Felddesorptions-Massenspektrometrie

Reduced fragmentation in liquid injection field desorption/ionization Fourier transform ion cyclotron resonance mass spectrometry by use of helium for the thermalization of molecular ions

RATIONALE

To exploit the softness of liquid injection field desorption/ionization (LIFDI), the molecular ions, M(+•), need to be transferred from their origin at the field emitter through the mass analyzer without disrupting their integrity. To preserve the molecular ions, ion-activating events like collisions must therefore be avoided. In hybrid quadrupole Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometers, however, multiple ion-guiding and ion-trapping events occur prior to mass analysis. The effects thereof compromised initial spectra from a LIFDI and electrospray ionization (ESI) combination (LIFDI-ESI) ion source and, thus, called for refined experimental conditions.

METHODS

A hybrid quadrupole FT-ICR instrument equipped with a new LIFDI-ESI combination ion source was used to obtain LIFDI spectra of polystyrene 1050, of 2,3,4-tridodecyloxybenzaldehyde, and of sewing machine oil as well as a field ionization (FI) spectrum of pentafluoroiodobenzene. The abundance of molecular ions, M(+•), was optimized, in particular by variation of the trapping conditions inside the instrument's accumulation RF-hexapole ion trap.

RESULTS

Ion-buffer gas collisions in the instrument's accumulation RF-hexapole ion trap were detrimental to the easy-to-fragment molecular ions of hydrocarbon species, whereas more robust even-electron ions were not affected. Exchanging the instrument's standard supply of argon buffer gas for helium resulted in a remarkable improvement. Together with further adjustments of potentials applied along the ion transfer path, hydrocarbon species could be analyzed.

CONCLUSIONS

The use of helium buffer gas remarkably improved LIFDI spectra, because the loss of molecular ions by dissociation during transfer from the LIFDI source into the ICR cell was significantly reduced. Hydrocarbon species could be analyzed while fragmentation of ions was avoided for the most part.

A liquid injection field desorption/ionization-electrospray ionization combination source for a fourier transform ion cyclotron resonance mass spectrometer

A new type of combination ion source has been devised. It unites two complementary ionization methods, i.e., liquid injection field desorption/ionization (LIFDI) and electrospray ionization (ESI). This LIFDI-ESI combination ion source has been constructed for a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The LIFDI-ESI combination ion source can be switched between the LIFDI and ESI modes of operation within 15 min without breaking the vacuum. The source design and its operation are described. LIFDI-FT-ICR spectra of the ionic liquid trihexyl(tetradecyl)-phosphonium tris(pentafluoroethyl)-trifluorophosphate, polyethylene glycol 600, 2,3,4-tridodecyloxy-benzaldehyde, and [60]fullerene are described.

Molecular ions of ionic liquids in the gas phase

Ionic liquids form neutral ion pairs (CA) upon evaporation. The softness of the gas-phase ionization of field ionization has been used to generate "molecular ions," CA(+*), of ionic liquids, most probably by neutralization of the anion. In detail, 1-ethyl-3-methylimidazolium-thiocyanate, [C(6)H(11)N(2)](+) [SCN](-), 1-butyl-3-methylimidazolium-tricyanomethide, [C(8)H(15)N(2)](+) [C(4)N(3)](-), N-butyl-3-methylpyridinium-dicyanamide, [C(10)H(16)N](+) [C(2)N(3)](-), and 1-butyl-1-methylpyrrolidinium-bis[(trifluormethyl)sulfonyl]amide, [C(9)H(20)N](+) [C(2)F(6)NO(4)S(2)](-) were used. The assignment as CA(+*) ions, which has been confirmed by accurate mass measurements and misassignments due to thermal decomposition of the ionic liquids, has been ruled out by field desorption and electrospray ionization mass spectrometry of the residues.

