Why does my fluoride method require a multi-line calibration?

The reasoning for multi-line on Fluoride has to do with the probe response at low levels. We have a stated MDL for the test of 0.01 ppm, and it is known that the slope for fluoride probes (and all ISE probes) deviates from the expected range and approaches zero as the fluoride concentration also approaches zero. We use multi-line to account for this changing slope at the low concentration levels to achieve the lowest detection limit possible. The ability to switch to linear without an effect on results depends on the range you are measuring.