Some tenets pertaining to electrospray ionization mass spectrometry

The field of electrospray ionization mass spectrometry is reviewed with emphasis placed upon advances in the elucidation of fundamental mechanistic aspects of the ionization process that have been reported over the past 10 years. The analytical consequences of these findings are also examined. Eight central conclusions or 'tenets' are presented, as deduced from the body of work contained in 80 references.     

Pneumatically assisted desorption/ionization: 1. Some thoughts on the possible ionization mechanism(s)

We have observed that spraying solvent droplets on a zopiclone tablet produced MH⁺ ions also in the absence of any electrical field and without the addition of organic acids to the sprayed solvent. The choice of a drug tablet as test bench has been done for the signal stability, higher than that observed when the drug is directly placed on a stainless steel surface. This behavior indicates that the formation of MH⁺ ions is mainly due to pneumatical effects and the results are discussed with respect to those obtained by other research groups. Different mechanisms contributing to MH⁺ production under these conditions are proposed and discussed. The local heating of the solvent thin layer present on the surface has been calculated and the small temperature increase (and the consequent small decrease of pK_a value) suggests that this effect can play only a minor role. However, different solvents have been employed for studying this aspect and, quite surprisingly, the best results have been obtained with acetonitrile (ACN). Experiments performed by spraying CD₃CN showed again the formation of MH⁺ and not MD⁺, and this excludes the role of ACN as protonating medium. A further thought was stimulated by the behavior observed by varying the sheath gas (N₂) flow, showing that the MH⁺ ion intensity increases by increasing the flow. Side effects related to the highest kinetic energy of the spraying droplets can be considered, but an active role of N₂ in the MH⁺ formation could be taken into account, by considering the possible ionization of N₂ by collisional phenomena. The N₂^+? ions could undergo a charge–exchange reaction with analyte molecules leading to a short‐lived odd electron ion which behaves as protonating media for neutral molecules. The above‐described mechanism does not require either the presence of electrical fields nor the addition of organic acid to the sprayer solvent and can give a rationale for what was observed when only pneumatical conditions are employed. Copyright © 2010 John Wiley & Sons, Ltd.     

Mechanistic investigation of ionization suppression in electrospray ionization

We show results from experiments designed to determine the relative importance of gas phase processes and solution phase processes into ionization suppression observed in biological sample extracts. The data indicate that gas phase reactions leading to the loss of net charge on the analyte is not likely to be the most important process involved in ionization suppression. The results point to changes in the droplet solution properties caused by the presence of nonvolatile solutes as the main cause of ionization suppression in electrospray ionization of biological extracts.     

Transmission mode desorption electrospray ionization

A new mode of operation for desorption electrospray ionization (DESI) analysis of liquids or solid residues from evaporated solvents is presented. Unlike traditional DESI, the electrospray is not deflected off of a surface but instead is transmitted through a sampling mesh at a 0° angle between the electrospray tip, sample mesh, and capillary inlet of a mass spectrometer. In this configuration, deposited samples can be analyzed rapidly without rigorous optimization of spray distances or angles and without the preparation time associated with solvent evaporation. The new transmission mode desorption electrospray ionization (TM-DESI) technique is not applicable to bulk materials, but instead is a method designed to simplify the sample preparation process for liquid samples and sample extracts. The technique can reduce analysis time to seconds while consuming only microliters of sample. The results presented summarize the optimization of the technique, highlight key figures of merit for several model compounds, and illustrate potential applications to high throughput screening of liquid mixtures in both extraction solvents and biological matrices.     

Cationization versus surface activity--the effect on electrospray ionization

Ethoxylated fatty alcohols, C(12)E(1), C(12)E(2), C(18)E(1) and C(18)E(2), were studied by electrospray ionization mass spectrometry (E is the ethoxylene unit OCH(2)CH(2)). For compounds containing two ethoxylene units, which form quite stable adducts with sodium cation, the abundances of [M + Na](+) ions were not affected by alkyl chain, so the hydrophobic effect was not observed. For the compounds containing one ethoxylene unit, forming rather unstable adducts with sodium, the hydrophobic effect was clearly seen since the [C(18)E(1) + Na](+) ion was more abundant than the [C(12)E(1) + Na](+) ion. Two ethoxylene units are not able to form stable adducts with potassium cations, therefore the hydrophobic effect was observed for the [C(12)E(2) + K](+) and [C(18)E(2) + Na](+) ions, the latter being more abundant than the former. For lithium cation adducts with C(12)E(1) and C(18)E(1), the hydrophobic effect was observed, but was less manifested than for sodium cations since lithium adducts are more stable than sodium ones. C(18)E(1) and C(18)E(2) gave more intense signals at higher cone voltage values than C(12)E(1) and C(12)E(2), respectively. However, this is not related to the hydrophobic effect but to the collisions being less effective for the former.     

