text: Detailed sample preparation depends on the type of material. Pure standards are most likely to be prepared by chemical synthesis and purification and characterized by determination of remaining impurities. This is often done by commercial producers. Natural matrix CRMs (often shortened to matrix CRMs) contain an analyte or analytes in a natural sample (for, example, lead in fish tissue). These are typically produced by homogenization of a naturally occurring material followed by measurement of each analyte. Due to the difficulty in production and value assignment, these are usually produced by national or transnational metrology institutes like NIST (USA), BAM (Germany), KRISS (Korea) and EC JRC ( European Commission Joint Research Centre).
  For natural materials, homogenization is often critical; natural materials are rarely homogeneous on the scale of grams so  production of a solid natural matrix reference material typically involves processing to a fine powder or paste. Homogenization can have adverse effects, for example on proteins, so producers must take care not to over-process materials. Stability of a certified reference material is also important, so a range of strategies may be used to prepare a reference material that is more stable than the natural material it is prepared from. For example, stabilizing agents such as antioxidants or antimicrobial agents may be added to prevent degradation,  liquids containing certified concentrations of trace metals may have pH adjusted to keep metals in solution, and clinical reference materials may be freeze-dried for long term storage if they can be reconstituted successfully.