Determination of particle size, particle size distribution using the microscopic method

Aim: Determination of particle size, particle size distribution using microscopic method.

Requirements:

(a) Chemicals: Any powder sample or diluted suspension.

(b) Glassware and apparatus: Microscope, eye-piece micrometer scale, standard stage micrometer scale, glass slide, cover slips etc.

Theory:

The need for particle size control in the manufacture of pharmaceuticals is becoming increasingly apparent for the solubility profiling. Precise particle size control helps in the development of novel drug delivery platforms. Thus, the need for highly reproducible particle size assessment techniques has grown significantly in the past decade.

Microscopy method is suitable for counting and characterizing particles of 1 µm and greater. With the increased resolving power of the microscope particles smaller than 1 µm can be detected and characterized. Microscopy method is useful for characterizing particles that are not spherical.

A microscope with sufficient magnification allows adequate characterization of the smallest particles to be classified in the sample under examination. All the elements of the optical system should be aligned and focused in accordance with the recommendations of the equipment manufacturer. Critical axial alignment is essential. Before use, it must be ensured that the microscope is stable and is placed on a surface protected from vibration.

The magnification and numerical aperture must be sufficiently high to allow adequate resolution of the images of the particles. The actual magnification should be determined using a calibrated stage micrometer to calibrate an ocular micrometer.

If the magnification is such that the image of the particle is at least 10 ocular divisions, errors can be minimized. Each objective should be calibrated separately.

To calibrate the ocular scale, the stage micrometer and the ocular scale should be aligned in order to enable a precise determination of the distance between ocular stage divisions. The use of different magnifications may be necessary to characterize materials of varying particle size in a sample.

Count all the particles having a maximum dimension greater than the prescribed size limit. For spherical particles define the size by diameter and for irregular particles, by the definitions of the different types of diameter stated in the Standard. In case of irregularly shaped particles the homogeneity of the powder must be checked using appropriate magnification.

Commonly used descriptions of shape are acicular, columnar, equant flake, lath, plate etc. The interest in particle size measurements is high particularly in view of FDA trends toward recommending more thorough descriptions of particle size distributions in submissions in which the emphasis of a drug product claim is based in a tightly controlled particle size.

Procedure:

1. Clean the stage, the light reflecting mirror, the eye-piece of the microscope, standard stage micrometer scale as well as slide, properly.

2. Adjust light for maximum intensity and mount standard stage micrometer scale on the stage. Fit eye-piece scale in eyepiece of microscope for its calibration

3. Calibrate eyepiece scale using standard stage micrometer scale as described under calculations and find relation of one division of eyepiece scale with respect to micrometer unit.

4. Remove standard stage micrometer scale from stage of microscope.

5. Sprinkle small amount of given powder on a lean slide. (Note or suspension is used, just mount a drop of sample on the slide and cover it with cover slip)

6. Place the slide on the stage of microscope and rotate it for better resolution.

7. Observe and count minimum of 300-500 particles for their size

Precautions:

1 .Use a good microscope.

2. Observe under combination of 10x eyepiece and 45x objectives.

3. Calibrate eyepiece scale properly.

4. Dilute suspension to avoid lumps or aggregation.

5. Avoid damage to standard stage micrometer.

6. Avoid entrapment of air bubble if cover slip is used