Roel H. Fokkens

High front- and back-end resolution MS/MS in Fourier transform ion cyclotron resonance mass spectrometry☆

Abstract It is demonstrated that each of the molecular ions of cyclopropylbenzene and di- n -propyl sulphide, having the same nominal mass m/z 118 and generated from a nearly 1:1 mixture by 70 eV electron impact in a Fourier transform ion cyclotron resonance mass spectrometer, can be selectively isolated in the cell by ejection of all other ions from the cell. This corresponds to a so-called front-end resolution of primary ion selection for MS/MS experiments in excess of 35 000 at m/z 118. Subsequent collisionally induced dissociation (CID) experiments on each of the mass-selected molecular ions have been performed using helium, introduced via a pulsed valve, as collision gas. A so-called back-end resolution of 205 000–475 000 has been obtained for the CID product ions over the mass range m/z 61–117, while the accuracy of the mass measurements on the corresponding ions is shown to be better than 1 ppm.

Characterization of Hydrogen‐Bonded Supramolecular Assemblies by MALDI‐TOF Mass Spectrometry after Ag+ Labeling

The high affinity of Ag+ ions for aromatic π donors and cyano groups is exploited in a novel MALDI-TOF mass spectrometric method for the identification of hydrogen-bonded assemblies. The interaction with the Ag+ ions-which, for example, can be complexed by two phenyl groups in a sandwich-type manner (see drawing on the right)-provides positively charged assemblies in a nondestructive way.

Ag+ labeling: a convenient new tool for the characterization of hydrogen-bonded supramolecular assemblies by MALDI-TOF mass spectrometry

Herein we describe our results on the characterization of a wide variety of different hydrogen-bonded assemblies by means of a novel matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) technique with Ag+ labeling. The labeling technique with Ag+ ions is extremely mild and provides a nondestructive way to generate charged assemblies that can be detected by mass spectrometry. Up to now more than 25 different single (1(3).2(3)), double (3(3).2(6)), and tetrarosettes (4(3).2(12)) have been successfully characterized by the use of this method. The success of the method entirely depends on the presence of a suitable binding site for the Ag+ ion. A variety of functionalities has been identified that provide strong binding sites for Ag+, either acting in a cooperative way (pi-arene and pi-alkene donor functionalities) or individually (cyano and crown ether functionalities). The method works well for assemblies with molecular weights between 2,000 and 8,000 Da, and most likely far beyond this limit.

In-source decay of hyperbranched polyesteramides in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Hyperbranched polyesteramides (DA2), prepared from hexahydrophthalic anhydride (D) and diisopropanolamine (A) have been characterized, by use of matrix-assisted laser desorption/ ionization time-of-flight mass spectrometry (MALDI-TOF-MS), field desorption (FD)-MS, and electrospray ionization (ESI)-MS. MALDI of polyesteramides produces protonated molecules. The spectra show a complex chemical composition distribution and end-group distribution which are mainly composed of two series of homologous oligomers DnAn+1 − mzH2O and DnAn − mH2O, where m = 1–2. Signals from protonated molecules DnAn+1 and DnAn are almost absent in the MALDI spectrum, whereas these ions are responsible for the base peak of DnAn+1 − mH2O and DnAn − mH2O (m = 1–2) clusters in the ESI spectrum. The absence of −OH end-groups signals in the MALDI spectrum is due to a metastable decay of protonated DnAn+1 and DnAn ions in the ion source of the MALDI mass spectrometer prior to ion extraction. In-source decay results in the formation of protonated lower DnAn+1 − mH2O and DnAn − mH2O oligomers and their corresponding neutrals, leading to wrong conclusions concerning the relative end-group distribution as a function of the degree of polymerization and the chemical composition.

