[jones123]

Hello,

In class we're currently studying sample preparation methods, but I am struggeling with the alkaline fusion method. Suppose, for example, you want to determine the Ti concentration in Egyptian black sand sediments (containing quartz, magnetite and rutile) by using ICP-MS, which means the used sample should be a solution, and sample homogenisation by the alkaline fusion method. Then I have two questions:

1) What actually happens by performing an alkaline fusion method? Why is this option better than just dissolving the sediment with acid?
2) The outcome is some kind of 'glass bead'. How should this by transformed into a soluble mixture suitable for ICP-MS?

Thanks in advance!

[Arkcon]

I don't know if an expert will be by soon with personal insights, but I would start with this article here:  http://www.sciencedirect.com/science/article/pii/S0009254197000983  It's comparing various methods of digesting geological samples before testing for platinum metals by ISP.  Not exactly what you're asking, but the discussion sections will likely mention other, similar studies on the topic.

[Borek]

From what I remember being told during lectures 30 years ago, whether you use acid or base is just a matter of which approach gives results. Some things dissolve in acid, other things dissolve in alkali.

[ScholasticSpastic]

Alkaline fusion is a useful alternative to HF for digestion of samples containing silicates (in this case, your Egyptian black sand).  The "alkaline" in its name does not refer to pH, but rather to the fact that the fusion flux often contains alkali or alkaline Earth metals.  Some fluxes are considered acidic or basic as a function of which matrices are best taken into the flux.

Depending on the flux used, the pellet can be dropped into your nitric acid solution either molten or after having solidified and will dissolve.  It is then possible to do elemental analysis on the resulting sample solution. 

Essentially, you're making a low-melting-point pool of a molten water-soluble glass-like substance in a platinum crucible.  If you're looking for alkali or alkaline Earth metals, it is probably not the best method to use.  I read one account which involved elemental analysis of cores taken from the sea floor aboard a survey vessel.  Flux fusion was preferred to HF digestion of these samples because spilling molten glass in high seas was regarded as much less hazardous than spilling HF.