Accuracy And Precision; Standard Additions - Hach DR/700 Procedures Manual

ACCURACY AND PRECISION

Accuracy is the nearness of a test result to the true value. Precision refers
to the agreement of a set of replicate results or repeatability. Although
good precision suggests good accuracy, precise results can be
inaccurate. The following paragraphs describe techniques to improve
accuracy and precision of analysis.

Standard Additions

Standard additions is a widely accepted technique for checking the
validity of test results. Also known as "spiking" and "known additions,"
the technique also can be used to check the performance of the reagents,
the instrument and apparatus, and the procedure.
Standard additions is performed by adding a small amount of a standard
solution containing a known amount of the component being measured
to an analyzed sample and repeating the analysis—using the same
reagent, instrument and technique. The amount of increase in the test
result should equal exactly the amount of component added.
For example, if testing shows a 25-mL water sample analyzed for iron
contains 1.0 mg/L, the result can be checked by adding 0.10 mL of a
50.0-mg/L iron standard solution to another 25-mL portion of the water
sample and repeating the analysis. The result of the analysis on the
second sample should be 1.2 mg/L iron because the standard added an
equivalent of 0.2 mg/L. For example:
0.10 mL x 50.0 mg/L
= 0.2 mg/L
25 mL
If 0.2 mg/L is recovered from the 0.2 mg/L addition, the analyst can
conclude the first answer was correct and the reagents, instrument and
method used are all working properly. Because the effect of incremental
volume additions is small, the sample volume used in the above equation
was 25 mL (not 25 + 0.1). Using 25 mL, instead of 25.1 mL, represents
less than 0.4% error. For 0.3 mL standard addition, the error would be
less than 1.2% error.
If the second analysis does not give the correct amount of increase in the
iron content, it must be concluded the first answer also may be incorrect.
The analyst must determine why the technique did not work. The source
of the problem can be determined by using a logical troubleshooting
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