Hach DR/700 Procedures Manual page 28

The apparent interference in Example B could be the result of an error
made in the standard addition. The analysis should be repeated.
The two examples illustrate chemical interferences which most certainly
mean the result of the first analysis of the water sample was incorrect.
When this type of interference occurs, the analyst should attempt to
analyze the sample with an alternate method which uses a different type
of chemistry.
Branch G
Examples of uniform increments between standard additions are given
below.
Example C:
mL Std.
Added
0
0.1
0.2
0.3
Plot C illustrates a common interference with a uniform effect upon the
standard and the substances in the sample. The four data points form a
straight line which may be extrapolated back through the horizontal
axis. The point intersection with the horizontal axis can be used to
determine the concentration of the substance in question. In the
example, the first analysis showed 0.4 mg/L. The result located
graphically should be much closer to the correct result: 0.8 mg/L.
Apparent interferences also may be caused by a defect in the instrument
or the standards. Before assuming the interference is chemical in nature,
check Branch B.
Example D:
mL Std.
Added
0
0.1
0.2
0.3
mg/L Std.
Added
0
0.2
0.4
0.6
mg/L Std.
Added
0
0.2
0.4
0.6
1-10
mg/L
Found
0.4
0.5
0.6
0.7
mg/L
Found
0
0.2
0.4
0.6