overview
The preparation of the sample before the particle size test is very important. Most of the problems encountered in the measurement are caused by insufficient sample preparation, such as sample agglomeration, dissolution, floating on the surface or not getting a typical sample, and the result will be wrong. It is therefore important to find effective ways to ensure adequate sample preparation. Once a suitable dispersion technique is found, the procedure needs to be standardized so that two samples can be compared.
typical sampling
When measuring extracted samples, make sure that the samples used are representative. If the sample is extracted from a bottle or container, it is necessary to ensure that the sample is fully mixed. If the sample is powdery, large particles tend to float on the surface of the container, and small particles tend to sink to the bottom. Most samples will have some large particles, and some small particles, but most of them are in the middle of the two extremes. Samples are taken from the surface of the container, and most of the measurements are large particles. If compared with samples taken from the middle of the container, Results will be significantly different.
If samples are stored in containers, the samples should be thoroughly mixed before measurement. Do not shake the container, as this will accelerate particle separation. Instead, hold the container with two hands and roll it gently, changing directions for 20 seconds, even better when the container is half full. Liquid samples stored in containers may also be separated, with large particles sinking to the bottom. To extract a typical sample the sample should be mixed thoroughly. It should be noted that when mixing liquid samples with a magnetic stirrer, due to centrifugal separation, large particles are easy to move out of the container, which easily leads to sample deviation.
Problems that should be paid attention to in dry samples
The first step in measuring a sample for the first time is deciding whether to analyze the sample wet or dry. This is determined by what sample is ultimately used. If samples are to be used or stored in dry form, dry analysis methods are preferred.
Some samples are prone to react with wet dispersants, such as may dissolve or swell when in contact with liquid, so they can only be measured in dry state.
Another consideration is whether the substance can flow freely in a dry state. Good performance is non-stick dry powder samples, which can be fully decomposed in the feeder, while highly viscous substances are easy to stick, causing measurement deviations.
Agglomerated samples only need to be dried in the oven. However, when fine substances are dried in an oven, the sample will be damaged. In order to remove moisture, the oven should be adjusted to the highest temperature, but not higher than the melting point of the sample.
If the oven has a significant effect on the sample, a desiccator can be used. New samples that have not been exposed to moisture in the air work well and often give good results. Hygroscopic samples should be packed into bags as soon as possible to avoid moisture absorption.
Wet samples should pay attention to the problem
The issues considered above are for the analysis of dry samples. More care should be taken when analyzing samples in the wet state because there are more options. The latter part of this chapter is concerned with the issues that should be paid attention to in wet sample preparation.
Dispersant selection and preparation
The first choice is the choice of the suspension medium (dispersant) when measuring wet samples. When analyzing the sample for the first time, it is better to check the dispersion in advance, add the selected dispersant (usually water for the initial measurement) to the beaker with a little sample and observe the result. The sample may be dissolving, this can be observed, if in doubt, the sample can be analyzed and the opacity observed, if a decrease in opacity is observed, the sample is dissolving. It is worth noting if the dispersant itself contains impurities or particles. The issue of safe sample handling after measurement also needs to be considered. Follow local policies and use proper procedures for handling samples and dispersants. Most local regulations prohibit the discharge of hazardous samples and dispersants into water.
Surfactants and Mixing Agents
The addition of surfactants and mixing agents is useful when problems such as sample floating on the surface of the dispersant are encountered, and the next section briefly explains the usefulness of such additions.
Surfactant
Sample preparation is aided by the addition of surfactants, which remove charge effects on the sample that would cause the sample to float or clump. The activator is added by small addition method, the standard is one drop per liter. If too much is added, foam will be produced when stirring or drawing the sample, which may be seen as particles in the system, which will affect the test results.
Ultrasonic use
In addition to the above process, ultrasonic waves can be used to help disperse whether or not it contains surfactants. When mixing samples in the suspension medium, it can be observed with the naked eye whether ultrasound is required. If there is a large amount of agglomeration of particles at the bottom of the beaker, the effect will be very obvious if the beaker containing the sample is dispersed in an ultrasonic tank for two minutes. If desired, ultrasound can also be used when the sample is added to the sample cell, which will prevent reagglomeration.
Notice
Use caution when using sonication on fragile particles, as sonication may detach the particles. If in doubt about the effect before and after the use of ultrasound, it can be observed with a microscope.
