. Biomonitoring program : standard operating procedures. Water quality bioassay; Freshwater biology. INTERPRETATION AND REPORTING OF MICROTOX RESULTS Test Description Microtox™ is the trade name for a particular acute toxicity test. The test is used as a toxics screening tool in addition to other, more traditional, methods of analysis. The Microtox™ analyzer uses freeze-dried luminescent bacteria as its test organisms. When re-hydrated, these bacteria emit light. To test a water sample for toxicity using Microtox™, an analyst prepares a series of dilutions of the sample and adds re-hydra
. Biomonitoring program : standard operating procedures. Water quality bioassay; Freshwater biology. INTERPRETATION AND REPORTING OF MICROTOX RESULTS Test Description Microtox™ is the trade name for a particular acute toxicity test. The test is used as a toxics screening tool in addition to other, more traditional, methods of analysis. The Microtox™ analyzer uses freeze-dried luminescent bacteria as its test organisms. When re-hydrated, these bacteria emit light. To test a water sample for toxicity using Microtox™, an analyst prepares a series of dilutions of the sample and adds re-hydrated bacteria to these. The light intensity of each sample dilution is measured at preselected time intervals over a 30-minute period and compared with that of a control (bacteria only). It is assumed that changes in light intensity are due to toxicant interference with the biochemical reaction that produces light. Toxicity is then measured as the percent decrease in light intensity of each of the sample dilutions compared with that of the control. Data Interpretations The most commonly used result from these tests is the 30-minute EC5Q. This is defined as the sample Concentration causing a 50% reduction in the measured Effect (light production) over a 30-minute time period. The relationship of the EC50 to toxicity is an inverse one; , the lower the EC5Q, the greater the toxicity of the sample. A useful conversion of the EC50 is the Toxic Unit. This is simply the inverse of the EC50 multiplied by a factor of 100: Toxic Units = 100 EC50 (%) Toxic Units approximate the amount of dilution a sample must undergo so as not to induce a toxic response in the test organisms (the Microtox™ bacteria). As Toxic Units increase, so does the relative toxic strength of a sample. The relationship of EC5q's, Toxic Units, and toxicity are demonstrated below: EC50 (^) Toxic Units Toxicity 0-5 200 High 100 10 1 Low. give the regi :oxicity - sample dilutions which induce a
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