Liquid injection field desorption/ionization of reactive transition metal complexes

Liquid injection field desorption/ionization (LIFDI) has been applied to identify transition metal complexes that are highly reactive to air and moisture by mass spectrometry. The complexes of nickel and rhodium were supplied as dilute solutions (approximately 0.2 mg ml(-1)) in toluene, tetrahydrofuran or acetonitrile, and were applied onto the field desorption emitter inside the vacuum of the ion source under inert conditions by means of the injection capillary unique to the LIFDI set-up. LIFDI mass spectrometry on a double-focusing magnetic sector instrument provided spectra exhibiting intense molecular ion peaks for the species investigated or signals that could easily be related to the target compound by assuming neutral loss of the weakest-bound ligand. Eventually, byproducts of the synthesis or other components resulting from incomplete reactions or some degree of decomposition were also detected.

Field desorption mass spectrometry of large multiply branched saturated hydrocarbons

Large multiply branched saturated hydrocarbons containing 67-103 carbon atoms (molecular masses 941.8-1446.8 Da) were analyzed by field desorption mass spectrometry (FD-MS) with a double-focusing mass spectrometer. FD-MS was found to have detection limits in the 100 fmol range. The FD mass spectra exhibited molecular ions of astonishingly low abundance. However, the fragment ions formed were closely related to the proposed molecular structure, allowing us to set up rules for straightforward structure elucidation of unknowns. In detail, (i) dehydrogenation, (ii) alkyl losses from molecular ions and (iii) subsequent alkene losses were observed. The influence of the electric field strength on dehydrogenation and C-C cleavages was examined by variation of the emitter potential. Additionally, ion dissociations in the ion source and in the first and second field-free regions, respectively, were compared to study the relative importance of field-induced and thermally induced processes.

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.

Desorption mass spectrometry of glycosphingolipids

Desorption mass spectrometry has become an important tool for sequencing and mapping of glycosphingolipids of natural, synthetic, or semi-synthetic origin. The appropriate combination of different desorption mass spectrometric techniques with other spectroscopic, enzymatic, chemical, and/or immunological methods represents the most direct and efficient way to establish frequent, yet unknown, molecular structure-function relationships.

[FAB (fast atom bombardment)-mass spectrometry. A new study method in the hands of the (forensic) toxicologist]

The FAB (Fast Atom Bombardment)-mass spectrometric ionization technique, which has now been available for about 1 year, has been successfully employed in forensic toxicology. The mass spectral behaviour of some representative drug-glucuronides (Codeine, p-Nitrophenol and 2-Phenyl-1-propanol) were studied by positive- and negative-ion-FAB-MS. The presented promising results may be of some interest, not only for the analytical toxicologist.

Identification and quantitation of methotrexate and methotrexate metabolites in clinical high-dose therapy by high pressure liquid chromatography and field desorption mass spectrometry

High-pressure liquid chromatography in combination with field desorption mass spectrometry as techniques of high specificity and sensitivity have been applied to the identification and quantitation of the anticancer drug methotrexate and its metabolites which occur in clinical high-dose therapy. Field desorption mass spectra of methotrexate and several methotrexate and folic acid derivatives, when investigated as free acids or ammonium salts, yield abundant protonated molecular ions and a consistent pattern of structurally significant fragments. High-pressure liquid chromatographic separation of methotrexate metabolites was performed on reverse-phase, C-18 columns using a volatile, ammonium bicarbonate/acetate containing mobile phase that was especially suited for the field desorption mass spectral analysis of isolated metabolites, and provided the definite identification of 7-hydroxymethotrexate and 4-[[2,4-diamino-6-pteridinyl]methyl]methylamino]-benzoic acid in serum and urine of patients treated with high-dose methotrexate. The high intensity and stability of the [MH]+ ions was found suitable for the quantitation of methotrexate and related folate analogues by field desorption mass spectrometry. A synthetic methotrexate derivative, methotrexate-gamma-(2-hydroxy)ethyl-amide was used as internal standard for the quantitative determination of methotrexate in serum and urine. In a study to comparatively assess the potential of specific quantitation methods, serum and urine levels of methotrexate and its major metabolite, 7-hydroxymethotrexate were determined by (i) an enzyme immunoassay, (ii) reverse phase high-pressure liquid chromatography and (iii) field desorption mass spectrometry. Results obtained from four patients with osteogenic sarcoma receiving high-dose methotrexate/leucovorin rescue therapy consistently show the sustained elimination of 7-hydroxymethotrexate over several days, thus indicating the utility of specifically monitoring this nephrotoxic metabolite, at massive methotrexate doses.

