Alkali ion adducts

Metal ions, in particular singly charged ions such as Na", K", Cs", and Ag are sometimes added to the matrix-analyte solution to effect cationization of the neutral analyte. [98] This is advantageous when the analyte has a high affinity to a certain metal ion, e.g., towards alkali ions in case of oligosaccharides. [6] Addition of a certain cation can also help to concentrate the ions in one species, e.g., to promote [Mh-K]" ions in favor of all other alkali ion adducts upon addition of a potassium salt. 

Despite these widespread applicahons, ILM is not equally well suited for all classes of analytes. Due to the need for increased laser energies/fluences for the ionizahon/desorption process, ILMs may only be of restricted suitability for some classes of analytes. For example for proteins, an extensive peak broadening caused potenhally by the combination of extended neutral losses (e.g., of ammonia or water) and alkali-ion-adduct formation can be observed. On the other hand, the increased tendency of the ILM to favor sodium and potassium adduct formation makes it ideally suited for the measurement of carbohydrates [38,40], whereas in proteomics, this tendency of adduct formahon is again an unwanted effect. 

In homogeneous, aqueous solution, alkali metal hydroxides react with carbohydrates to produce negatively charged carbohydrate species. Although the general feeling among chemists is that these species are free alcoholate anions, the possibility that they are composed, at least partially, of carbohydrate-hydroxide ion adducts cannot be dismissed. 

Alkali metal adducts often are observed, and even in negative ion mode (M + Na — 2H) are often abundant for monocharged ions. If they can be avoided, not only does the signal increase because it is not divided any more over several species but also better MS/MS spectra can be obtained. To eliminate these adducts, glassware should not be used during sample work-up. Then, addition of acid or, better still, ammonium acetate allows their interference to be reduced further. However, often alkali metal salts are added at low concentrations to suppress the protonated species. This is easier to achieve, but fragmentation of these adducts yields less sequence information than protonation. 

Firm identification of the [XeOF5]" ion in the solid state for the alkali fluoride adducts was based on more detailed analysis of the Raman spectra160). VSEPR theory predicts distortion from the pseudo octahedral (C4v) to Cs symmetry, the lone pair occupying an octahedral face adjacent to the axial fluorine (XVIII). For... 

Properties of the complexes of alkali metal cations with various bases are important in understanding ion-molecule interactions, solvation effects, biomedical and physiological phenomena related to ion channels and relevant in medical treatments. Reliable experimental bond dissociation enthalpies, and thereby gas-phase alkali ion affinities, could now be obtained using various mass spectrometry techniques such as the Fourier-transform ion cyclotron resonance (FT-ICR), collision-induced dissociation and photodissociation methods. However, these methods do not provide direct information on the adduct structures. 

Anthocyanins in the presence of acidic matrixes such as MAA are predominately in the aromatic oxonium ion form 23) and are detected as [M] ions by MALDI-TOF mass spectral analysis 23,24). However, anthocyanins 23), in the same manner as polyflavan-3-ols 21,22,25), may also associate with naturally abundant sodium [M + Na] and potassium [M + K] forming alkali metal adducts. To suppress the formation of alkali metal adducts, we deionized the anthocyanin-polyfIavan-3-ol/matrix solution prior to deposition on the target. This approach resulted in the detection of andiocyanin-polyflavan-3 ols in die oxonium ion form [M]. ... 

Other basic zeolites are those prepared by Baba et al. [79], consisting of low-valent Yb or Eu species introduced into alkali ion-exchanged Y zeolites by impregnation with Yb or Eu metal dissolved in liquid ammonia then heating under vacuum at ca 470 K. These materials catalyze the Michael addition of cyclo-pent-2-enone and dimethyl malonate at 303 K, without solvent, yielding, after 20 h reaction, 81 % Michael adduct with 100% selectivity [80,81]. 

Metal-Ion Adducts as Precursor Ions CID of the metal-ion adducts has been a useful tool to distinguish linkage positions in oligosaccharides. These applications have inclnded cationization with alkali [9,20,21,32], alkaline earth [32], and transition metals [33]. 

In some cases it is observed that, under the experimental conditions used (mobile phase composition, ionization and API interface parameters), more than one ionized form of the intact analyte molecule is observed, i.e. adduct ions of various kinds (see Section 5.3.3 and Table 5.2). An example is shown in Figure 9.6, in which a well known anticancer drug (paclitaxel, Figure 9.6(a)) was analyzed by positive ion ESI-MS (infusion of a clean solution). The first spectrum (Figure 9.6(b)) shows four different adducts (with H+, NH, Na+ and K+). Adjustment of the cone (skimmer) potential (Section 5.3.3a), to lower values in this case, enabled production of the ammonium ion adduct to dominate the MS spectrum (Figure 9.6(c)) in a robust fashion, and this ion yielded a useful product ion spectrum (that appeared to proceed via a first loss of ammonia to give the protonated molecule) which was exploited to develop an MRM method that was successfully validated and used. It is advisable to avoid use of analyte adducts with alkali metal ions (commonly Na+ and to some extent K+) since, when subjected to colli-sional activation, these adducts frequently yield the metal ion as the dominant product ion with only a few low abundance product ions derived from the analyte molecule. However, when feasible, both the ammonium adduct and protonated molecule should be investigated as potential precursor ions at least until it becomes clear that one will provide superior performance (sensitivity/selectivity compromise) than the other. 

