The detection limit of XRF would be to high to analyze As at least in plant samples. Why dont you just do a wet digestion with percloric acid (provided you have the safety measures in your lab). But als a classical aqua regia extraction give you nearly the total As content.

why you don't think for wet digestion method? if you are using AAS, then it works fine with that. I have been using it so long and it gives better data than microwave digestion. You can check the following paper;

Frank, A. 1976. Automated wet ashing and multi-metal determination in biological materials by atomic absorption spectrometry. Fresenius Zeitschrift für Analytische Chemie 279, 101-102.or our paper

Article Influence of silicon on arsenic uptake and toxicity in lettuce

Hello Beste:

Fusion approach would be great for determining majors and minors in geological material, etc..  However, for determining traces, especially for volatile elements such as As in plant material, etc.., I would prefer microwave, or simple hot-plate digestion.  Since you already have a microwave system, I think, the best direction you could go is to optimise the microwave digestion conditions. Following are few points you could keep in mind:

Irrespective of the aggressiveness of the conditions you use, you cannot beat the chemical or solubility equilibria.  The insoluble residue is most likely silica (plant material contains high silica).  Even if the sample is totally decomposed, silica can precipitate. 

1. In spite of the presence of un-dissolved silica, most of the analytes of your interest may be completely extracted into the solution.  Following your microwave digestion, you may centrifuge the solution to get rid of the residue, use a known portion of the extract, dilute and analyse. You may test the recoveries using few reference materials and see. 

2. If you really want everything to be dissolved, you may add a bit of HF (handle with great caution !!) into the cocktail for the microwave digestion, which should dissolve silica.

3. Also, you may reduce the amount of sample to acid ratio, which should encourage complete dissolution.

Best wishes,

Nimal

PS:  If you use HF, please use the minimum amount of HF.  Excess HF can attack, your glassware or glass components in the instrument (e.g. nebulizers, etc..)

Hello Beste,

If what you are asking is how can the fusion method (that makes the glass bead for XRF) can also be used for an AAS solution?  I have used the an alkali-fusion method to prepare high Si sediment solutions for ICP-ES and ICP-MS.  The reason that I used this method is that no matter how much HF that was used in the microwave, not all Si was being digested. After months of trials of optimization it was decided to go back to the older fusion method (this is how geologic samples were put into solution decades ago).  

I have no experience with the particular fusion instrument that you reference (Claisse) but use one from another company.  The theories are the same, with heat the alkali solution attacks the sample matrix and creates a molten "glass".  If you want an XRF bead or disc this is poured into a mold.  If you want a solution for AAS or ICP the molten glass is poured into a teflon beaker containing usually 10% HNO3.  Do NOT use glass or plastic.    The benefits of this method is that you don't have to deal with the strong acids (conc HNO3, HCl or HF) and that the Si bonds are completely broken.  If Si is of interest or if you do have high Si in your matrix, you will have to add a trace amount of HF, but a very little.  

As you mentioned, some of the problems with this method are the possibility of loosing some of the volatile elements.  You can compensate for this by adjusting the temperature profile and analyzing several reference materials to make sure you are not loosing analyte.  

There are several other pros and cons to this method vs using the conventional microwave digestion.  I was not convinced until I actually worked out this method, but it does work.  If you would like more information I would be happy to answer any questions.  Feel free to send me a private message.

You will have some losses of volatiles in fusion; it is hard to avoid. For high organic samples I prefer dry ashing prior to microwave digestion to eliminate the organic matrix. You can limit the loss of volatiles by controlling your temperature. We ash at 400 C, which leaves some refractory organics present but keeps you from losing most of the volatile elements, and then follow that with microwave digestion using aqua regia.