Secondary ionization techniques

By modifying the laser wavelength (and intensity) or using a second laser, a photoionization process may be included (LPI) to generate ions from the analyte that will be further separated and detected in the mass spectrometer. When the ion formation from the plume of the first laser is low, some other secondary ionization techniques such as Cl can also be used. 

The three techniques — laser desorption ionization, laser ablation with secondary ionization, and matrix-assisted laser desorption — are all used for mass spectrometry of a wide variety of substances from rock, ceramics, and bone to proteins, peptides, and oligonucleotides. 

In other articles in this section, a method of analysis is described called Secondary Ion Mass Spectrometry (SIMS), in which material is sputtered from a surface using an ion beam and the minor components that are ejected as positive or negative ions are analyzed by a mass spectrometer. Over the past few years, methods that post-ion-ize the major neutral components ejected from surfaces under ion-beam or laser bombardment have been introduced because of the improved quantitative aspects obtainable by analyzing the major ejected channel. These techniques include SALI, Sputter-Initiated Resonance Ionization Spectroscopy (SIRIS), and Sputtered Neutral Mass Spectrometry (SNMS) or electron-gas post-ionization. Post-ionization techniques for surface analysis have received widespread interest because of their increased sensitivity, compared to more traditional surface analysis techniques, such as X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), and their more reliable quantitation, compared to SIMS. 

Intact bacteria were first introduced into a mass spectrometer for analysis of molecular biomarkers without processing and fractionation around 1975.6 The ionization techniques available at the time limited analysis to secondary metabolites that could be volatilized, such as quinines and diglycerides, and vigorous pyrolysis of bacteria was explored as an alternative.7 Although biomarkers were destroyed in pyrolysis strategies, computer-supported cluster analysis was developed to characterize pure samples. 

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 is a branch of MS specially designed for dealing with the analysis of inorganic materials.[21,22] Different specific ionization techniques, such as inductively coupled plasma mass spectrometry (ICP-MS),[23] glow discharge mass spectrometry (GD-MS)[24] and secondary ion mass spectrometry (SIMS),[25] are available and they are widely used in cultural heritage applications. Their description is beyond the scope of this chapter. 

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. 

From Chapters 1 and 2, it should be clear that flame AAS and AFS, and FES are all secondary analytical techniques which depend upon a comparison of signals from samples with those from standards. To yield accurate results, it is imperative in all three techniques that the determinant in samples and standards behaves in exactly the same way. If it does not, erroneous results will be obtained, and we say that an interference has occurred. Interferences fall into four broad classes physical, chemical, ionization, and spectral. Each of these classes needs to be considered in turn, together with the methods used to combat the problems which they would otherwise cause. 

No matter which ionization technique or chromatographic method is used, three acquisition modes exist scanning, selected-ion monitoring (SIM) (not to be mistaken with SIMS, which means secondary ion mass spectrometry) and selected-reaction monitoring (SRM). 

There are other MS ionization techniques applied to analyze pyrolysates such as secondary ion mass spectrometry (SIMS) [66], californium plasma desorption [67], fast atom bombardment (FAB) [68]. 

The three techniques---laser desorption ionization, laser ablation with secondary ionization, and matrix—... 

Chemical, electron field desorption, laser desorption, photon, plasma desorption, spark, and thermal ionization are all used as primary ionization processes. Secondary ionization is the term used to describe a process in which ions are ejected from a surface as a result of bombardment by a primary beam of atoms or ions. If low energy or soft ionization techniques are used, the mass of the target molecule can be determined. Advances in soft ionization techniques have extended the use of MS to the direct measurement of peptide and protein mass. Ionization at higher energy results in more extensive fragmentation of target molecules. 

Early in the history of mass spectrometry (MS), large biomolecules were not analyzed because efficient methods to transport these molecules into the gas phase were unknown. Degradation typically occurred during vaporization of these nonvolatile molecules so that electron ionization of the intact molecular ion was not possible. Ionization by fast atom bombardment (FAB), field desorption (FD), secondary ionization mass spectrometry (SIMS), and plasma desorption (PD)" from the radioactive decay of Cf finally made the ionization and analysis of peptides possible. These latter techniques, although still used today, are not as popular as electrospray... 

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