Adduct ions ionization

Owing to soft conditions, the mass spectra obtained by this kind of ionization techniques are also characterized by the presence of adduct ions, i.e. ionic species formed by weak interactions between the ions and other chemical species (see below). 

If during the ionization the amount of energy deposited on the molecule is low, as occurs in soft techniques, i.e. Cl, ESI, DESI and MALDI, the mass spectrum is very simple. It is characterized by protonated/deprotonated molecules, and eventually few adduct ions but very few or no fragment ions. This implies that it is easy to obtain the molecular weight of the analyte under investigation, but structural information is missing. As an example, the ESI mass spectrum of a small molecule is reported in Figure 2.20. There are two main ions one at m/z 556 and another at m/z 578. As the mass spectrum has been obtained in positive... 

This 1,2-NH3 migration is the key step in the reorganization of the heavy atom skeleton which precedes pseudo-a-cleavage of ionized long-chain alkylamines (Scheme 3). The transition state for this process may be considered to resemble a tight complex of an ionized alkene (ethylene in the case of the archetypal /J-DI, CH2CH2NH3+) and NH3 (equation 9). This idea is consistent with the behaviour of adduct ions [C H2 NH3]+ generated by direct combination of the appropriate ionized alkene and NH3 58,59. 

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. 

Also, a brief note has appeared concerning electrospray ionization mass spectrometry of mixtures of -carotene with ft- and with y-cyclodexlrin in aqueous methanol solutions. Whereas negative ion ESI produced 1 1 adduct ions of -carotene with both of the cyclodextrin isomers, positive ESI gave these adducts only in the case of ft-cyclodextrin302. 

The analyte may be neutral or ionic. Solutions containing metal salts, e.g., from buffers or excess of noncomplexed metals, may cause a confusingly large number of signals due to multiple proton/metal exchange and adduct ion formation. [91] The mass range up to 3000 u is easily covered by FAB, samples reaching up to about twice that mass still may work if sufficient solubility and some ease of ionization are combined. 

When G2-OH is mixed with a fourfold molar excess of Cu + ions the spectrum in Fig. 9b results. These data indicate that each G2-OH can sorb at least four Cu + ions. Moreover, the separation between adjacent copper adducts is 62.5, which indicates that the oxidation state of Cu inside dendrimer during the MALDI-MS experiments is -1-1. Reflectron-mode MS also confirms this assignment the mass differences between the monoisotopic peaks of protonated dendrimers, singlecopper adducts, and double-copper adducts are 61.96 and 61.93, respectively, which is consistent with the assignment of the adduct ions as [Mis + Cu(l)]+ and [Mis + 2Cu(I)-H] +. We speculate that the presence of Cu+ is a consequence of the photochemical reduction of Cu + during ionization. Such photoreduction in MALDI MS measurements has been observed previously when polymers or peptides are used as ligands for Cu + [117,118]. 

In chemical ionization (Cl) mass spectra of 1-phenyl- and 1-heteroarylsilatranes289,210 (X = 2-furyl, 3-furyl, furfuryl, 2-thienyl and 3-thienyl) employing NH3 as the carrier gas, the main peak corresponds to the adduct ion [(M — X)NH3]+. 

The desorption of explosives from the passes was achieved by short wave infrared radiation. The vapors produced were drawn into a triple quadrupole mass spectrometer (MS/MS) and were monitored in the selected reaction monitoring (SRM). Ionization is carried out by corona discharge, followed by APCI. Ions formed from most explosives are M, [M-H]-, and adduct ions. One of these ions is selected to pass into the collision cell, to react with molecules of nitrogen, as a result of which a series of product ions are formed. In the SRM mode, one precursor ion and one product ion are chosen for each compound. The first and third quadruples are adjusted in order to enable SRM transition between these two ions. 

The ion chemistry of tetraalkoxysilanes is relevant to plasma processes. FT-ICR studies have shown a drive towards the growth of -Si-O-Si- units71. For example, the reaction of Si(OMe)3+, the major fragment ion from the El ionization of (MeO)4Si, leads to an adduct ion and to an addition-elimination product which undergoes further addition of (MeO)4Si (equation 27)71b 72. 

APCI has become a popular ionization source for applications of coupled HPLC-MS. Figure 1.33 shows an example of an application of HPLC-APCI coupling [79]. It shows the analysis obtained from extracts of maize plants. Six compounds are identified by mass spectrometry. These compounds have been identified as glucoconjugated DIMBOA (2,4-dihydroxy-7-methoxy-l,4-benzoxazin-3-one) and similar molecules that differ by the number of methoxy groups in the benzene ring and/or by the N-O methylation of the hydroxamate function. This example clearly shows the influence of the analyte on the type of observed molecular species. Indeed, the presence of an acidic group in the compound from peak 1 allows mainly the detection of deprotonated molecular ions, whereas the compound from peak 4 does not contain an acid group and thus leads only to the formation of adduct ions. 

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. 

Most investigations of the chemical ionization (Cl) mass spectral behavior of saccharides have employed methane, isobutane, ammonia, helium (e capture), or a combination of these gases. The reacting forms of these gases are CHs", C Hg, and NH, which decrease in their protonating capability in the order shown. Thus, fragment ions are quite abundant in the methane and isobutane spectra of saccharides, whereas the ammonia Cl spectra are dominated by ammonia duster and sample adduct ions (72TL4827),... 

The term "molecular ion" by definition refers to a radical cation or anion of an intact molecule. Molecular ions are odd-electron ions, which may thus be generated by El. Unfortimately, the term molecular ion is also frequently used to indicate the even-electron ionic species produced by electrospray and APCl. This obviously is not correct. In the soft ionization techniques, predominantly even-electron protonated molecules are generated in positive-ion mode, and deprotonated molecules in negative ions. In addition, various adduct ions may be generated (Table 2.2). These all are even-electron ions, and should therefore not be referred to as molecular ions. Alternatively, the term protonated molecular ions is used, which again is incorrect one cannot protonate a radical cation ... 

The molecular mass of the analyte can be determined from the m/z of the molecular ion in El, if it is observed in the mass spectmm. With a soft-ionization method, next to, or even instead of, the protonated molecule in the positive-ion mode or the deprotonated molecule in the negative-ion mode, a variety of adducts ions may occur in the mass spectrum (Table 2.2). The presence of adduct peaks can be often helpful in assigning the correct molecular mass. The use of both the positive-ion mode, resulting in m/z of the [M+H] ion, and the negative-ion mode, resulting in m/z of the [M-H] ion, if applicable, also leads to an unambiguously molecular-mass determination for an unknown compound. 

---