The Fluoride method requires a multi-line calibration for the probe response at low levels. We have a Minimum Detection Limit for fluoride analysis 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.
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