Maybe, the residues are silicates. You have to add HF for dissolving them. The problem with undissoved residues is that they can contain metals in the crystalline structure. So you can have a negatove bias in the analysis of total metals. But you have also to think that these metals in the crystals are not bioavailable and should not contribute to pollution or effects to living organisms

I think it may occur - the white precipitation is probably SiO2.  It is rather soluble in bases. I found quickly one paper, in which they precipitated SiO2 using  ca 10% sulphuric acid.

Based on photo attached, I am afraid, that, as you think your digestion was not complete - the supernatant is too yellow in my opinion - it should be incolorous. You can try to see, if longer digestion change this state (on some samples).  I have not sufficient knowledge to say, if such precipitated silica is dangerous for AAS or interferes with results- may be you should consult it with producer? Incompletely digestion may result in lower quantity of studied elements in resulting supernatant. Yellow colour clearly difficults use of colorimetric methods, for example determination of P with some Vanadium compounds.

Moreover, if it is soil, I think some remnants of mineral parts should be remaining undissolved - main part of my answer referred mainly to digested organic materials or similar substances.

Best regards.

I agree with prof Stefano Polesello

i go with prof Polesello, but the issue of using HF is the reaction vessel to use during digestion, because glass it self is majorly silica which can easily dissolve in HF.

It is normal situation. I perform microwave digestion of soil samples using Aqua Regia. Neither nitric acid nor Aqua Regia will digest soil completely. You will only get "pseudo total" content of elements. If you want to determine real totals you have to use hydrofluoric acid (HF). The precipitation visible on the photo are most probably the aluminiumsilicate complexes, which are insoluble. You should either filter or centrifuge the supernatant prior to analysis. You cannot say that your digestion is incomplete as this is normal situation. Best way to check if your procedure of digestion is correct is to use Certified Reference Material which has known content of elements determined with the method you are using. There are plenty of CRMs for soil available on the market.  Also, please refer to the commonly available literature. There are plenty of publication regarding analytical procedures for digestion and determination of elements in soil with the use of wide range of techniques.  

We can read on your vials that you put soil with nitric acid: nitric acid does not dissolve soils. You can safely use your supernatant and analyse it after a proper dilution, but various portions of elements (probably about 100% for silicon, ~0% for Zn) remain in the undigested solid (this is not a precipitate).

Yes.

 . I perform microwave digestion of  serpentine soil samples using 8 ml nitric acid, but i also used and 2 ml H2O2 ( hydrogen peroxide) and i had completed digestion.  I think that precipitation are most probably the aluminium silicate complexes, and a fraction of metals  exists in them . Like Rafael i think check if your procedure  is correct using Certified Reference Sample. 

I think soil samples can be completely digested using a mixture of HNO3 H2O2 an HF. Infact I used 6mL HNO3, 2 mL H2O2 and 1 mL of HF for digesting 0.05g soil in microwave digestor

You can safely use your supernatants in AAS, however you must be aware that the concentrations of elements obtained are not total concentrations but concentrations obtained with your method. Thus it depens on your research question (and the elements you are interested in) if you want to use these results or not. For example if you are interested in plant available concentrations of elements these results should be ok. In contrast if you want to show the total concentrations of elements in soil (elements like Ge that is often associated with Si) then you should consider to use HF.

Dear Oliver, I am interested in total concentration of arsenic in soil samples. Is it appropriate to use for this case?

According to Hudson-Edwards et al (2004) (Trends in Analytical Chemistry, Vol. 23, No. 10, the use of aqua regia  indeed is suitable as a proxy for total concentrations of As in soils and sediments (with a recovery of 70-100%). However it depends on the constitution of soil used. So, in my opinion you have three options: Firstly, you can use your method and everything will be fine if you want to compare concentrations within your experiment. However, there could be problems if you want to compare your results with that of other researchers. Second, you could digest and measure a suitable soil sample with certified concentrations to check  accuracy. Third, you could change to an digestion with HNO3 and HF.       

Using of HF is widely practiced to dissolve settled SiO2. If u r analysing ur samples by ICP-MS, try to avoid HF addition.otherwise it will cause damage to plasma.