Powders: Powder sampling and MCQs for GPAT, NIPER, Pharmacist and Drug Inspector Exam
When powders are poured into a pile, segregation occurs. Particles with small size (fine particles) will stay in the center of the pile, and coarser particles tend to concentrate at the periphery. Size segregation is unavoidable, during powder transportation, storage, or packing. Therefore, powder sampling is important for the quality control of the final product.
Table no. 1 – Summary of sampling methods
| Apparatus | Type of sampler | Method of sampling | Scale | Advantages | Disadvantages |
| Sample thief | Manual | Tubular steel retains a core sample when inserted into powder |
Large and small |
Good for free-flowing powders | Additional weight at bottom of bag may vary sample size; Can be hard to push into powder; Fines may lodge between tubes; Particles can fracture; Fines compact, impeding flow; Segregation may occur as fines percolate into sample more easily than coarse particles; A plug of powder can be pushed ahead of thief and surface material contaminates sample; Personal preference introduces bias for the area sample |
| Hand scoop | Manual | Cross-sectional sample from moving stream, bags, or barrels |
Large | Simple and cheap | Thin layer may remain on belt, leading to bias; Overfilling can lead to an excess of fines. |
| Shovel | Manual | Pits are drug in the powder bed and a shovelful taken from bottoms and sides |
Large, up to several tons |
Simple and cheap | Cannot be used with particles of more than 5 cm diameter |
| Cross-cut sampling |
Manual/ semiautomatic/ automatic |
Material is shovelled from the conveyor belt |
Large | Simple | Can leave a layer on the belt and result in bias |
| Pneumatic lance |
Semiautomatic | Air flow used on entry and exit of lance from powder bed |
Large | Disturbance of powder minimized over sample thief; Porous plate prevents too many fines due to strong air current |
Personal preference may bias sample. |
| Vacuum probe sampler |
Semiautomatic | Powder extracted by vacuum | Large | Simple | Difficult to sample below surface without contamination; Personal preference leads to bias; Fines more easily extracted than coarse particles. |
| Gravity-flow auger sampler |
Semiautomatic | Slotted tube in flowing powder rotates and worm screw carries out material |
Large and small |
Easy to use | Bias is still a problem here; Difficult to sample all of powder stream, therefore bias. |
| Sampling from a moving stream |
Manual/ automatic |
Powder is sampled as it falls off the conveyor |
Large | If carried out properly can be very good sampling technique; easily designed into a new plant. |
If overfilled, a greater number of fines than coarse particles are collected; Difficult and expensive to fit into an existing plant; Obtaining a fixed sample is difficult; Difficult to prevent dust escaping. |
| Full-stream trough sampler |
Automatic | Powder is sampled as it falls off the conveyor |
Large | Can be used to sample dusty material |
Must not overfill; Difficult and expensive to install into existing plant. |
| Arc path cutter |
Automatic | Chute moves through powder stream, and collects sample |
Large | No operator bias | Difficult to take more than one sample size; Must cover the whole of the stream to avoid bias. |
| Straight path cutter |
Automatic | Rectangular chute moves through powder stream |
Large | Different sample sizes can be taken easily |
Must cover the whole of the stream to avoid bias |
| Moving-flap sample divide |
Automatic | A flap in the streamsamples powder or allows it to be stored depending on its position |
Large | Efficient as covers the whole of the stream when sampling |
Resultant sample is large and subsampling needed. Bias may be introduced here; Bias is present due to one side being sampled more than the other. |
| Integrated automatic sampling plant |
Automatic | Primary sample is selected and repeatedly screened, resulting in the final sample |
Large | Quick | Variations in materials can cause problems |
| Chute splitter | Sub-sampler | A series of chutes split sample repeatedly |
Large | Can be repeated until desired sample size is achieved |
If segregation occurs the result can be misleading; Prone to operator bias. |
| Cone and quartering |
Sub-sampler | Powder poured through cone and divided into four equal parts. This is repeated until the desired sample size is reached |
Small | Simple | Prone to operator bias as fine particles remain in the center of the cone; Symmetry is difficult to achieve but essential for accuracy. |
| Spinning riffler | Sub-sampler | A steady stream of powder flows into a rotating basket of containers |
Large/ small |
Good for sub-sampling large samples; Good for powders with good flow properties; Minimal bias; More efficient than other samplers tested. |
Air currents may displace fines but can be avoided with a slower rotation speed; Expensive; Time-consuming; Segregation may be a problem. |
| Free-fall tumbler mixer |
Sub-sampler | A ladle in the lid of the mixer collects a good representative sample |
Large/ small |
Can be used with fines present; Representative sample produced in a short time period. |
|
| Hopper sample divider |
Sub-sampler | Hoppers oscillates and powder falls into two containers. Only one of the contents is kept. |
Small | Sample size can be controlled by monitoring time over each container |
Large number of increments needed for accuracy |
| Table sampler | Sub-sampler | Powder flows down inclined plane and prisms and holes split the powder |
Small | Simple | Very low accuracy; After each separation should be a complete mix to avoid errors. |
Multiple choice questions:
1.When powders are poured into a pile _____ occurs.
