Field Desorption Spectra Research Articles

Field evaporation and field desorption from HTSC single-crystal surfaces

The problems of field evaporation (FE) and field desorption (FD) from surface of oxide high-temperature superconductors (HTSC) of 123, 2212 and 2223 type are discussed. Wide-angle atom probe, usual time-of-flight atom probe and field ionization magnet mass-spectrometer have been used in these experiments. The main purpose of the investigation was to search for effects connected with phase transition. Therefore, the experiments were carried out over a wide range of temperatures: from T of solid nitrogen to room T. Composition of pure, contaminated and deep corroded surface and typical species of FE and FD spectra have been studied. The bond energies between some atoms (or clusters) and the surface have been estimated on the basis of experimental results and FE theory. From these estimations, and from discovering of correlated ion pairs, it was concluded that the redistribution of local bonds should be taken into consideration in this case. The increase of FE rate was observed at cryogenic temperatures. Various versions are discussed to explain these effects. Some of them are connected with phase transition. A detailed study of FD of water protonized clusters is performed and the catalytic activity of HTSC crystal surfaces is discussed.

FD and FAB mass spectra of some oligopeptides of the tryptophyllin family.

Mass spectra of some natural thryptophyllins and synthetic analogues have been obtained by using field desorption and fast atom bombardment as soft ionization techniques to overcome the low volatility of these oligopeptides. Field desorption spectra generally show the molecular ion as base peak; some fragments of thermal origin are present at higher emitter heating current, thus providing additional structural information. Fast atom bombardment mass spectra show a more regular fragmentation of the peptide backbone, which allow the complete amino acid sequence to be checked. The presence of two consecutive proline residues in some of the examined molecules affects their fragmentation pattern in both field desorption and fast atom bombardment mass spectrometry giving rise to spectral features which are useful from the diagnostic point of view. The two ionization methods are compared and the advantage of the combined use of both techniques is pointed out.

Application of field desorption and secondary ion mass spectrometry for glycolipid analysis.

Field desorption (FD) and secondary ion mass spectrometry (SI-MS) mass spectra of several glycolipids are presented to demonstrate their potential for the analysis of glycolipids. FD and SI-MS give useful information on molecular weight, ceramide structure and sugar sequence. In general, FD provides clearer fragment ion peaks for the analysis of sugar sequence than SI-MS. For underivatized acidic glycolipids such as gangliosides, sulfatide and seminolipid, SI-MS provides quasimolecular ions which are hardly produced by FD. In contrast to underivatized gangliosides, permethylated samples give molecular ion species of high intensity in both FD and SI-MS, but no fragment ions pertinent to carbohydrate sequence could be observed in FD spectra. SI-MS spectra of permethylated samples provide good information on sugar chain structure. Thus FD and SI-MS mass spectra complement each other, and the combination of these ionization methods will provide powerful tools for glycolipid analysis.

A 15N labelling, FD and FAB study of ammonia loss from protonated arginine molecules

AbstractThe positive ion field desorption (FD) spectrum of arginine taken at the best anode temperature only contains a peak due to [M+H]+ ions. At higher emitter temperatures a considerable amount of fragmentation is induced and the [M+HNH3]+ ions become most abundant. Specific 15N labelling reveals that the eliminated ammonia molecule, exclusively, contains one of the terminal nitrogen atoms of the guanidyl group. This also applies to the ammonia loss from metastably decomposing [M+H]+ ions. The positive ion fast atom bombardment (FAB) spectrum shows more fragmentation than the FD spectrum. In contrast with the FD results, the [M+H]+ ions generated upon FAB with ion lifetimes <10−6 s eliminate both ammonia containing one of the terminal nitrogen atoms of the guanidyl group and ammonia containing the α‐amino group in the ratio of 1.35, as found by 15N labelling. The metastably decomposing [M+H]+ ions, however, eliminate only the former ammonia molecule. In the negative ion FD and FAB spectra no other peak than that corresponding to the [MH]− ion is observed. Some attention has been paid to the thermal degradation of arginine on the basis of a few Curie‐point pyrolysis experiments.

Thermal, laser, and fast atom desorption

The desorption methods show similarities in their spectra and suggest common mechanisms. However, thermal desorption spectra indicate that preformed ions and cationized species can be generated by purely thermal means, while differences in fast atom bombardment and field desorption spectra indicate the importance of external or localized fields in the kinds of ions extracted from the condensed phase.

