Hach-Lange ISEF12103 User Manual - Manual (Page 13)

Message or symptom Possible cause Action
Below detection limit
Measurement is not quantifiable
with current saved calibration
(based on IUPAC-defined practical
detection limit). Perform a new calibration. Check that sample
concentration is bracketed between two standard
solution values (if within linear range).
Re-run calibration and measurement with the
Drinking Water or Pure Water default methods to
optimize meter settings for slope acceptance and
stabilization criteria at lower levels.
Re-run calibration and sample measurement with the
tips for low-level measurement.
Measurement value is outside of
range. Make sure sample is within the range of the probe.
Check probe response
To make sure there is a probe response, measure the probe potential (in mV) of two
Fluoride Standard Solutions that are above and below the expected sample
concentration. For example, use 1 and 10 mg/L Fluoride Standard Solutions. The two
solutions should have potentials (difference in mV readings) that are 59 mV apart at 25 ºC
(within the slope limits of the method is acceptable). Both solutions should be above
1 mg/L Fluoride.
Check accuracy of sample reading
To make sure the sample measurement is accurate, add a spike of Fluoride Standard
Solution with the volumetric pipet. Refer to
Table 3 and formulas to calculate the percent
of recovery.
Typically a percent of recovery of 100% ±5% is a good indication that the instrument,
technique and the sample do not contribute to measurement errors.
Table 3 Spike reference
Measured sample concentration Volume of standard at add Concentration of standard
1 to 2 mg/L 0.5 mL 100 mg/L
3 to 6 mg/L 1.0 mL 100 mg/L
7 to 15 mg/L 0.3 mL 1000 mg/L
15 to 30 mg/L 0.5 mL 1000 mg/L
30 to 60 mg/L 1.0 mL 1000 mg/L
Percent recovery
Use the following formula to calculate the percent recovery when the sample volume is
25 mL:
E = (C x V
1
) / V
2
R = (A / (E + S)) x 100
• S = mg/L of Fluoride in sample (before spike)
• C = concentration of standard used for spiking (mg/L)
• V
1
= spike volume (mL)
• V
2
= spike volume (mL) + 25 mL sample volume
• E = expected concentration of spike (mg/L)
• R = percent recovery
• A = actual reading on meter after spike (mg/L Fluoride)
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