Plasma desorption technique

After focusing the accelerating potential (V) is applied for a much shorter period than that used for ion production ca 100 nsec) so that all the ions in the source are accelerated almost simultaneously. The ions then pass through the third electrode into the drift zone and are then collected by the sensor electrode. The velocity of the ions after acceleration will be inversely proportional to the square root of the ion mass. With modern ion optics and Fourier transform techniques Erickson et al. (6) could sum twenty spectra per second for subsequent Fourier transform analysis. The advantage of the time of flight mass spectrometer lies in the fact that it is directly and simply compatible with direct desorption from a surface, and thus can be employed with laser desorption and plasma desorption techniques. 

An alternative method for producing mass spectra of solid samples which shows promise for the analysis of zeolites and related materials is the plasma desorption technique recently reported by Schwiekert et al. [71,72]. This technique uses a less energetic means of ionizing the solid than laser ablation, and initial indications are that negative ion spectra from materials like zirconium phosphate may reflect the connectivity as well as the stoichiometry of the solid analyzed to a greater degree than laser ablation. 

A mass spectrometic method for determining the sequence and molecular weight of protected oligonucleotides containing phosphodiester groups has been developed using califomium-252 plasma desorption techniques. It has been dem-... 

In both of these two cases, the heat of evaporation is greater than the energy of the thermal degradation of the molecules. Therefore, methods omitting evaporation prior to ionization had to be looked for. Several attempts were made to overcome this problem . Today, the most promising methods using ionization of the sample in the solid state directly from a probe surface are field desorption (FD), desorption chemical ionization (DCI), Cf-plasma desorption technique, and laser-induced desorption technique. 

While simple molecular weight measurements do not provide information on the amino acid sequences of peptides, they have been profoundly useful for verifying sequences which are inferred from the nucleic acid sequences of the genes encoding the peptide. In particular, they have been used to verify the sequences of peptides produced by recombinant techniques or by total chemical synthesis, or to reveal possible post-translational modifications. More specific information, however, can be obtained by comparative mass mapping of tryptic (or other enzymatic) digests. This approach is particularly useful when the molecular-ion mass exceeds the mass range of the plasma desorption technique. 

In the plasma desorption (PD) technique, liquid matrices have also been used, but most samples are prepared as a thin-film solid. A solution of the... 

The introduction of soft ionization techniques, such as plasma desorption (PD),[1] field desorption (FD)[2] and fast atom bombardment (FAB),[3] marked the beginning of a new era for MS. In fact, they allowed MS to extend its applications to wide classes of nonvolatile, polar, thermally unstable and high molecular weight analytes. This opened up new horizons for MS in many unexpected fields, such as biology, biomedicine and biotechnology, in which this methodology had not previously found any possible application. 

Over the years, a lot of desorption ionization techniques have been introduced to MS, such as plasma desorption, field desorption, laser desorption, secondary ion mass spectrometry, fast atom bombardment, matrix assisted laser desorption and desorption electrospray ionization. Most of them are actually no longer used. In the following paragraphs, both matrix assisted laser desorption (MALDI) and desorption electrospray ionization (DESI) will be discussed. 

There are many different ionization techniques available to produce charged molecules in the gas phase, ranging from simple electron (impact) ionization (El) and chemical ionization (Cl) to a variety of desorption ionization techniques with acronyms such as fast atom bombardment (FAB), plasma desorption (PD), electrospray (ES), and matrix-assisted laser desorption ionization (MALDI) (Mano and Goto 2003). 

Earlier DI techniques include fast atom bombardment (FAB), secondary ionization mass spectrometry (SIMS), plasma desorption, and field desorption. Since their applications are primarily qualitative, they will not be discussed here. 

The mechanisms that lead to such laser desorption are now believed to be collective, non equilibrium processes in the condensed phase (26). In this respect they are closer to processes that must be assumed to lead to ion generation in SIMS and plasma desorption rather than to the thermal laser induced ion generation discussed above, even though the spectra are often indistinguishable for all different laser techniques. The recently reported observation of metal ion (Cu, Ag, Mg etc.) attachement for desorption with high power, short pulse lasers (10, 11, 12) also points to the similarity with SIMS. 

Plasma-desorption mass spectrometry is another technique that has been applied successfully to the detection of readily removable fatty acyl substituents in intact glycolipids and their acylated derivatives. The specific location of the fatty acyl substituents in the ring of the glycosyl residues, as in LOS antigens, is determined by methylation under nonbasic conditions (see Section II.lb), followed successively by O-deacylation, ethylation of the exposed hydroxyl groups, and GC-MS analysis of partially alkylated alditol acetates21 ethyl groups denote the sites of previous O-acylation. 

Plasma Desorption Ionization. Plasma desorption ionization is a highly specialized technique used almost exclusively with a time of flight mass... 

Any MS experiment begins with ionization of molecules of analyte. Numerous ionization techniques (electron ionization, fast atom bombardment, plasma desorption, electrospray ionization, etc.) allow MS analysis of a wide range of organic molecules. In most cases the characterization of combinatorial libraries means analysis of crude compounds i.e. one can expect not only the intended compound to be present in the analyte, but also products of side... 

More sophisticated mass spectrometric methods have been found in the electrospray (ES-MS) and plasma desorption (PD-MS) techniques which have successfully been applied directly to nonvolatile pteridines. A small peak can be detected with folic acid at m/z = 441 together with the mono-, di-, and trisodium species. The dihydro- and tetrahydro derivatives also give the expected results <83Mi 718-05). Fast atom bombardment (FAB) works also with folic acid in special matrices and is another tool for structural studies <83MI 718-08). Even in a molar mixture of 5-methyl-5,6,7,8-tetrahydropterin and tris(pentane-2,4-dionato)iron(III), the metal-pterin complex could be detected by ES-MS <92HCA1955>. 

Mass spectrometry (MS) is a gas-phase technique in which atoms or molecules present in the spectrometer chamber are ionized, and follow a trajectory through applied electric and magnetic fields which separates them according to their mass/charge ratio. A number of procedures have been developed to enable MS to be used for analysing species in the liquid and solid phases, and are based on species extraction into the gas phase. These include plasma desorption, ion bombardment, thermospray and electrospray ionization, and laser desorption. In this section we concentrate on techniques useful to electrochemistry. 

The rarest of these specialized techniques, californium-252 plasma desorption mass spectrometry, has not been applied extensively to inorganic systems, though in a 1983 review (115) Macfarlane quotes several examples, such as polymeric platinum blue, with molecular ions extending to m/z 3000. At present there are only 10 functioning systems and this certainly limits its growth. 

Plasma desorption (PD) was introduced by Mcfarlane and Torgesson [15]. In this ionization technique, the sample deposited on a small aluminized nylon foil is exposed in the source to the fission fragments of252Cf, having an energy of several mega-electronvolts. 

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