The best way to ensure correct results is to analyze a nitrate standard solution in place of the sample. Adjust the shaking technique if theresults are higher or lower than the standard concentration.
- Be sure to measure a reagent blank and subtract this reading from your test results. The medium and high range nitrate methods especially have high reagent blank values.
- Consider alternate nitrate methods which have detection limits well below the sample concentration or nitrate methods which don't use cadmium.
Another way to improve accuracy involves running a standard along with the samples and doing a few calculations. This is described below:
Each time samples are analyzed a standard solution should be analyzed at the same time. Be sure to shake the sample cells with the same consistency by putting them in a rack and shaking with both hands together. This is very important. If shaking causes the standard to be measured incorrectly by a certain percentage , then assume that the unknown sample will also be measured incorrectly by the same percentage. The calculations are as follows:
Given the following:
A = True Concentration of your unknown sample
B = Concentration of the Reagent Blank (what you get when you run deionized water as a sample)
C = Observed Concentration of the sample
D = True Concentration of the standard
E = Observed Concentration of the standard
A = (C - B) * [D / (E - B)]
Details and Example:
To improve accuracy, run a reagent blank for the new lot of reagent. This is done by running the test on deionized water as if it were a sample. It is often best to do this in triplicate, noting the average as your Reagent Blank, or B in the equation above.
For this example, use 0.5 mg/L as the value of the Reagent Blank.
Then, take a nitrate standard solution and run it through the test along with the sample.
For this example, say a 10 mg/L standard was used and a value of 12 mg/L was measured. The True Concentration of the standard, D in the equation, would be 10 mg/L. The Observed Concentration of the standard would be 12 mg/L. For this example, say the sample was measured at 15 mg/L. This is Observed Concentration of the sample.
Now, using the equation above:
A = (15 - .5) * [10 / (12 - .5)]
A = (14.5) * [10/11.5]
A = 14.5 * .87
A = 12.6 mg/L
In conclusion, the sample, which initially read 15 mg/L, has a true concentration of 12.6 mg/L once the reagent blank and the effect of shaking has been accounted for.