Alex B. Young

Interferences and contaminants encountered in modern mass spectrometry

With the invention of electrospray ionization and matrix-assisted laser desorption/ionization, scientists employing modern mass spectrometry naturally face new challenges with respect to background interferences and contaminants that might not play a significant role in traditional or other analytical techniques. Efforts to continuously minimize sample volumes and measurable concentrations increase the need to understand where these interferences come from, how they can be identified, and if they can be eliminated. Knowledge of identity enables their use as internal calibrants for accurate mass measurements. This review/tutorial summarizes current literature on reported contaminants and introduces a number of novel interferences that have been observed and identified in our laboratories over the past decade. These include both compounds of proteinaceous and non-proteinaceous nature. In the supplemental data a spreadsheet is provided that contains a searchable ion list of all compounds identified to date.

A hybrid BEQQ mass spectrometer for studies in gaseous ion chemistry

Abstract A hybrid mass spectrometer of BEQQ geometry (B, magnetic sector; E, electric sector; Q, quadrupole mass filter), designed for fundamental and applied studies in gaseous ion chemistry, is described. The high-resolution (BE) stage is followed by a deceleration lens, which also shapes the ion beam, an r.f.-only quadrupole collision cell, and a quadrupole mass analyzer. This assembly allows collision processes to be studied over the laboratory energy range from 0 to 500 eV with selection of the reactant ion at high mass resolution. The instrument is also equipped with dual collision cells and a deflector electrode in the field-free region between B and E for the study of neutralization—reionization reactions as well as single collision processes at 1–8 keV collision energy. The use of the instrument is illustrated with a variety of examples involving charge stripping, charge inversion, fast neutral ionization, and neutralization—reionization processes at high collision energies, as well as collision-induced dissociation reactions at both high and low collision energies. In addition, a number of experiments involving simultaneous scanning of the electric sector and the QQ stage are reported.

The Ni(II)-binding properties of the metallochaperone SlyD.

Metallochaperones are essential for the safe and targeted delivery of necessary yet toxic metal cofactors to their respective protein partners. In this study we examine the nickel-binding properties of the Escherichia coli protein SlyD, a factor that contributes to optimal nickel accumulation in this organism. This protein is also required for E. coli energy metabolism because it participates in the nickel insertion step during [Ni-Fe]-hydrogenase metallocenter assembly. Our study demonstrates that SlyD is a multiple nickel ion binding protein. The analysis of noncovalent metal-protein complexes via electrospray ionization mass spectrometry revealed that SlyD binds up to seven nickel ions in a noncooperative manner with submicromolar affinity (<2 microM, upper limit) and that the protein exists in a dynamic mixture of metalloforms that is dependent on the availability of nickel ions in solution. Structural analysis indicates that this metallochaperone undergoes small but distinct changes in the structure upon metal binding and that the nickel-binding sites are assembled through beta-turn formation. Although the C-terminal metal-binding domain is primarily responsible for metal chelation, we find that metal binding also perturbs the structure of the N-terminal domains. An investigation of the nickel sites by using X-ray absorption spectroscopy shows that SlyD binds nickel ions by adapting several different geometries and coordination numbers. Finally, the characterization of SlyD mutants demonstrates that the cysteine residues in the C-terminal domain confer tighter affinity as well as increased binding capacity to SlyD. On the basis of the presented data a model for nickel binding to SlyD as well as its role in nickel homeostasis is discussed.

Fragmentation of Doubly-Protonated Pro-His-Xaa Tripeptides: Formation of b22+ Ions

When ionized by electrospray from acidic solutions, the tripeptides Pro-His-Xaa (Xaa=Gly, Ala, Leu) form abundant doubly-protonated ions, [M+2H]2+. Collision-induced dissociation (CID) of these doubly-protonated species results, in part, in formation of b22+ ions, which fragment further by loss of CO to form a22+ ions; the latter fragment by loss of CO to form the Pro and His iminium [immonium is commonly used in peptide MS work] ions. Although larger doubly-charged b ions are known, this represents the first detailed study of b22+ ions in CID of small doubly protonated peptides. The most abundant CID products of the studied doubly-protonated peptides arise mainly in charge separation involving two primary fragmentation channels, formation of the b2/y1 pair and formation of the a1/y2 pair. Combined molecular dynamics and density functional theory calculations are used to gain insight into the structures and fragmentation pathways of doubly-protonated Pro-His-Gly including the energetics of potential protonation sites, backbone cleavages, post-cleavage charge-separation reactions and the isomeric structures of b22+ ions. Three possible structures are considered for the b22+ ions: the oxazolone, diketopiperazine, and fused ring isomers. The last is formed by cleavage of the His-Gly amide bond on a pathway that is initiated by nucleophilic attack of one of the His side-chain imidazole nitrogens. Our calculations indicate the b22+ ion population is dominated by the oxazolone and/or fused ring isomers.

O− and OH− chemical ionization of some fatty acid methyl esters and triacylglycerols

O− and OH− react with fatty acid methyl esters (FAMES) under chemical ionization conditions both as Bronsted bases to form [M - H]− and as nucleophiles to form the carboxylate ion RCOO−. O− shows a much greater tendency to react as a nucleophile than does OH−. The [M - H]− ions fragment by elimination of CH3OH, with unsaturation in certain positions in the fatty acid hydrocarbon chain promoting this elimination for unknown reasons. The reaction of O− and OH− with triacylglycerols leads to [M - H]−, characteristic of the molecular mass, and to carboxylate ions characteristic of the fatty acid(s) present in the lipid. The presence of the three ester functions in the lipids greatly enhances the formation of carboxylate ions compared to the FAMES.

Studies in gaseous ion chemistry using a hybrid BEQQ mass spectrometer

Abstract A hydbrid mass spectrometer of BEQQ geometry (B, magnetic sector; E, electric sector; Q, quadrupole mass filter), designed for fundamental and applied studies in gaseous ion chemistry, is described. The high-resolution BE stage is followed by a deceleration lens system, an r.f.-only quadrupole collision cell and a quadrupole mass analyzer. The arrangement allows high-energy collisional processes to be studied by the mass-analyzed ion kinetic energy spectroscopy (MIKES) technique using the B and F sectors. The BEQQ configuration allows low-energy collisional processes to be studied over the laboratory energy range 0–500 eV with selection of the reactant ion at high resolution. The use of the instrument in gaseous ion chemistry studies is illustrated with a variety of examples primarily involving use of the quadrupole portion of the instrument.

A compact high pressure ion source for high and low energy collision-induced dissociation studies of cluster ions on a VG analytical ZAB-2FQ

A compact, field-free high pressure ion source designed to replace, with minimum disruption, the electron impact/chemical ionization ion source of a VG Analytical ZAB-2FQ hybrid BEqQ mass spectrometer is described. This ion source may be operated at temperatures from ≈40 to 250 °C and at pressures up to 4–5 torr and, thus, is capable of producing proton-bound cluster ions up to hexamers in good yields. Examples of high energy collision-induced dissociation, low energy collision-induced dissociation, and neutralization-reionization studies of proton-bound cluster ions produced in this source are presented.