What is the best way to prevent contamination when doing TKN and Ammonia Lachat methods?

Document ID

Document ID TE8014

Version

Version 3.0

Status

Status Published

Published Date

Published Date 05/23/2019
Question
What is the best way to prevent contamination when doing TKN and Ammonia Lachat methods?
Summary
TKN and Ammonia contamination
Answer
Contamination can be minimized by doing the following:
  • Wash all reagent bottles with 1:1 HCl before using.
  • Do not place reagents that contain ammonia near the digestor or instrument.
  • Nitrate in excess of 10 mg/L can oxidize ammonia released from the digested organic nitrogen, producing N2O and resulting in a negative interference. If high nitrate is expected, spike the samples with ammonium prior to digestion.
  • If preserving samples, use H2SO4, not HgCl2, because it will interfere in the determination.
  • Do not use ammonia-containing cleaning solutions near the laboratory.
  • Use fresh DI water so that it does not have any ammonia.
  • Decreasing the temperature of the digestion will not decrease contamination. USEPA method 351.2 requires the digestion to be maintained at 380 degrees for at least a half an hour.
  • Glassware and boiling stones should be rinsed with DI water prior to adding samples to digestion tubes. The process of drying the tubes can contribute to contamination. Water droplets pick up ammonium chloride from the air, they evaporate and deposit the salts on the walls of the tubes.
  • Minimize the amount of time digestion solutions are exposed to the atmosphere.

 
When running low-level ammonia analysis, also consider the following: 
-There are many common sources of ammonia and it could be present in lab air. This can be difficult to eliminate depending on the environment. Some labs even have an ammonia "clean room" if they are intent on measuring low levels of ammonia.
-Acid is used to clean because acid is a huge trapper of ammonia. This means that care must be taken with acidified carrier as well as acidified standards/samples. Ammonia is very volatile, and if it is in the air and contacts an acidic solution it will turn into ammonium and become part of the solution.
-One obvious source of concern is the ammonium chloride buffer used for nitrate analysis. For low level ammonia work it may be desirable to not run nitrate at the same time. Another possible solution is to parafilm all reagent containers (and waste containers) to avoid ammonia leaving nitrate reagent and waste as well as entering carrier or other ammonia reagents. 
-One option is to put large beakers on stir plates with small amounts of concentrated acid in them around the area when doling low level ammonia. This is to create ammonia "sinks" in an effort to soak up ambient ammonia from the air. There may be ammonia in solutions such as floor cleaner or other cleaners being used in the lab. Try to find out if this is the case and eliminate it if possible. Any background ammonia is going to cause concerns if you want analyze low levels of ammonia.
-If you use a hood in the laboratory, it is not a clean place when it comes to airborne contaminants. Many people have the misconception that it is a clean place to prepare solutions but if you think about it a hood will draw in the surrounding air from the lab. This is not to say you cannot prepare solutions for ammonia analysis in a hood, but you need to be aware of what else is going on in the lab when you are working with ammonia solutions and samples.

 

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