Identification of conjugation and cleavage products in the thiolytic metabolism of the anticancer drug 4'-(9-acridinylamino)methanesulfon-m-anisidide

Conjugation and cleavage products in the thiolytic metabolism of the anticancer drug 4'-(9-acridinylamino)methanesulfon-m-anisidide were identified primarily by high-pressure liquid chromatography in combination with field desorption mass spectrometry. The spontaneous metabolic pathway of the drug, as related to its susceptibility to nucleophilic attack by endogenous thiols at the 9-carbon atom of the acridine moiety, has been studied. Among the metabolite fraction of 4'-(9-acridinylamino)methanesulfon-m-anisidide excreted in rat bile after administration of a therapeutic dose, a conjugate was identified as the 9-acridinyl thioether of glutathione. This conjugation product and the corresponding 9-acridinyl conjugates were formed spontaneously after incubation of 4'-(9-acridinylamino)methanesulfon-m-anisidide with glutathione, cysteine and N-acetylcysteine in sodium phosphate buffer and other aqueous media, as established by high-pressure liquid chromatography and field desorption mass spectra. The thiolytic pathway results in the release 4-amino-3-methoxymethanesulfonanilide which was identified in all in vitro experiments and in rat serum after intravenous 4'-(9-acridinylamino)methanesulfon-m-anisidide. 9(10H)-Acridone, 9-aminoacridine and other acridine derivatives which occur as minor products during the thiolytic cleavage in vitro were identified by field desorption and partially by high resolution electron impact mass spectrometry.

Comparative electron impact, chemical ionization and field desorption mass spectra of some thioether metabolites of acetaminophen

The mass spectra of several thioether conjugates of the widely used analgesic, acetaminophen (4'-hydroxyacetanilide), have been recorded under various ionization conditions. Conjugates were obtained from both in vitro and in vivo sources and purified by high performance liquid chromatography. Some standards were chemically synthesized. Of the thioethers examined, only the methylthio and mercapturic acid conjugates provided parent ions under electron impact conditions. In the chemical ionization mode, using isobutane as the reagent gas, the cysteinyl conjugate gave a pseudomolecular ion as well, although relatively large quantities (10-20 micrograms) of this amino acid adduct were required. Because of the highly polar nature and thermal instability of the cysteinyl and glutathionyl conjugates, these two thioethers were most successfully analyzed by field desorption techniques. Field desorption mass spectrometry was well suited for direct analysis of these two adducts where prominent [M]+, [MH]+ or [M + Na]+ ions were observed. Furthermore, by application of the field desorption/collision induced dissociation and linked (B/E) scan technique, structurally informative fragmentation patterns were generated. In addition, field desorption mass spectrometry was used successfully to characterize the glucuronide conjugate of acetaminophen but not the sulfate conjugate.

Polypeptide sequencing by liquid chromatography mass spectrometry

Key steps in a proposed automated system for polypeptide sequencing utilizing a liquid chromatograph mass spectrometer computer system have been tested with mixtures containing up to six model oligopeptides. At the low nanomole level it was possible to obtain complete sequence information for all components in many, but not all, of the mixtures tried. Interpretation of the results is complicated by the presence of numerous side-products formed in the derivatization process. Minimization of such impurities will be necessary to reduce the ambiguity of the sequence information resulting from more complex mixtures, such as those expected from the degradation of larger polypeptides, and to reduce sample requirements to the subnanomole level. However, the present system appears to have unique advantages over other proposed automated methods.