Exopeptidase digestion in combination with field desorption mass spectrometry for amino acid sequence determination

Numerous studies have been devoted in the last years to the development of mass spectrometric methods for the sequence determination of peptides [ 1,2]. Most advanced among this work has been so far the rigorous chemical derivatization of oligopeptides to achieve sufficient volatility for the application of conventional (electron impact, EI) mass spectrometry [2-41. For example, the analysis by gas chromatography-mass spectrometry (GC-MS) of mixtures of oligopeptide fragments derivatized after chemical or enzymatic hydrolysis of polypeptides has been successfully used for sequence determinations [2,5]. Major limitations of this approach are that only small peptide derivatives are amenable to GC-MS, and the relatively large amount of original peptide needed. More recently, mass spectra of underivatized peptides have been obtained by field desorption (FD) mass spectrometry [6], a technique that does not require any volatility of the sample to be ionized. It was shown that FD mass spectra of oligopeptides usually exhibit molecular ions (e.g., protonated, MH’ ions) of high intensity, with relatively little fragmentation [7,8]. While studies of thermally or collisioninduced fragmentation in FD spectra of peptides [9-l l] so far indicate only limited utility for obtaining structural information, the possibility of analysing peptide mixtures by means of the predominant molecular ions has been suggested [ 121. FD mass spectrometry in combination with Edman degradation-subtractive analysis has been reported [ 131. the available enzymes (e.g., contamination with endopeptidase or different types of exopeptidase activity), and the problem of determining repetitive amino acid residues have limited their application so far. Attempts to overcome these difficulties have been made such as the addition of endopeptidase inhibitors [ 14,151 and solid-phase sequencing with exopeptidases [ 161. However, the characteristic feature of the inhomogeneous kinetics of exopeptidase digestion which, in principle, should match well the capability of FD mass spectrometry to analyse peptide mixtures prompted our interest to test the combination of both methods for peptide sequencing. Here, fast and unequivocal sequence determinations were obtained for 2 oligopeptides by direct FD mass spectral analysis of aliquots taken from partial aminoand carboxypeptidase digests The finding that FD mass spectra of exopeptidase digests provided reliable sequence data without any separation step required, and with amounts of material at the sub-nanomole level indicate that this combination may prove to be a valuable approach among the instrumentarium for the sequence determination of peptides.

Electron impact and field desorption mass spectrometry of some macrocyclic‐acyclic polyether‐ester compounds

AbstractThe electron impact mass spectrometric behaviour of a series of macrocyclic‐acyclic polyether‐ester compounds containing an isoxazole, benzene or pyridine ring is discussed in detail with the aid of exact mass measurements and metastable ions (linked scans B/E and B2/E). The nature of unusual [M+H]+ species, present in the crown moieties only, is elucidated by means of low electron energy measurements and labelling experiments. The field desorption spectra show only intense peaks corresponding to [M+H]+ species and rule out the presence of dimeric moieties.

The mass spectra of some silver salts

AbstractFifteen of twenty‐four silver(I) carboxylates examined give useful electron impact mass spectra. The compounds vaporize at moderate temperatures, apparently mainly as dimer with traces of higher oligomer in only a few cases. The molecular ion for the dimer is generally weak or absent, with the most abundant silver containing ion being [Ag2(O2CR)]+ in most cases. Metastable defocusing and deuterium labeling experiments on silver acetate have established some of the fragmentation pathways. The reported loss of carbon dioxide from perfluorocarboxylates to give intense peaks for organosilver ions was not observed in this study. Attempts to obtain spectra on the silver salts of organic materials other than carboxylic acids were successful in several cases. Silver trifluoromethanesulfonate, although much less volatile, gives a spectrum and fragmentation very much like the carboxylates, whereas silver trifluoromethanethiolate gives a complex spectrum which suggests tetramer as a major gas phase species. Of three compounds examined which have silver to nitrogen bonding only silver(II) phthalocyanine is sufficiently volatile to give a spectrum without decomposition. The field desorption spectra of the four compounds examined all show the ions AgnXn − 1 for X=acetate (n=1 − 6), X=p‐chlorobenzoate (n=1 − 4), X=methanesulfonate (n=1 − 7) and X=p‐toluenesulfonate (n=1 − 5).

Identification of ginsenosides from Panax ginseng in fractions obtained by high-performance liquid chromatography by field desorption mass spectrometry, multiple internal reflection infrared spectroscopy and thin-layer chromatography

The high-performance liquid chromatographic (HPLC) determination of the ginsenoside saponins from Panax ginseng C.a. Meyer (Araliaceae) was used to investigate the possibilities and limitations of peak identification with on-line and off-line methods. Thin-layer chromatography was used to separate the ginsenosides with a running distance of 6.5 cm, the detection limit being 0.2 μg. By means of multiple internal reflection infrared (IR) spectroscopy, IR spectra were obtained with 20 μg of ginsenoside. Field desorption (FD) mass spectrometry permitted not only the identification and determination of the molecular weights of underivatized ginsenosides, but also gave important information about the sequence of the sugar moities in the molecule. Less than 1 μg of ginsenoside is needed to produce an FD spectrum. The time required for the FD investigation of an HPLC fraction containing a ginsenoside is about 1 h, including sample preparation. FD measurements, data processing, output, evaluation and interpretation.

