Biomedical Engineering Reference
In-Depth Information
ionized by ESI or APCI. Theron et al. [
61
] compared ESI and APPI ionization for the
analysis of venlafaxine and its metabolite
O
-desmethylvenlafaxine in plasma and
water solutions. They concluded that the linearity of response in water, which is lost
at high concentrations with ESI due to droplet surface saturation or limited amount
of excess charge on the droplet, could be increased with APPI ionization. Moreover,
APPI was less susceptible to ion suppression than ESI, with which signal response
and calibration range was dramatically reduced in plasma compared to water, while
similar results were obtained with APPI in both experiments. Regardless of the
ionization method, positive ionization mode was employed in all cases, except for
sulfate conjugates, which are more efficiently ionized in negative mode [
45,
73
] .
4.2
Mass Analyzer
With regard to the mass analyzer, quadrupoles (Q) were usually employed due to its
suitability for quantitative analysis and its relatively low prices, affordable for most
laboratories. Ion trap mass spectrometers (IT) were employed in some but few
LC-MS methodologies [
65,
69,
74,
75
]. Although these analyzers are less robust for
quantitative analysis than Q, MS
n
spectrums can be obtained by successive selection
and fragmentation of specific product ions. Franceski et al. [
94
] used Q-IT tandem
mass spectrometer, combining the advantages of both analyzers, for the determina-
tion of fluoxetine and its metabolite norfluoxetine. A time-of-flight (TOF) mass
analyzer was used for fragmentation pathway elucidation of some TCAs [
58
] and
sertraline [
62
], structural elucidation of bupropion metabolites in urine [
45
] , and
fl uoxetine quantitation [
95
]. TOF analyzers allow accurate mass measurement with
an assignation of four decimal digits, which dramatically reduces the possible
elemental formula of the detected analyte; however, TOF is more expensive and has
a narrower linear dynamic range than that achieved with Q and, therefore, the later
is preferable for quantitative analysis of target analytes.
5
LC-MS Applications for Antidepressant
Quantitative Analysis
Quantitative LC-MS analysis is usually performed by selection of the pseudomo-
lecular ion of the analyte of interest in the selected ion monitoring (SIM) mode. In
single quadrupole instruments, selection of fragment ions (
m/z
) is also possible by
promoting collision induced dissociation (CID) reactions in the entrance to the mass
spectrometer. Tandem LC-MS/MS instruments enhance selectivity and signal-
to-noise ratio compared to single quadrupole instruments by operating on multiple
reaction monitoring (MRM) mode. In MRM mode, specific precursor-to-product
ion transitions can be monitored by fragmentation of the analyte in the collision cell
located between the two analyzers, and subsequently monitoring a selected