Investigation on the role of pneumatic aspects in electrospray, desorption electrospray surface ionization and surface activated chemical ionization

This review reports the results of some studies carried out by us on the role of pneumatic aspects in electrospray and desorption electrospray surface ionization, with the aim to propose some relevant aspects of the mechanisms involved in these ionization methods. Electrospray ion sources, with the exception of the nano- electrospray source, operate with the concurrent action of a strong electrical field and a supplementary coaxial gas flow. The electrical field is responsible for electrospraying of the analyte solution but the use of a coaxial gas flow leads to a significant increase of the analyte signal and allows the use of higher solution flows. However, by employing capillary voltages much lower than those necessary to activate the electrospray phenomenon, analyte ions are still observed and this indicates that different mechanisms must be operative for ion production. Under these conditions, ion generation could take place from the neutral pneumatically sprayed droplet by field-induced droplet ionization. Also in the case of desorption electrospray ionization (DESI), and without any voltage on the spraying capillary as well as on the surface of interest, ions of analytes present on the surface become detectable and this shows that desorption/ionization of analytes occurs by neutral droplets impinging the surface. Consequently, the pneumatic effect of the impinging droplets plays a relevant role, and for these reasons the method has been called pneumatic assisted desorption (PAD). Some analogies existing between PAD and surface activated chemical ionization (SACI), based on the insertion of a metallic surface inside an atmospheric pressure chemical ionization source operating without corona discharge, are discussed.     

Influence of boric acid on electrospray ionization efficiency

Liquid chromatography electrospray ionization mass-spectrometry (LC-ESI-MS) was used to analyze 9-fluorenylmethylmethoxycarbonyl chloride (Fmoc-Cl) and diethyl ethoxymethylenemalonate (Deemm) derivatives of three amino acids and five other compounds. Influence of boric acid on their ionization was investigated and dramatic impact on the signal was observed. The strongest signal suppression (6% of signal remains) was observed for the Deemm derivative of beta-Alanine (with ammonium acetate in eluent). With only formic acid as the eluent pH modifier, signal enhancement was observed, being largest for Fmoc-Cl derivative of Phenylalanine, 267%. Investigation of the influence of boric acid shows that it is a possible signal enhancer for LC-ESI-MS analysis.     

Studies on electrochemical oxidation mechanisms of polyamides containing N-methylpyrrole and N-methylimidazole by electrospray ionization tandem mass spectrometry

Electrochemical oxidization mechanism of polyamides containing N-methylpyrrole and N-methylimidazole was investigated by electrospray ionization tandem mass spectrometry (ESI-MS(n)). The spectra of the oxidization products unambiguously revealed that one or two oxygen atoms are added to the polyamide after the bulk electrolysis. It was found that electrochemical oxidation preferably takes place on the imidazole ring of the polyamide with both imidazole and pyrrole to produce a carbonyl group on the rings. ESI tandem mass spectrometry has proven to be an excellent method for the structural identification of electrochemical oxidation products of DNA-recognizing polyamides.     

Microfabrication of polydimethylsiloxane electrospray ionization emitters

Microfabricated polydimethylsiloxane (PDMS) emitters for electrospray ionization mass spectrometry (ESI-MS) were implemented as tips along the edge of the PDMS device by three methods which utilize soft lithography processes. These microfabrication methods for producing PDMS emitters as an integral part of a microfluidic device will facilitate development of more complex microfluidic analysis systems using ESI-MS.     

Redox transformations in desorption electrospray ionization

Redox changes occur in some circumstances when organic compounds are analyzed by desorption electrospray ionization mass spectrometry (DESI-MS). However, these processes are limited in scope and the data presented here suggest that there are only limited analogies between the redox behavior in DESI and the well-known solution-phase electrochemical processes in standard electrospray ionization (ESI). Positive and negative ion modes were both investigated and there is a striking asymmetry between the incidence of oxidation and of reduction. Although in negative ion mode DESI experiments, some aromatic compounds were ionized as odd-electron anion radicals, examples of full reduction were not found. By contrast, oxidation in the form of oxygen atom addition (or multiple oxygen atom additions) was observed for several different analytes. These oxidation reactions point to chemically rather than electrochemically controlled processes. Data is presented which suggests that oxidation is predominantly caused by reaction with discharge-created gas-phase radicals. The fact that common reducing agents and known antioxidants such as ascorbic acid are not modified, while a saturated organic acid like stearic acid is oxidized in DESI, indicates that the usual electrochemical redox reactions are not significant but that redox chemistry can be induced under special experimental conditions.     

