Ionization efficiency spectrum

Figure 6. Ionization efficiency spectrum, obtained jrom a neutrai gyrovibronic intermediate state by scanning with photon hvt, either jrom a second laser, monochromator or edge filter.

Figure 1. Sketch of the ionization efficiency curves and the "70 eV" mass spectrum for the electron impact ionization of carbon monoxide.

Recording the MALDI spectrum of a mixture of two polymers having different backbones, one finds that MALDI peak intensities reflect in a distorted manner the abundances of the chains and the composition of the blend. In some cases, the distortion is small and thus MALDI is semiquantitative. The main cause is that the ionization efficiency (i.e., the probability of ion production) for the two polymers is not the same. Furthermore, it has been shown that instrumental parameters can affect peak intensities, thus falsifying the composition of the blend. For instance, some authors [5] studied an equimolar mixture of PEG and PMMA, recorded the MALDI spectrum of the mixture and found, on changing instrumental parameters, that the apparent blend composition changed from 100/0 to 50/50 to 0/100. 

Unfortunately, there are also some disadvantages i) the decreased ionization efficiency at 12-15 eV also means a significant loss of sensitivity (Fig. 5.4), ii) low ion source temperatures cause long-lasting memory of previous samples due to slow desorption from the surfaces that have been in contact with the sample vapor, and iii) a weak molecular ion peak may well be enhanced, however, a spectrum showing no molecular ion peak at 70 eV will not turn into a spectrum exhibiting a strong molecular ion peak at 12 eV. 

In summary, preliminary experiments have demonstrated that the efficiency and outcome of electron ionization is influenced by molecular orientation. That is, the magnitude of the electron impact ionization cross section depends on the spatial orientation of the molecule widi respect to the electron projectile. The ionization efficiency is lowest for electron impact on the negative end of the molecular dipole. In addition, the mass spectrum is orientation-dependent for example, in the ionization of CH3CI the ratio CHjCriCHj depends on the molecular orientation. There are both similarities in and differences between the effect of orientation on electron transfer (as an elementary step in the harpoon mechanism) and electron impact ionization, but there is a substantial effect in both cases. It seems likely that other types of particle interactions, for example, free-radical chemistry and ion-molecule chemistry, may also exhibit a dependence on relative spatial orientation. The information emerging from these studies should contribute one more perspective to our view of particle interactions and eventually to a deeper understanding of complex chemical and biological reaction mechanisms. 

An early theory of the IT value was proffered by Spencer and Fano [44], based on the degradation spectrum. Another method, the Fowler equation, was employed by Inokuti [47] for electron irradiation, based on the approximation that there is only one ionization potential and that the ionization efficiency is unity. These restrictions can be relaxed. The main result of Inokuti s analysis may be given as follows. 

Each mass spectrum has a story to tell. The molecular ion, M+ , tells us the molecular mass of an unknown. Unfortunately, with electron ionization, some compounds do not exhibit a molecular ion, because M+ breaks apart so efficiently. However, the fragments provide the most valuable clues to the structure of an unknown. To find the molecular mass, we can obtain a chemical ionization mass spectrum, which usually has a strong peak for MHH. 

Figure 1. Versions of photoionization spectroscopy wherein not only the dependence of the multiphoton ionization efficiency on the laser wavelength is subject to measurement, but also the mass spectrum of photons and energy spectrum of photoelectrons (a) energy-level diagram (b) collision of a neutral particle with laser photons.

More often, as described earlier, compound purity is reported taking into account the purities determined from the UV, ELSD, and CLND detectors. In some instances, purity assessment has been made based on the intensity of the expected ion in the mass spectrum relative to the sum of the intensities of all ions in the spectrum. This method, however, is only a very crude estimate of purity, because ionization efficiencies for compounds can vary widely within and between classes of compounds. Though LC/MS (with UV and/or ELSD detection) has been adopted as the method of choice for assessing the quality and quantity of material prepared by parallel synthesis techniques, a decision still needs to be made by each respective organization as to what constitutes acceptable quality before submitting a sample for biological testing. 

Fig. 5 shows a comparison based on extracted ion current profiles of ionization efficiencies for Nifedipine. Included are positive ion APCI and ESI, along with negative ion chloride ion attachment APCI. As predicted. Nifedipine can be readily ionized by all three LC/MS ionization processes. The mass spectra of Nifedipine obtained using these three ionization processes are shown in Fig. 6. Positive ion ESI and APCI both show [M + H] at m/z 347 (note the [M + Na] ion at m/z 369 in the ESI spectrum) with some fragmentation (the ion at m/z 315 likely results from loss of methanol from one of the methyl ester groups). The negative ion chloride ion attachment spectrum shows the anticipated [M + Cl] at m/z 381, along with a [M + Cl - HCl] ion at m/z 345. Note that there is also a relatively small [M + TFA] (TFA) ion at m/z 459. If TFA is present in the HPLC...

The ICP has proliferated as a method of converting chemical compounds into their elemental constituents which subsequently emit light of characteristic wavelengths. Accordingly, ICP has been used extensively as an emission source for optical detection systems in order to perform elemental analysis. Since each element can emit hundreds of optical lines, the use of ICP/AES for multiple element analysis, or for the detection of elements in unknown or concentrated matrices, can suffer from interferences due to spectral overlap. By contrast, ICP-MS provides inherently simpler spectral Information. An example of such a spectrum is demonstrated in Figure 2 showing a typical ICP-MS scan for a 10 ug ml"l solution of mixed transition metals. The demonstrated sensitivity here is 10 to 10 counts s l per ug m1"l and, coupled with the nearly universal ionization efficiency of the ICP ion source, provides typical detection limits in a narrow range between 0.1 to 10 ng.ml" for most elements. In fact over 90% of the elements in the periodic table are accessible for such analytical determinations. 

Monks, P. S., L. J. Stief, D. C. Tardy, J. F. Liebman, Z. Zhang, S. -C. Kuo and R. B. Klemm Discharge flow-photoionization mass spectrometrie study of HOI Photoionization efficiency spectrum and ionization energy, / Phys Chem, 99 (1995) 16566-16570. 

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