Highly Sensitive Pharmaceutical and Clinical Analysis Using Selective Solid-Phase Extraction Coupled to Micro flow Liquid Chromatography and Isotope-Dilution Mass Spectrometry - LC-MS in Drug Bioanalysis - page 88

Fig. 3 Comparison of mLC/MS/MS chromatograms of budesonide (BUD) in plasma after ( a ) a
generic SPE extraction and ( b ) a selective SPE extraction. The concentration of BUD was 20 pg/
mL and the injection volume was 5 mL (Reproduced with permission from American Chemical
Society)
adjusted to pH 3.2 using formic acid, noticeably suppressed both the [M + Na] + and
[M + K] + ions, and the[M + H] + ions became predominant. Therefore, this acidified
2 mM ammonium acetate-formate mixture was selected as the mobile phase
modifier. The collisionally activated dissociation (CAD) spectra of the four analytes
were individually investigated, and the major fragments are shown in Table 1 [ 4 ] .
The results are in agreement with previous publications, and the most abundant
product ions were chosen for the MRM of each corticosteroid. The MS/MS condi-
tions for each transition were optimized in order to achieve the maximum S/N, and
the position of the spray needle was adjusted to achieve the optimal S/N for all
compounds. The optimized collision energies for the MRM transitions are shown in
Table 1 for each corticosteroid.
It was demonstrated previously that smaller inner-diameter (I.D.) columns with
lower flow rates dramatically improve the sensitivity of LC/ESI-MS/MS. However,
smaller I.D. columns also have proportionally lower loading capacities and may