68
If iB > 0, then the signal for the limit of detection will be
XWD=XB +3aB
2 Analytical Properties
Interpolation of this signal into the calibration curve provides the relation between sensitivity and the limit of detection
XWD(= XB + 3aB) = XB + SCWD
3aB
C WD = - S
Because S is not constant along the initial portion of the curve, this expression is
only approximate. The relationship between CWD and S depends on the value of
the blank.
The limit of quantitation is defined as the analyte concentration C WQ which
produces a signal XWQ that can be considered the lower limit of the linear range.
Its mathematical expression is also based on statistical processing of blanks:
It is therefore a value above the limit of detection (C WQ > CWD ). Its relationship
to sensitivity can be established similarly to that of the limit of detection:
lOaB
C WQ = - -
S
A comprehensive study of sensitivity relies on statistical hypothesis testing, a
detailed description of which is beyond the scope of this book. Interested readers
are referred to the book by Kateman and Buydens (suggested reading no. 4) for
an extensive justification of the previous definitions.
Some authors use the so-called limit of decision, Cw , the value of which lies
in between those of the previous two parameters.
X w = X + 6aB
Figure 2.12 illustrates in graphical form the concepts of blind, decision, detection
and quanti tat ion region, which arise in placing the three types of limit on an
analyte concentration scale that is related to the analyte signal scale and the
blank signal scale via the previous expressions .
• In the blind region (0 < C < CWD ), the analyte can be neither detected nor
determined as the signal obtained is not statistically different from the blank
signal or background noise from the measuring instrument.
• In the detection region (CWD < C > C WQ ), the analyte can be detected but not
quantified with accuracy since C falls outside the linear range.
If iB > 0, then the signal for the limit of detection will be
XWD=XB +3aB
2 Analytical Properties
Interpolation of this signal into the calibration curve provides the relation between sensitivity and the limit of detection
XWD(= XB + 3aB) = XB + SCWD
3aB
C WD = - S
Because S is not constant along the initial portion of the curve, this expression is
only approximate. The relationship between CWD and S depends on the value of
the blank.
The limit of quantitation is defined as the analyte concentration C WQ which
produces a signal XWQ that can be considered the lower limit of the linear range.
Its mathematical expression is also based on statistical processing of blanks:
It is therefore a value above the limit of detection (C WQ > CWD ). Its relationship
to sensitivity can be established similarly to that of the limit of detection:
lOaB
C WQ = - -
S
A comprehensive study of sensitivity relies on statistical hypothesis testing, a
detailed description of which is beyond the scope of this book. Interested readers
are referred to the book by Kateman and Buydens (suggested reading no. 4) for
an extensive justification of the previous definitions.
Some authors use the so-called limit of decision, Cw , the value of which lies
in between those of the previous two parameters.
X w = X + 6aB
Figure 2.12 illustrates in graphical form the concepts of blind, decision, detection
and quanti tat ion region, which arise in placing the three types of limit on an
analyte concentration scale that is related to the analyte signal scale and the
blank signal scale via the previous expressions .
• In the blind region (0 < C < CWD ), the analyte can be neither detected nor
determined as the signal obtained is not statistically different from the blank
signal or background noise from the measuring instrument.
• In the detection region (CWD < C > C WQ ), the analyte can be detected but not
quantified with accuracy since C falls outside the linear range.