Several Problems in Microwave Digestion of Hydrofluoric Acid and Sulfuric Acid Systems

Hydrofluoric acid matrix         

HF matrixA hydrofluoric acid-resistant system must be needed. In fact, the funding is insufficient, so the atomizer must be PFA . Hydrofluoric acid-resistant systems often have more serious memory effects and are expensive. However, as long as the sample contains Si-related substances, HF is necessary, and certain elements can only be stable in the HF system. Of course, more elements cannot be stabilized at a percentage content of HF matrix under the premise of ppb level concentration.       

There are quite a difference between the conditions of hydrochloric acid and there are not many elements that can stabilize in the HF matrix. Five elements of the first main clan; nine elements of the fourth, fifth and sixth sub clans, plus Mn and Fe; seven elements that intersect between metal and non-metal on the periodic table: B, Al, Si, Ge, Sn, Sb, Te.     

Please note that once a large amount of HF exists, it is difficult to stabilize precious metals and rare earth elements even if hydrochloric acid is added.     

Here is a message: the quick digestion method, using water regia to extract Hg in the soil can easily be accurate, but adding HF is not accurate. Some teachers think that Hg has run away. In fact, I have done an experiment. Even if the acid is rushing to the point where it is near dryness at 120 degrees, the loss of Hg is less than 20%. In fact, I think it is the formation of fluoride precipitation, which leads to a low result.     

Having written this, I must make an explanation: In the semiconductor field or similar applications, due to the extremely low content (there is no absolutely insoluble substance), the container quality is good (the inner wall of the container is closely related to adsorption and precipitation), the instrument is clean (similar to the container), and it is ready to be used (even the formation of precipitation takes time), many cases can be done even when the substrate is unstable. However, once a certain factor is not satisfied, there will definitely be problems with the stability of the solution.     

In addition: HF is necessary when preparing nitrides, such as Si3N4.     

Hydrofluoric acid is very annoying. It is more harmful to people than nitric acid and hydrochloric acid, so don’t use it if it can be used, and don’t use it if it can be used less.

Sulfate matrix     

Putting aside sulfuric acid itself, it is necessary to determine the element content, sulfuric acid is also an important acid for digesting the sample. For example, in the determination of rare earth elements in soil, due to the formation of precipitation of fluoride, it is necessary to use the high boiling point of sulfuric acid to redissolve the rare earth elements.     

The elements marked with the colors in the picture above can be dissolved in sulfuric acid at higher concentrations.① The two elements of Hg and Au cannot exceed 100ppm, and should be stored in a glass container.② As mentioned when writing about the nitrate hydrochloric acid matrix, Ag must not contain trace amounts of Cl.③ F means that a small amount of HF is required to be stable, and HF means that a large amount of HF is required. This is consistent with the nitric acid and hydrochloric acid systems.④ Cl and T represent: using hydrochloric acid and tartaric acid can stabilize sulfuric acid. It can be found that it is still different from the HF system. Even if you add hydrochloric acid to the hydrofluoric acid system, it cannot guarantee the stability of elements such as precious metals.     

Finally, I would like to remind you: ICP-MS test acidity linearity and sample matching as much as possible, and the acidity should not be too large. You can use hydrogen peroxide to reduce acid dependence, and consider organic injection, alkaline solution and other methods.The article is reproduced from the official account " ICPMS enthusiast ".