Quality control in ensuring your product, or scientific result falls in between a set of guidelines determined by the necessary amount of accuracy and precision required. It allows a customer to know how reliable a scientific result is.
For instance, say you want to analyze some water samples from off the coast of Louisiana to check if there is any lead contamination because the battery factory was flooded and damaged from a hurricane. To start with, you might want to take samples in various locations and record some type of data like distance from shore and maybe depth of sample taken along with the pH of the water. To tell that the guy or girl sampling the water hasn't contaminated it themselves, they'll need to take some DI water up with their water sampling equipment and bottle that up to go with all the sea water samples.
Then, it'll get back to the lab and it will have to be prepared to be analyzed, the preparer will include another blank with the batch of sea water samples to ensure that he or his lab did not contaminate the samples. These blank samples are representative of the whole batch of samples, if they have any lead in them, then all the other sea water samples would be expected to have the same amount. That's why you gotta keep it all clean, with a mass spec you're usually looking for pretty low amounts.
To be continued... as a series or possibly just a trilogy.