This standard details the procedure for assessing powder particle size distribution through optical microscopy. It outlines comprehensive steps including sample preparation, microscope configuration, magnification choice, particle counting, and computation of size distribution by numerical and mass fractions. It serves as a crucial reference for professionals requiring accurate sizing of particles ranging from sub-micron scale up to approximately 150 microns.
Overview
This standard details the procedure for assessing powder particle size distribution through optical microscopy. It outlines comprehensive steps including sample preparation, microscope configuration, magnification choice, particle counting, and computation of size distribution by numerical and mass fractions. It serves as a crucial reference for professionals requiring accurate sizing of particles ranging from sub-micron scale up to approximately 150 microns.
Audience
Contents
Structure
This section defines the scope of the method and includes essential conversion factors for particle size measurement methods such as sieve, projected, and Stokes diameters. It provides tables with size class ranges, mean sizes, and weighting coefficients, and explains combined sieve and microscope size analysis.
Clarifies key terms including definitions of particle sizes by different measurement principles, the use of shape factor conversion tables, and the categorization of size classes with associated weighting factors.
Details the essential microscope components such as illumination sources, focusing mechanisms, substage condensers, mechanical stages, objective lenses covering the full particle size range, and types of microscopes suitable for the method.
Describes procedures for preparing representative powder samples, including dilution, particle concentration adjustment, and adherence to standards for sample representativeness to ensure accurate microscopic analysis.
Outlines the setup of the microscope including illumination type, diaphragm adjustment, use of filters, and recommendations for monochromatic light when measuring very fine particles.
Explains how to choose appropriate objective lenses and magnifications based on particle size, numerical aperture requirements, and graticule matching to size classes.
Provides guidelines on the dimensions and calibration of graticules, including geometric progression of reference circles and accuracy requirements for grid patterns.
Details the methodology for selecting fields of view and sampling patterns to ensure uniform coverage and representative particle counting across the slide.
Describes the formulas and procedures for determining particle size distribution by number, including particle counts, field areas, and computation of percentages along with standard error evaluation.
Details the approach and formulas for calculating size distribution by weight, including limits on standard error and examples to ensure reliable measurements.
Discusses criteria for reproducibility of particle size data, including control size class definition, standard error limits, and sampling requirements for consistent results.
Summarizes the stepwise procedures for size distribution calculations, integration of sieve and microscope data, and recommendations for size class usage.
Defines the control size class concept, minimum particle counts needed for statistical validity, and size class selection guidelines.
Explains how to maintain constant sample area across different size classes by adjusting field areas and number of fields, ensuring uniform counting conditions.
Provides conversion factors and guidance for correlating particle sizes measured by sieve, projected, and Stokes methods, including considerations for particle shape effects.
Frequently Asked
As per the standard, the smallest detectable particle size is governed by the numerical aperture (NA) and magnification of the objective lens. Typically, the minimum particle size (in microns) is approximately 1.5 divided by the NA, except for apochromatic objectives with NA equal to or exceeding 1.3. Dry objectives use the smaller between the rated NA and 1.0, while oil immersion objectives with refractive index above 1.3 use the rated NA. The smallest recommended size class generally ranges from 1.5 to 2 times the minimum particle size. The minimum total magnification is determined based on objective type and NA. Higher NA objectives allow sizing of finer particles, with oil immersion lenses enabling detection down to about 0.6 microns.
The control size class is identified as the particle size category containing the greatest proportion by particle count, usually the smallest size class. If unknown, the smallest present size class is selected. It is denoted with subscript 'o' indicating mean size, particle count, and sample area. For weight-based analysis, it is defined as the largest size class with over 5% by weight. A minimum of 25 particles must be counted in this class to ensure statistical confidence, with greater counts required if it exceeds 10% by weight. This class forms the basis for standard error calculations and sample area determinations.
The standard mandates counting at least 625 total particles across all size classes with a minimum of 25 particles in the control size class to achieve statistical validity. If the control size class constitutes more than 10% by weight, a larger count is recommended. For the control size class, a minimum of 96 microscope fields should be examined, while other size classes require at least 12 fields, preferably 24. These thresholds ensure that the standard error remains below 2%, guaranteeing reliable and reproducible particle size distributions.
To prevent agglomeration and ensure proper dispersion, powders should be suspended in a viscous liquid medium whenever possible. If such a fluid is unavailable, a more fluid liquid combined with a dispersing agent should be employed, selected based on powder characteristics through experimentation. The particle concentration on the slide must be adjusted so that approximately 3 to 6 particles of the size class under examination appear per field of view, avoiding overcrowding and facilitating accurate counting. Additionally, particles should be distributed evenly in a systematic pattern across the slide to represent the sample uniformly.
Particle size distribution by number is determined by counting particles in each size class using the microscope and expressing each class’s count as a percentage of the total particles counted. For weight (or volume) distribution, particle volumes are computed assuming uniform shape by cubing the diameter, then calculating the volume percentage each size class contributes relative to the total volume. If particle density is consistent, volume percentages correspond to weight percentages. Quality control involves analyzing multiple samples, comparing percentage ranges against acceptable limits, and grouping results if necessary to ensure accuracy.
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