Electron ionization fragment ions

Metastable ions yield valuable information on fragmentation in mass spectrometry, providing insight into molecular structure. In electron ionization, metastable ions appear naturally along with the much more abundant normal ions. Abundances of metastable ions can be enhanced by collisionally induced decomposition. 

In addition to the searchable library compilations, several compendial books on the electron ionization fragmentation behavior of compounds have been published [34-36]. They are dated, but nevertheless effectively capture the collective fragmentation information prior to their publication. All of these information sources discuss electron ionization spectra. El fragmentation rules, however, can be of limited assistance in interpreting soft ionization and MS-MS product ion spectra. 

The emitter was loaded by the dipping techniques from an acetone solution. An emitter current of 20 milliamperes was used to obtain the spectrum. The spectrum shews a molecular ion, M+, at m/e = 252 and a MH+ at m/e = 253. The peak at m/e = 258 is due to acetone used for instrument tuning purposes. Several low intensity fragment ions are found and are related to the electron-ionization fragmentation mechanism described above. 

Practical Aspects of Electron Ionization Fragmentation of Organic Ions and Interpretation of El Mass Spectra Electron ionization the classical key to organic MS and indispensable part of every introductory course... 

It is well known that the electron-impact ionization mass spectrum contains both the parent and fragment ions. The observed fragmentation pattern can be usefiil in identifying the parent molecule. This ion fragmentation also occurs with mass spectrometric detection of reaction products and can cause problems with identification of the products. This problem can be exacerbated in the mass spectrometric detection of reaction products because diese internally excited molecules can have very different fragmentation patterns than themial molecules. The parent molecules associated with the various fragment ions can usually be sorted out by comparison of the angular distributions of the detected ions [8]. 

In the ideal case for REMPI, the efficiency of ion production is proportional to the line strength factors for 2-photon excitation [M], since the ionization step can be taken to have a wavelength- and state-mdependent efficiency. In actual practice, fragment ions can be produced upon absorption of a fouitli photon, or the ionization efficiency can be reduced tinough predissociation of the electronically excited state. It is advisable to employ experimentally measured ionization efficiency line strengdi factors to calibrate the detection sensitivity. With sufficient knowledge of the excited molecular electronic states, it is possible to understand the state dependence of these intensity factors [65]. 

Electron ionization occurs when an electron beam crosses an ion source (box) and interacts with sample molecules that have been vaporized into the source. Where the electrons and sample molecules interact, ions are formed, representing intact sample molecular ions and also fragments produced from them. These molecular and fragment ions compose the mass spectrum, which is a correlation of ion mass and its abundance. El spectra of tens of thousands of substances have been recorded and form the basis of spectral libraries, available either in book form or stored in computer memory banks. 

The positive-ion electron-ionization spectra of BFB and DFTPP must exhibit molecular and specified fragment ions, the relative abundances of which must fall within a predefined range. Ion abundance criteria for BFB and DFTPP are shown in Table 41.1. 

Chemical ionization produces quasi-molecular or protonated molecular ions that do not fragment as readily as the molecular ions formed by electron ionization. Therefore, Cl spectra are normally simpler than El spectra in that they contain abundant quasi-molecular ions and few fragment ions. It is advantageous to run both Cl and El spectra on the same compound to obtain complementary information. 

In the few microseconds that molecular ions (M ) spend in an ion source following electron ionization, many have sufficient energy to decompose to give fragment ions (F, . .., F +). 

A normal, routine electron ionization mass spectrum represents the m/z values and abundances of molecular and fragment ions derived from one or more substances. 

Physical Chemical Characterization. Thiamine, its derivatives, and its degradation products have been fully characterized by spectroscopic methods (9,10). The ultraviolet spectmm of thiamine shows pH-dependent maxima (11). H, and nuclear magnetic resonance spectra show protonation occurs at the 1-nitrogen, and not the 4-amino position (12—14). The H spectmm in D2O shows no resonance for the thiazole 2-hydrogen, as this is acidic and readily exchanged via formation of the thiazole yUd (13) an important intermediate in the biochemical functions of thiamine. Recent work has revised the piC values for the two ionization reactions to 4.8 and 18 respectively (9,10,15). The mass spectmm of thiamine hydrochloride shows no molecular ion under standard electron impact ionization conditions, but fast atom bombardment and chemical ionization allow observation of both an intense peak for the patent cation and its major fragmentation ion, the pyrimidinylmethyl cation (16). 

