Biomedical Engineering Reference
In-Depth Information
accuracy is simply termed trueness [
60
]. While this approach is still accepted in
forensic toxicology, it is preferably to use a control material (certified reference
material, CRM) with a specific target concentration, which will allow independent
verification of calibration accuracy [
56
], as also assessed by EC Guidelines. If no
CRM is available, instead of trueness, the recovery can be determined [
57
] .
7.2.2
Recoveries
Recovery means the percentage of the true concentration of a substance recovered
during the analytical procedure. It can be determined as follows: select 18 aliquots
of a blank material and fortify six aliquots at each of 1, 1.5, and 2 times the mini-
mum required performance limit or 0.5, 1, and 1.5 times the permitted limit, analyze
the samples and calculate the concentration present in each sample, calculate the
recovery for each sample, calculate the mean recovery and the coefficient of varia-
tion (CV) or the relative standard deviation (RSD) from the six results at each level
[
57
]. Nevertheless in toxicological analysis, as assessed by Peters et al. [
60
] , a high
value for recovery is not mandatory, as long as the data for LLOQ, precision and
accuracy (bias) are acceptable. It can be calculated as the percentage of the analyte
response after sample workup compared to that of a solution containing the analyte
at a concentration corresponding to 100 % recovery. In the validation of LC-MS-
(MS) methods, it is therefore more appropriate to perform the recovery experiments
together with ion suppression/enhancement experiments [
60
] .
7.2.3
Precision
Precision should be measured using a minimum of five determinations per concen-
tration. A minimum of three concentrations in the range of expected concentrations
is recommended. The precision determined at each concentration level should not
exceed 15 % of the coefficient of variation (CV) except for the LLOQ, where it
should not exceed 20 % of the CV. Precision is further subdivided into within-run,
intrabatch precision or repeatability, which assesses precision during a single ana-
lytical run, and between-run, interbatch precision or reproducibility, which mea-
sures precision with time, and may involve different analysts, equipment, reagents,
and laboratories. Since it is not always easy to obtain data about the reproducibility
in the strict sense it often makes sense to use intermediate precision: this expresses
within-laboratories variations on different days, by different analysts, and on differ-
ent equipment, etc. [
60
] .
7.2.4
Selectivity
Proving the lack of response in blank matrix is the way to establish method selectiv-
ity. It needs to analyze blank samples and check for interferences (signals, peaks,
ion traces) in the region of interest where the target analyte is expected to elute.
