Modifying the Acidity of Charged Droplets

Mari Ojakivi, Jaanus Liigand and Anneli Kruve

Chemistry Select 2018, 3, 335 – 338

DOI: 10.1002/slct.201702269

The concept of acidity in confined spaces is up to date poorly understood; especially, in case of media violating electroneutrality. Here, we describe the acidity of charged droplets via their ability to protonate simple nitrogen bases and we propose ways to modify the protonation efficiency with the help of additives. We observed that the protonation of compounds in charged water droplets is independent of solution-phase acidity; instead, it can be adjusted with the help of additive type. On the other hand, the extent of protonation in charged methanol droplets can be adjusted with the
conventional approach of changing the pH.

pH Effects on Electrospray Ionization Efficiency

Jaanus Liigand, Asko Laaniste and Anneli Kruve

J. Am. Soc. Mass Spectrom. 2017,28( 3), 461 – 469

DOI: 10.1007/s13361-016-1563-1

Electrospray ionization efficiency is known to be affected by mobile phase composition. In this paper, a detailed study of analyte ionization efficiency dependence on mobile phase pH is presented. The pH effect was studied on 28 compounds with different chemical properties. Neither pKa nor solution phase ionization degree by itself was observed to be sufficient at describing how aqueous phase pH affects the ionization efficiency of the analyte. Therefore, the analyte behavior was related to various physicochemical properties via linear discriminant analyses. Distinction between pH-dependent and pH-independent compounds was achieved using two parameters: number of potential charge centers and hydrogen bonding acceptor capacity (in the case of 80% acetonitrile) or polarity of neutral form of analyte and pKa (in the case of 20% acetonitrile). It was also observed that decreasing pH may increase ionization efficiency of a compound by more than two orders of magnitude.

Think Negative: Finding the Best Electrospray Ionization/MS Mode for Your Analyte

Piia Liigand, Karl Kaupmees, Kristjan Haav, Jaanus Liigand,Ivo Leito, Marion Girod, Rodolphe Antoine and Anneli Kruve

Anal. Chem. 2017, 89( 11), 5665 – 5668

DOI: 10.1021/acs.analchem.7b00096

For the first time, the electrospray ionization efficiency (IE) scales in positive and negative mode are united into a single system enabling direct comparison of IE values across ionization modes. This is made possible by the use of a reference compound that ionizes to a similar extent in both positive and negative modes. Thus, choosing the optimal (i.e., most sensitive) ionization conditions for a given set of analytes is enabled. Ionization efficiencies of 33 compounds ionizing in both modes demonstrate that, contrary to general practice, negative mode allows better sensitivity for 46% of such compounds whereas the positive mode is preferred for only 18%, and for 36%, the results for both modes are comparable.

The Evolution of Electrospray Generated Droplets is Not Affected by Ionization Mode

Piia Liigand, Agnes Heering (Suu), Karl Kaupmees, Ivo Leito, Marion Girod, Rodolphe Antoine and Anneli Kruve

J. Am. Soc. Mass Spectrom. 2017, 28 (10), 2124-2131

DOI: 10.1007/s13361-017-1737-5

Ionization efficiency and mechanism in ESI is strongly affected by the properties of mobile phase. The use of mobile-phase properties to accurately describe droplets in ESI source is convenient but may be inadequate as the composition of the droplets is changing in the plume due to electrochemical reactions occurring in the needle tip as well as continuous drying and fission of droplets. Presently, there is paucity of research on the effect of the polarity of the ESI mode on mobile phase composition in the droplets. In this paper, the change in the organic solvent content, pH, and droplet size are studied in the ESI plume in both ESI+ and ESI– ionization mode. We introduce a rigorous way – the absolute pH (pHabsH2O) – to describe pH change in the plume that takes into account organic solvent content in the mobile phase. pHabsH2O enables comparing acidities of ESI droplets with different organic solvent contents. The results are surprisingly similar for both ionization modes, indicating that the dynamics of the change of mobile-phase properties is independent from the ESI mode used. This allows us to conclude that the evolution of ESI droplets first of all proceeds via the evaporation of the organic modifier and to a lesser extent via fission of smaller droplets from parent droplets. Secondly, our study shows that qualitative findings related to the ESI process obtained on the ESI+ mode can almost directly be applied also in the ESI– mode.

