Ion characteristics

7 Characteristic ions Care should be taken when using tables of characteristic ions and neutral losses as the values listed represent only a minor fraction of the fragmentations possible. Table A.8. Characteristic ion series and neutral losses  

From benzoyl 51,77, 105 aromatic aldehydes, ketones, carboxylic acids and derivatives 

In Unknown 5.13, chemical ionization and exact mass measurements show that mjz 119 is formed by the loss of C2H5 from the molecular ion such small neutral losses are indicative of the structure. Did you identify both important ion series  

Thus the next step in the interpretation of the spectrum is to note the possible structural significance of all important peaks. Where more than one possible interpretation exists, be sure to note all of them if other evidenee in the spectrum eliminates one or more of these possibilities, these remaining will be much more meaningful. Possible assignments for ions are given as part of the ion series of Table A.6. Mass Spectral Correlations (McLafferty and Venkataraghavan 

1982) shows what elemental compositions and structures are possible for a peak, and what the relative probability is for each of these postulations. The Important Peak Index of the Registry of Mass Spectral Data (McLafferty and Stauffer 1991) provides a listing of compounds which give a particular mjz value as the most important, second most important, etc., peak in their spectra. 

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By using a laser with less power and the beam spread over a larger area, it is possible to sample a surface. In this approach, after each laser shot, the laser is directed onto a new area of surface, a technique known as surface profiling (Figure 2.4c). At the low power used, only the top few nanometers of surface are removed, and the method is suited to investigate surface contamination. The normal surface yields characteristic ions but, where there are impurities on the surface, additional ions appear. 

Some characteristic ions in the mass spectra of quinolines include m/z 102, 128, 156, and so on. Again, as in alkyl pyridines, RCN is lost if the alkyl group is attached to the carbon atom adjacent to the nitrogen atom. 

Table 9.2. Characteristic ions of TBDMS derivitized amino acids...

Alkyl benzenes (C7H7) (m/z 104 and/or 117 are also characteristic ions)... 

McLafferty rearrangement A molecular rearrangement that occurs under certain ionization conditions which results in the production of characteristic ions in the mass spectrum of the analyte from which it has been generated. 

The apparatus for the PFAM film coating on the slider surface is shown in Fig. 1 (a). The film thickness was measured by the TOF-SIMS as shown in Fig. 1 (b). It used a pulsed primary Ga+ ion beam to impact the surface of the PFAM film with an inset energy of 15 keV, an extractor current of 2 fj,A, beam current of 600 pA, a pulse width of 17.5 ns, and a frequency of 10 kHz, respectively. The positive TOF-SIMS spectra on the slider surface is shown in Fig. 2 where the peaks at m/z 31, 50, 69, 100, and 131 in Fig. 2(a) correspond to the positive secondary ion fragments of CF+, CFj, C2F4, and C3F5, respectively. The peak at m/z 469 apparent in Fig. 2(b) corresponds to the ion C12H7F 15O2H+ which is the characteristic ion of PFAM molecules. Therefore, the positive TOF-SIMS spectra demonstrates the existence of PFAM film [24,25]. The thickness of the PFAM film can be determined... 

After dosing methyl radicals and chlorine molecules onto CuaSi samples which were cooled to 180 K, mass spectrometry was used to identify the gas phase reaction products upon heating. The silane products have been identified by monitoring their characteristic ions, which include SiCU" " (m/e=168), CHaSiCla (m/e=148), SiCla" " (m/e=133), (CHa)2SiCl2+ (m/e=128), CHaSiCl2+ (m/e=113), (CHa)2SiCl+ (m/e=93), SKCHala" " (m/e=73). All of these ions are detected. On the other hand, no CHaCl (m/e=53) or SiH4+ (m/e=32) are observed. 

Principles and Characteristics Ion mobility spectrometry (IMS) is an instrumental technique for the detection and characterisation of organic compounds as vapours at atmospheric pressure. Modern analytical IMS was created at the end of the 1960s from studies on ion-molecule chemistry with mass spectrometers and from ionisation detectors for vapour monitoring. An ion mobility spectrometer (or plasma chromatograph in the original termininology) was first produced in 1970 [272],... 

Fig. 8. The most characteristic ions in the mass spectral fragmentation of atropine (26).

Figure 8.7 Low mass (choline phosphate) region of a LD mass spectrum of blood from a P. vivax infected human patient (estimated parasitemia approximately 32,000 parasites/pl). Protocol C is used for sample preparation. A commercial LD TOF system is used laser wavelength 337nm. All one hundred single laser shot spectra, obtained from linear scanning of an individual well, are averaged (no data smoothing). The characteristic ions of detected choline phosphate are denoted.

GC-MS operated in electron impact (El) mode was only sporadically used for the determination of some UV filters such as 4-MBC, EHMC, and OC. Separation was achieved on a 60 m x 0.25 mm i.d. DB-5 column, with 0.25-pm film thickness. For quantification of the compounds, data acquisition was performed in selected ion monitoring (SIM) mode recording three characteristic ions per compound. GC-MS allowed the differentiation between the two isomers (cis/trans) for 4-MBC and EHMC. 

Figure 3.7 Scheme of fragmentation for lupane type molecules that leads to the formation of the characteristic ion fragment at m/z 189... 

The presence of characteristic peaks from palmitic and stearic acids is consistent with the hypothesis of the use of oil as binding media. The lack of any characteristic ions issued from egg tempera means that ToF-SIMS does not allow detection of egg tempera in this sample. However, it could be present but is not detected due to high degradation occurring in very old egg tempera. The presence of short chain fatty acids, which are not detected in the new reference sample, is attributed to oil ageing. The distribution of fatty acid ions in the cross-section is well correlated with the distribution of lead. The ions are not detected in the ground layer (Figure 15.9). 

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