From ions in solution to ions in the gas phase - the mechanism of electrospray mass spectrometry

M(NO 3 ) xϪ ions are generated by electrospray ionization (ESI) of metal solutions in nitric acid in negative ion mode. Collision-induced reactions of these ions are monitored in a tandem mass spectrometer (MS) of quadrupole-octopole-quadrupole (QoQ) geometry. For Group 1and 2 elements, the M(NO 3 ) x Ϫ ions dissociate into NO 3 Ϫ and neutral metal nitrate molecules. These elements also form some M x (NO 3 ) xϩ1 Ϫ clusters, especially Li ϩ . Metal nitrate ions from transition elements and Group 13 elements fragment into oxo products and also undergo internal electron transfer to leave the M atom in a lower oxidation state. To calibrate the collision energy, the dissociation energy of O-NO 2 Ϫ is found to be 5.55 eV, about 0.76 eV lower than a value derived from thermochemistry. Most studies of small inorganic ions have been done in positive ion mode [14 -18]. Many metal-solvent clusters, e.g., M xϩ (H 2 O) n , n ϭ 1 to 6, are typically formed under "soft" ion extraction conditions [7,17,18]. Highly charged metal ions tend to react with solvent molecules to yield hydroxy or methoxy ions when energetic collision conditions are used to remove solvent during ion extraction. Preserving the original form of the metal ion in positive mode generally requires element-specific optimization of ion extraction conditions and leads to a variety of ion-solvent clusters.Previous work from our group [19] showed that an excess of nitric acid can be used to produce M(NO 3 ) x Ϫ ions in negative ion mode. A single set of ion extraction conditions yields these ions for many elements. This early work was done with only one stage of MS. The present work reports collision-induced dissociation (CID) reactions of these M(NO 3 ) x Ϫ ions in a triple "quadrupole" MS, actually a QoQ instrument. Experimental Samples and Sample PreparationConcentrated nitric acid was purchased from J. T. Baker (Phillipsburg, NJ) and used without further purification. Aliquots of the concentrated acid were mixed with HPLC grade methanol and deionized water (18 M⍀, Barnstead Nanopure, Newton, MA) to form a solvent of 0.05% nitric acid in 25% water:75% methanol. Aliquots of aqueous, acidic metal stock solutions (1000 ppm, Spex Certiprep, Metuchen, NJ ) were diluted with solvent to the desired concentrations.

Section: Ms and Cid Of Nitrate Complexes Of Group 1 And 2 Metalsmentioning

confidence: 99%

“…A fter the pioneering studies of Yamashita and Fenn [1], ESI-MS has revolutionized the analysis of biological molecules [2][3][4][5]. This method also provides valuable information about inorganic and organometallic compounds [6 -8] and the interactions and binding of metal ions to biological molecules [9 -11].…”

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confidence: 99%

Tandem mass spectrometry of metal nitrate negative ions produced by electrospray ionization

Byers

Houk

2003

“…Field desorption (FD) mass spectrometry has proved useful for polysiloxane analysis [9]. Recent development of soft ionization techniques such as matrix-assisted laser desorption/ionization (MALDI) [10 -12] and electrospray ionization (ESI) [13,14] make mass spectrometry an even more valuable analytical tool enabling better fundamental understanding of organosilicon chemistry [15][16][17][18][19][20].…”

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confidence: 99%

Endgroup-assisted siloxane bond cleavage in the gas phase

Chen

2003

ϩ . The formation of SiC 5 H 16 NO ϩ involves dissociation of a Si™O bond and formation of an O™H bond through rearrangement. The fragmentation mechanism was investigated utilizing ab initio calculations and Fourier transform ion cyclotron resonance (FTICR) mass spectrometry in combination with hydrogen/deuterium (H/D) exchange reactions. Sustained off-resonance irradiation collision-induced dissociation (SORI-CID) studies of the fully deuterated ion D 2 N(CH 2 ) 3 SiOSi(CH 2 ) 3 ND 3 ϩ provided convincing evidence for a backbiting mechanism which involves hydrogen transfer from the terminal amine group to the oxygen to form a silanol-containing species. Theoretical calculations indicated decomposition of H 2 N(CH 2 ) 3 SiOSi(CH 2 ) 3 NH 3 ϩ through a backbiting mechanism is the lowest energy decomposition channel, compared with other alternative routes. Two mechanisms were proposed for the fragmentation process which leads to the siloxane bond cleavage and the SORI-CID results of partially deuterated precursor ions suggest both mechanisms should be operative. Rearrangement to yield a silanol-containing product ion requires end groups possessing a labile hydrogen atom. Decomposition of disiloxane ions with end groups lacking labile hydrogen atoms yielded product ions from direct bond cleavages. (J

“…Evaporation increases the surface charge density to a point where fission occurs. The disintegration process triggered in this way is highly asymmetric, leading to the formation of several small progeny droplets that carry away only a small fraction of mass, but a disproportionately high amount of charge [13,14]. High-speed imaging is an important tool for gaining insights into the breakup mechanism [15,16].…”

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confidence: 99%

A simple model for the disintegration of highly charged solvent droplets during electrospray ionization

Konermann

2009

This work uses a minimalist model for deciphering the opposing effects of Coulomb repulsion and surface tension on the stability of electrosprayed droplets. Guided by previous observations, it is assumed that progeny droplets are ejected from the tip of liquid filaments that are formed as protrusions of an initially spherical parent. Nonspherical shapes are approximated as assemblies of multiple closely spaced beads. This strategy greatly facilitates the calculation of electrostatic and surface energies. For a droplet at the Rayleigh limit the model predicts that growth of a very thin filament is a spontaneous process with a negligible activation barrier. In contrast, significant barriers are encountered for the formation of larger diameter filaments. These different barrier heights favor highly asymmetric droplet fission because the dimensions of the filament determine those of the ejected droplet(s). Substantial charge accumulation occurs at the filament termini. This allows each progeny droplet to carry a significant fraction of charge, despite its very small volume. In the absence of a long connecting filament, relieving electrostatic stress through progeny droplet emission would be ineffective. The model predicts the prevalence of fission events leading to the formation of several progeny droplets, instead of just a single one. Ejection bursts are followed by collapse back to a spherical shape. The resulting charge depleted system is incapable of producing additional progeny droplets until solvent evaporation returns it to the Rayleigh limit. Despite the very simple nature of the model used here, all of these predictions agree with experimental data. E lectrospray ionization (ESI) mass spectrometry (MS) has evolved into one of the most versatile and widely used analytical techniques. The ESI process itself has been subject of numerous studies (see, e.g., [1][2][3][4][5][6][7][8][9][10][11][12] and references therein). Analyte solution is passed through a metal capillary to which high voltage has been applied, and solvent droplets emanate from the tip of a Taylor cone at the capillary outlet [11]. Each droplet carries a net charge due to protons or other cationic species that reside at the air/liquid interface. Solvent evaporation and droplet fission are fundamental elements of the ESI process. Evaporation increases the surface charge density to a point where fission occurs. The disintegration process triggered in this way is highly asymmetric, leading to the formation of several small progeny droplets that carry away only a small fraction of mass, but a disproportionately high amount of charge [13,14]. High-speed imaging is an important tool for gaining insights into the breakup mechanism [15,16]. More recently, the rapid solidification of charged nanodroplets using sol-gel techniques has provided a means to capture transient droplet shapes and study them by microscopy [17]. Also, molecular dynamics simulations represent an interesting approach in this area [9,10]. All those studies reveal that disintegrati...