Electron impact, chemical ionization and field desorption mass spectrometry of substituted dihydroxyphenyl-benzodioxins isolated from insect cuticle. Occurrence of thermal decomposition and oligomerization reactions in the mass spectrometer.

The electron impact, field desorption and chemical ionization mass spectra of seven dihydroxyphenyl-benzodioxins isolated from insect cuticle are discussed. Reproducible electron impact and chemical ionization spectra are obtained as a result of thermal decomposition and either electron impact or chemical ionization. In some of the chemical ionization spectra dimerization of the thermal degradation products is observed. Structure determination is possible based upon the electron impact and field desorption spectra.

Mass spectrometry of quaternary ammoniohexanoates: Intermolecular alkyl transfer during field desorption

AbstractIntermolecular alkyl transfer occurs during field desorption of quaternary ammoniohexanoates, resulting in mass spectra containing structurally diagnostic adduct ions. Methyl, ethyl and propyl groups attached to nitrogen readily undergo intermolecular transfer to give [M+CH3]+, [M+C2H5]+ and [M+C3H7]+ ions, respectively. Evidence is presented that alkyl groups even as large as C10H21 can transfer intermolecularly at high emitter temperatures. In addition to the alkyl ion adducts, the field desorption spectra of \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm C}_{10} {\rm H}_{21} \mathop {\rm N}\limits^ + \left({{\rm CH}_3 } \right)_2 \left({{\rm CH}_2 } \right)_5 {\rm COO}^ - $\end{document} show several other adduct and fragment ions whose relative intensities depend strongly on emitter current. The field desorption results are compared with earlier pyrolysis electron impact results on similar compounds.

High-pressure liquid chromatography and field desorption mass spectrometry of heme a, Heme a dimethyl ester and acetyl heme a dimethyl ester

Field desorption mass spectroscopy was shown to be a valid technique for determining the molecular weights of hemins and hemin esters, as well as of porphyrins. The observed base peaks of ligand-free protoheme IX, protoheme IX dimethyl ester, and protoporphyrin IX dimethyl ester correspond well to the molecular weights of these compounds and the base peak for hematoporphyrin corresponds to the molecular weight of this porphyrin minus two molecules of water. The technique was employed to confirm the molecular weights of heme a, heme a dimethyl ester, and acetyl heme a dimethyl ester. Heme a dimethyl ester was prepared by reaction of heme a with trimethyloxonuim tetrafluoroborate and purified by high-pressure liquid chromatography. The isolated product was converted to the acetylated derivative by reaction with acetic anhydride and was subsequently purified by high-pressure liquid chromatography. A field desorption spectrum of heme a shows a base peak at 582 which is in agreement with previous deductions of the structure of this prostetic group. Base peaks of the heme a ester and its acetylated derivative demonstrate that the two carboxyl groups have been methylated and the single hydroxyl group has been acetylated without alteration of the molecule.

Field ionization mass spectrometry. I--Field desorption spectra of nucleotides--experimental problems.

Instrumental and substance inherent problems typical for field-desorption spectra of highly sensitive polar substances are discussed and illustrated with examples from the nucleotide field.

An emission-controlled field desorption and electron impact spectrometry study of some N-substituted propane and butane sultams.

It is shown that the production of field desorption mass spectra with an emission-controlled emitter heating device is a useful tool in obtaining information about molecular structures. Since the structurally significant signals are sometimes missing in the corresponding electron impact spectra but not in the field desorption spectra the use of field desorption mass spectrometry appears to be more advantages. This will be especially true for substances that are thermally labile, unvolatile or unstable upon electron impact (both at high and low electron energy), the latter being the case for some of the compounds presently investigated which are derivatives of a drug with antitumour and antiepileptic effects. Further, emission-controlled desorption improves the reproducibility of the spectra obtained and thus appears to be a prerequisite for quantification in biomedical and pharmacokinetic studies.

