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Further checks you can do:
The blank should be the cuvette with reagent and no sample (not even deionised water) and then the sample. The sample cuvette must be pH 1-11 and 20ºC for full colour development – outside these limits then a poor colour may cause low bias. Dependent on the sample itself, if there are any interferents above the level shown on p1 a low bias could result and as such give a negative reading (a negative reading means there is no fluoride in the sample at all and the absorbance of the sample is lower than the water used to calibrate the photometer).
First pass you should check the expiry date of the reagent is not expired and if OK run a standard at for example 1mg/L and identify if the reagent batch/photometer/pipette is good.
The next pass would be to check that the pipette is giving the correct volumes-if they weigh a fixed volume of water from the pipette, based on a density of 1g/cm3 does the pipette give the correct volume?
The third pass is to run a spike test to identify if there is interferent – if they test the 3ml sample and then run a second cuvette using 1.5ml of sample plus 1.5ml of known standard eg 1mg/L they would expect (0.5x value sample alone + 0.5x value standard) result. If they do not see the correct value there is a chemical interferent in the sample affecting the colour development (it will not identify which and to remove this all they can do is dilute the sample to dilute the interferent and re-run the test then multiply the result by the dilution factor. This however may dilute the fluoride lower than the test can measure.
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