Agriculture Reference
In-Depth Information
physicochemical screening techniques will be covered. For initial method validation,
at least one analyte from each known chemical class or subclass should be selected.
For example, in the case of quinolones, one acidic compound and one amphoteric
compound could be chosen for the validation study. Even when analytes have similar
physicochemical properties, they are likely to have different retention times and
therefore coelute with different concentrations of matrix coextractives. Thus, they can
become subject to different ion suppression or ion enhancement effects. For this
reason, it is preferable to test all analytes and not just a selection. It should be noted
that there are a number of different terminologies pertaining to veterinary drug residue
analyses compared with pesticide residue analyses.
There are two commonly used approaches to determining speci
city/selectivity
and detection capability (CC
): the
“
classical approach
”
and the
“
alternative matrix-
β
comprehensive approach.
”
3.6.2 Determination of Speci
city/Selectivity and Detection Capability (CC
β
)
Using the Classical Approach
The degree of con
dence required and the ratio between the screening target
concentration (STC) and the action/regulatory limit (AL) should be used to determine
the number of “screen-positive” control samples (samples spiked at the STC) that
should be tested. Speci
c examples of the use of these values are given below:
If the STC is set at 50% of the AL or lower, the occurrence of one, or no, false-
compliant result from the analysis of 20 replicate samples spiked at the STC is
considered to be suf
cient to demonstrate that CC
is less than the AL.
β
If the STC is set between 50 and 90% of the AL, the occurrence of no more than
two false-compliant results from the analysis of 40 replicate samples spiked at
the STC is considered to be suf
cient to demonstrate that CC
is less than the
β
AL.
If the STC is set between 90 and 100% of the AL, the occurrence of no more than
three false-compliant results from the analysis of 60 replicate samples spiked at
the STC is considered to be suf
cient to demonstrate that CC
is
fit for purpose.
β
If the method is to be applied to a single matrix, for example, muscle, then the
replicate sample spikes can be split into separate animal species, such as bovine,
porcine, and poultry.
3.6.3 Establishment of a Cutoff Level and Calculation of CC
β
Validation of screening methods requires identi
cation of a cutoff level at, or above,
which the sample would be deemed to be “screen positive” and liable to further
conformational analysis using the additional speci
city of a physicochemical method.
The STC at which the blank matrix samples are to be spiked should ideally be at
50% of the AL. If this is not possible, a concentration between 50 and 100% of the AL
