Sulfate migration in oligosaccharides induced by negative ion mode ion trap collision‐induced dissociation

Migration of sulfate groups between hydroxyl groups was identified after collision‐induced dissociation (CID) of sulfated oligosaccharides in an ion trap mass spectrometer in negative ion mode. Analysis of various sulfated oligosaccharides showed that this was a common phenomenon and was particularly prominent in sulfated oligosaccharides also containing sialic acid. It was also shown that the level of migration was increased when the sulfate was positioned on the flexible areas of the oligosaccharides not involved in the pyranose ring, such as the extra‐cyclic C‐6 carbon of hexoses or N‐acetylhexosamines, or on reduced oligosaccharide. This suggested that migration is dependent on the spatial availability of the sulfate in the ion trap during collision. It is proposed that the migration is initiated when the negatively charged ‐SO₃^– residue attached to the oligosaccharide precursor becomes protonated by a CID‐induced proton transfer. This is supported by the CID fragmentation of precursor ions depleted of acidic protons such as doubly charged [M – 2H]^2– ions or the sodiated [M + Na – 2H]^– ions of oligosaccharides containing one sulfate and one sialic acid in the same molecule. Compared to the CID fragmentation of their monocharged [M – H]^– ions, no migration was observed in CID of proton depleted precursors. Alternative fragmentation parameters to suppress migration of sulfated oligosaccharides also showed that it was not present when sulfated oligosaccharides were fragmented by HCD (High‐Energy C‐trap Dissociation) in an Orbitrap mass spectrometer. Copyright © 2011 John Wiley & Sons, Ltd.     

Top‐down proteomics with a quadrupole time‐of‐flight mass spectrometer and collision‐induced dissociation

With slight modifications of the instrumental parameters, we demonstrate that satisfactory top‐down data can be obtained with collision‐induced dissociation (CID) tandem mass spectrometry on a quadrupole time‐of‐flight (qTOF) instrument not originally designed for this purpose. Protein identification is achieved with both N‐ and C‐terminal sequence tags and BLAST database searches. The accurate mass measurement of multiply charged fragment ions (mostly y and b‐type) supplements the limited set of cleavage sites and provides a high degree of sequence coverage (90–100%). Post‐translational modification issues can be addressed too. This approach might help those mass spectrometry (MS) core facilities that are not able to afford very high‐resolution instruments, thus expanding the benefits of top‐down protein analysis over the worldwide MS community. Copyright © 2009 John Wiley & Sons, Ltd.     

Comparison of in‐source collision‐induced dissociation and beam‐type collision‐induced dissociation of emerging synthetic drugs using a high‐resolution quadrupole time‐of‐flight mass spectrometer

In‐source collision‐induced dissociation (CID) is commonly used with single‐stage high‐resolution mass spectrometers to gather both a molecular formula and structural information through the collisional activation of analytes with residual background gas in the source region of the mass spectrometer. However, unlike tandem mass spectrometry, in‐source CID does not involve an isolation step prior to collisional activation leading to a product ion spectrum composed of fragment ions from any analyte present during the activation event. This work provides the first comparison of in‐source CID and beam‐type CID spectra of emerging synthetic drugs on the same instrument to understand the fragmentation differences between the two techniques and to contribute to the scientific foundations of in‐source CID. Electrospray ionization–quadrupole time‐of‐flight (ESI‐Q‐TOF) mass spectrometry was used to generate product ion spectra from in‐source CID and beam‐type CID for a series of well‐characterized fentanyl analogs and synthetic cathinones. A comparison between the fragmentation patterns and relative ion abundances for each technique was performed over a range of fragmentor offset voltages for in‐source CID and a range of collision energies for beam‐type CID. The results indicate that large fragmentor potentials for in‐source CID tend to favor higher energy fragmentation pathways that result in both kinetically favored pathways and consecutive neutral losses, both of which produce more abundant lower mass product ions relative to beam‐type CID. Although conditions can be found in which in‐source CID and beam‐type CID provide similar overall spectra, the in‐source CID spectra tend to contain elevated noise and additional chemical background peaks relative to beam‐type CID.     

