Ionization efficiency (IE) of different compounds in electrospray ionization (ESI) source differs widely, leading to widely differing sensitivities of ESI-MS to different analytes. An approach for quantifying ESI efficiencies (as logIE values) and setting up a self-consistent quantitative experimental ESI efficiency scale of organic compounds under predefined ionization conditions (ionization by monoprotonation) has been developed recently. Using this approach a logIE scale containing 62 compounds of different chemical nature and ranging for 6 orders of magnitude has been established. The scale is based on over 400 relative IE (DeltalogIE) measurements between more than 250 different pairs of compounds. To evaluate which molecular parameters contribute the most to the IE of a compound linear regression analysis logIE values and different molecular parameters were carried out. The two most influential parameters in predicting the IE in ESI source are the pK(a) and the molecular volume of the compound. This scale and the whole approach can be a tool for practicing liquid chromatographists and mass spectrometrists. It can be used in any mass-spectrometry laboratory and we encourage practitioners to characterize their analytes with the logIE values so that a broad knowledge base on electrospray ionization efficiencies of compounds would eventually develop.

Formation of sodium adducts in electrospray (ESI) has been known for long time, but has not been used extensively in practice, and several important aspects of Na(+) adduct formation in ESI source have been almost unexplored: the ionization efficiency of different molecules via Na(+) adduct formation, its dependence on molecular structure and Na(+) ion concentration in solution, fragmentation behaviour of the adducts as well as the ruggedness (a prerequisite for wider practical use) of ionization via Na(+) adduct formation. In this work, we have developed a parameter describing sodium adducts formation efficiency (SAFE) of neutral molecules and have built a SAFE scale that ranges for over four orders of magnitude and contains 19 compounds. In general, oxygen bases have higher efficiency of Na(+) adducts formation than nitrogen bases because of the higher partial negative charge on oxygen atoms and competition from protonation in the case of nitrogen bases. Chelating ability strongly increases the Na(+) adduct formation efficiency. We show that not only protonation but also Na(+) adduct formation is a quantitative and reproducible process if relative measurements are performed.

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Rationale The first comprehensive quantitative scale of the efficiency of electrospray ionization (ESI) in the positive mode by monoprotonation, containing 62 compounds, was published in 2010. Several trends were found between the compound structure and ionization efficiency (IE) but, possibly because of the limited diversity of the compounds, some questions remained. This work undertakes to align the new data with the originally published IE scale and carry out statistical analysis of the resulting more extensive and diverse data set to derive more grounded relationships and offer a possibility of predicting logIE values. Methods Recently, several new IE studies with numerous compounds have been conducted. In several of them, more detailed investigations of the influence of compound structure, solvent properties, or instrument settings have been conducted. IE data from these studies and results from this work were combined, and the multilinear regression method was applied to relate IE to various compound parameters. Results The most comprehensive IE scale available, containing 334 compounds of highly diverse chemical nature and spanning 6 orders of magnitude of IE, has been compiled. Several useful trends were revealed. Conclusions The ESI ionization efficiency of a compound by protonation is mainly affected by three factors: basicity (expressed by pKaH in water), molecular size (expressed by molar volume or surface area), and hydrophobicity of the ion (expressed by charge delocalization in the ion or its partition coefficient between a water–acetonitrile mixture and hexane). The presented models can be used for tentative prediction of logIE of new compounds (under the used conditions) from parameters that can be computed using commercially available software. The root mean square error of prediction is in the range of 0.7–0.8 log units.

The ability of the Conductor-like Screening Model for Realistic Solvation (COSMO-RS) computational method to model hydrogen bond (HB) formation in solution is examined by comparing computational data with experimental data from literature. This is the first study of this kind where mixed solvents are also involved. Hydrogen bond formation is examined between neutral molecules, between acids and their anions, and between various anion receptor molecules and different anions in a number of aprotic solvents. HB formation equilibrium constants, the corresponding Gibbs' free energies and, when available from the literature, enthalpies were calculated. The supermolecule (SM) approach and the contact probability (CP) approach were used. Both in the case of the SM and CP approach, good to very good correlations between the experiment and computations are found for complexes formed from neutral species, enabling quantitative predictions. When the HB acceptor is an anion, the correlations are poor and in some cases even qualitative predictions fail.