Homemade nitrogen assays versus commercial test kits
Jo Long — Fri, 2012-06-08 18:56
Over the past couple of weeks, I have been putting together protocols and other instructional material for using the colorimeter. While we want to make the colorimetric assays as instructional as possible, we also understand that not everyone will have access to chemicals and labware. Using off-the-shelf kits (which generally do not require any extra chemicals or labware) is very convenient and the ones we have tested so far have worked well with the colorimeter. However, if you are anything like me, it can be frustrating to use a commercial product where the contents of a solution are not listed. To this end, we are planning to also develop assay instructions where the user makes their own solutions from chemicals they can easily buy from Sigma and other vendors.
So here is the plan:
- A lab-based assay: requires making your own solutions but is more instructional as the materials are open;
- A home kit assay: uses commercial off-the-shelf test kits. Less instructional in that solution contents are often not disclosed by the manufacturer but does not require making any solutions.
Below are the results from a comparison of an ammonia lab-based assay versus a commercial kit.
Lab-based method vs. Commercial Kit
The lab based assay is the ammonia-salicylate test. The salicylate method involves a three-step reaction sequence. The first reaction step involves the conversion of ammonia to monochloroamine by the addition of chlorine. The monochloroamine then reacts with salicylate to form 5-aminosalicylate. Finally, the 5-aminosalicylate is oxidized in the presence of sodium nitroferricyanide (a catalyst) to form a blue-green colored dye that absorbs light at 650nm.
The commercial kit is the API ammonia test kit (freshwater), which you can buy from most pet stores, aquarium store or online. The kit is also based on the ammonia-salicylate test. The kit costs around $8, and does not require any extra solutions or equipment.
To compare the two methods, I prepared a standard curve of ammonia. The methods for the lab-based assay are described in more detail here. The API test kit was carried out as per the manufacturers instructions. In both cases, the color developed quickly at room temperature and was left to fully develop for 5-10 minutes. Absorbance was measured with the colorimeter using the red LED. The data was plotted and is shown below.
As you can see from the graphs, both methods produced very nice results, with a clear linear relationship between absorbance and ammonia concentration. The lab-based assay (where we made our own solutions) gave slightly greater sensitivity over the range tested. This may be expected as the solutions were prepared fresh that day.
In the end, each user should weigh the advantages and disadvantages of the two approaches, and decide which assay best suits their needs.
For a quick, easy test at home - we recommend the commercial kits;
For an educational lab (high school or undergraduate labs) - we recommend the lab-based assay.