APCI spectra

The nature of the APCI plasma varies widely as both solvent and nebuhzing gas contribute to the composition of the Cl plasma, and thus, APCI spectra can resemble PICI, CECI, NICI, or EC spectra (Chap. 1.2-1 A) depending on the actual conditions and ion polarity. The influence of solvent components, temperature and other parameters explains why APCI conditions suffer from comparatively low reproducibility as compared to other ionization methods. 

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APCI spectra can contain ions from adducts of the analyte with the HPLC mobile phase or organic modifiers, such as ammonium acetate, that may be present. The presence of ions such as (M - - NH4)+ and (M - - CHbCOO)" may hinder interpretation of the spectra obtained. 

Electrospray and APCI spectra consist predominately of molecular species and the ways in which structural information may be generated from analytes ionized by these techniques have been considered in some detail. 

Whereas the components of (known) test mixtures can be attributed on the basis of APCI+/, spectra, it is quite doubtful that this is equally feasible for unknown (real-life) extracts. Data acquisition conditions of LC-APCI-MS need to be optimised for existing universal LC separation protocols. User-specific databases of reference spectra need to be generated, and knowledge about the fragmentation rules of APCI-MS needs to be developed for the identification of unknown additives in polymers. Method development requires validation by comparison with established analytical tools. Extension to a quantitative method appears feasible. Despite the current wide spread of LC-API-MS equipment, relatively few industrial users, such as ICI, Sumitomo, Ford, GE, Solvay and DSM, appear to be somehow committed to this technique for (routine) polymer/additive analysis. 

Both ESI and APCI spectra can look relatively simple in most cases, just showing the pseudo-molecular ion MH or adduct ion in the positive mode, and deprotonation or adduct ions in the negative mode. With API techniques we are dealing with even-electron (non-radical) MH ions as opposed to odd-electron M species that result from electron ionisation. Once an ion has achieved an even-electron state, it is unlikely to revert to an odd-electron state, as this is energetically unfavourable. This means that fragmentations from MH should... 

Figure 5. Positive (left) and negative (right) APCI spectra of cyclohexyl MPA...

Pseudomolecular Ions. In contrast to the traditional MS, the highest mass peaks in ESI/APCI spectra are not always the molecular ion of interest. Instead, pseudomolecular ions, or noncovalent complex ions, are commonly observed. The pseudomolecular ions are generally formed by the analyte-adduct interaction in the solution system that is preserved as a result of the soft ionization of the ESI/APCI process. These ions are also formed by analyte-adduct gas-phase collisions in the spray chamber [49]. The exact mechanisms of how the analyte adducts are formed in ESI/APCI still remain unresolved at this point. More often than not, the adduct ion formation is a major cause for the low detection limit for ESEAPCI MS. However, these associative processes have also created interest in the study of drug-protein/ drug-oligonucleotide gas-phase complexes that benefit from the ability of ESI/APCI MS analysis. 

Note that accurate mass measurements can be obtained for El and Cl as well as ESI and APCI spectra. This relatively simple example gives some idea of the power of modem MS for extractables/leachables testing, and trace organic analysis in general. Using state-of-the-art instruments, such as hybrid TOF and Fourier Transform mass... 

Jewett et al. [24] compared the nse of positive-ion and negative-ion ESI and positive-ion APCI in the analysis of AES, i.e., CH3(CH2)x(0CH2CH2)n0S03 Na. In positive-ion ESI, the sodiated sodinm salt [M+Na] and desulfated molecule [M-NaS03+2H] were observed for each AES component, while in positive-ion APCI only the latter is observed. The positive-ion ESI and APCI spectra of the industrial AES mixture Dobanol 23PES04 are shown in Figure 8.2. In negative-ion... 

Deprotonated molecule peaks for BPA (and NP) were predominant in the ESI spectra, while APCI spectra indicated slight thermal fragmentation. " " Signal intensities and signal-to-noise (S/N) values, based on mass chromatograms for [M-H] of each analyte, were 50 to 100 times larger in the ESI mode than those obtained in the APCI mode. This indicates that ESI is preferred to APCI for accurate quantification and sensitive detection of the target compounds. " ... 

Electrospray appears to be useful especially when dealing with compounds that give poor APCI spectra due to thermal degradation (such as glucuronide conjugates). Meanwhile, electrospray and its derivative, nanoflow electrospray , have developed a niche in the analysis of proteins. They are especially useful as an ionization method combined with capillary electrophoresis, and for offline analysis of spots retrieved from chromatographic plates. 

In addition to the analysis of TATP, the synthetic impurities (i.e., oligomeric peroxides) are also observed by APCI. Figure 16.7 contains the APCI spectra, positive ion mode, from direct infusion of an HCl-catalyzedTATP synthetic product mixture. The sample was infused as a solution in methanol with ammonium acetate additive. The m/z 240 ion corresponds to [TATP + and the series of ions separated by 74 Da, m/z 422-866, correspond to [H(00C(CH3)2) 00H + NH4] (n = 5-11) [60]. Experimental parameters for direct infusion APCI analysis of TATP are given in Table 16.2. 

FIGURE 16.7 APCI spectra, positive ion mode, of an HCl-catalyzed TATP synthetic product mixture. 

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