1. Use FRESH Standard 2 in the APA6000. In warm and humid locations the standard is subject to infestation by nitrogen-consuming microbes. These microbes then ‘eat up’ ammonia in the standard, dropping its concentration. The damage to the standard occurs after the standard is opened, and can occur within just a few days. When the analyser calibrates and sees low recovery of ammonia, this results in an artificially high calibration gain, and subsequently high monochloramine and total ammonia readings.
2. Use FRESH Reagent 3 in the APA6000. Reagent 3 is a solution of about 400 ppm free chlorine. Free chlorine is very volatile, and it can be lost even while the bottle is still sealed. Shelf life of this reagent is 6 months; you should view the expiration date on the label as a ‘drop dead’ date. If this reagent is past (or even near) the expiration date, it is best to assume it has gone bad. If Reagent 3 is exposed to sunlight, stored at an elevated temperature, subject to vibration - it will go bad even quicker. The end result is low recovery of ammonia during calibration, and artificially high calibration gain.
3. Be sure the CL17 is well maintained and the colorimeter is not too old. CL17’s can lose sensitivity as certain parts age, and this can result in low readings. Most people just assume the CL17 factory calibration is forever accurate, and never bother to calibrate – not such a good assumption, especially if the CL17 is several years old.
4. DR850 – same as the CL17, things age.
5. The sample can also be tested in the DR850 using APA6000 reagents; the method for doing so is attached. If this test is used, how do the APA6000 and DR850 results compare?