Preparation

When analysis is to be performed on small specimens representative of a large amount of sample material, this material must first be homogenized. Furthermore, when a trace analysis is to be performed the sample matrix should be separated. In general, these operations are more difficult for solid than for liquids, and more difficult for inorganic than for biological materials.

Solid bulk materials can be fragmented and the small pieces then be ground down in a mill or mortar. The fine powder thus prepared is put in a solution of water and ethanol. A fairly homogeneous suspension can be achieved by shaking, stirring or ultrasonication. Aliquots can be used as specimens for TXRF. The solid materials can also be dissolved. Different inorganic and organic solvents are suited for that purpose.

Biomaterials can be subjected to various methods of sample preparation. Such methods are chosen to decompose the sample matrix and transform it into a clear solution.

Samples can be digested in either an open or a closed vessel. The material is usually placed into a Teflon or quartz vessel and a strong acid or mixture of acid is added. Usually HNO₃ is chosen and also H₂O₂, HF and HCl. Percloric and sulfuric acid are not recommended for TXRF since they do not evaporate afterwards. In the case of closed vessels, microwave assisted digestions is recommended.

In many cases the resulting solutions after mineralization of the samples have to be concentrated. First of all a liquid can be concentrated by evaporation at high temperatures or by freeze drying. Another simple method is based on volatilization of the matrix, where a chemical reaction is conducted whereby the matrix evaporates as a gaseous compound.

The extraction of traces via different separating phases (liquid-liquid extraction) has also been successfully performed. A more generally applicable technique is the separation of traces using a chelating agent. Chelates are filtered or extracted from the matrix solution and re-dissolved in minute volumes of mineral acids.

Due to the high absolute sensitivity of the method, it must be verified that the concentration of impurities in the used reagents does not introduce additional contamination to the sample. Even when using high purity reagents, blank samples reproducing all the steps in sample preparation shall be prepared and measured.