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Low OD for Nitrite/Nitrate
Posted: 05 June 2018 11:09 AM

Hi, I seem to be having an issue with nitrite/nitrate method having low OD’s for the RBL’s. The OD’s for NO2 are negative @ 0.0013 and the OD’s for the NO3 side are at 0.0016 - 0.0057.  I have remade all of the reagents and standards. We purchased new Sulfanilamide for the color reagent, but that didn’t seem to help. The Ammonium Chloride is from the same lot I have been using for quite some time, as well as the EDTA. I replaced cuvettes on all of our SmartChems on 5-17-18.  I guess I’m just wondering which chemical(s) is most likely to give problems so I know where to start. 
thank you
Diane Holdreith

Posted: 05 June 2018 01:34 PM   [ # 1 ]

Hi Diane,
I have a few suggestions on how to proceed with troubleshooting here. 
~Firstly, are you running efficiency checks on the Cd reduction coil to ensure the efficiency is >90%?  If you are getting a low Cd coil efficiency, I would try reactivating/reconditioning the coil and/or
  replacing the Cd coil depending on age and usage. 
~When is the last time the nitrate module peristaltic pumps tubes were replaced as this can affect the efficiency of Cd coil?  It is recommended these are changed every 2-3 months and please check that they are 100 mm in length.
~The pH of the ammonium chloride-EDTA buffer is critical with this method.  Te pH of the reagent buffer and reservoir buffer needs to be pH=8.5 (unless samples are preserved with H2SO4 to pH<2, then pH=9.1).  Note: the pH of the buffer needs to be determined with a pH meter and not pH paper to ensure an accurate reading.
~Please ensure the color reagent is stored in dark, amber glass and refrigerated as it degrades when exposed to warmer temperatures and light.  You can even cover the 40 mL reagent bottle used for the color reagent with aluminium foil to further protect from light and temp degradation.
~You mentioned having multiple SmartChems.  When running the same batch of reagents and standards as comparison on different SmartChems are the OD’s low on all?  Such a comparison study should help identify if it is a chemistry or instrument specific issue.

I hope this helps.  Please let me know if any further questions arise and what you determine the cause of the issue to be.

Thank you,
Colleen Rochelle


Colleen Rochelle\nChemist / Applied Technology Manager\nUnity Scientific

Posted: 12 June 2018 12:25 PM   [ # 2 ]

Hi Colleen,
Sorry for the delay in getting back to you.
So I came across an error message that I had gotten on May 31st on the instrument in question, that said” NO3 Failure- Restart
If problem persist call technical support” I restarted the instrument at that time, and things seemed ok right after that. This was on a Thursday, and I didn’t have any samples on Friday and then tried to get things going on the following Monday, and that is when things seemed to start going south. I had forgotten about his message by that time, and was distracted with the method not running, so sorry for not mentioning it.  Things seemed to have settled down right now, And I haven’t gotten this message again. Hopefully it was just a computer glitch and turning the instrument off and restarting it has gotten rid of it. in answer to your question though:
Yes we run efficiency checks for the Cd coil daily to ensure efficiency between 90-110%.
The tubes for the instrument in question were changed on April 2, 2018. I make sure the tubes are at that length, but the company sent replacement tubes that are 4” in length, so they need to be cut.
The pH of the reagent is at 8.5 and is tested on a meter, which is calibrated before use. We have never used the pH at 9.1 for the acid preserved samples, and after reading you message I made up the ammonium chloride-EDTA at that pH and ran a calibration for this method. It was a mess, so not sure what is going on with that. Could be the settings on the instrument (?) I remember when we first got the instrument (2006) we were adding ammonium hydroxide to the samples before running them, but somewhere along the way this was stopped and I really don’t remember why. Seems like we started using SO4 in the reagent water and diluent. I think through the years, when there is an issue, we call in and get someone who gives advice and then things get changed and they continue that way. Anyway we are currently using the same reagents as the NO3 method for drinking water for the acid preserved samples.
Color reagent is stored in foil wrapped bottle in a fridge. We just bought new chemicals to make this to ensure no contamination.
I tried running on the other smartchems in our lab. We have 3, but didn’t get a calibration to pass, but I think that is due to not running the method consistently on the other instruments. By the time I got the Cd coils reactivated/reconditioned things seemed to clear up on the one that was giving me problems.
I do have one other question, what is it method for de-chlorination of samples with sodium thiosulfate? We tried this and it seemed to deactivate the Cd coil. The same thing for the addition of EDTA, there are no amounts listed in the Smartchem method
Thank you,