sample preparation

What is important?

Before comminuting, it is important to determine whether the sample material can be processed directly or whether pretreatment is necessary. Factors such as moisture, agglomerations, uneven distribution, or foreign matter can hinder the process and lead to inaccurate results.

The required sample quantity must also be considered: How much material is needed for the subsequent analysis, and how large is the original sample? If necessary, a representative subsample must be taken whose composition corresponds to the total sample.

Drying

Comminution of moist or wet sample material—for example, in jaw crushers, rotor mills, or cutting mills—is often difficult. Moisture can quickly lead to blockages in the grinding chamber, cause ring and bottom sieves to swell, and ultimately block the device. This not only causes material loss but also increases cleaning effort.

Exceptions include processes such as colloidal grinding, in which liquids are deliberately added to ball mills, and the homogenization of fresh fruit and vegetables, where hardly any material is lost.

In most cases, however, it is necessary to dry moist samples before grinding. When selecting a suitable drying method, in addition to the boiling point, potential hazards such as reactivity, dust explosions, or the temperature sensitivity of certain substances must be considered. Appropriate safety precautions are particularly essential for substances such as polychlorinated biphenyls (PCBs) or dioxins.

Embrittlement with liquid nitrogen or dry ice

Cooling the material to be ground often improves its fracture behavior. Intensive, direct cooling is therefore required for the pre-grinding and fine grinding of temperature-sensitive samples, such as many plastics. One option is to embrittle the sample material in liquid nitrogen (N₂, LN₂) before grinding. Alternatively, it can be cooled with dry ice, so that the material becomes brittle at low temperatures and is easier to grind.Cryogenic grinding)

Cooling methods are also used when volatile components in the sample need to be preserved. The reason for this is the low temperature at which, for example, moisture in the sample material solidifies into ice and cannot escape during the grinding process. After grinding, the frozen water thaws again at room temperature. Therefore, it is always important to check whether the chosen method leaves the composition of the sample unchanged.