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What is the Max acid ratio (FM) and why is it important?
The EPA method 445.0 is available on the Application page.  This method explains in detail all the variables involved in determining Chlorophyll using the acidification method.  Pay close attention to section 10.0 for detailed explanations.  This is the method we used in our labs to calibrate and/or determine chlorophyll concentrations.

The FM value or max acid ratio value is the ratio of the response of Chlorophyll (Standard or Sample) before acidification to the response of the same (Standard or Sample) after acidification.  Typically, in natural waters you will see Fm values vary from 1 to 2.  Values near 1, indicate that you have a high concentration of pheopigments in your system.  Values near 2 indicate a lower concentration of pheopigments.  In a perfect world, the highest max acid ratio you will get is 2.1.

Why you would want to check the acid ratio of your Standard?

To determine if the Standard hasn't been degraded or converted to pheopigments (i.e. a double check to see if your calibration standard is good).