Analysis of Sizes and Shapes of Particles Using Microscope
Introduction:
There are many ways to analyse the shapes and sizes of the particles. In many pharmaceutically relevant cases, particles deviate from circularity and sphericity. Thus, the use of a single equivalent diameter measurement may be inappropriate. It is more appropriate to use the concept of characterizing a particle using more than one dimension. For example, a powder consisting of monosized fibrous particles would appear to have a wider distribution according to statistical diameter measurement. Besides, the use of equivalent diameter may be misleading as the shape of particle may be acicular. Therefore, by returning to the concept of characterizing particle using more than one dimension, we can obtain the breadth of the fibre, di and the fibre length could be measured using the a projected circle circumscribed around the fibre, dc . The ratio of di / dc can be used as a simple shape factor to provide information about the circularity of a particle. The ratio of di / dc is 1 for a circle and diminish as the particle becomes more acicular.
To measure the particle size, we can use many different methods based on different criteria, which include size range of analysis, wet or dry methods, manual or automatic methods, and speed of analysis. The examples of the particle size analysis methods include sieve method, coulter counter, laser light scattering method, dynamic light scattering method, sedimentation method and lastly,microscope method. In this experiment, we will do the size and shape analysis using the microscope method, which is by observing the sand particles under the microscope.
Objective :
To observe and analyse the sizes and shapes of different types of sand samples by using a microscope.
Procedure:
By using the microscope, 5 samples of different sizes of sand are observed in terms of their sizes and shapes. First and foremost, a small amount of sand is put on the microscope slide. Then the microscope is transferred to the stage on the microscope. Then it is observed under the microscope. The samples observed are drawn. The steps are repeated in the other four samples of sand.
Observation:
Discussion :
Microscope method is an excellent technique in the development of analysing particle size and shape. We can directly look at the particles to determine the presence of agglomeration. This technique increase the manufacturing efficiency and improve product performance. By the way, use of microscopy and image analysis can be considered as the most reliable technique to characterize particle shape, size and volume distribution. From this practical, it is found that the overall shape of the sand is asymmetrical and oval. One of the methods used to measure a particle is the projected area diameter which is measured based on the equivalent area to that of projected image of that particle. Another method is the projected perimeter diameter which is based on the circle having the same perimeter as the particle. Both of the methods are non-dependent on the 3 dimensional shape of particle (orientation). They only consider the 2 dimensions of the particle, which is inaccurate for unsymmetrical particle. During the experiment, we put different types of sand on slide to be directly observed them using a light microscope. The specimen prepared for light microscopy must be adequately dispersed on the slide to avoid analysis of agglomerated particles. Principle of this measurement is that the size analysis is carried out on two-dimensional image of particles which are generally assumed to be randomly oriented in 3-dimensional and they are carried out accepting that they are viewed in their most stable orientation. Feret’s and Martin’s diameter that considering the orientation of the particles is one of the methods to measure the diameter of particle. Feret’s diameter is the mean distance between two parallel tangents to the projected particle perimeter while Martin’s diameter is the mean chord length of the projected particle perimeter. The best statistical method is Feret’s and Martin’s diameter because both of them use statistical diameter which are the average over many different orientations to produce a mean value for each particle diameter. Since we need to access the three-dimensional image of particle, we use the electron microscope that considering the orientation and shape of the image. However, microscope method is slow and need elaborate sample preparation.
Questions :
1. Explain briefly on the various statistical methods that can be used to measure
the diameter of a particle.
Most powders contain particles of a range of different equivalent diameters. To define a size distribution or compare the characteristics of two or more powders consisting of particles with different diameters, the size distribution can be broken down into size ranges and presented in the form of histogram. A histogram allows different particle size distributions to be compared. There are generally three methods used to measure the diameters of the particles. First, we can represent our data in a normally distribution which is fully symmetrical about the central value. The peak frequency value is known as the mode that separates the normal curve into two identical halves. Besides, there are also skewed distribution as a statistical method to measure the diameter of the particle as not all particle populations are characterized by normal distribution curve. There are two types of skewed curve, which are positively skewed and negatively skewed. Positively skewed curve is the curve such that the elongated tail towards higher size ranges while negatively skewed curve is the reverse case. In some size distribution, there may be more than one mode occurs. We can represent the data in a bimodal frequency distribution.
2. State the best scientific method for each sample that you used.
For 800 micrometer sand particles, sieve method is the best scientific method. The range of analysis for sieve method is within 45 to 1000 micrometer. Besides, sieve method is cheaper and readily usable for large particles. For other particles, microscope method is preferred due to their small size range. Besides, microscope method can be used to detect the presence of agglomeration and their two-dimensional images are analyzed according to the desired equivalent diameter which is Feret’s diameter,dF and Martin’s diameter,dm.
Conclusion:
In conclusion, microscope method is a size analysis method carried out on the two-dimensional images of particles which are generally assumed to be randomly oriented in three-dimensions but they must be in most stable orientation. It can be used to measure the 2-D images according to the desired equivalent diameters including projected area diameter, da; projected perimeter diameter, dp; Feret’s diameter, dF and Martin’s diameter, dM.
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