Field desorption study, fragment ions

Mass spectrometry (MS) in its various forms, and with various procedures for vaporization and ionization, contributes to the identification and characterization of complex species by their isotopomer pattern of the intact ions (usually cation) and by their fragmentation pattern. Upon ionization by the rough electron impact (El) the molecular peak often does not appear, in contrast to the more gentle field desorption (FD) or fast-atom bombardment (FAB) techniques. An even more gentle way is provided by the electrospray (ES) method, which allows all ionic species (optionally cationic or anionic) present in solution to be detected. Descriptions of ESMS and its application to selected problems are published 45-47 also a representative application of this method in a study of phosphine-mercury complexes in solution is reported.48... 

The techniques of field desorption (FD) and electrohydrodynamic ionization (EHD) differ from SIMS, LD, and FAB in their physical basis and in features seen in the spectra. For example, the diquaternary ammonium salts discussed above yield intact doubly-charged ions, and fragmentation is less extensive. Nonetheless, the three classes of ions described for molecular SIMS are generally seen in FD (50). An EHD study of a series of diquaternary ammonium salts (51) led to the conclusion that the amount of internal energy deposited in EHD is less than in SIMS and FAB. The same study also indicated that FAB (liquid matrix) deposits less energy than SIMS (solid matrix), so in this case the order of energy deposition is SIMS > FAB > EHD. 

Chemical ionization (CI)-MS can be used to study alkaloids that are not amenable to examination by electron impact (EI)-MS. For example, the quaternary alkaloid thalirabine (Section II,C, 123), undergoes fragmentation under the conditions of EI-MS and does not show a parent ion however, the CI-MS shows a double Hofmann elimination product which retains the skeletal atoms (32). Field desorption (FD)-MS has similar utility, as in the case of cycleanine IV-oxide (Section II,C,17) for which FD-MS shows the parent ion not detectable by EI-MS (65). Desorption/CIMS (D/CIMS) was used on dihydrosecocephar-anthine (Sec. H,C,30) and related bases (80,292a). 

FAB is most often compared to the soft ionization method known as field desorption (FD) mass spectrometry, a technique in which the sample, deposited on an emitter wire coated with microcrystalline carbon needles, is desorbed under the influence of a high electric field gradient. As usual, bioorganic systems are best represented by both techniques (21, 33). Though FAB is the easier of the two, they are complementary, FAB being particularly suited for the case of extreme thermal lability and FD for the case of chemical lability or matrix interference. Cerny et al. (33) compare the two techniques for the study of coordination complexes and conclude FD is better for molecular-ion determination, while FAB provides better fragmentation information, which is useful in elucidating structures. 

Schulten (110, 111) has used laser-assisted field desorption mass spectrometry to study some inorganic and organometallic systems. This method is intermediate between LAMMA and simple FD. Metal cations predominate from inorganic salts. The technique also showed clusters of the type reported from both FAB and SIMS studies. By carefully controlling the laser, a chlorophyll molecular ion could be obtained as well as fragments relating to its structure. 

Regrettably, alkyne-substituted cluster complexes seem particularly prone to fragmentation and very few accurate mass spectroscopic studies have been reported. A recent exception has been the field-desorption and electron-impact mass spectral investigation of mono-and oligo-nuclear ferracyclic ring systems of the from Fei(CO)J,(C2R2)2 (x = 1, 2, 3 y = 6, 8) (384). These species show intense molecular ion peaks, which enable ready recognition of the molecular composition. 

When using field desorption, however, it is occasionally difficult to distinguish the fragment ions from impurities and the products of various reactions (surface reactions, pyrolysis). The study of uni- and bimolecular decomposition spectra is very useful, since it enables one to go beyond the simple identification of artifacts to the elucidation of molecular structures. 

