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How to Understand Nominal Concentration and Measured Concentration in Aquatic Toxicity

Little Pro on 2019-04-14 Views:  Update:2019-04-14

In aquatic toxicity testing, it is very important to understand the difference between nominal concentration and measured concentration and know which one shall be used to report test result. In this article, we will share some basic technical knowledge with you.

Definition of Nominal Concentration and Measured Concentration

Nominal concentration is the theoretical concentration of a testing substance when you prepare a test solution. For example, if you add 100mg of a test substance to 1L water, the nominal concentration will be 100mg/L. Ideal situation is that the concentration of the test substance is maintained at this level during the whole test duration so that you can observe if the substance causes any adverse effects (i.e, death, growth inhibition) to aquatic organisms (i.e, fish, algae) at this concentration.

However, it is not always easy to maintain the nominal concentration of the test substance during the whole test. Some substances are poorly soluble in water, making it difficult to achieve the nominal concentration of 100mg/L, a typical concentration used in limit test (pre-test). In reality, the highest concentration of the test substance in a pre-test should not exceed its solubility in water. Some substances degrade or hydrolyze in water during the test. In the end of the test, actual concentrations may be much lower than initial nominal concentrations. The actual concentrations need to be measured and monitored and they are known as measured concentrations. Sometimes, the measured concentrations can also be higher than nominal concentrations due to the natural error of analytical methods.

OECD aquatic toxicity test guidance usually requires that the concentration of the test substance be measured, as a minimum, at the highest and lowest test concentration, at the beginning and end of the test. 

How to Report Test Results Using Measured Concentrations

OECD TG requires that results be based on measured concentrations. However, if evidence is available to demonstrate that the concentration of the test substance has been satisfactorily maintained within 20 percent of the nominal or measured initial concentration throughout the test, then the results can be based on nominal or measured initial values.

Let’s assume that we have conducted a limit test on fish at a nominal concentration of 100mg/L. At this level, no fish died during the whole study (24, 48, 72 and 96 hours). Measured initial concentration is 80mg/L. Measured concentration at the end of this study is 60mg/L. Because the measured concentrations are not maintained within 20 percent of the nominal concentration, we can only use measured concentrations to report test result. We will need to calculate the mean value of the measured concentrations first (70mg/L) and then report the test result as LC50 > 70mg/L.

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