Surface-induced dissociation of molecular ions in a quadrupole ion trap mass spectrometer

A method is reported by which surface-induced dissociation is used to activate ions stored in a quadrupole ion trap mass spectrometer. The method employs a short (< 5 μs), fast-rising (< 20-ns rise time), high voltage direct current (dc) pulse, which is applied to the endcaps of a standard Paul-type quadrupole ion trap. This is in contrast to the application of an alternating current (ac) signal normally used to resonantly excite and dissociate ions in the trap. The effect of the dc pulse is to cause the ions rapidly to become unstable in the radial direction and subsequently to collide with the ring electrode. Sufficient internal energy is acquired in this collision to cause high energy fragmentations of relatively intractable molecular ions such as pyrene and benzene. The dissociations of limonene are used to demonstrate that high energy demand processes increase in relative importance in the dc pulse experiment compared with the usual resonance excitation method used to cause activation. The fragments are scanned out of the ion trap using the conventional mass-selective instability scan mode. Simulations of ion motion in the trap provide evidence that surface collisions occur at kinetic energies in the range of tens to several hundred electronvolts. The experiments also demonstrate that production of fragment ions is sensitive to the phase of the main radiofrequency drive voltage at the point when the dc is initiated.     

Investigations into the use of a reverse scan in a quadrupole ion trap mass spectrometer

The forward scan (i.e. an increasing RF voltage ramp for the mass-selective instability scan) is commonly used as an analytical scan for ion detection with quadrupole ion trap instruments. A number of phenomena have been observed while using this scan technique. These include space charge effects resulting in the delayed ejection of ions from the ion trap, and the fragmentation of fragile ions producing very broad peaks. Here the use of a reverse scan (i.e. a decreasing RF voltage ramp) is examined to determine the effect of the above phenomena in this acquisition method. With regard to space charge effects, the apparent reduction of the carbon isotope spacing below one Thomson (for singly charged ions) that is observed with the forward scan is now replaced by an apparent increase in this spacing. The reverse scan, which optimizes at lower axial modulation ejection voltages than the forward scan, allows for the intact ejection of fragile ions under its typical operating conditions whereas the forward scan results in fragmentation. Reducing the axial modulation voltage for the ejection of ions in the forward scan results in less dissociation of the fragile ions during ion ejection, but with the observation of ghost peaks due to incomplete ejection of all of the ions at the resonance ejection condition. While performing the reverse scan experiment, the formation of product ions from dissociation of the MH(+) ion has also been observed.     

Competition between resonance ejection and ion dissociation during resonant excitation in a quadrupole ion trap

The competition between ion dissociation and ion ejection during resonant excitation in a quadrupole ion trap is investigated. Ions of similar mass but with a range of critical energies for the onset of dissociation have been examined. The effects of the amplitude and duration of the resonant excitation, the well depth in which the ions are trapped, and the pressure and nature of the collision gas are explored. Once the onset of ion ejection is reached, the rate of ion ejection increases with increased amplitude of the resonant excitation signal. The rate of ejection decreases or stays constant as a function of the duration of the resonant excitation, depending upon the ion species being excited. Increasing the trapping well depth increases the relative amount of dissociation versus ejection as does increasing the pressure of the bath gas. Adding heavier bath gases lowers the onset of ion dissociation and raises the onset of ion ejection.     

