Requirements
A minimum 5mg sample is required as samples are run in duplicate and a further 2mg if O is required. Use screw-cap tubes to preserve your sample. If samples are difficult to remove due to electrostatic charge etc, then you'll need at least twice that amount. Samples must be clean (no foreign particles) and dry. If you send samples by post, they should be in plastic containers with sealed lids.
Preparation
Many materials are often inhomogeneous, and an appropriate homogenisation procedure is therefore required.
Note: the reproducibility of results is independent of the sample size but strongly dependent of the sample homogeneity.
Dry and soft materials
Samples of cereals, dry bread, plant tissues, etc: grind the sample in a rotor mill equipped with a 1mm sieve, possibly with trapezoidal holes. According to AOAC 992.03 method, the suitable fineness of grind is that which gives relative standard deviation (RDS) < 2% for 10 successive determinations of nitrogen.
Dry mixtures of hard, soft and tough materials
Samples of rock-containing soil, shale, mining slags, etc: first use a crushing mill to crush the hard particles and then mix them with the rest of the sample; in some cases you'll need to carry out a second grinding process in a ball mill.
Dry or wet soft materials
Meat, sausages, fish meat, etc: grind in a knife mill with liquid nitrogen. Place the sample in the mill, add liquid nitrogen, wait about one minute, add liquid nitrogen again and start the mill. Thanks to the action of liquid nitrogen, the sample is immediately frozen at -195°C and becomes very brittle and fragile. A mild grinding action is then enough for the complete destruction of the texture of the sample, and the heat generated by the mill action won't affect the water content as it will be adsorbed by the nitrogen evaporation.
Rape-seeds and other oil-containing seeds, which become smeary when ground at room temperature, form a homogeneous dry powder. Meat, fresh plant tissues and similar wet products form a homogeneous paste.
When liquid nitrogen isn't available, it's possible to freeze the sample (at least -25°C, minimum 1 hour) and to grind it when deeply frozen.
Materials containing volatile compounds
Sometimes it's important that the material to be ground doesn't lose volatile constituents or take up moisture. Liquid nitrogen, which is added to the container, is quite dry, and all moisture in the sample is immediately frozen and can't escape. When liquid nitrogen is volatilised from the container, the sample is still so cold that no volatile constituents are volatilised. After shaking, the container is allowed to warm up to room temperature unopened so that no atmospheric moisture can condense on the cold sample. The homogenised sample has the same moisture content and the same content of volatile constituents as the original material.
| Tool |
Sample type |
|---|
| Mortar (agate or porcelain) |
Coals, soils |
| Coffee mill |
Cereals, barley, meal |
| Hammer mill |
Textiles, paper |
| Ball mill* |
Solids (coals, cokes) |
| Ball mill + liquid nitrogen |
Hard materials (bones, textiles, hair), wet products (meat, fresh vegetables) |
| Rotor speed mill** |
Cereals, plants, plastics, synthetic resins, soils, animal feeds, drugs, grain, coal, fertilisers |
| Scissors, blades |
Leaves, fibres, plastic products, rubbers |
* The microdismembrator is a very strong, longitudinally shaking ball mill. The materials to be ground are placed into an egg-shaped Teflon container together with 2 to 4 steel balls, which should have different diameters, usually 10mm and 7mm.
** The samples are ground by the impact and shearing stresses from the high-speed rotor and are forced through the sieve by the ultra-centrifugal force. The ground material is collected in a collecting vessel. It's possible to grind up to 0.5-1mm particle size.
