Agriculture Reference
In-Depth Information
two product ions formed are unequivocal characteristics of a substance. In Directive
2002/657/EC, the concept of identi
cation points (IP) was set to strengthen the MS
con
rmation of the presence of a substance. The need of a minimum of 3 IP
(substance group B
=
MRL permitted veterinary drugs) or 4 IP (substance group
A
banned veterinary drugs) was put into force. Thanks to HRMS technology, 2 IP
for a precursor ion and 2.5 IP for a product ion could be achieved. So DDA experiment
in HRMS leads to suf
=
cient IP to con
rm group A or group B substances.
7.3.2 Nontargeted Analysis Using HRMS: Screening for Unknown
Compounds
Identi
cation of untargeted analytes in biological sample matrices from high-
resolutionMSrawdataisnotaneasytaskduetothepresenceofanenormous
number of endogenous matrix ions. This dif
culty can be overcome by data
processing through specialized software that can help in
finding biomarkers or
drug metabolites. These specialized software packages have the capability of
comparing different MS raw data
files. Generally, mass spectrometry-based
drug metabolomic studies are carried out by comparing acquired MS data of
both the control samples and the drug-incurred samples with the help of chemo-
metric software tools. In the incurred samples, the statistically obtained differences
in the mass-to-charge ratio (
m
/
z
) of the detected ions can assist in the identi
cation
of substances or metabolites. However, in this process, it must be veri
ed that the
observed
m
/
z
differences truly represent differences in the sample and are not
simply due to analytical variations. But even with
<
3 ppm mass accuracy data, it is
possible that a few dozen chemical formulas
ed mass-to-charge ratio. It
then requires a lot of effort to deduce the reliable formula among all the likely
candidates. In this pursuit, preliminary data mining can be implemented in order to
choose compatible structures using the increasing number of databases that can be
accessed from university web sites and elsewhere on the World Wide Web. The
greater the instrument
tthespeci
s mass resolution or the smaller the mass error, the fewer the
compatible chemical formulas, which increases the con
'
dence with which any
ed. Furthermore, the suspected ions can be
subjected to MS/MS analysis to obtain the fragmentation pattern with accurate
mass and ring double bond equivalents that can help in the structure elucidation.
Interpretation of product ion spectra of unknown analytes requires extensive
knowledge and experience. In addition, manual interpretation is a time-consuming
process and assigning of all product ions in a spectrum is dif
unknown molecule can be identi
cult. In order to
simplify the interpretation of the MS/MS spectra, some research groups are working
on the development of algorithms to interpret high-resolution MS/MS spectra of
unknown compounds or metabolites [23
25]. With these general targeted and
nontargeted strategies in mind, we discuss two speci
-
c applications of LC
-
MS and
LC
MS/MS to analyze the veterinary medicine degradation products or their
metabolites in different sample matrices. The following examples provide a
glimpse of the extraordinary variety of applications for which these instruments
have been used.
-
