Sampling Methods in Analytical Chemistry

Instructor: Sarah Pierce

Sarah has a doctorate in chemistry, and 12 years of experience teaching high school chemistry & biology, as well as college level chemistry.

This lesson is an introduction to the sampling methods using in analytical chemistry. Different sampling methods of homogeneous and heterogeneous materials are discussed.

Is All Chocolate The Same?

When you go to a grocery store and look at the aisle with candy, there are lots of different types of chocolate. Are all those chocolate bars the same? No way. You probably are already thinking about your favorite chocolate that you would buy and your least favorite that you wouldn't eat even if it was given to you for free!

Sampling Methods

Now let's imagine you have a friend who can't have very much caffeine, an ingredient in chocolate. You want to analyze lots of different chocolate bars to see which one your friend can eat. Think about how you would analyze the amount of caffeine in chocolate. First, you would need to get a small amount to test, called a sample.

How would you prepare samples of chocolate?

How do you think you would go about obtaining a sample? You would need to make sure it was a representative sample that mirrors the consistency of each chocolate bar a person finds in the grocery store. What is the best way to do that?

The first thing you would do is take a bulk sample from a lot. A lot is the total amount of material that you are going to take your sample from. For example, the lot may be a carton of chocolate at the grocery store. The bulk sample is a large sample that is first taken from the lot. In our chocolate example, it could be several candy bars from the carton.

The bulk sample would need to represent the entire lot, or be a representative sample, so you would probably take a bar from the top of the carton, the middle of the carton, and the bottom of the carton. You would then want to obtain a laboratory sample that could be tested. To do this, you might grind all three bars up, mix them together, and then take a small portion known as an aliquot for testing.

A flow chart showing the steps of sampling from lot to aliquot
Sampling Flow Chart

Homogeneous Sample

Imagine all the different chocolates that you encounter. Not all of them are the same. Some have nuts or fruit, while other are pure chocolate. You would treat these sample differently based on their composition. The pure chocolate bars are an example of a homogeneous sample or a sample that has the same consistency throughout. Homogeneous samples are the easiest type of sample to prepare. After obtaining your sample, you can just go to the laboratory and begin to analyze it.

Heterogeneous Sample

In a heterogeneous sample, the material does not have a uniform consistency throughout. An example of a heterogeneous sample would be a chocolate bar with fruit and nuts. The fruit and nuts are different from the chocolate, making the sample heterogeneous. There are two types of heterogeneous materials: random heterogeneous materials and segregated heterogeneous materials.

Random Heterogeneous Material

Random heterogeneous material randomly varies in its composition on a small scale. For example, imagine you are trying to measure the amount of iron in the dirt in a field and you know the amount varies through the field. To get a representative sample, you would need to divide the field into small regions. Then, you would take random samples, which are basically just samples that are chosen randomly, from the different regions.

One way to take a random sample is number the regions from 1 to 100 in the field and then draw several numbers out of a hat. You would then take samples from those numbered areas. You could then combine all the samples from the different regions to give you a representative bulk sample.

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