Ionization mechanisms related to negative Ion APPI, APCI, and DART

Abstract

A recent report found that negative ion atmospheric pressure photoionization (Ni-APPI) and direct analysis in real time (Ni-DART) ionize compounds by electron capture, dissociative electron capture, proton abstraction, and anion adduction. The authors of this report suggested that the common ionization of Ni-APPI and Ni-DART demonstrated that these techniques ionize a wider array of compounds than negative ion atmospheric pressure chemical ionization (Ni-APCI). Here we show that Ni-APCI, using the atmospheric sample analysis probe (ASAP) technique, in the absence of solvent vapors, ionizes the same and similar compounds by the same reported mechanisms. These results are supported by previous publications, which show that each mechanism is active for Ni-APCI. This work demonstrates that irrespective of the initial method of ionization, at atmospheric pressure, similar ion/ electron-molecule chemistries prevail. (J Am Soc Mass Spectrom 2009, 20, 1518 -1521

Direct analysis in real time (DART) mass spectrometry of nucleotides and nucleosides: elucidation of a novel fragment [C5H5O]+and its in-source adducts

“…A homogenous mixture (1:1 molar ratio) of dAMP and 13 C, 15 N-labeled dAMP was introduced into the DART mass spectrometer under the same conditions as described above. The addition products resulting from the recombination between 13 C, 15 N-labeled and non-isotope labeled fragments was detected. Observed accurate m/z values, along with theoretical values, of ions and their relationships with the corresponding elemental compositions are listed in Figure 3.…”

“…Observed accurate m/z values, along with theoretical values, of ions and their relationships with the corresponding elemental compositions are listed in Figure 3. In particular, the ion with m/z 221.1048 represents the adduct formed between non-isotope labeled adenine (A) and 13 C, 15 N-labeled fragment 1 (1 iso ) and the ion with m/z 226.0899 represents the adduct formed between 13 C, 15 N-labeled adenine (A iso ) and 1 (For the adduct structures, see Scheme S-2 of the Supporting Information). The observation of these two peaks clearly indicates that nucleobases in the DART ion source directly react with fragment 1, validating the first reaction mechanism.…”

“…The addition reaction between a nucleobase and 1 was also confirmed by the spectrum obtained using DART mass spectrometry of a mixture of dAMP and 13 C, 15 N-labeled dTMP (1:1 molar ratio) (Figure S-6 of the Supporting Information). The ion with m/z 221.1040 represents the adduct (Scheme S-3 of the Supporting Information) between adenine that results from the fragmentation of dAMP and 1 iso that is produced from 13 C, 15 N-labeled dTMP. In this study, we were not able to determine whether the adduct formation occurs according to the second proposed mechanism (Scheme 2, Mechanism B) although such reactions are definitely possible.…”

“…The major breakthrough/advantage of this technique is that it adopts a new ion source that does not operate in vacuum as many classic mass spectrometer ion sources do [11]. Recent reports show that ionization mechanisms in the DART source are complicated [12][13][14]. The ionized analyte molecules generated in either positive-ion or negative-ion mode can be detected by an atmospheric pressure ionization mass spectrometer (i.e., Accu-TOF-LC).…”

Atmospheric Pressure Photoionization Mass Spectrometry of Fullerenes

“…Some authors have used C 60 as a test analyte to study the performance of APPI,99 reporting that fullerene ionization occurred through a solvent mediated ionization mechanism 100 yielding the radical molecular ion [M] -• as the base peak of the mass spectrum. [29][30][31] Kawano et al 32 proposed a LC-APPI-MS method for the determination of C 60 This paper evaluates the applicability of negative ion APPI for the analysis of a wide range 109 of fullerene compounds (C 60 , C 70 , C 76 , C 78 and C 84 ) and reports in-depth the study of their APPI MS 110 spectra. These spectra are compared with those found when using other atmospheric pressure 111 ionization (API) sources and possible discrepancies are discussed, taking into account the different 112 ionization mechanisms.…”