On-target deuteration technique in time-of-flight 252Cf plasma desorption mass spectrometry

A new on-target deuteration technique in plasma desorption mass Spectrometry (PDMS) is proposed, involving replacing labile hydrogen atoms in a molecule with deuterium atoms. This technique is very simple and allows direct comparison of spectra before and after deuteration for the same probe. The utility of the deuteration procedure in PDMS has been demonstrated in obtaining additional information on the structure of a molecule and its fragments and on the mechanism of quasi-molecular ion formation.     

252Cf plasma desorption mass spectrometry of some group 8 metal-containing coordination compounds

²⁵²Cf plasma desorption mass Spectrometry was applied to some series of ionic coordination compounds and was demonstrated to yield better results than fast atom bombardment mass Spectrometry. Intra-ligand fragmentadon was observed regularly, and evidence was also obtained for a fission fragment-induced methylene transfer reaction.     

Quantitative variability in the 252Cf plasma desorption mass spectra of triglycerides and waxes

A series of mixtures of solid triglycerides and waxes are examined using ²⁵²Cf plasma desorption mass spectrometry. The relative abundances of diagnostic ions are found to vary with sample preparation, handling, and composition. It is proposed that the less efficiently packing, shorter chain, compounds are forced to the analysis site, i.e. the surface. This takes place at varying rates that reflect the bulk properties of the mixture as revealed by its phase diagram and thermal history. Mixtures that remain liquids during analysis show ion abundances correctly reflecting their composition. Mixtures of isotopomeric glycerides also show normal ion abundances.     

Characterization of synthetic N-acetylcysteine conjugates by positive- and negative-ion 252Cf plasma desorption mass spectrometry

N-Acetylcysteine and nine N-acetylcysteine conjugates of synthetic origin were characterized by positive- and negative-ion plasma desorption mass Spectrometry. For sample preparation the electrospray technique and the nitrocellulose spin deposition technique were applied. The fragmentation of these compounds, which are best seen as S-substituted desaminoglycylcysteine dipeptides, shows a similar behaviour to that of linear peptides. In the positive-ion mass spectra intense protonated molecular ion peaks are observed. In addition, several sequence-specific fragment ions (A⁺, B⁺, [Y + 2H]⁺, Z⁺), immonium ions (I⁺) and a diagnostic fragment ion for mercap-turic acids (R_M⁺) are detected. The negative-ion mass spectra exhibit deprotonated molecular ions and in contrast only one fragment ion corresponding to side-chain specific cleavage ([R_XS]^?) representing the xenobiotic moiety. In the case of a low alkali metal concentration on the target, cluster molecular ions of the [nM + H]⁺ or [nM - H]^? ion type (n = 1-3) are observed. The analysis of an equimolar mixture of eight N-acetylcysteine conjugates shows different quasi-molecular ion yields for the positive- and negative-ion spectra.     

Quantitative 252Cf plasma desorption mass spectrometry for pharmaceuticals: A new approach to coupling liquid chromatography with mass spectrometry

A ²⁵²Cf plasma desorption mass spectrometer has been set up to analyse thin layers of non-volatile organic samples. Molecular and fragment ions were produced and their mass was determined by a time-of-flight measurement. A novel interface combines a high-performance liquid chromatograph with the ²⁵²Cf plasma desorption mass spectrometer in a twofold way: introducing the effluent continuously through a capillary inlet in the on-line liquid chromatography mass spectrometry mode or transferring already prepared samples through a vacuum lock into the mass spectrometer in the off-line liquid chromatography+mass spectrometry mode. The off-line mode has been applied for the quantitative analysis of pharmaceuticals in blood using stable isotope labelled standards.     

Field desorption mass spectrometry of amino acids

The mass spectra of some amino acids have been studied using the field desorption method. All amino acids yield molecular or quasi-molecular ions, even in the case of arginine and cystine, where these ions cannot be detected with the electron-impact (EI) or chemical ionization (CI) methods. The fragmentation is reduced as compared to EI, CI and FI.     