Ionization Efficiency of Doubly Charged Ions Formed from Polyprotic Acids in Electrospray Negative Mode

Piia Liigand, Karl Kaupmees and Anneli Kruve

J. Am. Soc. Mass Spectrom. 2016, 27 (7), 1211-1218

DOI: 10.1007/s13361-016-1384-2

The ability of polyprotic acids to give doubly charged ions in negative mode electrospray was studied and related to physicochemical properties of the acids via linear discriminant analysis (LDA). It was discovered that the compound has to be strongly acidic (low pKa1 and pKa2) and to have high hydrophobicity (logPow) to become multiply charged. Ability to give multiply charged ions in ESI/MS cannot be directly predicted from the solution phase acidities. Therefore, for the first time, a quantitative model to predict the charge state of the analyte in ESI/MS is proposed and validated for small anions. Also, a model to predict ionization efficiencies of these analytes was developed. Results indicate that acidity of the analyte, its octanol-water partition coefficient, and charge delocalization are important factors that influence ionization efficiencies as well as charge states of the analytes. The pH of the solvent was also found to be an important factor influencing the ionization efficiency of doubly charged ions.

Establishing Atmospheric Pressure Chemical Ionization Efficiency Scale

Riin Rebane, Anneli Kruve, Piia Liigand, Jaanus Liigand, Koit Herodes and Ivo Leito

Anal. Chem. 2016, 88( 7), 3435 – 3439

DOI: 10.1021/acs.analchem.5b04852

Recent evidence has shown that the atmospheric pressure chemical ionization (APCI) mechanism can be more complex than generally assumed. In order to better understand the processes in the APCI source, for the first time, an ionization efficiency scale for an APCI source has been created. The scale spans over 5 logIE (were IE is ionization efficiency) units and includes 40 compounds with a wide range of chemical and physical properties. The results of the experiments show that for most of the compounds the ionization efficiency order in the APCI source is surprisingly similar to that in the ESI source. Most of the compounds that are best ionized in the APCI source are not small volatile molecules. Large tetraalkylammonium cations are a prominent example. At the same time, low-polarity hydrocarbons pyrene and anthracene are ionized in the APCI source but not in the ESI source. These results strongly imply that in APCI several ionization mechanisms operate in parallel and a mechanism not relying on evaporation of neutral molecules from droplets has significantly higher influence than commonly assumed.

Transferability of the Electrospray Ionization Efficiency Scale between Different Instruments

Jaanus Liigand,Anneli Kruve, Piia Liigand, Asko Laaniste, Marion Girod, Rodolphe Antoine and Ivo Leito

J. Am. Soc. Mass Spectrom. 2015, 26( 11), 1923 – 1930

DOI: 10.1007/s13361-015-1219-6

For the first time, quantitative electrospray (ESI) ionization efficiencies (IE), expressed as logIE values, obtained on different mass-spectrometric setups (four mass analyzers and four ESI sources) are compared for 15 compounds of diverse properties. The general trends of change of IE with molecular structure are the same with all experimental setups. The obtained IE scales could be applied on different setups: there were no statistically significant changes in the order of ionization efficiency and the root mean of squared differences of the logIE values of compounds between the scales compiled on different instruments were found to be between 0.21 and 0.55 log units. The results show that orthogonal ESI source geometry gives better differentiating power and additional pneumatic assistance improves it even more. It is also shown that the ionization efficiency values are transferable between different mass-spectrometric setups by three anchoring points and a linear model. The root mean square error of logIE prediction ranged from 0.24 to 0.72 depending on the instrument. This work demonstrates for the first time the inter-instrument transferability of quantitative electrospray ionization efficiency data.

Unified pH Values of Liquid Chromatography Mobile Phases

Agnes Suu, Lauri Jalukse, Jaanus Liigand, Anneli Kruve, Daniel Himmel, Ingo Krossing, Marti Rosés and Ivo Leito