Characterization of glycopeptides by combining collision‐induced dissociation and electron‐transfer dissociation mass spectrometry data

Structural characterization of a glycopeptide is not easily attained through collision‐induced dissociation (CID), due to the extensive fragmentation of glycan moieties and minimal fragmentation of peptide backbones. In this study, we have exploited the potential of electron‐transfer dissociation (ETD) as a complementary approach for peptide fragmentation. Model glycoproteins, including ribonuclease B, fetuin, horseradish peroxidase, and haptoglobin, were used here. In ETD, radical anions transfer an electron to the peptide backbone and induce cleavage of the N–Cα bond. The glycan moiety is retained on the peptide backbone, being largely unaffected by the ETD process. Accordingly, ETD allows not only the identification of the amino acid sequence of a glycopeptide, but also the unambiguous assignment of its glycosylation site. When data acquired from both fragmentation techniques are combined, it is possible to characterize comprehensively the entire glycopeptide. This is being achieved with a mass spectrometer capable of alternating between CID and ETD on‐the‐fly during an LC/MS/MS analysis. This is demonstrated here with several tryptic glycopeptides. Copyright © 2008 John Wiley & Sons, Ltd.     

Origin of mass shifts in the quadrupole ion trap: dissociation of fragile ions observed with a hybrid ion trap/mass filter instrument

A novel hybrid tandem mass analyzer, coupling a quadrupole ion trap with a quadrupole mass filter, has been constructed to permit mass analysis of ions ejected from the ion trap. The initial application of this instrument is the investigation of the origin of mass shifts in the ion trap due to ion fragility. We hypothesize that fragile ions undergo mass shifts, characterized by peak fronting, due to early ejection from the quadrupole ion trap. As these ions come into resonance with the ejection frequency, they gain kinetic energy, collide with buffer gas molecules and thus can dissociate to produce fragment ions. These fragment ions will not be stable within the ion trap as they are situated past the stability boundary at q(z) = 0. 908. Consequently the fragment ions are ejected prematurely. This results in an apparent mass shift due to peak fronting. The experiments reported here clearly document the production of fragment ions as the origin of mass shifts during the resonant ejection of fragile ions. Copyright 2000 John Wiley & Sons, Ltd.     

Diastereochemical differentiation of bicyclic diols using metal complexation and collision‐induced dissociation mass spectrometry

Metal complex formation was investigated for di‐exo‐, di‐endo‐ and trans‐2,3‐ and 2,5‐disubstituted trinorbornanediols, and di‐exo‐ and di‐endo‐ 2,3‐disubstituted camphanediols using different divalent transition metals (Co²⁺, Ni²⁺, Cu²⁺) and electrospray ionization quadrupole ion trap mass spectrometry. Many metal‐coordinated complex ions were formed for cobalt and nickel: [2M+Met]²⁺, [3M+Met]²⁺, [M–H+Met]⁺, [2M–H+Met]⁺, [M+MetX]⁺, [2M+MetX]⁺ and [3M–H+Co]⁺, where M is the diol, Met is the metal used and X is the counter ion (acetate, chloride, nitrate). Copper showed the weakest formation of metal complexes with di‐exo‐2,3‐disubstituted trinorbornanediol yielding only the minor singly charged ions [M–H+Cu]⁺, [2M–H+Cu]⁺ and [2M+CuX]⁺. No clear differences were noted for cobalt complex formation, especially for cis‐2,3‐disubstituted isomers. However, 2,5‐disubstituted trinorbornanediols showed moderate diastereomeric differentiation because of the unidentate nature of the sterically more hindered exo‐isomer. trans‐Isomers gave rise to abundant [3M–H+Co]⁺ ion products, which may be considered a characteristic ion for bicyclo[221]heptane trans‐2,3‐ and trans‐2,5‐diols. To differentiate cis‐2,3‐isomers, the collision‐induced dissociation (CID) products for [3M+Co]²⁺, [M+CoOAc]⁺, [2M–H+Co]⁺ and [2M+CoOAc]⁺ cobalt complexes were investigated. The results of the CID of the monomeric and dimeric metal adduct complexes [M+CoOAc]⁺ and [2M–H+Co]⁺ were stereochemically controlled and could be used for stereochemical differentiation of the compounds investigated. In addition, the structures and relative energies of some complex ions were studied using hybrid density functional theory calculations. Copyright © 2009 John Wiley & Sons, Ltd.