Diphenylthiirene 1-oxide and several thiirene 1,1-dioxides show very weak molecular ions by electron impact mass spectrometry, but the molecular ions are much more abundant in chemical ionization mass spectrometry (75JHC21). The major fragmentation pathway is loss of sulfur monoxide or sulfur dioxide to give the alkynic ion. High resolution mass measurements identified minor fragment ions from 2,3-diphenylthiirene 1-oxide at mje 105 and 121 as PhCO" and PhCS, which are probably derived via rearrangement of the thiirene sulfoxide to monothiobenzil (Scheme 2). 

The possible mechanism of ionization, fragmentation of studied compound as well as their desoi ption by laser radiation is discussed. It is shown that the formation of analyte ions is a result of a multi stage complex process included surface activation by laser irradiation, the adsoi ption of neutral analyte and proton donor molecules, the chemical reaction on the surface with proton or electron transfer, production of charged complexes bonded with the surface and finally laser desoi ption of such preformed molecules. 

The mass spectral fragmentations of 9,10-dimethoxy-2,3,4,6,7,ll/)-hexa-hydro-l//-pyrimido[6,l-n]isoquinolin-2-ones 140 and -2,4-diones 141, under electron ionization (at 70 eV) were examined by metastable ion analysis, a collosion-induced dissociation technique and exact mass measurement (97RCM1879). Methyl substituent on N(3) in 140 (R = Me) had a larger effect on both the fragmentation and on the peak intensities, than a methyl substituent on C(6) (R = Me). The ionized molecules of 140 (R = H) were rather stable, whereas 4-phenyl substitution on C(4) of 140 (R = Ph) promoted the fragmentations of the molecular ions. The hexahydro-1//-pyrimido[6,l-n]isoquinoline-2,4-diones 141 were more stable, than the hexahydro-l//-pyrimido[6,l-n]isoquinolin-2-ones 140, and the molecular ions formed base peaks. 

Thus we think of the chemical ionization of paraffins as involving a randomly located electrophilic attack of the reactant ion on the paraffin molecule, which is then followed by an essentially localized reaction. The reactions can involve either the C-H electrons or the C-C electrons. In the former case an H- ion is abstracted (Reactions 6 and 7, for example), and in the latter a kind of alkyl ion displacement (Reactions 8 and 9) occurs. However, the H abstraction reaction produces an ion oi m/e = MW — 1 regardless of the carbon atom from which the abstraction occurs, but the alkyl ion displacement reaction will give fragment alkyl ions of different m /e values. Thus the much larger intensity of the MW — 1 alkyl ion is explained. From the relative intensities of the MW — 1 ion (about 32%) and the sum of the intensities of the smaller fragment ions (about 68%), we must conclude that the attacking ion effects C-C bond fission about twice as often as C-H fission. 

Chlornitrofen and nitrofen conditions for GC/MS column, cross-linked methyl silicone capillary (12 m x 0.22-mm i.d., 0.33- am film thickness) column temperature, 60 °C (1 min), 18 °C min to 265 °C inlet, transfer line and ion source temperature, 260, 200 and 200 °C, respectively He gas column head pressure, 7.5 psi injection method, splitless mode solvent delay, 3 min electron ionization voltage, 70 eV scan rate, 0.62 s per scan cycle scanned mass range, m/z 100-400. The retention times for chlornitrofen and nitrofen were 11.8 and 11.3 min, respectively. The main ions of the mass spectrum of chlornitrofen were at m/z 317, 319 and 236. Nitrofen presented a fragmentation pattern with the main ions at m/z 283, 202 and 285. ... 

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