I have a Hach DR3900 and am running their LCK323 Fluoride Kit.
After blanking the instrument the test itself gave a negative value, what is the likely cause?
It sounds like the DR3900 needs updating with the latest software parameters for the DR3900
If you have a USB stick you can get the latest settings from the Hach website:
You will need the last item on the page- DR3900 spectrophotometer version 1.09/08/18 or greater.
Once you have them saved to the flash drive you can download the data to the DR 3900 via its USB port.
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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