INTRODUCTION:- Taking a large amount of powder for analysis is expensive and wasteful, and so it must be reduced. Sampling in its strict sense is therefore a simple mass reduction.
The purpose is to collect a manageable amount of the powder which is representative of the batch as a whole. Many small samples are taken from all parts of the whole, which, together, are representative. Theoretically, every particle should have an equal chance to be selected for examination, and this should be attained with minimum disturbance to the system. The composition of the original powder must therefore be retained during sampling. Powder characteristics change under an applied load and attrition and segregation may occur in transfer. Representative sampling is essential for the relevance of any subsequent testing and is defined by the selection method, which must be accurate and reproducible. To produce reliable samples, a correctly designed sampler is required.
Important rules should be followed to optimize sampling:-
- Sample from a moving stream of powder.
- Sample the whole stream for equal periods of time, rather than part of the stream for all of the time.
- Sample Thief:- This tool can be useful for free-flowing powders, with two concentric tubes, one enclosing the other. The outer tube is pointed, with holes cut in corresponding positions in inner and outer tubes. The holes are opened or closed through the rotation of the inner tube to capture material.Sample thieves
(A) Separate holes along the tube, useful for segregation determination.
(B) Full length chamber, useful for spot sampling.
(C) Chamber at pointed end only to gain an average from many samples.
Another variation is a concentric, inner slotted tube with a shoulder at the lower, pointed end. The outer tube has no holes and sits on the shoulder of the inner tube in the closed position. The thief is pushed in thepowder diagonally and the outer tube is raised to expose the slot, which must face upward. The material falls into the container and the tube was removed, slowly opening the tube to collect a representative sample from all parts of the powder bed.
An alternative technique, which reduces segregation, is a unit-dose compacting sample thief. Bias is still likely with this method because of the invasive nature of any sample thief.
The unit-dose compacting sample thief. The sample is collected in the ‘‘S’’ section and the outer rod rotates to open and close the thief.
- Hand Scoop:- A scoop can take a cross-sectional sample from a bag, barrel, or flowing stream. From the powder stream, a single movement of the scoop completely across the stream collects the sample. Opposite directions for each collection should be used. The scoop width should be at least 2.5–3 times the largest dimension of the particles to prevent overfilling leading to an excess of fines, and care must be taken not to leave a thin layer of powder on the belt. With fines, a larger scoop minimizes moisture loss, but the volume may be too large and using a shovel would be more convenient. This method is widely used in manufacturing industries as it is simple and cheap.
- Sampling with a Shovel:- In this method, an imaginary grid is applied to a pile of material. At the intersections, pits are dug, roughly 30 cm deep, and a shovelful is taken from the bottoms and sides of each hole. This method is only applicable to large amounts (up to several tons) of material, but it cannot be used with particles larger than 5 cm in diameter.
- Cross-Cut Sampling:- Using a conveyor belt, the belt is stopped and a sample is taken by hand or an automatic or semiautomatic device. A suitable size for sampling can be designated by markings on the conveyor belt and all material within markings was collected. If an automatic system is used, a mechanically operated head moves across the material at a preset interval and moves a sample into the collection point.
- Pneumatic Lance:- This method is used for bulk powder. A gentle flow of air out of the nozzle allows the probe to move through the powder bed. When at the site, air is very slowly reversed to draw up a sample, which is collected against a porous plate at the end of the probe. The porous plate prevents great numbers of fines being present in the sample if the air current is too strong. The pneumatic lance minimizes powder disturbance and therefore has an advantage over a sample thief, although bias is still a problem.The pneumatic lance
- Vacuum Probe Samplers:- Large samples can be extracted from bins or holds using a vacuum cleaner principle. Contamination is a problem when sampling below the surface, fines are extracted preferentially to coarse particles, and bias is a problem.
- Gravity-Flow Auger Sampler:- A slotted tube is rotated in a flowing mass, and the material collected is carried out of the tube by a worm screw. It is difficult to sample the entire stream, and bias remains to be a problem.
SAMPLING FROM A MOVING STREAM:- With these methods, great care should be taken to avoid the effect of segregation. The powder is taken as it falls from the conveyor and can display two types of segregation: –
- If the powder has been charged on the conveyor, fines concentrate at the center of the belt while coarse particles roll to the outer edges such as the segregation seen when sampling from a heap.
- If the powder is exposed to vibration on the belt, percolation causes larger particles to rise to the top of the powder bed and fines remain at the bottom.
The whole of the powder must be collected for a short period of time, and care must be taken moving the sampler in and out of the stream. Various methods exist for sampling from a moving stream. The width of the receiver should be greater for smaller particles than for larger ones because of the tendency for fines to drift during the movement through air. To minimize error, the ratio of box width to particle diameter should be at least 20 : 1, larger when sampling fines. The depth should be large enough to prevent the receiver from becoming full while sampling. If it overfills, fines will percolate through the heap that forms, while coarse particles are lost. The receiver length should be sufficient to sample the full length of the powder stream.
- Full-Stream Trough Sampler:- This can be used for sampling dusty material. Sampling is only carried out on the return stroke. The trough must not overfill.
- The full-stream trough sampler
- Bag Sampling:- When many tons of material need sampling from sacks, several sacks should be selected systematically, i.e., 100th, 200th, 300th, etc., or through use of a random number table or rootn – 1 (9 of 100). Each sample is individually assessed to determine the variation and if it is acceptable, or combined to give an average. The thief sampler is recommended, but this can lead to bias. It is preferable to use a spinning riffler; however, this is expensive to install. It does avoid rejecting good material and accepting poor material which would occur with the above, inexpensive technique.
- Sampling from Wagons and Containers:- This is almost impossible to do satisfactorily because of the major segregation in the filling and vibrations in transport. Sampling should not be carried out in the top 30 cm to avoid the segregation in the surface layer that will have occurred as a result of vibration. When removing the samples, no surfaces on which particles can slide should be introduced. This can be achieved using a sample thief. Extraction should be carried out at eight points in the bed. This method of sampling is not satisfactory but may have to be used in certain circumstances.
Locations for sampling from wagons and containers
- SAMPLING FROM HEAPS:- This should not be carried out because marked segregation occurs, with fine particles concentrating in thecenter of the cone. Powder should preferentially be sampled from a moving stream of powder instead.
- AUTOMATIC SAMPLING:- A predetermined amount of powder is sampled at regular intervals or continuously. Because of the inherent bias, regular inspections are needed. All the material should be sampled at a constant rate, and clogging of the material should be avoided.
- Arc Path Cutter:- A chute at an angle is mounted on a vertical shaft, which rotates to periodically move the chute through the stream of powder for a set time. By increasing the speed of rotation, an increase in the number of increments is taken, but this does not alter sample size.
- Straight Path Cutters:- A rectangular chute moves through the powder stream and rests outside it for a variable time period. This allows different sample sizes to be taken relatively easily. The whole of the streammust be covered to avoid bias.
- Moving-Flap Sample Divider:- A flap pivoted about a horizontal axis can rest in either of two positions. The powder stream flows to storage or to be sampled and the time for the flap to be in either position can be controlled automatically. If the sample is too large, a table sampler is used and an extra powder is returned to the store. This method would be efficient if it was not for the use of the table sampler which causes error. A slight bias is involved as one side of the stream is sampled more than the other.
Encyclopedia of pharmaceutical technology