Anal. Chem. 2015, 87( 5), 2623–2630

DOI: 10.1021/ac504692m

This work introduces a conceptually new approach of measuring pH of mixed-solvent liquid chromatography (LC) mobile phases. Mobile phase pH is very important in LC, but its correct measurement is not straightforward, and all commonly used approaches have deficiencies. The new approach is based on the recently introduced unified pH (pHabs) scale, which enables direct comparison of acidities of solutions made in different solvents based on chemical potential of the proton in the solutions. This work represents the first experimental realization of the pHabs concept using differential potentiometric measurement for comparison of the chemical potentials of the proton in different solutions (connected by a salt bridge), together with earlier published reference points for obtaining the pHabs values (referenced to the gas phase) or pHabsH2O values (referenced to the aqueous solution). The liquid junction potentials were estimated in the framework of Izutsu’s three-component method. pHabs values for a number of common LC and LC–MS mobile phases have been determined. The pHabs scale enables for the first time direct comparison of acidities of any LC mobile phases, with different organic additives, different buffer components, etc. A possible experimental protocol of putting this new approach into chromatographic practice has been envisaged and its applicability tested. It has been demonstrated that the ionization behavior of bases (cationic acids) in the mobile phases can be better predicted by using the pHabsH2O values and aqueous pKa values than by using the alternative means of expressing mobile phase acidity. Description of the ionization behavior of acids on the basis of pHabsH2O values is possible if the change of their pKa values with solvent composition change is taken into account.

Negative Electrospray Ionization via Deprotonation: Predicting the Ionization Efficiency

Anneli Kruve, Karl Kaupmees, Jaanus Liigand and Ivo Leito

Anal. Chem. 2014, 86( 10), 4822–4830

DOI: 10.1021/ac404066v

Electrospray ionization (ESI) in the negative ion mode has received less attention in fundamental studies than the positive ion electrospray ionization. In this paper, we study the efficiency of negative ion formation in the ESI source via deprotonation of substituted phenols and benzoic acids and explore correlations of the obtained ionization efficiency values (logIE) with different molecular properties. It is observed that stronger acids (i.e., fully deprotonated in the droplets) yielding anions with highly delocalized charge [quantified by the weighted average positive sigma (WAPS) parameter rooted in the COSMO theory] have higher ionization efficiency and give higher signals in the negative-ion ESI/MS. A linear model was obtained, which equally well describes the logIE of both phenols and benzoic acids (R2 = 0.83, S = 0.40 log units) and contains only an ionization degree in solution (α) and WAPS as molecular parameters. Both parameters can easily be calculated with the COSMO-RS method. The model was successfully validated using a test set of acids belonging neither to phenols nor to benzoic acids, thereby demonstrating its broad applicability and the universality of the above-described relationships between IE and molecular properties.

Effect of Mobile Phase on Electrospray Ionization Efficiency

Jaanus Liigand,Anneli Kruve,Ivo Leito, Marion Girod and Rodolphe Antoine

J. Am. Soc. Mass Spectrom. 2014, 25( 11), 1853 – 1861

DOI: 10.1007/s13361-014-0969-x

Electrospray (ESI) ionization efficiencies (IE) of a set of 10 compounds differing by chemical nature, extent of ionization in solution (basicity), and by hydrophobicity (tetrapropylammonium and tetraethylammonium ion, triethylamine, 1-naphthylamine, N,N-dimethylaniline, diphenylphthalate, dimethylphtahalate, piperidine, pyrrolidine, pyridine) have been measured in seven mobile phases (three acetonitrile percentages 20%, 50%, and 80%, and three different pH-adjusting additives, 0.1% formic acid, 1 mM ammonia, pH 5.0 buffer combination) using the relative measurement method. MS parameters were optimized separately for each ion. The resulting relative IE data were converted into comparable logIE values by anchoring them to the logIE of tetrapropylammonium ion taking into account the differences of ionization in different solvents and thereby making the logIE values of the compounds comparable across solvents. The following conclusions were made from analysis of the data. The compounds with pKa values in the range of the solution pH values displayed higher IE at lower pH. The sensitivity of IE towards pH depends on hydrophobicity being very strong with pyridine, weaker with N,N-dimethylaniline, and weakest with 1-naphthylamine. IEs of tetraalkylammonium ions and triethylamine were expectedly insensitive towards solution pH. Surprisingly high IEs of phthalate esters were observed. The differences in solutions with different acetonitrile content and similar pH were smaller compared with the pH effects. These results highlight the importance of hydrophobicity in electrospray and demonstrate that high hydrophobicity can sometimes successfully compensate for low basicity.

Sodium adduct formation efficiency in ESI source

Anneli Kruve, Karl Kaupmees, Jaanus Liigand, Merit Oss and Ivo Leito

J. Mass Spectrom. 2013, 48( 6), 695-702

DOI: 10.1002/jms.3218

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.