Sticky, tough or elastic samples are generally difficult to pulverize. Cheese, for example, can be ground with a knife mill but only to a final fineness of about 1 – 2 mm which is not small enough for most analysis techniques. Sweets, on the other hand, often consist of various components like elastic foamy sugar with a sticky, liquid filling.
If these types of sample are not embrittled before grinding, they tend to clog the mill. Cryogenic treatment, however, improves the breaking properties of cheese or candy so that even elastic materials can be successfully pulverized.
A solid sample material should always be sufficiently prepared by size
reduction and homogenization before it is subjected to chemical or physical analysis. Care should be taken that the analysis sample fully represents the original material and that the sample preparation process is carried out reproducibly. Only then are meaningful results guaranteed. Most sample materials can be reduced to the required analytical fineness at room temperature by choosing a mill with a suitable size reduction principle (impact, pressure, friction, shearing, cutting).
From cell disruption to homogenization and pulverization of a great variety of biological samples
Biological samples exist in all shapes and sizes: hard bones, tough and fibrous plants, tough and viscous sputum, soft muscles, tumor or liver tissue. Not to mention the millions of cells such as yeast, bacteria or algae, which have to be disrupted for applications such as DNA or RNA isolation or protein extraction. For research in genomics, transcriptomics or metabolomics, all kinds of biological samples are prepared. Sample preparation is the first step of every analytical process. Retsch offers a range of mills and grinders for easy and reproducible pulverization of solid sample materials some of which are also suitable for cell disruption and homogenization of biological sample materials.
In the beverage industry plastic bottles – or rather their performs - are analyzed for their acetaldehyde content to prevent any health risk for consumers. The acetaldehyde must be extracted from the preforms prior to analysis. To ensure correct results, the bottles first have to be crushed and homogenized by laboratory mills. Care must be taken that the volatile substance acetaldehyde does not escape during the process due to heat build-up inside the grinding chamber. To preserve the acetaldehyde, part of the sample preparation is carried out under cryogenic conditions.
How to turn a laboratory sample into a representative part sample with homogeneous analytical fineness -
Food occurs in a great variety of consistencies and is often inhomogeneous. Food testing labs require representative samples to produce meaningful and reproducible analysis results. Therefore, food samples must be homogenized and pulverized to the required analytical fineness, ideally with as little time and effort as possible. Furthermore, reliable analytical results can only be obtained if the entire sample preparation process is carried out reproducibly.
Plastic is an inherent part of our everyday lives; it is used in a huge variety of things such as, for example, packaging, furniture, clothing or electronic devices. Though the utility of the material is undoubted, consumers are increasingly unsettled by recurring news about hazardous substances detected in plastics. Substances such as plasticizers, which are not firmly bound in the material, are absorbed via the skin and can influence the hormonal balance. Plasticizers contained in food packaging, for example, penetrate into the food and thus into the human body when the food is eaten. Plasticizers in toys are a particularly serious problem; children tend to take toys into their mouths thus absorbing the dangerous chemicals. Equally hazardous are polycyclic aromatic hydrocarbons (PAH). The family of PAH comprises more than 100 compounds most of which have been found to be carcinogenic.
Outdoor activities such as hiking, biking or climbing are very popular. As a result, the textile industry offers a huge variety of functional outdoor clothing made from synthetic high-tech materials which are wind- and waterproof as well as breathable. In 2013 Greenpeace published a study proving that hazardous substances such as perfluorinated and polyfluorinated chemicals (PFC) were used in weatherproof clothing as protection against water or dirt.
This article describes the sample preparation process with cutting mills and cryogenic mills of high-tech fibres.
Reliable and accurate analysis results can only be guaranteed by reproducible sample preparation. This consists of transforming a laboratory sample into a representative part sample with homogeneous analytical fineness. Retsch offers a comprehensive range of the most modern mills and crushers for coarse, fine and ultra-fine size reduction of almost any material. The product range also comprises a wide choice of grinding tools and accessories which helps to ensure contamination-free preparation of a great variety of sample materials. The selection of the correct grinding tool depends on the sample material and the subsequent method of analysis. Different grinding tools have different characteristics, such as required energy input, hardness or wear-resistance.
Considerable care must be taken when analyzing a sample like rice in order to achieve an accurate result. The major source of error when analyzing a bulk material comes not from the analytical measurement itself, but from the sample handling, i.e., sampling, sample division, grinding, digestion, etc.
In the analysis of solid material, the popular adage that “bigger is better” certainly does not apply. The goal is to produce particles that are sufficiently small to satisfy the requirements of the analysis while ensuring that the final sample accurately represents the original material. The “particles” of interest to the analyst generally range from 10 µm to 2mm. Additionally there are many application, where even finer sizes are needed. One example are active ingredients, where it is necessary to grind in the submicron range. Finally for DNA or RNA extraction mechanical cell lysis is well-established.
Materials differ widely in their composition and physical properties. Hence, there are many different grinding principles that can be applied, and this, together with other variables such as initial feed or “lump” size, fineness needed and amount of sample available, results in a wide range of models available to the researcher.
A variety of methods can be used to analyze solid materials. What they all have in common is the necessity to use a representative, homogeneous analysis sample which needs to have a particular fineness, depending on the analytical method used. The size reduction and homogenization of solids is usually carried out with laboratory crushers and grinders.
A faultless and comparable analysis is closely linked to an accurate sample handling. Only a sample representative of the initial material can provide meaningful analysis results. Rotating dividers and rotary tube dividers are an important means to ensure the representativeness of a sample and thus the reproducibility of the analysis. Correct sample handling consequently minimizes the probability of a production stop due to incorrect analysis results. Thus correct sample handling is the key to effective quality control.
Some sample materials have properties which make size reduction at ambient temperature impossible. If, for example, very elastic materials need to be ground or volatile components have to be preserved for further analysis, it is essential to carry out cryogenic grinding. The use of liquid nitrogen helps to embrittle the sample, thus improving its breaking properties, and preventing volatile substances from escaping due to the frictional heat produced by the grinding process.
The detection of illegal drugs and pharmaceuticals plays a role in various fields, for example in forensic science, road traffic accidents, in competitive sports or at the workplace. Chemical substances can be detected in blood, saliva, urine and in hair. Hair has the great advantage of storing the substances for a long period, which means that detection is still possible several months after consumption of the drug. In addition to the detection of drugs, hair samples are also used for DNA analysis as well as for the analysis of heavy metals and minerals.
Polycyclic aromatic hydrocarbons, short PAH, are usually a by-product of combustion and can be found, for example, in cigarette smoke or oil-based products. Mineral oil containing PAH is often used in rubber products as plasticizer, especially in black-coloured products such as car tyres, rubber grips of tools or rubber shoe soles. It was discovered that some polycyclic aromatic hydrocarbons are carcinogenic, so that maximum permissible values have been determined for the concentration of PAH in consumer products.
Why Size Reduction? A reliable and accurate analysis can only be guaranteed by reproducible sample preparation. The "Art of Milling" describes the process of turning a laboratory sample into a representative part sample with homogeneous analytical fineness.