Hach DR/700 Procedures Manual page 27

graph as A and B show that the four data points do not lie on a straight
line. Plot A illustrates an interference becoming progressively worse as
the concentration of the standard increases. This type of interference is
not common and may be caused by an error or malfunction of the
procedure, reagents or instrument. It is recommended Branch B be
performed to verify the supposed interference.
Plot B illustrates a common chemical interference which becomes less
or even zero as the concentration of the standard increases. The graph of
the example shows the first standard addition was consumed by the
interference and the remaining additions gave the correct incremental
increase of 0.2 mg/L.
Figure 2 Multiple Standard Additions Graph
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