Wang, H.-Y., Yim, W.-L., Kluner, T, Metzger, J.O. (2009) ESIMS Studies and Calculations on Alkali-metal Adduct Ions of Ruthenium Olefin Mefathesis Catalysts and Their Catalytic Activity in Metathesis Reactions. Chem. Eur. J. 15 10948-10959. 

Singly charged ions encompass radical ions, protonated/deprotonated molecules, products of alkali ion additions, or complex ions with other charge carriers. In the case of singly charged radical ions, the molecular weight of an analyte molecule approximately equals to the m/z value of that ion (one electron affects the measurement by only 0.00055 u). In the case of protonated or deprotonated molecules, the m/z values are expressed as m -I- 1 or m - 1, respectively. Alkali metal adducts are also commonly observed in MS for example, m -l- 23 (sodium adducts) or m -i- 39 (potassium adducts). The alkali ions are mostly contaminants, which are very difficult to remove from sample vials, solvents, or sample plates. However, some analytes such as carbohydrates can only be ionized by association with alkali ions [5,6]. 

Alkali metal ion addncts reported in Dl and ESI mass spectra, however, were considered nuisances in some cases [45 7]. Cationization takes place rrrrfavorably, especially in matrix-assisted laser desorption/ionisation (M/VLDI) and ESI. The rm-avoidable presence of sodinm and potassiirm salts as imprrrities produces cation addncts and often comes up with a considerable decrease in sensitivity in Dl methods. Fmthermore, the presence of protonated species together with alkaU ion adducts can appreciably compUcate the identification of the components of a complex mixtnre. 

Owing to its compatibility with solution samples, ESI is preferred over other ionization methods in many MS fields. Applications of metal ion adducts have been reported for ESI [55-57]. For example, ESI can be used to produce alkali-metal adducts of antibiotics that do not form abundant [M+H]+ ions. Informative adducts between alkali-metal ions and peptides have been observed under a variety of conditions of electrospray ionization mass spectrometry (ESI-MS). It should be noted, however, that the presence of salt ion adducts cause the signal suppression and interference with the interpretation of the mass spectra, particularly in analytical MS of proteins and other biological molecules. 

Observations of alkali-metal ion adducts of the type [M+Li]+ [M+Na]+ etc. are common in the desorption ionization (DI) mass spectra of a variety of polar molecules. In fact, alkali-metal ion association reactions are observed with FD ionization, FAB ionization, Cf plasma desorption (PD), secondary ion mass spectrometry (SIMS), MALDI, and ESI. Ion yields can be greatly enhanced by addition of alkali-metal salts to the sample. Particularly for the MALDI analysis of synthetic polymers, metal cations are often intentionally added to enhance signals. A qualitative description of the current understanding of formation mechanism of alkali-metal ion complexes from the condensed phase was presented [75]. Knowledge of the ionization mechanisms is important and helpful from the perspective of increasing the analytical utility of the method. 

An example of this behavior is shown in Fig. 4.2, where the positive-ion and negative-ion ESI-MS spectra of a compoimd are shown with two catboxyhc acid functions, analyzed as di-potassium salt in a mobile phase containing ammonium acetate. In positive-ion mode, both the ammoniated molecule [M+NHJ+ and the potassiated molecule [M+K]" (or the H+/K -exchange product [(M-H+K)+H]+) are observed, whereas in negative-ion mode, the H+/K+-exchange product [(M-H+K)-H] is observed next to the deprotonated molecule [M-H] . Thus, apparent alkali metal ion adducts may be observed in negative-ion ESI-MS as well. 

Another development of methodology leading to cationization of organic molecules as a method of ionization is dne to the work of Roellgen and cowoikers [4], They developed a technique using a two-filament desiga In a specially constracted ion source, two filaments are in close proximity. The first filament is loaded with a mixture of silica gel and alkali salt (LiCI, Nal, and/or KI), which, when heated, emits alkali ions. Thermally labile compounds are present on the second filament These molecules thermally desorb and form adducts with the alkali ions from the first filament. This method produces simple mass spectra from which molecular weight information can be easily derived. 

Nozaki K, Tami A, Osaka I, Kawasaki H, Arakawa R. Elimination technique for alkali metal ion adducts from an electrospray ionization process using an on-line ion suppressor. Anal Sci. 2010 26 715-8. 

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