2.3 Errors in Analytical Chemistry
51
Result
Specific uncertainty
R
±
u
Gross errors
Systematic errors
Random errors
Fig. 2.6. Expression of a result (and'its uncertainty) and relationships to the different types of
error and to the analytical properties accuracy and precision
This brief introduction to errors in Analytical Chemistry is the basis for
defining accuracy (a capital analytical property) and precision (a basic analytical property). Both were mentioned earlier in this section. Figure 2.6 shows
the relationships of gross and systematic errors to the analytical result, R, via
accuracy, and that of non-specific uncertainty, U, to random errors via precision.
Such a categorical difference is not factual: as shown in forthcoming sections,
accuracy is impossible in the absence of precision.
Box 2.4
Let us illustrate the previous definitions of errors in Analytical Chemistry with the fol lowing
example: four different methods (A. B. C and DJ are applied n times each to aliquots of the
same sample in order to determi ne the concentration of an analyte for which the value held
Errors
Method
x ~
Random Systematic
A
• • • .1. • • • •
High
High
Not accurate
(-j
(Bias)
not precise
Xi
+
B •• 1.
low
High
Not accurate
..
H
(Bias)
p,«i ..
~ Xc
C
• • • •
I
• • • •
High
low
Accurate
r
not prt
•
PD
•••••
low
Low
ACC\lnlt
'r'
precise
0
X'
51
Result
Specific uncertainty
R
±
u
Gross errors
Systematic errors
Random errors
Fig. 2.6. Expression of a result (and'its uncertainty) and relationships to the different types of
error and to the analytical properties accuracy and precision
This brief introduction to errors in Analytical Chemistry is the basis for
defining accuracy (a capital analytical property) and precision (a basic analytical property). Both were mentioned earlier in this section. Figure 2.6 shows
the relationships of gross and systematic errors to the analytical result, R, via
accuracy, and that of non-specific uncertainty, U, to random errors via precision.
Such a categorical difference is not factual: as shown in forthcoming sections,
accuracy is impossible in the absence of precision.
Box 2.4
Let us illustrate the previous definitions of errors in Analytical Chemistry with the fol lowing
example: four different methods (A. B. C and DJ are applied n times each to aliquots of the
same sample in order to determi ne the concentration of an analyte for which the value held
Errors
Method
x ~
Random Systematic
A
• • • .1. • • • •
High
High
Not accurate
(-j
(Bias)
not precise
Xi
+
B •• 1.
low
High
Not accurate
..
H
(Bias)
p,«i ..
~ Xc
C
• • • •
I
• • • •
High
low
Accurate
r
not prt
•
PD
•••••
low
Low
ACC\lnlt
'r'
precise
0
X'