Fragmentation elemental composition

In one instrument, ions produced from an atmospheric-pressure ion source can be measured. If these are molecular ions, their relative molecular mass is obtained and often their elemental compositions. Fragment ions can be produced by suitable operation of an APCI inlet to obtain a full mass spectrum for each eluting substrate. The system can be used with the effluent from an LC column or with a solution from a static solution supply. When used with an LC column, any detectors generally used with the LC instrument itself can still be included, as with a UV/visible diode array detector sited in front of the mass spectrometer inlet. 

Gases and volatile materials can be swept into the center of an argon plasma flame, where they are fragmented into ions of their constituent elements. The m/z values of ions give important information for identification of the elemental composition of a sample, and precise measurement of ion abundances is used to provide accurate isotope ratios. 

Accurate mass measurement on a molecular ion of any substance gives directly the molecular formula for fragment ions, similar measurement gives their elemental compositions. 

A double-focusing mass spectrometer can mea.sure mass accurately to several decimal places, thus enabling the determination of molecular formulae and elemental compositions of fragment ions. 

This method can sometimes be used for determining the probable elemental composition of fragment ions. However, it is not as generally applicable and does not replace accurate mass measurement for determining molecular formulae and elemental compositions. 

B. Characteristic Fragment Ions The most characteristic cleavage is the loss of carbons 15,16, and 17, together with the side chain and one additional hydrogen. It is possible to determine the elemental composition of the side chains of steroids by the difference in the mass between the molecular ion and an intense ion more than 15... 

Ionization and fragmentation of materials by a variety of means, principally by electron bombardment, or the softer techniques of chemical ionization, field ionization or fast atom bombardment. Analysis of the range of mass fragments produced. Elemental composition of non-volatile materials by application of an RF spark. 

Unknown 1. Try to identify a compound with the spectrum represented in Fig. 5.1. The exact molecular mass of the compound is 60.0211 Da, which defines its elemental composition as C2H4O2. At this stage pay attention only to the most abundant peaks in the spectrum m/z 60 (molecular ion) and primary fragment ions of m/z 45, m/z 43, m/z 28, and m/z 15. Use the masses of elements from the periodic table of chemical elements. 

There are certain rules determining fragmentation of organic compounds in a mass spectrometer. That is why on the basis of the fragmentation pattern it is possible to define the molecular mass, elemental composition, presence of certain functional groups, and often the structure of an analyte. There are a lot of similarities in the mass spectrometric behavior of related compounds. This fact facilitates manual interpretation of a mass spectrum, although it requires some experience. It is also worth mentioning that mass... 

Determination of the elemental composition should be started with the M + 2 peak. Chlorine, bromine, sulfur, and silicone are easily detected due to characteristic signal multiplicity for each of these elements. There is a simple mle to check the presence of the main A + 2 elements. If the intensity of the M + 2 peak constitutes less than 3% of the intensity of the M peak, the compound does not contain chlorine, bromine, sulfur, or silicon atoms. This rule is valid for the fragment ions as well, while its applicability is confirmed by the data summarized in Table 5.4. 

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. 

To be reliable the scheme should represent ions of known elemental composition, while the mentioned fragmentation pathways should be confirmed by metastable ion... 

Note It is important to assign the correct charge and radical state to all species encountered and to carefully track them through a fragmentation scheme. Otherwise, impossible fragmentation pathways may be formulated, thereby misleading the assignment of elemental composition and molecular constitution. 

The molecular ion peak directly provides valuable information on the analyte. Provided the peak being of sufficient intensity, in addition to mere molecular mass, the accurate mass can reveal the molecular formula of the analyte, and the isotopic pattern may be used to derive limits of elemental composition (Chaps. 3.2 and 3.3). Unfortunately, the peak of highest m/z in a mass spectrum must not necessarily represent the molecular ion of the analyte. This is often the case with El spectra either as a result of rapidly fragmenting molecular ions or due to thermal decomposition of the sample (Chaps. 6.9 and 6.10.3)... 

Although convenient at first sight, the lack of fragment ion peaks in FI spectra also means a lack of structural information. If more than an estimate of the elemental composition based on the isotopic pattern is desired, collision-induced dissociation (CID, Chap. 2.12.1) can deliver fragment ions for structure elucidation. Fortunately, the fragmentation pathways of IVT ions in CID are the same as in EI-MS (Chap. 6). 

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