High resolution field desorption mass spectrometry V: Mixtures of amino acid phenylthiohydantoins and Edman degradation products

Using photographic detection and high resolution, the potential of field desorption mass spectrometry for mixture analysis is exemplified by means of synthetic mixtures of up to 15 amino acid phenylthiohydantoins (PTH amino acids). The high molecular ion intensities, low fragmentation, and relatively small intermolecular interaction allow the easy discrimination of individual components of these mixtures. The sensitivity and selectivity of the field desorption method is tested on PTH amino acids obtained from automated Edman sequenator degradations of a ribosomal protein. The field desorption spectra show the molecular ions and significant fragment ions of the PTH derivatives of all 10 degradation steps investigated. Even in cases where conventional electron impact mass spectrometry fails to show the molecular ions and only characteristic fragment ions are found, the field desorption method gives rise to the molecular ions in high yields (e.g., PTH-arginine and PTH-(ϵ-PTC)-lysine). Therefore the use of the method as a complementary technique for the confirmation of PTH amino acids released in the Edman sequenator appears to be advantageous.

Field ionization and field desorption mass spectra of polymeric sulfur nitride

Field ionization and field desorption mass spectra of polymeric sulfur nitride, (SN) x, are reported. The results show that (SN) x vaporizes as a (SN) 4 species isomeric in structure to cyclic S 4N 4. Field desorption spectra, obtained by subliming a sample of (SN) x on to the field emitter, contain only the (SN) 4 + ion. In this communication we report the field ionization and field desorption mass spectra obtained for polymeric sulfur nitride, (SN) x. The polymer (SN) x has a number of unusual properties [1–4]. It is highly anisotropic; it is a conductor and becomes a superconductor at 0.3°K; and it may be sublimed to form (SN) x films of structure identical to the crystals from which it is sublimed. This unique set of properties has made (SN) x the subject of intensive study [1–5]. The vapor-phase species are of particular interest because of their role in the production of (SN) x films. While the chemical and physical properties of the (SN) x crystals and films have been documented [1–4], the vapor phase species, which repolymerize as (SN) x films on cool surfaces, have been a subject of speculation. Recently, however, we used phase-angle mass spectrometry [5] to determine that an (SN) 4 species is the major gas-phase component of (SN) x vapors. Analysis of electron-impact mass spectra and appearance potential measurements of (SN) x vapors, cyclic S 4N 4, and S 2N 2 suggested that the (SN) 4 species sublimed from (SN) x is a structural isomer of cyclic S 4N 4.

Chemical ionization and field desorption mass spectrometry of the gentamicins.

Gentamicin sulphate employed in therapeutics consists of a mixture of three major aminoglycoside components, the gentamicins C1, C1a and C2. In the electron impact mass spectra of these components weak parent ions may be observed but they are of no diagnostic value in the commercial mixture. Chemical ionization and field desorption spectra of individual gentamicin components and commercial mixtures are reported and discussed. Isobutane chemical ionization spectra exhibit some fragmentation, but under optimum field desorption conditions little glycosidic cleavage is observed and [M + H]+ ions dominate the spectra.

A computer-controlled heating device used in field desorption mass spectrometry

A computer-controlled heating device is described for direct and indirect heating of a field anode in a field desorption mass spectrometer. Using this heating device connected to a small on-line computer, various field desorption spectra can be detected at different temperatures with a single loading of the field anode.

Applications of high-pressure liquid chromatography and field desorption mass spectrometry in studies of natural porphyrins and chlorophyll derivatives

Mixtures of porphyrins derived from natural sources can be readily separated by high-pressure liquid chromatography both analytically and on a preparative scale. A variety of procedures have been developed not only for the esters but also for free acids, and on the analytical scale quantitation is easily achieved by visible absorption. The retention times are largely characteristic of the number of carboxylic acid side chains (or other polar groups) but further information can be obtained by mass spectrometric studies of the various fractions. Field desorption mass spectrometry is particularly useful for this purpose because the emitter wire can be dipped directly into the eluates. The field desorption spectra of porphyrin free acids and esters as well as their metal complexes give essentially molecular ions with little or no fragmentation in most cases, whereas electron-impact mass spectrometry, particularly of free acids, is impeded by the low volatility of porphyrins. Mixtures can also be analysed by field desorption mass spectrometry, and this provides not only a rapid qualitative assessment of the components of a mixture, but also a check on the subsequent chromatographic separations.

Comparative study of ion kinetic energy, chemical ionisation and field desorption spectra

AbstractA comparison of the fragmentation processes operating in the first field free region was made with the fragmentation in the ion source under electron ionisation, chemical ionisation and field desorption conditions for some selected compounds which undergo retro‐DielsI‐Alder fragmentation in the ion source under electron ionisationconditions. The fragmentation processes which com‐ete with retro‐Diels‐Alder fragmentation under chemical ionisation and field desorption conditions are discussed. The complementary nature of these techniques are illustrated.