Agriculture Reference
In-Depth Information
sought routinely. These analytes might be nonapproved chemicals that are unexpected
but could be present due to misuse.
3.2.1 Selectivity of Mass Spectrometry-Based Methods
Typically, quantitative MRMs are based on nominal mass quadrupole technologies,
either GC
MS operated in selective ion monitoring (SIM) mode or tandem quadru-
poles (MS/MS) operated in selected reaction monitoring (SRM) mode. Physico-
chemical qualitative screening multiresidue methods (QSMRMs) often employ high-
resolution/accurate mass time-of
-
flight (ToF-MS), ion trap, or hybrid instruments
operated in full-scan mode. Although quantitative MRMs provide identi
cation of
individual analytes, only those analytes that have ions preprogrammed into the
method will be detected. In contrast, qualitative MRMs do not provide suf
cient
information to meet quanti
cation AQC criteria, but can, at least in principle, detect
any analyte that is included in a spectral library or database of compounds that is
linked to the mass spectrometer. In practice, the ef
ciency of operation of the method
is largely dependent on the performance of the data processing software package
associated with the library.
Methods involving the use of GC
MS/MS most often base
analyte identification on the generation of precursor and product ions. The “identifi-
cation power
-
MS(/MS) and/or LC
-
of the method is then dependent on selectivity. The measure of
selectivity is the probability of any compound in a sample extract showing the same
precursor ion, product ions, and retention time as an analytical standard of the same
compound in the same extract matrix that has been analyzed using the same operating
conditions. It is therefore obvious that the selection of speci
”
c characteristic ions is
essential for unambiguous identi
cation. This aspect of the method has not been
addressed in any detail in any method validation guideline document. Berendsen
et al. [4] have recently published a paper that describes how the probability of co-
occurrence of a compound showing the same characteristics in LC
MS/MS can be
estimated from empirical models, derived from three databases that included data on
precursor ion mass, product ion mass, and retention time. This approach describes
how the
-
MS/MS method operated in MRM
acquisition mode can be determined. A further paper by Kmellár et al. [5] similarly
details the effects of operational parameters on pesticide residue analysis by LC
“
identi
cation power
”
of an LC
-
-
MS/
MS. The in
ers on retention time and detector
response was found to be important. The use of mixed analytical standards containing
150 different pesticides was also found to produce signi
uence of different mobile phase modi
cant suppression or
enhancement of coelution effects.
3.2.2 Con
rmatory Methods
A screening method is normally qualitative or semiquantitative and it must be
validated to a certain minimum target analyte concentration to allow results to be
reported. The method must also be validated to ensure that possible false-positive and
false-negative results are minimized. To this end, tentative identi
cations from
