Our publications

Electrospray ionization studies

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, Anneli Kruve
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.

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
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.


Predicting ESI/MS Signal Change for Anions in Different Solvents
Anneli Kruve, Karl Kaupmees
Doi: 10.1021/acs.analchem.7b00595

LC/ESI/MS is a technique widely used for qualitative and quantitative analysis in various fields. However, quantification is currently possible only for compounds for which the standard substances are available, as the ionization efficiency of different compounds in ESI source differs by orders of magnitude. In this paper we present an approach for quantitative LC/ESI/MS analysis without standard substances. This approach relies on accurately predicting the ionization efficiencies in ESI source based on a model, which uses physicochemical parameters of analytes. Furthermore, the model has been made transferable between different mobile phases and instrument setups by using a suitable set of calibration compounds. This approach has been validated both in flow injection and chromatographic mode with gradient elution.

Adduct Formation in ESI/MS by Mobile Phase Additives
Anneli Kruve, Karl Kaupmees
Doi:10.1007/s13361-017-1626-y

Adduct formation is a common ionization method in electrospray ionization mass spectrometry (ESI/MS). However, this process is poorly understood and complicated to control. We demonstrate possibilities to control adduct formation via mobile phase additives in ESI positive mode for 17 oxygen and nitrogen bases. Mobile phase additives were found to be a very effective measure for manipulating the formation efficiencies of adducts. An appropriate choice of additive may increase sensitivity by up to three orders of magnitude. In general, sodium adduct [M + Na]+ and protonated molecule [M + H]+ formation efficiencies were found to be in good correlation; however, the former were significantly more influenced by mobile phase properties. Although the highest formation efficiencies for both species were observed in water/acetonitrile mixtures not containing additives, the repeatability of the formation efficiencies was found to be improved by additives. It is concluded that mobile phase additives are powerful, yet not limiting factors, for altering adduct formation.

pH Effects on Electrospray Ionization Efficiency
Jaanus Liigand, Asko Laaniste, Anneli Kruve
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.

Influence of mobile phase, source parametersand source type on electrospray ionization efficiency in negative ion mode

Anneli Kruve

Doi: 10.1002/jms.3790

The effect of organic solvent content on ionization efficiency (sensitivity) is compared for different ESI sources in negative mode. It was observed that ionization efficiency in ESI with thermal focusing (such as Jet Stream source) is little affected by the organic solvent content while in conventional ESI ionization efficiency can be significantly (by an order of magnitude) increased with increasing organic solvent content (both acetonitrile and methanol). However pure acetonitrile is not recommended for such measurements as it yields poor repeatability. Additionally though organic solvent content increase results in higher ionization efficiency it unfortunately also increases ionization suppression.

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

Piia Liigand, Karl Kaupmees, Anneli Kruve

Doi:10.1007/s13361-016-1384-2

The factors influencing observation of doubly charged ions in mass spectra was studied on the example of small acidic (carboxylic acids, phenols, sulphonic acids) compounds with at least two ionisable sites in ESI negative mode. It was observed that being only a strong acid (meaning that the compound is present in solution as two valent anion) is not sufficient for observing doubly charged ion in mass spectra. Additionally two compound needs to be sufficiently hydrophobic. Also, if sufficiently hydrophobic compounds which are not expected to be present as divalent anions in solution, may give doubly charged ions in mass spectra.

Establishing Atmospheric Pressure ChemicalIonization Efficiency Scale

Riin Rebane, Anneli Kruve, Piia Liigand, Jaanus Liigand, Koit Herodes, Ivo Lieto

Doi: 10.1021/acs.analchem.5b04852

Transferability of the Electrospray IonizationEfficiency Scale between Different Instruments

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

Doi: 10.1007/s13361-015-1219-6

Influence of source and mass analyser type on ionization efficiency scales was studied on the example of ESI positive mode. It is demonstrated that ionization efficiency scales can be successfully transferred between different instruments.

Effect of Mobile Phase on ElectrosprayIonization Efficiency

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

Doi:10.1007/s13361-014-0969-x

The mobile phase pH and organic solvent content effect on ionization efficiency (sensitivity) of basic or neutral compounds in ESI positive mode was studied. It was observed that ionization efficiency changes with pH for compounds which have pK_a in the range of varied pH. Compounds that are permanently charged in solution or are not charged at all in solution will not change their ionization efficiency in ESI. Also organic solvent content influences ionization efficiency.

Negative Electrospray Ionization viaDeprotonation: Predicting the Ionization Efficiency

Anneli Kruve, Karl Kaupmees, Jaanus Liigand, Ivo Leito

Doi: 10.1021/ac404066v

Introducing an ionization efficiency scale for electrospray negative ionization. Altogether ionization efficiencies (related to sensitivity) of 64 acidic compounds (carboxylic acids, phenols, imides, sulphonic acids, sulphonamides, etc) was measured and modelled. It was observed that ionization efficiency can be explained by the ionization degree in solution and charge delocalization of the formed anion. The missprediction of the proposed model was 3 times while the measured ionization efficiencies varied over six orders of magnitude.

Sodium adduct formationefficiency in ESI source

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

Doi: 10.1002/jms.3218

The sodium adduct formation efficiency (SAFE) scale was introduced and showed that the order compounds is unaffected by the sodium content in the mobile phase. Also it was observed that SAFE can not be explained by the solvent/vacuum distribution coefficient of formed species nor with strength of the formed adduct species (described by partial charged on the heteroatom associated with sodium and bond length between the hetero atom and sodium).

Electrospray IonizationEfficiency Scale of Organic Compounds

Merit Oss, Anneli Kruve, Koit Herodes, Ivo Leito

Doi: 10.1021/ac902856t

Towards the electrosprayionization mass spectrometry ionization efficiency scale of organic compounds

Ivo Leito, Koit Herodes, Merito Huopolainen, Kristina Viroo, Allan Künnapas, Anneli Kruve, Risto Tanner

Doi: 10.1002/rcm.3371