 Many analytical problems begin with objects that are not suitable for a laboratory experiment. The object might be a human tissue, a 2000 year old urn, a lake full of water, or a train load full of ore. To perform a meaningful chemical analysis, you have to obtain a small sample whose composition is representative of the larger object. Think of a glass of water. Its composition is considered to be the same all the way through. We call this homogeneous. However, a glass of water and a lake are completely different. Can you think how? Pause the video and continue when ready. The answer is scowl on what the water is in contact with. A glass of water is very small and is only in contact with the glass, but even a shallow lake which is much larger is likely to contain different layers of water. We call these layers heterogeneous, which means something well mixed. So these layers will have differing characteristics. The topmost layer of water in a lake is in contact with the atmosphere and the bottom layer of water is in contact with sediment. The temperature and solute content of the water will vary according to where you take a sample and as a result of these differences the lake prevents rapid mixing of the layers creating a heterogeneous mixed composition. Let's say we were to analyse a large object which is called the lot. Then a representative bulk sample needs to be prepared. This is usually achieved by dividing the lot up into segments. In this case let's think about an agricultural example. A farmer wants to analyse why some plants are growing better in one place compared to another. The farmer decides to look for nutrients absorbed by the crops grown in one field and will then compare this with a second field. The biggest problem is that the field is huge, so how can the farmer do this? Let's say the farmer has a 100 meter by 200 meter sized field. He could divide the field into 200,000 small patches that are 100 centimetres on the side. After assigning a number to each patch he could use a computer program to pick a thousand numbers at random from 1 to 200,000. By harvesting crops from these 1000 patches a representative bulk sample can be prepared. Very heterogeneous materials like a crop may need to be sampled in great bulk, amounting to 1% or more of the total. The resulting sample then needs to be reduced in size by some means that preserves its representative character. Coning and quartering is one approach. The original sample is formed into a cone-shaped pile and then flattened into a disc. The disc is divided into four quadrants. Two opposite quadrants are shoveled into a second pile, mixed together and then coned and quartered again. This sequence continues until the selected material has been reduced to a size small enough for a useful laboratory sample. The laboratory sample and individual test samples taken from the field are then prepared by dissolving the sample in water. Now the farmer has a laboratory sample. Small test portions called aliquots of the laboratory samples are used for individual analyses. From these analyses the representative amount of nutrients are known and the farmer can compare these with the individual samples taken from different parts of the same field. This confirms that the nutrients from fertilizers have been spread evenly through the field. How do you think you'll use this sampling technique in the second field? Pause the video and think about this. Resume when ready. The answer is that the process would be repeated in the second field and the two fields compared. Comparison of laboratory samples from the two fields will tell the farmer if there is a discrepancy in the fertilizer mix between the two fields.