Tandem mass spectrometry of prostaglandins: A comparison of an ion trap and a reversed geometry sector instrument

Abstract

The three-dimensional quadrupole ion trap was first described by Paul and Steinwedel several years ago. ' The development of the mass-selective instability mode of operation, in combination with the use of a lowmolecular-weight buffer gas at pressures around lop3 Torr, has led to significant improvements in mass range, resolution and sensitivity.2 Recently, it has been demonstrated that chemical ionization (CI) and MS/MS are possible with the same basic electrode s t r u~t u r e ,~'~ without the need for specific ion sources or additional analyzers. Depending on the nature of the parent ion, collisionally activated dissociation efficiencies approaching 100% can be a~h i e v e d .~ Recent attempts to study selectivity effects in GC/MS/MS assays of prostaglandins using a reversedgeometry sector instrument (referred to as the B-E instrument) were hindered by the low efficiency of the CAD process. This limited the instrumental response obtainable in certain selected-reaction-monitoring (SRM) experiments on biological samples. Continuing evaluations of how selectivity of analysis was affected by the type of fragmentation chosen, as well as other parameters, were limited. It was therefore decided to investigate the MSIMS analysis of prostaglandins using the ion trap mass spectrometer (ITMS). The types of fragmentation observed and the daughter ion intensities were compared to results obtained on a B-E sector mass spectrometer.' Sensitivity in the GC/MS/MS mode was also evaluated using an ammonia-CI source in the first MS stage. Preliminary results are reported herein. EXPERIMENTALThe derivatization of prostaglandin E2 as the methyl ester, 0-methoxime, bis-TMS ether derivative (PGE2-MeMoxTMS2) and MS/MS analysis on the B-E instrument [ZAB-2F, Vacuum Generators, Altrincham, UK] have already been described.' A description of the ion trap mass spectrometer [ITMS, Finnigan-MAT, San Jose, CA, USA] prototype has recently been p u b l i~h e d .~ The instrument used for these studies is a modification of the prototype, and is described in detail in publications from the manufacturer. The basic methods of preparing and admitting the sample are the same for the ZAB-2F and the ITMS. In both cases, the ' Author to whom correspondence should be addressed.daughter ion spectra were obtained by direct probe analysis of standards. In the case of the B-E instrument, a 2 pg sample (based on initial PGE2 concentration) was analyzed. On the ITMS, about 100ng was used with a constant probe temperature of 60°C. The ion trap cell was kept at 105°C. G C conditions were: 15m DB-5 (0.25pm film), He carrier gas (30cm/s), 1 pL splitless injection, 60" (2 min), then 30"/min to 310" (10 min).On the ITMS, MS/MS analyses were carried out with a new scap function (Fig. 1). The stability diagram is shown in Fig. 2. It is important to remember that higher masses have lower qz values and that stable trajectories (ions stored) correspond to the cross-hatched region.Electron or ammonia-chemical ionization at relatively low-amplitude, radiofrequency (RF, 1....

Determination of daughter ion formulas by multiple stages of mass spectrometry

“…The instrument was operated in the mass-selective instability mode (i.e., radio frequency (rf) only) [12] except for the parent ion isolation step. For parent ion isolation, a direct current voltage was applied to the ring electrode along with the rf voltage (mass-selective stability mode of operation) [13]. Following mass selection, the parent ion was kinetically excited, leading to collision-activated dissociation (CAD), by application of a supplementary rf voltage to the endcaps of the ion trap at the resonant frequency of the parent ion [14].…”

Combined Ion/Molecule Reactions and Multi-stage Tandem Mass Spectrometric Techniques in an Ion Trap Mass Spectrometer for the Characterization of Conjugated Ethylenic Epoxides. 1—Protonating Agents

“…Isolation of particular parent ions was performed using the QISMS software according to either (i) RF-DC apex isolation 31,32 or (ii) resonance ejection/broadband ejection. 33,34 Collision-induced dissociation (CID) was achieved by resonant excitation 35,36 under the following conditions: q z , 0.2, 0.5-0.7 V (optimized), 5 ms.…”

Mass‐selected reagent ion chemical ionization and multiple tandem mass spectrometry techniques in an ion trap mass spectrometer for structural analysis

“…Two methods were used for parent or reagent ion isolation: (a) RF-DC Apex Isolation (apex mode) 15,16 in which isolation is achieved by moving the working point of the ion first by the RF drive (q = 0.78) and then by a negative DC pulse to the vicinity of the upper apex of the stability diagram. (b) Resonance Ejection/ Broadband Ejection (RF/BB mode), 17,18 a two-step sequence with (i) the lower mass ions being ejected by resonance scanning using a supplementary frequency of 485 KHz (Axial Modulation) and (ii) higher mass ions resonantly ejected by applying a broadband multifrequency waveform (20-400 KHz) while the parent ion is placed at q = 0.84.…”