High resolution and tandem fourier-transform mass spectrometry with californium-252 plasma desorption

For ions formed by plasma desorption (PD) in a Fourier-transform mass spectrometer, high resolution measurements are demonstrated, such as 65,000 (FWHH) for the protonated molecular ion of gramicidin S (MW 1140.7). Resolution is substantially improved by delaying measurements until a significant ion concentration has built up in the cell, and by collisionally deactivating the orbital kinetic energy of the ions. This also makes the ions available for subsequent dissociation steps, so that tandem mass spectrometry can be demonstrated for PD ions. With this for larger ions, collisionally activated dissociation (CAD) is affected with> 85% efficiency. The CAD spectra of (M + Na)⁺ and of fragment ions from the PD of gramicidin S provide structurally useful information.     

Electrospray ionization mass spectrometric study of encapsulation of amino acids by cyclodextrins

Electrospray ionization (ESI) mass spectrometry has been used to study inclusion (host-guest) complexes of cyclodextrins (CDs) with amino acids. Host-guest complexes formed in solution are stable for characterization by ESI mass spectrometry: The relative abundances and the stoichiometry of the complexes formed in solution can, thus, be determined in the gas phase. The studies verified that β- and γ-cyclodextrin better accommodate protonated amino acids than α-cyclodextrin, and that chemically modified cyclodextrins such as heptakis(2,6-di-O-methyl)-β-cyclodextrin (DM-β-CD) may show profound improvement in complexation. The preferential formation of DM-β-CD-aromatic amino acid over DM-β-CD-aliphatic amino acid complexes is confirmed by the experiments, and the relative gas-phase stabilities determined by repeller-collimator collision-induced dissociation show an identical trend to the complexation in solution. Although molecular mechanics studies also may predict the encapsulation preference of protonated amino acids by cyclodextrins, only small differences in the total complexation energies are obtained because of the inability of the calculations to consider hydrophobic interactions. An experimental approach based on ESI mass spectrometry is, therefore, more reliable in predicting host-guest interactions that involve cyclodextrins and amino acids than the theoretical calculations that employ molecular mechanics models.     

Combined liquid chromatography time-of-flight mass spectrometry: An application of 252Cf fission fragment induced desorption mass spectrometry

A ²⁵²Cf fission fragment induced desorption mass spectrometer has been coupled on-line to a high-performance liquid chromatograph. The polar effluent (MeOH? H₂O) is introduced directly into a rough vacuum stage, where a thin sample of non-volatile compounds is collected in a vacuum-drying process (<1 cm³ min^?1). The interface which spans the difference in operating pressure between the collection site and the ion source consists basically of a sample changing disk. Twelve discrete samples are collected consecutively and analysed, typically one per minute. The system has been tested for the quantitative analysis of some antitumor drugs in serum using homologous compounds as internal standards.     

252Cf-plasma desorption and cesium-ion liquid secondary-ion mass spectrometric analysis of recombinant proteins.

The 35 keV Cs+ liquid secondary-ion mass spectrometry (LSIMS) and 252Cf-plasma desorption (PD) mass spectra of recombinant proteins in the 10-25 kDa mass range are compared. Both techniques showed comparable mass accuracy and sensitivity, and in the case of LSIMS, remarkably short analysis time. Analysis by the PD/nitrocellulose method demonstrated slightly higher sensitivity and relatively lower dependence on the salt and buffer content of the protein sample.     

Electrospray ionization tandem mass spectrometric and electron impact mass spectrometric characterization of mycosporine-like amino acids

Positive-ion mass spectral fragmentations of seven mycosporine-like amino acids (MAAs) are reported and discussed. The MAAs studied are small compounds composed of a cycloheximine ring substituted with amino acid or amino alcohol units. Techniques used include electron impact (EI) and electrospray ionization (ESI) with tandem mass spectrometry (MS/MS). ESI-MS/MS showed unusual small radical losses, generally resulting from the loss of a methyl group with the exception of shinorine and porphyra for which the initial losses were 30 and 44 Da, respectively. As expected from structural similarities, porphyra, shinorine and palythinol displayed similar fragmentation patterns, while palythenic acid and palythene fragmented in a similar manner. Overall, the ESI-MS/MS fragmentations at m/z <200 exhibited a distinctive pattern for all seven MAAs with characteristic ions at m/z 137, 168, 186, and 197 or 199. Several ions were observed for each of the MAAs analyzed, and together provide a useful and potentially diagnostic pattern for identification of MAAs and as an aid in structure elucidation of novel MAAs. For GC/EI-MS analysis, trimethylsilyl (TMS) derivatives were made. The EI-MS fragmentation patterns of TMS-MAAs showed many features typical of TMS-derivatized alpha-amines. The precursor TMS-MAA ion was not detected, but a [M-90](+ radical) ion was the highest-mass intense peak observed for palythine, palythinol and shinorine, while palythene gave a [M-116](+ radical) ion. Besides determining the number of acidic hydrogens, EI-MS of TMS-derivatized MAAs will aid in structure elucidation of novel MAAs.     

