In the previous post, we learned that getting an appropriate representative sample for analysis is a very important step in getting accurate results.In this post, we will further discuss the concept of sampling.
|Sample size||Type of analysis|
|0.001 to 0.1g||Semi microanalysis|
|0.0001 to 0.001g||Microanalysis|
|<10-4 g||Ultra microanalysis|
How do we visualize these quantities? Consider 1gram gold-
When we consider 0.1g quantity, we are roughly looking at size 1/10th of the size of 1g gold! The macro analysis samples are this small! The ultra microanalysis deals with samples that are very small -0.00001g only. Techniques for handling very small samples are quite different from those for treating macro samples.
Based on the level of analyte in the sample ,there can be the following constituent types in samples –
|Analyte level||Type of constituent|
|1% to 100%||Major|
|0.01%(100ppm) to 1%||Minor|
|1ppb to 100 ppm||Trace|
The trace and ultratrace constituents are very difficult to sample especially if the constituents are heterogenous.
e.g– Iron is present in our bodies as a trace element. However, it is a very important constituent of haemoglobin and thus there are instances where finding the amount of iron in blood is crucial.
Physical form of samples
The method of sampling also depends on the physical form of the sample i.e whether the bulk material is solid, liquid, or gas. Sometimes the bulk consists of mixed phases. e.g. Gas dissolved in liquids or particles suspended in a liquid. Sometimes the bulk is in one form and it needs to be converted to another form for analysis.e.g.- Molten steel can be analyzed only after casting into a solid form.
Solids are usually heterogeneous and thus they have to be sampled very carefully. Solid samples can be in various forms –
- Monolithic Samples
- Particulate solids
- Sheet materials
- Tissue samples
Solids can be monolithic (huge or massive e.g.– an ore) or particulate ( particle form). Sampling monolithic samples take more effort as the sample size is huge. For example, sampling a huge ore underground may be very difficult. If these solids are compact, then they can be sampled using various equipment like an auger, hand drill, chisel, shovel, post hole digger, etc.
Sampling particulate solids is a comparatively easier process as these are dry and free-flowing. However even in particulate solids, if the bulk material is heterogeneous, there may be difficulty in sampling. In the case of samples having lumps, the entire sample is crushed to powder first using a grinder or a ball mill. Then the sampling is done by a powder thief tube. How does a powder thief work? Watch this video – https://youtu.be/GBkjMwauEzE
For underground sampling drilling is also used as a sampling technique as follows – https://youtu.be/i9eQcc7ilVw
For more information on sampling solids watch this – https://youtu.be/DWKmI7HSDxs
If the size of the bulk is more then size reduction methods like coning and quartering are used. In this method, a cone is made out of the heap of bulk and then it is flattened and quartered to get a sample for analysis as shown below-
Pure and homogenous liquids are generally sampled by simple methods like pouring, siphoning, etc. Low or medium viscosity liquids(less viscose means liquids that flow easily) can be sampled by a liquid master sample. How does it work? Watch – https://youtu.be/PL-OFFP0Fu8
A cup sampler is also used to sample liquids – https://youtu.be/LNlSUwBgKO8
A multiple layer sampler is used to take samples from flowing liquids as it helps to get samples without any bias – https://youtu.be/prCiWff47mI
Sampling liquids from underground e.g.- wells/river beds/ocean beds involve ‘down-hole’ sampling devices or electric pumps.
The sampling plan for gases totally depends on the subject under consideration. The sampling method used for one gas may not be applicable to others. For gaseous substances sampling is done in three forms, namely – SPOT, CONTINUOUS, or REPRESENTATIVE.
SPOT SAMPLING – Spot samples are taken one at a time at one point. The sample is then collected in a sample cylinder and taken to a laboratory for analysis.e.g.A pivot tube is inserted in a pipeline to collect gas.
CONTINUOUS SAMPLING – In this type of sampling, samples are continuously taken from a line and fed into an analyzer. In many cases, the sample is inserted into a flowing stream, where a number of operations can be carried out on the gas before the analysis is done.
For analyzing gases, the sample gas is also adsorbed on a solid surface or absorbed into a liquid. However, then the analysis is carried using solid or liquid sampling techniques. e.g.- O2, CO2 dissolved in the bloodstream is an example of gas in a liquid.
REPRESENTATIVE SAMPLING – In this type of sampling, representative samples are collected over a period of time, and then, the gross sample is analyzed later. It is essential that the sample collected should not react with the materials of the container in which it is stored and the physical and chemical properties of the gas should NOT change during storage. Care should be taken to reduce changes due to volatilization during storage.
A general design of a gas sampling apparatus consist of –
1) Sample probe , 2) Sample delivery line and 3) container.
Video of a gas sampling probe – https://youtu.be/TC2R26UbnCM
In this post, I have shared lot of links as watching different sampling instruments is so much better than just describing them in words. With this post, we complete discussing the fundamental concepts in analytical chemistry. From the next post onwards, we shall start learning a new physical chemistry chapter. Till then, be a perpetual student of life and keep learning….
Good day !
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