Ionization spectra

Johnson P M 1976 The multiphoton ionization spectrum of benzene J. Chem. Rhys. 64 4143-8... 

Molecular Identification. In the identification of a compound, the most important information is the molecular weight. The mass spectrometer is able to provide this information, often to four decimal places. One assumes that no ions heavier than the molecular ion form when using electron-impact ionization. The chemical ionization spectrum will often show a cluster around the nominal molecular weight. 

Temperature Effects. The chemical ionization spectra of three paraffins (n-decane, 2,2,3,3-tetramethylhexane (compound 13), and 2,2,5,5-tetramethylhexane (compound 14) have been determined at several different temperatures of the mass spectrometer ionization chamber, and the relative intensities obtained for the MW — 1 ions are give in Table VII. The relative intensities decrease for all three compounds as the temperature increases, in accordance with the behavior found for the chemical ionization spectrum of ethyl-/3-chloropropionate... 

These calculations have been conducted on the basis of RHF optimized geometries, considering the 6-31G basis set for the n-alkane compounds (11), and the 6-31G basis set for the polyacene series (12). In both cases, the basis set contention has been checked by comparison with more thorough investigations on small compounds, such as ADC[3] calculations (11a) on n-butane based on the 6-31IG, 6-31G and 6-31G basis, or the MRSDCI ionization spectrum of ethylene as obtained by Murray and Davidson (33) using a 196-CGTO basis set. 

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. 

The use of ammonia for the protonation of nitroarenes leads frequently to formation of aduct ions, e.g. [M + NH4]+, but not to the protonated species (MH+)112,113. The ammonia chemical ionization spectrum of nitrobenzene shows, in addition to a series of adduct ions, a dominant signal corresponding to the anilinium ion (m/z 94)112114115. Evidence for the isomerization of the [M + NR ]"1" adduct followed by successive loss of NO and OH or NH3 to give ions corresponding to the substitution products, e.g. the anilinium ion, has been given115 see Scheme 41. 

Gilbert, R., and Child, M. S. (1991), Effects of Polarization in the Field Ionization Spectrum of H20, Chem. Phys. Lett. 187, 153. 

Quite often, under electron impact (El), recognition of the molecular ion peak (M) poses a problem. The peak may be very weak or it may not appear at all how can we be sure that it is the molecular ion peak and not a fragment peak or an impurity Often the best solution is to obtain a chemical ionization spectrum (see below in this section). The usual result is an intense peak at M + 1 and little fragmentation. 

The most intense peak in a mass spectrum is called the base peak. Intensities of other peaks are expressed as a percentage of the base peak intensity. In the electron ionization spectrum in Figure 22-4, the base peak is at mlz 141. 

In the chemical ionization spectrum in Figure 22-4, MH+ at mlz 227 is the second strongest peak and there are fewer fragments than in the electron ionization spectrum. 

Figure 22-4 Mass spectra of the sedative pentobarbital, by using electron ionization (left) or chemical ionization (right). The molecular ion (M, m/z 226) is not evident with electron ionization. The dominant ion Itom chemical ionization is MH. The peak at m/z 255 in the chemical ionization spectrum is from M(C2H5). [Courtesy Varian Associates, Sunnyvale. CA.J...

For qualitative analysis, two detectors that can identify compounds are the mass spectrometer (Section 22-4) and the Fourier transform infrared spectrometer (Section 20-5). A peak can be identified by comparing its spectrum with a library of spectra recorded in a computer. For mass spectral identification, sometimes two prominent peaks are selected in the electron ionization spectrum. The quantitation ion is used for quantitative analysis. The confinnation ion is used for qualitative identification. For example, the confirmation ion might be expected to be 65% as abundant as the quantitation ion. If the observed abundance is not close to 65%, then we suspect that the compound is misidentified. 

Why is the two-photon ionization spectrum so broad Is the spectrum mainly homogeneously broadened or inhomogeneously broad-... 

The two-photon ionization spectrum of the Na2o cluster is broadened by many possible vibrational transitions. So the width is not related to the decay time of the resonance. 

Figure 14. (a) Transient two-photon ionization spectrum of Na3 recorded with transform-limited 60-fs pulses. (b) The corresponding Fourier transform shows only the breathing mode of the relating B state [13]. 

The field decreases less rapidly than n 4 because of the increasing number of high m states with increasing n. This method allows a unique transformation from the field ionization spectrum to an n distribution. 

Figure 3. Chemical ionization (methane) GC/MS of the acetylated final product derived from periodate oxidation of the myoinositol ring in PSL-I. (a) Total ion chromatogram of co-injected mixture of the unknown dideuterated alcohol product and the authentic erythritol. (b) Chemical ionization spectrum of peak indicated by an arrow in (a). Inset diagrams depict the fragmentation.

The electron Impact Ionization spectrum of melphalan Is shown In Figure 6. It was obtained by direct-probe introduction of the sample on a Hewlett-Packard 5995A mass spectrometer (70 eV). The relative intensities of the most prominent fragments are given In Table III the abundance cut-off was 1Z. 

The electron impact ionization spectrum is given in Figure 2. A LKB model 9000 mass spectrometer was used with ionization potential of 70eV and accelerating voltage of 3.5kv. The mass spectrum shows a molecular ion at 356 (m/e) with two relatively intense peaks at 341 and 233 (m/e). 

Figure 4. Medium resolution low energy (10 eV) electron ionization spectrum of tobacco smoke tar desorbed from a heated solids probe.

Fig. 4.4. (a) Excitation-ionization spectrum of the H atom Balmer series around the ionization limit in a static homogenous magnetic field, (b) Fourier-transformed time domain spectrum of the spectrum shown in (a). The square of the absolute value is plotted. The time scale is given in units of the cyclotron period Tc = 2 k/u c. Reprinted from Main, Holle, Wiebusch, and Welge (1987). 

No ionization band corresponding to 3,4-dimethyl-l,2-diazetine 44a was observed. Even at higher temperature (725 °C), no ionization band of HCN (13.63 eV) was seen. However, the ionization band of HCN as well as of 43a becomes prominent at 975 °C. Similarly, the ionization spectrum showed a band of imines 43b and 43c in the pyrolysis of 42b and 42c. In none of the cases was the intermediacy of the diazetidines detected by PS. 

The secondary maximum observed in the phase lag near /+ = 355.4 nm has been attributed to a weak transition to a vibrationally excited Rydberg state not visible in the ionization spectrum. Examples of a minimum in SS13, ... 

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