Cosmo-chemistry video

Extracting Be-10 and Al-26 from clean quartz cores is a process that often sounds like magic to many Earth-scientists, as we are usually not familiar with the chemistry tools involved in this process (e.g. chromatography columns).

It usually takes 3 to 6 weeks for a batch of 10 to 20 samples. Here, the process has been accelerated into an 8-minute video. Below you can find a summary of the different steps.

Wet-chemistry for Be-10 and Al-26 analysis

Sample weights and carrier addition
Empty bottles and bottles + samples (clean quartz grain cores) are weighted using a precision balance. Samples are then spiked with solutions containing known amounts of Be-9 and Al-27 (carrier addition). All bottles and containers used during the whole process are labelled using stickers. After this stage, samples are ready to be dissolved in hydrofluoric acid.

Addition of hydrofluoric acid
Spiked samples are dissolved using concentrated hydrofluoric acid (HF) in high-density polyethylene (HDPE) bottles. It is important to use narrow-mouth HDPE bottles to avoid HF leaks. The HF is added using a special pump designed to minimize the risk of spills and then transferred to the shaker table in the small fume-hood.

Dissolution (fume-hood) [the video starts here]
A shaker table is used to accelerate the dissolution process. Although HF is contained in the bottles, this process is carried out in a fume-hood to minimize the risk of releasing HF vapour in the lab. After 48 hours, the samples are dissolved and ready for chloride conversion.

Chloride conversion (fume hood)
Dissolved samples are transferred to Polytetrafluoroethylene (PTFE) beakers on a hotplate in a fume-hood. PTFE is the only heat-resistant material that can contain hydrofluoric acid (HF). The HF is evaporated and substituted with hydrochloric acid (HCl). During this process, all the oxygen (O) and most of the silicon (Si) of the quartz are fumed out. At the end of the chloride conversion, the samples are ready for anion exchange (AX).

Anion exchange
Anion exchange (AX) columns are used to remove the iron (Fe) from the solutions after chloride conversion. The samples are loaded in the AX columns and eluted using hydrochloric acid (HCl). Columns are stored in ultrapure water and conditioned before loading new samples. The conditioning process elutes the iron accumulated from previous samples. Iron-free samples are then transferred to glass beakers for sulphate conversion.

Sulphate conversion (fume-hood)
Iron-free samples dissolved in hydrochloric acid are evaporated in borosilicate glass beakers and re-dissolved in sulphuric acid (H2SO4). At the end of the sulphate conversion, the samples are ready for cation exchange (CX).

Cation exchange
Cation exchange columns are conditioned before loading the samples to avoid cross-contamination. Samples are loaded in the columns. Titanium fraction is eluted using sulphuric acid (H2SO4). Beryllium fraction is eluted using diluted hydrochloric acid (HCl). Aluminium fraction is eluted using strong hydrochloric acid (HCl). Columns are stored in ultrapure water. Beryllium, and sometimes aluminium fractions, are then precipitated as hydroxides.

Precipitation of hydroxides
Beryllium and aluminium are precipitated as hydroxides. The acid solution is taken to a slightly alkaline pH by adding ammonia (HNO4·OH). Two drops of methyl red are added to the acid solution to see a change in the colour of the solution when the acid-alkaline transition happens. A balance is used to monitor the amount of ammonia added. The final pH is checked using pH paper. Solutions containing suspended hydroxides are then centrifuged.

Centrifuges
Solutions containing suspended hydroxides are centrifuged to separate the solid from the supernatant liquid and then transferred to quartz crucibles for evaporation. The centrifuges are also used after chloride and sulphate conversions to avoid loading solid residues into the anion and cation exchange columns.

Evaporation of hydroxides (fume-hood)
Hydroxides are transferred to quartz crucibles and dried down on a hot plate to remove any water. When they are completely dry, the crucibles in the quartz tray are covered with quartz lids and transferred to the furnace.

Oxide conversion (furnace)
Dried hydroxides in quartz crucibles are heated up to 900 Celsius degrees. This triggers a polymorphic transition from hydroxides to oxides (e.g. Be(OH)2 → BeO + H2O). After this, the samples are ready to be mixed and pressed into the AMS targets.

Target pressing
Oxides are mixed with metal powder in the quartz crucibles and pressed into the copper cathodes that will be loaded into the AMS source to measure the cosmogenic/stable isotopic ratios.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: