Preparing sample
To obtain an accurate estimate of the cell density of your sample the cells need to be evenly suspended in your culture vessel before you remove an aliquot for counting. The method will vary depending on your culture method. For algae, you might be sampling from a flask or test tube. For cell cultures you probably are sampling from a Petri dish. To resuspend cells in a flask or test tube you simply swirl the flask or invert the test tube and then quickly remove your aliquot of cells. For a cell culture growing in a Petri dish, you need to resuspend the cells growing on the bottom of the dish by gently using a pipette to remove cells and media from a dish and then gently expelling them back into the dish. This aspiration of the contents of the petri dish should be completed several times, each time expelling the cells and media while moving the pipette across the bottom of the dish to gently discharge cells growing on the entire surface. After resuspension the aliquot is quickly removed from the vessel before the cells settle to the bottom again.
For cell culture applications cells are often stained with Trypan Blue so that dead cell can be distinguished from live cells. For example, a 0.5 ml suspension of cells would be removed from the Petri dish and mixed with 0.5 ml Trypan Blue solution in an Eppendorf or small test tube. Trypan Blue is a stain that selectively stains dead cells. For algal cells, Lugol’s solution is commonly used to stain an aliquot of cells in a small test tube. The Lugol’s solution will immobilize and kill the algal cells. The amount of stain used to dilute the cell suspension must be measured and recorded so that you can apply it as your dilution factor in the final calculation.