Ionic liquid-assisted electrospray ionization of polysaccharides

In this work, we give the report of significant detection sensitivity improvement of electrospray ionization (ESI) mass spectra of polysaccharides by adding various ionic liquid compounds into samples. Mass spectra obtained were greatly simplified and appeared to be similar to spectra from matrix-assisted laser desorption/ionization due to the narrow charge number distribution. Mass spectra of polysaccharides with the attachment of either anion or cation of ionic liquid compounds were observed. No protonated or deprotonated polysaccharide ions were detected when ionic liquid compounds were added into samples. Little alkali-attached polysaccharide ions were observed. Ionic liquid-assisted ESI (ILA-ESI) mass spectrometry has significantly improved the detection sensitivity of large neutral polysaccharide compounds.     

Ferrocenylalkylation processes under electrospray ionization conditions

The electrospray ionization behavior of some ferrocenylalkylazoles CpFeC₅H₄CH(R)Az (AzH are derivatives of imidazole, pyrazole, triazole and their benzo analogs; R = H, Me, Et, Ph), ferrocenylalkanols CpFeC₅H₄CH(R)OH (R = H, Me), and mixtures of the latter with azoles was studied. The electrospray ionization mass spectra of these compounds, in addition to the molecular ion [M]^+·, the protonated molecule [M + H]⁺, and ferrocenylalkyl cation [FcCHR]⁺ peaks, exhibit also intensive peaks for the binuclear ions [(FcCHR)₂X]⁺ (X = Az or O), resulting from ferrocenylalkylation of the initial compounds with the ferrocenylalkyl cations. Electrospray ionization of an equimolar mixture of ferrocenylmethanol FcCH₂OH and imidazole gives the protonated ferrocenylmethylimidazole molecule [FcCH₂Im + H]⁺ and the [FcCH₂(Im)₂ + H]⁺ dimer, apart from the ions typical of each component, i.e., ferrocenylalkylation of azoles with the ferrocenylalkylcarbinols, known in the chemistry of solutions, takes place under electrospray conditions. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1318–1321, August, 2006.     

Desorption electrospray ionization of aerosol particles

We have applied desorption electrospray ionization to aerosol particles. Ions were formed from aerosols by merging suspended dry particles with an electrospray of solvent in a modified ion trap mass spectrometer. Dry aerosol particles were generated using a fluidized bed powder disperser and directed toward the inlet of the mass spectrometer. A nanospray source was used to create a spray of solvent droplets directed at the inlet and at a right angle with respect to the aerosol. Ions generated by the interaction of the particles and electrospray were transferred into the ion trap mass spectrometer. Using this method, pure samples of caffeine and erythromycin A were analyzed. In addition, commonly available food and drug powders including instant cocoa powder, artificial sweetener and ibuprofen were analyzed.     

Synchronized dual-polarity electrospray ionization mass spectrometry

This work describes the synchronized dual-polarity (DP) electrospray ionization (ESI) method and demonstrates the first DP ESI mass spectra obtained using two mass spectrometers. Stable double Taylor cones were produced by applying two counter electric voltages with opposite polarities to one electrosprayer. The development of double Taylor cones required higher extraction voltages than conventional ESI, but DP ESI worked effectively at liquid flow rate range three times wider than conventional ESI. Using pure methanol, the emission currents of the two cones were neutralized and no current was drawn from the sprayer. Synchronized DP mass spectra were obtained using electrospray calibrants dissolved in methanol solution of low water content. For bovine insulin with conventional electrospray solution, the gas-assisted electrospray delivered satisfactory sensitivity and stability for routine mass analyses.     

Internal energy distribution in electrospray ionization

Internal energies and energy distributions were studied using the 'survival yield' method developed previously. In addition to conventional benzylpyridinium salts, protonated esters (fragmenting by rearrangement) and protonated leucine enkephalin were also used, extending the validity of the technique. Fragmentation processes were studied in the cone voltage region and modeled by the RRKM-based MassKinetics program. The results show that the shapes of the energy distributions are similar to thermal distributions. The mean internal energies are very similar for all compound classes studied, and show a linear increase with collision energy in the 10-50 eV region.