Powders-Lec 2

Question 1

Particle size and distribution

Answer 1

• There are many pharmaceutical methods available to evaluate particle and distribution: microscopy; laser scanning; sieving
• These can be used to characterise the particle system
• See BB for particle sizing results- distribution bell curve

Question 2

particle sizing: laser diffraction- MOST COMMON

Answer 2

• the laser is aimed at the material and scatter pattern is recorded
• For a specific material, the scattering pattern (laser diffraction) of a particle is unique for its size
• The interaction between the particles and light is mainly dependant on: particle shape, size, surface roughness, refractive indices of material and dispersing medium (This means when you measure the size, most of the time is in water so if different we need to account for that)
• Using the sample scattering pattern the particle size and distribution can be calculated
• Materials can be characterised in the range of 0.04 to 2000 microns and dispersion can be made in water, organic liquid as well as air
• Larger angle of lazer diffraction= larger particle size

Question 3

particle sizing

Answer 3

• Techniques such as laser scattering will give you an average particle size and distribution
• What if a specific range is needed?-

Question 4

Particle size control

Answer 4

• For controlled systems, you will require a certain size range
  1) Reduce particle size so that all particles fall below the required size range
  2) separate particles at or below required size range form outsized particles

Question 5

Aim of size separation and methods available

Answer 5

• To classify powders into separate particle size ranges
  1) sieving
  2) fluid classification: sedimentation; centrifugation
  3) elutriation methods
  4) cyclone separation methods

Question 6

1) Sieving

Answer 6

British standard sieves should be

• Uniform circular cross sections
• Strong enough to avoid distortion
• Resistant to chemical action with any material being sifted
• Commonly made of stainless steel (Has to be of a set quality and grade of steel regulated by regulatory bodies)
• Size range: 5-10,000 microns
• Very cost effective

Question 7

standards for powders based on serving

Answer 7

• Standard for powder used pharmaceutically are provided in the BP and pharmaceutical index
• BP- ‘the degree of coarseness or fineness of a powder is differentiated and expressed by reference to nominal mesh aperture size fo the sieves used’- MUST REMEMBER

Question 8

powder grades specified in BP
1= coarsest sieve diameter (microns)
2= sieve diameter through which no more than 40% of powder must pass (microns)

Answer 8

Coarse: 1=1700; 2=355
moderately coarse: 1=710; 2=250 
moderately fine: 1=355; 2= 180 
fine: 1-= 180
very fine: 1=125
-MUST REMEMBER

Question 9

2) sedimentation methods

Answer 9

• Particles of different diameter settle (sediment) at different rates (bigger particles= quickier sedimentation)
• The velocity at which a particle sediments is directly proportional to its diameter
• The bigger the particle the faster it sediments

Question 10

Sedimentation classification

Answer 10

• A chamber containing a suspension of solid particles in a liquid
• At predetermined time intervals, particles less than a certain diameter can be removed by a pump placed at a fixed distance below the surface of the liquid
• Larger particles will drop further in the same amount of time as a smaller particle

Question 11

Continuous settling chamber

Answer 11

Fluid inlet comes in, containing the particles of differing size
The larger particles will settle quicker, therefore, will be collected in the hopper closer to the fluid inlet and smaller particle will be further away
Particles separated into different hoppers can be removed continuously
Size range: 3-1000 microns

Question 12

Elutriation method

Answer 12

• Very similar to sedimentation separation except for fluid flow in the OPPOSITE direction to the sedimentation movement
• Therefore depending on the fluid flow larger particles will settle downwards and smaller particles will move upwards with the fluid flow
• The resistance that light and large particles will be different larger particles will need greater fluid flow so will settle in a different location
• Size range: 10-600 microns

Question 13

Cyclone-separation methods- Similar to spray drying

Answer 13

• The most common type used is the reverse-flow cyclone
• Particles are introduced at the top of the cyclone
• Larger particles are pushed to the outer section of the vortex
• Finer particles remain entrapped in the fluid stream and removed via the vortex finder

Question 14

Selection of size separation process

Answer 14

• Based on pharmacopeial requirements, efficiency and water solubility
• Each separation method is most efficient over particular size range
• Oversized material removed via these processes can be returned for further size reduction

Question 15

Centrifugation techniques

Answer 15

• Suspension fluid inlet
• Particles enter spinning chamber
• Increasing the centrifugation speed will decrease the particle size which is recovered at the fluid outlet
• Size range: 0.3-3 microns