Seminars: Numerical Problems
95
The uncertainty is determined by assessing the precision for samples
containing 1.0 ]lglL copper; absorbances over the range 0.453 ± 0.020 AU
are obtained at the 95 % probability level. Then, a series of samples
containing 1.0 ]lg/L copper and increasing concentrations of nickel from
o to 3 Jlg/L is prepared that provides the following results:
Sample no.
I
[Cu H ] (JlglL)
1.0
[NiH ] (JlglL)
0.0
Absorbance (AU)
0.452
500
• Types of interferences involved:
- positive
- same mechanism
- chemical origin
- additive
2
3
1.0
1.0
0.5
1.0
0.457 0.462
540
• The limits of confidence (in absorbance units) are:
0.433
0.473
4
5
1.0
1.0
1.5
2.0
0.473 0.489
nm
• If a response above 0.473 is obtained, then an interference will occur.
• A detailed study reveals that
Experiment
I
2
3
4
5
[Cu2+]
1.0
1.0
1.0
1.0
1.0
[Ni 2+]I[ Cu 2 +]
0
0.5
1.0
2.5
2.0
AU
0.453
0.457
0.462
0.473
0.489
Systematic error
No + yes
• Ni 2 + is thus tolerated at concentrations 1.5 times higher than is Cu 2 +.
6
1.0
3.0
0.504
6
1.0
3.0
0.504