The log P ow value of bathocuproinedisulfonic acid is very similar to substances that did not give doubly charged species and probably this is the main reason that the prediction is false negative. In calculation of physicochemical properties of bathocuproinedisulfonic acid, zwitterionic conformers were included and observed to be the most favored conformers in the liquid phase.
It has been shown by Teesch et al. Consequently, it can be presumed that log P ow is not the best parameter to describe distribution of zwitterionic compounds such as bathocuproinedisulfonic acid on the surface of the droplet. Span of the log IE scale and order of the substances in the scale was found to depend on solvent pH.
In acidic solvent, the range of the log IE scale for doubly charged species is one logarithmic unit wider than in basic solvent. Therefore, the pH is a suitable tool to increase or decrease MS sensitivity for doubly charged species.
The order of the substances is also dependent on solvent pH. Generally the ionization efficiencies for doubly charged species are about one logarithmic unit lower in basic solvent than for acidic solvent. The ionization efficiency values for singly charged species are less influenced by solvent pH see Figure 1.
Ionization efficiencies of analytes in acidic and basic solvents, where log IE 1 is the log IE of the molecule via formation of singly charged ion; log IE 2 of the molecule via formation of doubly charged ion.
In basic solvent, more doubly charged species are formed in the solution phase p K a2 is substantially lower than solvent pH allowing the second protonation step and, therefore, the signal corresponding to doubly charged species and corresponding log IE values are significantly higher. In acidic solvent, the formation of doubly charged ions is not favored and, therefore, more singly charged ions are formed.
As a result, the IEs of doubly charged ions are lower than in basic solvent. In basic solvent only bromothymol blue and eosin Y give higher IE for singly charged species than for doubly charges species. As mentioned before, the changes between acidic and basic solvent for singly charged species are, according to t -test, statistically insignificant. In both solvents, bromothymol blue has the highest log IE value for singly charged species. This is probably because it has the highest hydrophobic character and, therefore, the singly charged species will move to the surface of the droplet easily, even if not a lot of them are formed in solution.
The lowest log IE value belongs to tiron, which is a relatively small molecule with high hydrophilic character compared with other studied compounds. Also, another similar small and hydrophilic molecule, sulfosalicylic acid, has a low log IE value.
In acidic solvent, higher log IE 1 for singly charged species were observed for analytes that 1 have higher molecular volume, and 2 give doubly charged species with lower log IE 2 bromothymol blue, bromocresol green, and bromophenol blue. Among the previously mentioned compounds, the highest log IE 1 value belongs to bromothymol blue, which also has the highest hydrophobicity. Substances with lower log IE values either give doubly charged ions with high log IE 2 value or are relatively hydrophilic substances, such as sulfosalicylic acid and tiron.
This phenomenon can be explained that if the compound forms extensively doubly charged species, there is only a limited amount of compound left for forming singly charged species. Relatively large eosin Y has medium ionization efficiency and is different from others in hat it does not contain sulfo groups.
To formulate a model for predicting log IE 1 via formation of singly charged ions, different parameters were tested that had the best correlation with log IE 1 p K a1 , WAPS , and hydrogen bonding capacity for neutral substance and also their combinations. It seems that charge delocalization is most important.
All in all, it can be seen that log IE 2 values in basic and acidic solvents differ for doubly charged species, which shows that log IE values depend on pH values. Solvent pH is therefore an important parameter to take into account to form a universal log IE prediction model and also to reach higher selectivity in analysis. Also, it can be seen from previous the discussion that formation of singly and doubly charged species are strongly related processes. Also, a model was developed for predicting ionization efficiencies log IE 1 corresponding to singly charged species of multiply chargeable substances based on charge delocalization WAPS parameter.
It was also shown that double charging is most influenced by acidity of a substance, its hydrophobicity octanol-water partition coefficient , and charge delocalization. It was observed that solvent pH affects the ionization efficiencies of analytes and its optimal choice is a good tool to enhance selectivity and sensitivity.
Therefore, the factors influencing the ionization efficiency of doubly charged ions are similar to the ones that have been reported to influence ionization efficiencies of singly charged analytes. Cech, N. Mass Spectrom. Douglass, K. Huffman, B. Wong, S. Schnier, P. Smith, R. Felitsyn, N. Loo, J. Cole, R. Wiley, Hoboken Book Google Scholar. Iavarone, A. Ganisl, B. Efficient manipulation of negative ion net charge in electrospray ionization of proteins and nucleic acids.
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