Chemistry Reference
In-Depth Information
tAble 4.5
calculation of dl and Ql
conc. (ng/ml)
response
1.5
5250
3
7950
7.5
16650
15
31800
30
58950
R Square
0.9995
Standard error of intercept
380.71
Slope
1892.87
Note:
From table values: DL = 3.3(380.7)/1,892.87 = 0.66 ng/mL.
From
table values:
QL = 10(380.71)/1,892.87 = 2.01 ng/mL.
4.3.5 Q
uAntItAtIon
l
ImIt
The quantitation limit (QL, also sometimes called the limit of quantitation) is defined as
the lowest concentration of an analyte in a sample that can be quantitated with accept-
able precision and accuracy under the stated operational conditions of the method.
QL can be determined by some of the same procedures used to determine the
DL, either by S/N or on a calculation based on
the standard deviation of the response
and the slope of a calibration curve. For QL, the S/N ratio of 10:1 is used as a rule of
thumb because actual QL determinations must take into account the method objec-
tives of accuracy, precision, and the desired quantitative level. Typically, the signal is
measured from baseline to peak apex and divided by the peak-to-peak noise deter-
mined from a blank injection. It is important that the noise is measured in the blank
chromatogram during the same elution window as the peak of interest.
Calculations based on the standard deviation of the response and the slope of the
calibration curve are based on the following formula:
QL = 10*σ/S
where σ is the standard deviation of the response and S is the slope of the calibration
curve. The slope may be estimated from the calibration curve of the analyte, or a sep-
arate curve approaching the QL may be prepared. The value of σ may be determined
based on the standard deviation of blank injections, the residual standard deviation
of response, or the standard deviation of y-intercepts of the regression lines of the
calibration curve. Table 4.5 provides a simple example of determining the QL using
this formula where the response was determined at five levels (minimum number of
levels for linear curve [Section 4.3.6]).
Determination of σ for the standard deviation of blank injections is performed
by analyzing an appropriate number of blank samples for the magnitude of analyti-
cal background response and calculating the standard deviation of these responses.
As with DL, when using the calibration curve calculation, the standard error of the
