Serial Dilution

∞ generated and posted on 2016.01.23 ∞

Routine means of reducing concentrations without employing extremely large volumes or having to measure extremely small volumes.

Serial refers to events occurring in series, that is, one after another. With serial dilutions, dilutions take place one after another, with each subsequent dilution building upon the previous. This allows one to generate substantial levels of dilution without needing to employ very large volumes of diluent nor very small to-be-diluted volumes.

For example, it is quite simple to generate a ten-billion-fold dilution in only a handful of steps. If only a single step, however, then one would have to dilute 1 μl (one-thousandth of an ml) to 10 liters of diluent. With serial dilutions, however, this same total dilution requires only five one-hundred-fold dilutions, such as 1 ml to 99 ml or 0.1 ml to 9.9 ml or even 0.01 ml (10 μl) to 0.99 ml, repeated five times and with the last four steps serving to further dilute the previously diluted volumes. That is,

100×100×100×100×100 = 10,000,000,000 = 10 billion (= 100⁵ = 10¹⁰)

It is important to make sure that volumes are well mixed before drawing volumes to generate the next dilution, and that neither transfer devices nor vessels are reused prior to thorough rinsing or cleaning as well as, if necessary, sterilization.

In the above example, one could take 0.1 ml (100 μl) and add that volume to 9.9 ml of dilute. This mixture is then mixed well and 0.1 ml is removed from it into 9.9 ml of fresh diluent. Together this makes a 100 × 100-fold dilution, or 10,000-fold dilution. Repeat three more 100-fold dilutions and you have a 10,000,000,000-fold dilution, i.e., 10 billion.

Typically the phrase "Serial dilution" is used to describe the repeating of the same dilution, e.g., 100-fold followed by 100-fold followed by 100-fold, or 10-fold followed by 10-fold followed by 10-fold, etc., rather than mixing of different-fold steps.