Charakterisierung von supramolekularen Wasserstoffbrücken-Aggregaten durch MALDI-TOF-Massenspektrometrie nach Markierung mit Ag+

Die hohe Affinitat von Ag+-Ionen fur aromatische π-Donoren und Cyangruppen wird bei einer neuen MALDI-TOF-MS-Methode zur Identifizierung von H-Brucken-Aggregaten genutzt (MALDI-TOF-MS=Matrix-unterstutzte Laser-Desorptions-Ionisierungs-Flugzeit-Massenspektrometrie): Durch das Ag+-Ion, das z. B. sandwichartig von zwei Phenylgruppen komplexiert werden kann (siehe rechts), lassen sich die Aggregate - ohne Zersetzung - mit einer positiven Ladung versehen.

Non-covalent synthesis of multiporphyrin systems

Non-covalent synthesis was used in the controlled assembly of metallodendrimers containing up to 12 porphyrins on the surface. A porphyrin containing four Pd-Cl complexes was synthesized to assemble dendrimers with a porphyrin in the nucleus. 1H NMR, ES-MS and MALDI-TOF mass spectrometry were used to characterize the nanometer-size assemblies.

Noncovalent synthesis of water-soluble SCS PdII pincer assemblies

This article describes the noncovalent synthesis of water-soluble coordination assemblies based on SCS PdII pincer moieties. Two neutral solubilizing moieties, one based on a linear carbohydrate chain and the other on tetraethylene glycol residues, have been functionalized with pyridine and phosphane ligands. The coordination of the resulting molecules to various hydrophobic, cationic mono- and multimeric SCS PdII pincer systems has been investigated by 1H and 31P NMR spectroscopy, and by MALDI-TOF mass spectrometry. In general, the assemblies with tetraethylene glycol chains display a higher solubility in water than the ones containing linear sugar moieties. A hexapincer core decorated with six linear carbohydrates forms a hydrogel. Finally, a noncovalent water-soluble metallodendrimer having 18 peripheral tetraethylene glycol groups was constructed in a convergent manner.

Artifact formation due to ethyl thio-incorporation into silylated steroid structures as determined in doping analysis

Trimethylsilylation of target substances in a mixture of N-methyl-N-trimethylsilyltrifluoroacetamide (MSTFA), ammonium iodide and ethanethiol is frequently applied for the application of gas chromatography-mass spectrometry (GC-MS) in steroid analysis. However, artifacts were formed when using this mixture to silylate the steroids androsterone and etiocholanolone obtained from a urine matrix. The artifacts were identified as ethyl thio-containing products of the respective trimethylsilyl derivatives. The conversion of the studied products increased slowly as a function of time, was dependent on the presence of the urine matrix and was significantly accelerated by adding diethyl disulfide to the reagent before incubation. Also ethyl thio-incorporation into testosterone and epitestosterone was established. A mechanism for ethyl thio-incorporation is proposed. The conversion achieved after 120-h sample storage at room temperature was insufficient to significantly influence the analysis of androsterone and etiocholanolone under the studied conditions. However, the results provide fundamental insight into the mechanism of silylation and the occurring side-reactions. Moreover, when investigating the formation of new metabolites, the ethyl thio-incorporation can lead to misinterpretation.

Isolated excited electronic states in the unimolecular gas-phase ion dissociation processes of the radical cations of isocyanogen and cyanogen

Abstract The unimolecular gas-phase chemistry of CNCN +· and NCCN · has been investigated by electron-impact mass spectrometry, mass-analysed ion kinetic energy mass spectrometry and a theoretical density-functional method. The CN + formation appears to compete with the C +· 2 formation in the microsecond time scale, in spite of an energy gap of 3.5 eV (CNCN) and 3.0 eV (NCCN) between the two processes. This observation is explained by assuming that the fragmentation to CN + proceeds via an isolated excited electronic state of the molecular ion. The C +· 2 fragment ion must be formed from both isomeric C 2 N 2 +· parent ions by a rearrangement reaction. While NCCN +· is more stable than CNCN +· by 0.6 eV, the kinetic energies released in both rearrangements and theoretical calculations indicate that the corresponding intermediates are not only much closer in energy, but even reversed in energetic order with respect to their precursor ions.

Formation of a two-centre, three-electron, sulphur–sulphur bond in the gas phase

The dimer formed in the gas-phase reaction of ionized di-isopropyl sulphide with its neutral analogue is shown to possess a strong sulphur–sulphur bond.