High-energy collision-induced dissociation by MALDI TOF/TOF causes charge-remote fragmentation of steroid sulfates

A method for structural elucidation of biomolecules dating to the 1980s utilized high-energy collisions (~10 keV, laboratory frame) that induced charge-remote fragmentations (CRF), a class of fragmentations particularly informative for lipids, steroids, surfactants, and peptides. Unfortunately, the capability for high-energy activation has largely disappeared with the demise of magnetic sector instruments. With the latest designs of tandem time-of-flight mass spectrometers (TOF/TOF), however, this capability is now being restored to coincide with the renewed interest in metabolites and lipids, including steroid-sulfates and other steroid metabolites. For these metabolites, structure determinations are required at concentration levels below that appropriate for NMR. To meet this need, we explored CRF with TOF/TOF mass spectrometry for two groups of steroid sulfates, 3-sulfates and 21-sulfates. We demonstrated that the current generation of MALDI TOF/TOF instruments can generate charge-remote fragmentations for these materials. The resulting collision-induced dissociation (CID) spectra are useful for positional isomer differentiation and very often allow the complete structure determination of the steroid. We also propose a new nomenclature that directly indicates the cleavage sites on the steroid ring with carbon numbers.

Separation of Isobaric Compounds Using a Spiral Orbit Type Time-of-Flight Mass Spectrometer, MALDI-SpiralTOF

The development of a MALDI-TOF mass spectrometer that utilizes spiral ion trajectory, SpiralTOF, is reported. The total flight path length was 17 m, which is five times longer than that in commonly used reflectron ion optical system. The SpiralTOF reduced the dependence of the mass resolving power on the mass of the analyte, while improving the accuracy of the mass measurements. Furthermore, SpiralTOF has two advantages that can be exploited for the separation of minor abundant isobaric components in mass spectra. One is the reduction in chemical background due to the post source decay (PSD), which is achieved through PSD ion elimination by electrostatic sectors contained within the SpiralTOF. The other is that the stabilities of peak positions are improved during mass spectrum accumulation. The peak drift caused by the fine structure of matrix crystals and the small irregularities on the sample surface can be reduced by extending the flight path. In this study, these advantages are demonstrated via the analysis of a block copolymer and mass spectrometry imaging (MSI) of lipids.

Tandem time-of-flight mass spectrometer with high precursor ion selectivity employing spiral ion trajectory and improved offset parabolic reflectron

In this study, we have developed a tandem time-of-flight mass spectrometry (TOF/TOF) technique involving the use of a matrix-assisted laser desorption/ionization ion source that exhibits high precursor ion selectivity. An ion optical system with a 17 m spiral ion trajectory was used in the first time-of-flight mass spectrometer. High precursor ion selectivity was achieved by realizing a 15 m flight path, which is considerably longer than that of the conventional MALDI-TOF/TOF before the precursor ion selection by an ion gate; monoisotopic ions could be selected properly up to m/z 2500. Furthermore, the first time-of-flight mass spectrometer was composed of electrostatic sectors and could eliminate post-source decay (PSD) ions. Precursor ions with 20 keV kinetic energy were selected and injected into a collision cell, leading to the generation of fragment ions by high-energy collision-induced dissociation (HE-CID). The optimized second time-of-flight mass spectrometer included a post-acceleration region and an offset parabolic reflectron to record product ion spectra in the entire mass range. Our system could generate a simple HE-CID product ion spectrum because each fragment pathway could be observed as a single peak by the selection of monoisotopic ions of all precursor ions and HE-CID fragment pathways could be predominantly observed by the PSD ion elimination.

Design of a new multi-turn ion optical system 'IRIS' for a time-of-flight mass spectrometer

A new multi-turn ion optical system 'IRIS' has been designed for use with a high-performance time-of-flight (TOF) mass spectrometer, which satisfies the new design concepts of time focusing and phase space stability. It has an elliptical flight path composed of four toroidal electric sectors, with a flight path length for one lap of 0.974 m. Dimensions and voltages of sector electrodes have been optimized to satisfy theoretical requirements by simulations using surface charge method. Generally, multi-turn instruments require an injection and ejection system to inject and eject ions. On the basis of this ion optical study, we have designed an injection and ejection ion optical system, which achieves time focusing for the total system. Furthermore, we have designed novel field-adjusting electrodes (FAEs) for the perforated sectors in the injection and ejection systems, which accurately correct the electric potential around the perforated sector's hole. We have also used simulations to evaluate mass resolving power and ion transmissions for various lap numbers or flight path lengths. Through these we have confirmed that mass resolving powers of over 100,000 can be achieved with reasonable ion transmissions for a given set of initial conditions. Usually a multi-turn TOF mass spectrometer with a closed optic axis has mass range limitations from overtaking ions. To solve this problem, a TOF segmentation method is proposed that identifies all peaks in a TOF spectrum, including those from overtaking ions.