the most precise and versatile method

Disadvantages: expensive and requires significant expertise to build and manage the plumbing/optical system

2) Sealed capillary tubes

I don’t know much about this method but from what I understand, dry ice (solid CO2) is weighed into a glass tube which is then sealed using an acetylene (or similar) gas torch. I’m told this method works pretty well, but it can be challenging to precisely add the correct amount of dry ice to the tube for the desired density and then seal it without losing a little CO2 vapor.

Advantages: Cheap, fast, and relatively easy to make

Disadvantages: possible limitations regarding precision

3) Synthetic fluid inclusions

Various experimental labs make synthetic fluid inclusions by drilling out small (~1 cm long x ~3 mm diameter) cores from inclusion-free quartz, fracturing the cores by thermal shock, and annealing the quartz with a carbonated solution in a platinum capsule in externally-heated cold-seal bombs. The end product is a polished quartz wafer (i.e. a section from the annealed quartz core) with CO2-bearing fluid inclusions. The inclusions contain an H2O-rich liquid + a CO2-rich vapor, and the density of CO2 in the vapor phase can be calculated from the phase-relations of the H2O-CO2 system.

More details about the synthesis process can be found here: https://doi.org/10.1016/0016-7037(84)90314-4.

Advantages: probably the most convenient method, and not too expensive

Disadvantages: possible limitations regarding precision

4) Secondary standards:

These would just be some CO2-bearing fluid or melt inclusions, which have been analyzed previously using Raman spectroscopy or microthermometry. It’s not ideal, but it should get the job done!

Advantages: fast, cheap, easy