SAFETY IN FUSION SOLUTION
How some ICP/AA solution preparations by fusion may go wrong
While fusion is most commonly used for preparing solid bead samples for XRF analysis, there is a growing effort to leverage the speed and thoroughness of fusion to prepare liquid solutions.
In many cases, the hot melt produced by the fusion reaction is poured directly into a dilute acid, where the thermal shock and a swirling motion quickly dissolve the solids. There are, however, specific conditions to be aware of where such a preparation can create a violent burst, possibly projecting liquids.
Based on our experience, such bursts are in fact very rare and occur in specific conditions only, due to human error, or when the fusion parameters are grossly outside of their normal range. It is important to know and avoid those conditions, with any kind of fusion machine, or when preparing fusions by hand.
Even within controlled procedures, fusion operators shall always wear protective gear:
- face shield (or goggles)
- lab coat
The vapor effect
The actual mechanisms that generate the violent bursts are not completely understood, but they all seem to have one point in common: the production of large amounts of fumes/gases by the fusion mass, right before pouring. These gases can come from three sources: the halogenated non-wetting agents (NWA), the flux, and the sample itself.
Our understanding is that when the red-hot melt produces such fumes at a high rate while being poured, these gases form an insulating layer around the melt as it plunges into the aqueous solution. This gas layer prevents the direct contact of the cold solution with the hot melt. Then, this vapor layer shrinks and collapses due to the rapid cooling, and creates an initial burst at the interface between the cold acid and the hot melt. If this effect is strong enough, it disrupts the shape of the hot melt and dramatically increases its contact surface with the cold solution. In a cascading effect, this evaporates the solution so quickly that it produces a violent reaction.
In the absence of an insulating vapor layer at the hot/cold interface, the hot melt immediately interacts with the surface of the cold solution, without penetrating deeper into the aqueous solution; so, there would be a very limited risk of bursting, just a harmless fizzy heat transfer.
The Non-Wetting Agent (NWA)
The non-wetting (or “releasing”) agent is typically a halogen compound; the most common are LiBr and LiI. At high temperatures, like those encountered in a fusion preparation, those compounds may start to decompose, and the halogen would then be emitted as a gas. This gas production rate is approximately proportional to the amount of NWA used, and since analysts are using larger amounts of NWA when preparing solutions, one could end up with conditions that are favorable to a violent burst upon pouring, if the NWA amounts are excessive.
Our guideline: Never use any more than ~0.25 g of LiBr per crucible.
The flux and other additives
Similarly, some fluxes and other chemicals can also evaporate. In particular, sodium-based fluxes and additives (like sodium borate and sodium carbonate) have a lower boiling point than their lithium counterparts. So, special care must be taken when using sodium compounds.
Our guideline: Fusions using sodium-based flux should not be directly poured into an aqueous solution. Let the hot melt cool down in the crucible and then dissolve the solid into the dilute acid.
Most traditional samples do not evaporate at all during fusion: glass, cement and oxides are generally very stable in a fusion procedure.
Problems may arise when using large amounts of samples such as those rich in alkalis and/or halogens. Examples include: chlorides and bromides, potash, and some plant ashes.
Our guideline: Never go higher than a 1 : 5 ratio (sample : lithium flux) with such samples.
Things to keep in mind:
- Wear appropriate protective equipment: face shield, lab coat and gloves
- Use an adequately low fusion temperature for your ingredients (flux, sample and additives), to prevent the mixture from fuming
- Beware of the quantity of sample and NWA used
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