Organophosphonium salts are a general class of compounds which are difficult to analyse via conventional EI-MS Thus several reports have appeared in the literature which describe the use of alternative ionization methods to analyse this important class of compounds " Early studies, which centered around the use of field desorption (FD), demonstrated that the intact phosphonium ion is often the most abundant ion. A comparison was made between the use of FD and FAB as ionization methods for the analysis of the diphosphonium salt FD produces high abundances of the intact phosphonium ions [M - Br ] and [M - 2Br ], whereas these ions are only minor peaks in the FAB mass spectrum using glycerol as the matrix. Instead, the fragment ion 35 is the base peak in the FAB spectrum. 

Electron impact (El), the method outlined in Section 9.6.1, is the conventional method of ionizing samples for MS. However, El is not free from disadvantages, such as inability to provide molecular weight information from thermally labile samples, difficulty for study of low-volatility materials, complex spectra arising from interference between molecular and fragment ions, etc. In order to overcome these problems, a large number of alternative ionization methods, collectively known as soft ionization methods have been developed. Reviews of the available techniques have been compiled by Milne and Lacey (1974) and Games (1978). Chemical ionization (Cl), field ionization (FI) and field desorption (FD)... 

In chemical ionization (Cl) studies the sample is mixed with an excess of reagent gas and this gas is ionized. This initiates ion-molecule reactions, through which the sample itself is eventually ionized and the ions produced are subsequently analysed (Field, 1972). Field ionization (FI) and field desorption (FD) are closely related methods of ionization in FI (Beckey, 1972) the molecule in the gas phase is subjected to a powerful electric field (10 -10 V/cm), and this results in removal of an electron with a much smaller amount of internal energy being supplied, hence fragmentation is reduced. FD (Beckey and Schulten, 1975) differs from other ionization methods in that the sample is ionized without prior vaporization, and hence is an even softer method of ionization. Applications of these three methods to lipid analysis may be found in Games (1978) some of them will also be described in Section 9.6.3. 

The field-desorption spectra of several aldoses and ketoses ionized by attachment of potassium ions led to the identification of the characteristic fragments formed by loss of small molecules, and related secondary ion analysis was carried out on various purine and pyrimidine nucleosides. Also in the nucleoside field a set of mathematical procedures has been applied to the spectra derived from 125 compounds and has led to pattern recognition and interpretation. Underivatized nucleosides have been studied by a method based on pulsed laser and fission fragment-induced desorption, and also by a chemical ionization technique dependent on a direct exposure probe. ... 

By field ion mass spectroscopy of NH3 on Fe at room temperature, 4 -10"" torr, N Hy species and FeN Hy species are detected [510]. Secondary ion mass-spectroscopy of NH3/Fe(110) at 130 K shows FCnNH " with n, m = 1, 2, with n = 0,1,2, 3,4, H2 and Fe" [545]. The spectrum is interpreted as fragments of molecularily adsorbed NH3 [545]. When the temperature is increased, FeNH and FeNHl decrease smoothly, while Fe increases smoothly, illustrating the decrease in surface coverage and the reaction of NH3(g) with the surface at higher temperatures [545]. The intensity of NH3 with NH2 decreases steeply above 300 K consistent with the temperature programmed desorption studies [545]. Both ions are probably formed from adsorbed species [545]. The intensity of NH" decreases more slowly than NH2 and NH3 the reason is probably that NH" is formed from both NH3(g) and NH [545]. 

Since multiphoton excitation in mass spectrometry takes place in the more or less tight laser focus, which can easily be shifted in space and time or be subject to other variations, it can be combined with different ion optical or mechanical arrangements (e.g. sources of neutral molecular systems) without the need for much additional hardware. Thus, by combination with chromatography (particularly gas chromatography), species selection has successfully been realized. Another very promising combination, which has frequently been applied in the recent past for the study of involatile molecules (e.g. polycyclic aromatics, biomolecules), is that of laser desorption of neutral molecules and resonance enhanced multiphoton ionization. All the benefits of multiphoton mass spectrometry, such as soft ionization, selective ionization, controllable fragmentation or secondary excitation for tandem mass spectrometry, may be used in this field. 

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