Biomedical Engineering Reference
In-Depth Information
Fig. 6
Main MS cleavage for amphetamine and methylenedioxyamphetamine
urinary metabolites (glucuronide or sulfate conjugates). Signal is dependent not on
the
absolute amount
of analyte entering the source, but the
concentration
of the
analyte (i.e., the concentration of analyte in the injected volume) and the eluent
fl ow-rate [
73,
74
] .
Atmospheric pressure chemical ionization (APCI), is more efficient than ESI for
nonpolar (hydrophobic) analytes, such as steroids, which do not readily form ions
in solution [
61
]. Since APCI requires use of a heated nebulizer, thermally labile
compounds may decompose in the ionization source.
Some modern instruments are supplied with dual-ionization sources, with the
ability to rapidly switch between ESI and APCI during a chromatographic run or
even carry out both processes simultaneously [
75
] .
Since majority of illicit drugs are basic compounds, ESI in positive ionization
mode is the most widely used source for these analysis, although negative ionization
mode is also used in some cases.
8.2
MS Detection
Illicit drugs and their metabolites can have similar fragmentation pattern, so the use
of multiple MRM transitions whenever possible, can also help to differentiate iso-
baric interferences.
In literature are reported fragmentation studies for most of known illicit drugs,
here are two examples.
Derivatives of phenylethylamine
: The chemical structure of phenylethylamine is
shown in Fig.
3
. Fragmentation can occur by cleaving of the bond in a or b amino
nitrogen atom. The major route of fragmentation is in a to nitrogen, leading to the
formation of a carbocation and a secondary amine, in fact this fragment appears to
be the base peak for most of the CID spectra of these substances [
76
] .
Molecules such as amphetamine and methamphetamine are different in their
amino substitutions on nitrogen and, following the fragmentation pattern seen
above, it is clear how these molecules have a common fragment of
m
/
z
119 amu as
can be seen in Fig.
6
. The other fragment that appears in the CID spectra of these
two substances falls at
m
/
z
of 91 amu, due to tropylium ion fragment, characteristic
