Molecular Ion and Fragmentation Patterns

When a molecule is iouized by electrou impact, it uudergoes the reactiou 

a molecular iou is always a radical catiou, usually with a siugle positive charge. It is evideut that if au orgauic molecule loses au electrou, it must be left with au unpaired electron (i.e., it is a radical ion), as shown by the dot representing the unpaired electron. This radical ion with an unpaired 

However, the molecule may fragment in such a way as to leave a pair of electrons behind on one ion. This is an even electron (EE) ion. Such an ion arises by fragmentation and cannot be the molecular ion. Hence, the molecular ion is never an EE ion. As an example, CH OH may ionize and fragment as follows  

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Preliminary structural characterization was carried out on the soluble products of treatment with BF3/CH3OH (or LiAlH (8), in order to verify the similarity of our samples to materials studied previously (8-11). Gas chromatography-mass spectrometry (GC-MS) (Finnigan 3300 spectrometer) was used to establish the molecular ion and fragmentation patterns solution-state 13C NMR (IBM Instruments WP-200 spectrometer) was employed for quantitation of CH2, CH2OH, and CHOH moieties. 

The purpose of the MS techniques is to detect charged molecular ions and fragments separated according to their molecular masses. Most flavonoid glycosides are polar, nonvolatile, and often thermally labile. Conventional MS ionization methods like electron impact (El) and chemical ionization (Cl) have not been suitable for MS analyses of these compounds because they require the flavonoid to be in the gas phase for ionization. To increase volatility, derivatization of the flavonoids may be performed. However, derivatization often leads to difficulties with respect to interpretation of the fragmentation patterns. Analysis of flavonoid glycosides without derivatization became possible with the introduction of desorption ionization techniques. Field desorption, which was the first technique employed for the direct analysis of polar flavonoid glycosides, has provided molecular mass data and little structural information. The technique has, however, been described as notorious for the transient... 

Aryl-l,3-oxathianes have also been examined under El conditions and show pronounced molecular ions and fragment ions due to simple and complex fragmentation patterns <73AJC2009>. The El mass spectra of a variety of dithiaspiranes as well as 2-aryl-l,3-dithianes have been reported <95MI 608-02). 

The molecular ion and the pattern of fragment ion peaks are unique for each compound. A mass spectrum, therefore, is like a fingerprint of the compound. A positive identification of a compound can be made by comparing its mass spectmm with that of a known sample of the compound. 

The relative intensities of the [M]+ , [M+l]+ and [M+2]+ ions exhibit a characteristic pattern depending on the elements that make up the ion. For any molecular ion (or fragment) which contains one bromine atom, the mass spectrum will contain two... 

Any molecular ion (or fragment) which contains 2 bromine atoms will have a pattern of M M+2 M+4 with signals in the ration 1 3 1 and any molecular ion (or fragment) which contains 2 chlorine atoms will have a pattern of M M+2 M+4 with signals in the ration 10 6 1. 

The first conventional mode of MS involves El ionization, in which the neutral flavonoid is impacted in the gas phase with an electron beam of 70 to 100 eV. Resulting mass spectra of the flavonoid aglycones are characterized by molecular ion peaks (M ), and fragment ions from both the A and B rings. The use of a reactant gas in the ionization chamber. Cl, normally results in the production of a more abundant molecular ion and simpler fragmentation patterns. General information about mass spectra of flavonoids recorded by these methods has been published by several authors. More specific mass spectra analyses... 

As revealed by the data available, the type of compound closest to the ideal for structural analysis of monosaccharides is the class of dialkyl dithioacetals or their acetates their mass spectra contain a considerable peak due to molecular ion, and their fragmentation patterns are simple enough (due to the absence of a sugar ring) and specific enough to permit determination of the position of substituents on the basis of the position of peaks. Thus, elimination is characteristic of the C-2-substituents, whereas substituents at C-3 tend to be retained, producing a peculiar difference between the mass spectra. However, the mass spectra of dialkyl dithioacetals provide almost no information regarding the stereochemistry of the monosaccharide molecule. 

The vast majority of fragments will have only a single positive charge (i.e., e = 1) thus die mle of a given ion corresponds to the mass of the ion in atomic mass units. It is very important to remember that only ions (cations or radical cations) are detected—neutral species (closed-shell molecules or radicals) are not detected because they are not accelerated and they are not influenced by the applied field. Thus MS yields information about the mass of the molecular ion and the masses of fragment ions produced from the molecular ion. This so-called cracking pattern provides information about connectivity in the molecule that can be used to reconstruct the intact precursor molecule. 

LC-MS uses different types of soft chemical ionization that produces molecular ions and no fragmentation pattern. In MS/MS instruments the molecular ions can be fragmented by collision with a gas for example, He. This fragmentation can be used for identification of a compound. No mass spectral libraries exist for LC-MS hence identification of unknown compounds is more time-consuming than for GC-MS. For known compounds LC-MS is a very sensitive and specific method, using LC-MS/MS systems the analytical performance can be increased even more. LC-MS analysis is especially suitable for non-volatile POMs such as non-ionic surfactants in house dust samples (Clausen et al., 2003). 

The mass spectra of 3,5-diaryl-l,2,4-selenadiazoles 79 show a molecular ion and the base peak arising from ArCN+. In general, the first fragmentation step is the loss of arenenitrile. The proposed fragmentation pattern of the studied compounds is shown in Scheme 1. 

The volatility of GAs is increased prior to GC by forming the methyl esters with diazomethane. Hydroxylated GAs are often converted to trimethylsilyl (TMS) ethers after methylation. The mass spectra of GA methyl esters TMS ethers frequently contain intense molecular ions and characteristic fragmentation patterns, which are easier to interpret than those of the free hydroxy compounds. When recovery of GAs is required after GC, GA TMS ether esters are a convenient derivative since the free GA can easily be recovered after hydrolysis in water. 

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