a)segregation
b)agglomeration
c)sticking
d)all of these
2.Size segregation is unavoidable, during
a)powder transportation
b)storage
c)packing
d)all of these
3.Tubular steel retains a core sample when inserted into powder in which of the following?
a)Sample thief
b)Straight path cutter
c)Free-fall tumbler mixer
d)Table sampler
4.Which of the following is/are advantage of sample thief?
a)Good for free-flowing powders
b)Efficient as covers the whole of the stream when sampling
c)Can be used to sample dusty material
d)Simple
5.In Hand apparatus which of the following sampling methods are used?
a)Cross-sectional sample from moving stream
b)Cross-sectional sample bags
c)Cross-sectional sample from barrels
d)all of these
6.Which of the following is/are disadvantages of hand scoop method of sampling?
a)Thin layer may remain on belt, leading to bias
b)Overfilling can lead to an excess of fines
c)both of these
d)only b
7.Shovel apparatus is
a)automatic
b)manual
c)semi automatic
d)sub sampler
8.Cross-cut sampling apparatus is
a)automatic
b)manual
c)semi automatic
d)all of these
9.Which of the following is/are disadvantages of Pneumatic lance?
a)Large number of increments needed for accuracy
b)Very low accuracy
c)After each separation should be a complete mix to avoid errors
d)Personal preference may bias sample
10.Vacuum probe sampler have following advantage
a)Good for free-flowing powders
b)Efficient as covers the whole of the stream when sampling
c)Can be used to sample dusty material
d)Simple
11.Arc path cutter is based on which of the following sampling methods?
a)Chute moves through powder stream, and collects sample
b)A steady stream of powder flows into a rotating basket of containers
c)A ladle in the lid of the mixer collects a good representative sample
d)Powder flows down inclined plane and prisms and holes split the powder
12.In which of the following rectangular chute moves through powder stream?
a)Arc path cutter
b)Straight path cutter
c)Chute splitter
d)all of these
13.Cone and quartering apparatus is
a)automatic
b)semi automatic
c)manual
d)sub sampler
14.Which of the following is/are advantages of Free-fall tumbler mixer?
a)Can be used with fines present
b)Representative sample produced in a short time period
c)Both of these
d)None of these
15.Powder flows down inclined plane and prisms and holes split the powder in which of the following apparatus?
a)Table sampler
b)Hopper sample divider
c)Spinning riffler
d)Integrated automatic sampling plant
Solutions:
- a)segregation
- d)all of these
- a)Sample thief
- a)Good for free-flowing powders
- d)all of these
- c)both of these
- b)manual
- d)all of these
- d)Personal preference may bias sample
- d)Simple
- a)Chute moves through powder stream, and collects sample
- b)Straight path cutter
- d)sub sampler
- c)Both of these
- a)Table sampler
References:
- Remington Essentials of Pharmaceutics, 1st edition 2013, page no. 429-431.
List of Successful GPATINDIAN CANDIDATES
Participate in Online FREE GPAT TEST: CLICK HERE
Participate in Online FREE Pharmacist TEST: CLICK HERE
Participate in Online FREE Drug Inspector TEST: CLICK HERE
Participate in CSIR NET JRF Mock Test