Soft ionization types

It is therefore not surprising that the interest in PyMS as a typing tool diminished at the turn of the twenty-first century and hence why taxonomists have turned to MS-based methods that use soft ionization methods such as electrospray ionization (ESI-MS) and matrix-assisted laser desorption ionization (MALDI MS). These methods generate information-rich spectra of metabolites and proteins, and because the molecular ion is seen, the potential for biomarker discovery is being realized. The analyses of ESI-MS and MALDI-MS data will still need chemometric methods, and it is hoped that researchers in these areas can look back and learn from the many PyMS studies where machine learning was absolutely necessary to turn the complex pyrolysis MS data into knowledge of bacterial identities. 

Quite often a normal electron ionization mass spectrum appears insufficient for reliable analyte identification. In this case additional mass spectral possibilities may be engaged. For example, the absence of the molecular ion peak in the electron ionization spectrum may require recording another type of mass spectrum of this analyte by means of soft ionization (chemical ionization, field ionization). The problem of impurities interfering with the spectra recorded via a direct inlet system may be resolved using GC/MS techniques. The value of high resolution mass spectrometry is obvious as the information on the elemental composition of the molecular and fragment ions is of primary importance. 

In 1974, Comarisov and Marshall60 developed Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). This technique allows mass spectrometric measurements at ultrahigh mass resolution (R = 100000-1000000), which is higher than that of any other type of mass spectrometer and has the highest mass accuracy at attomole detection limits. FTICR-MS is applied today together with soft ionization techniques, such as nano ESI (electrospray ionization) or MALDI (matrix assisted laser/desorption ionization) sources. 

Attempts to identify nonvolatile compounds present in water extracts by using soft-ionization mass spectrometry have been reported (9). In general, however, the extracts were found to be too complex to be analyzed without using some type of preseparation technique. 

They are still the workhorses of coupled mass spectrometric applications, as they are relatively simple to run and service, relatively inexpensive (for a mass spectrometer), and provide unit mass resolution and scanning speeds up to approximately 10,000 amu/s. This even allows for simultaneous scan/ selected ion monitoring (SIM) operation, in which one part of the data acquisition time is used to scan an entire spectrum, whereas the other part is used to record the intensities of selected ions, thus providing both qualitative information and sensitive quantitation. They are thus suitable for many GC-MS and liquid chromatography-mass spectrometry (LC-MS) applications. In contrast to GC-MS with electron impact (El) ionization, however, LC-MS provides only limited structural information as a consequence of the soft ionization techniques commonly used with LC-MS instruments [electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI)]. Because of this limitation, other types of mass spectrometers are increasingly gaining in importance for LC-MS. 

Another important development in the field of biopolymer analysis is the introduction of matrix-assisted laser desorption ionization (MALDl), which is a rather recent soft ionization technique that produces molecular ions of large organic molecules. In combination with time-of-flight (TOP) mass spectrometry, it was proposed as a valuable tool for the detection and characterization of biopolymers, such as proteins, peptides, and oligosaccharides, in many types of samples.The use of these recently developed techniques has not decreased the use of chromatography in determinations of biopolymers. Some efforts on the adaptation of the separation abilities of HPLC to the high potential of MALDl-TOF for the sensitive determination of additives in biocomposites are currently being carried out. 

Mass spectrometry is widely used for the qualitative analysis of unknowns samples, and in particular, for the identification and characterization of biological macromolecules. Recent decades have seen the introduction and optimization of the so-called soft ionization methods that provide intact, vapor-phase biomolecular ions for separation and detection. This chapter considers MS fundamentals, ionization methods, and applications to biological macromolecules. Conventional mass spectrometers used for low volatile molecular weight samples that are introduced in the vapor phase are called single-focusing mass spectrometers, and use an electron-impact ion source.1 Figure 15.1 shows a diagram of this type of instrument. 

Mass spectrometry is generally useful only to obtain information on the molecular weight and to identify the type and functional groups bound to the terpenoid core. Soft ionization techniques must be employed (FAB, DC1, TSP). The spectra of Winterstein esters are characterized by a prominent peak at m z 210, and those of the X,W-dimethylphenylisoserine esters at 226, corresponding to the aminoacidic side chain [58],... 

The most popular soft ionization technique for MSI is MALDI. Instead of an ion or aerosol beam, a pulsed and focused laser beam is fired against the sample surface in order to initiate the desorption and ionization process (Figure 1A). However, in contrast to the former ionization methods, MALDI requires the sample to be covered with a matrix compound, usually a small organic and aromatic acid. Different matrices enable the ionization/desorption of different types of biomolecules, as summarized in Table 3. 

The ESI and APCI are both soft ionization techniques, hence the spectra obtained using these methods are usually very simple and consist predominantly of the pseudomolecular ion. When more information is required, tandem mass spectrometry is a popular option. Several different types of tandem LC/MS systems can be found, with triple quadrupole and ion trap being the most popular choices. Hybrid systems including magnetic sector-quadmpole, magnetic sector-TOF, quadrupole-TOF (Q-TOF) and ion trap-TOF have been described. The most successful among these hybrids is the Q-TOF instrument. 

Since MALDl is a soft ionization method, fragmentation is generally not pronounced in the mass spectra. However, xmder specific instrumenfal and experimental conditions, different types of fragments can be observed in MALDI-TOF spectra. ... 

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