Field desorption cluster ions

Veith, H.J. Field Desorption Mass Spectrometry of Quaternary Ammonium Salts Cluster Ion Formation. Org. Mass Spectrom. 1976,77,629-633. 

Two scientific aspects in cluster science have been investigated using pulsed-laser field desorption technique with a considerable degree of success.85 One is the critical numbers, which are the smallest numbers of atoms in multiply charged cluster ions when the ions can still resist being... 

M ammonium acetate, which yields [NH4] +, clustered [NH4] + and [CH3COO] ions protonation to [M + H] + or ammonium addition to [M + NH4] + is observed and the spectra are similar to ammonia Cl or field desorption spectra327,374 possible reactions between gas-phase ions and analyte molecules have been investigated374. The ionization mechanism is still not entirely understood, nor is the influence of a number of experimental parameters such as the nature and concentration of the electrolyte, the flow rate and the thermospray temperature375. Thermospray ionization exhibits better sensitivity than conventional Cl and is widely used in pesticide residue analyses327,336,337,340. 

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. 

Ofrier soft ionisation techniques include field desorption, field ionisation, atmospheric pressure ionisation, and fast atom bombardment and each increases the rmolecular weight but because of the production of cluster ions such as (M + Na), (M + 2Na), 2M, etc., they are not used in the investigation of unknowns. 

Older soft ionization methods that are related to MALDl—such as fast atom bombardment (FAB), hquid secondary ion mass spectrometry (LSIMS), or field desorption (FD)— in principle have potential for the generation of intact gas-phase ions from noncovalent complexes. In particular, the extensive clustering often found in FAB-MS, which is generally thought of as a nuisance, can be viewed as evidence for this. However, the limitation of these methods lies in their inabiUty to ionize very large molecules. It is generally very difficult or impossible to obtain useful mass spectra from compounds with molecular weights above a few thousand daltons. For this reason, FAB-MS, LSIMS, and FD-MS only play a very minor role in this field. 

Also of interest are models from closely related fields, such as Matrix Assisted Laser Desorption/Ionization (MALDI) and Electro Spray Ionization Mass Spectrometry (ESI-MS). MALDI in particular attracts attention as this technique records the large molecular ion emissions resulting from the localized deposition of energy occurring on laser irradiation of the solid s surface. Indeed, there appear to be many similarities in the recorded emissions from SIMS and MALDI. Examples of areas of interest include processes described within the Cluster-based mechanisms and the pooling mechanism. These along with their applicability to SIMS are discussed in Section 3.3.4.2. 

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