Mass spectrometry of labile onium ions and carbocation salts. Study by desorption-ionization mass spectrometry: Fast atom bombardment,field desorption and 252Cf plasma desorption time-of-flight mass spectrometry

In contrast to the accepted notion that mass spectrometry is impractical as an analytical tool in condensed-phase preparative carbocation chemistry, it has been shown that desorption ionization techniques (fast atom bombard-ment, field desorption and²⁵²Cf plasma desorption time-of-flight mass spectrometry) provide rapid and reliable methods for relative molecular mass determination for a wide variety of highly unstable, fragile carbocation salts and onium ions. In addition, interesting fragment ions and cluster ions (two cations and one anion) are observed. Application of these methods to study several dication ether salts has also been demonstrated.     

Small molecules as substrates for adsorption/desorption in 252CF plasma desorption mass spectrometry

Department of Chemistry, Texas A & M University, College Station, Texas, USA We present a new approach to substrate selection for californium-252 plasma desorption mass spectrometry (²⁵²Cf_PDMS) in which small volatile molecules that are water insoluble are used as matrices in place of the polymeric substrates used in previous studies. The desirable features of analyte adsorption are combined with the concept of using a volatile matrix to reduce the level of internal excitation of a desorbed analyte and to assist in ionization during the desorption process. Derivatives of anthracene were found to meet these requirements and to perform satisfactorily as substrates in ²⁵²Cf-PDMS. Spectra were obtained for bovine insulin (m I z 5734) adsorbed onto 9-anthroic acid and 2-aminoanthracene and compared with spectra using a nitrocellulose substrate. Sharper peaks and lower backgrounds are observed when the 9-anthroic acid matrix is used, indicating reduced levels of internal excitation and initial kinetic energy for the desorbed molecular ion of insulin. A comparison of the performance of 9-anthroic acid and 2-aminoanthracene shows the influence of substrate functional groups on desorbed protein yields. Finally, the versatility of the small-molecule matrix concept is discussed with respect to selection of a range of functionality, solubility, and hydrophilicity.     

Analysis of amino acid mixtures by plasma desorption mass spectrometry

Plasma desorption mass spectrometry (PDMS) was investigated as a means of analysing mixtures of three, four and five amino acids in both positive- and negative-ion modes. Fifteen mixtures were tested; each mixture contained equimolar amounts of selected amino acids. The PD mass spectra exhibited MH⁺ and [M – H]^? molecular ions for all the aminoacids with different desorption–ionization yields. The spectra were more easily interpreted in the negative- than the positive-ion mode. The desorption order of the amino acids was progressively established by comparing the molecular ion desorption–ionization yields for each mixture. This desorption order was well correlated in both the positive- and negation-ion modes with the acid–base thermodynamic data for the amino acids in the gas phase. This observation gives some insight into the desorption–ionization mechanisms under PDMS conditions.     

Electrospray ionization mass spectrometric observation of ligand exchange of zinc pyrithione with amino acids

Zinc pyrithione (ZnPT) is widely used as an antidandruff or antifouling reagent. However, this compound is considered toxic, such as the teratogenic effect, to aquatic lives, and it is important to clarify the mechanism of its toxicity. In this study, the interactions between ZnPT and amino acids were observed using electrospray ionization mass spectrometry (ESI‐MS) in order to obtain information on the activity of ZnPT within the living body. The ZnPT complex ([ZnPT‐ligand+Amino acid]⁺), in which the ligand of ZnPT was exchanged by the amino acid, was detected in ZnPT solutions mixed with one of 20 amino acids by ESI‐MS. Histidine and cysteine, in particular, showed a high reactivity with ZnPT, while serine and glycine showed a low reactivity. The complexes of ZnPT and a peptide were also observed by the ESI‐MS measurement of the solution containing ZnPT with the peptide. These results would be useful to understand the mechanism of ZnPT toxicities to living creatures. Copyright © 2009 John Wiley & Sons, Ltd.