Inorganic and Organic Clusters Formed upon Surface-Assisted Laser Desorption/Ionization

Examples of cluster formation from inorganic and organic compounds upon surface-assisted laser desorption/ionization have been considered. It has been shown that the cluster formation is especially efficient upon ionization of silver halides. The addition of silver salts to salts of other metals, in particular, lead and nickel salts and sodium stannate, leads to the formation of mixed clusters. The peak intensities of the ions of such clusters are higher than those of the clusters of initial salts, thereby indicating a higher efficiency of their formation. Nonpolar amino acids and short-chain peptides predominantly form adducts with alkali-metal ions.     

Matrix suppression and analyte suppression effects of quaternary ammonium salts in matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry: an investigation of suppression mechanism

In the matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI TOF MS) analysis of some quaternary ammonium salts (QASs), very clean spectra of the quaternary ammonium ions were recorded with a strong matrix suppression effect (MSE). The QASs also showed a considerable analyte suppression effect (ASE). It was demonstrated that the MSE and ASE of the QASs can be explained well by the cluster ionization model. According to this model, MALDI ions are formed from charged matrix/analyte clusters. Various analyte ions and matrix ions might coexist in the cluster, and they will compete for the limited number of net charges available. If enough quaternary ammonium ions are present in the cluster, they will take away the net charges, thus resulting in the MSE and ASE. Our results also suggest that ‘the cluster ionization model’ is not in conflict with ‘the theory of ionization via secondary gas‐phase reactions’. The initial MALDI ions produced from charged matrix/analyte clusters will collide with other molecules or ions in the MALDI plume. Depending on the properties of the initial ions and the composition of the MALDI plume, secondary gas‐phase reactions might result from these collisions. The final ions observed are the combined results of ‘cluster ionization’ and ‘ionization via secondary gas‐phase reactions’. Copyright © 2009 John Wiley & Sons, Ltd.     

Cluster ions of diquat and paraquat in electrospray ionization mass spectra and their collision-induced dissociation spectra

Cluster ions such as [Cat+X+nM](+) (n = 0-4); [Cat-H+nM](+) (n = 1-3); and [2(Cat-H)+X+nM](+) (n = 0-2), where Cat, X, and M are the dication, anion, and neutral salt (CatX(2)), respectively, are observed in electrospray ionization (ESI) mass spectrometry of relatively concentrated solutions of diquat and paraquat. Collision-induced dissociation (CID) reactions of the clusters were observed by tandem mass spectrometry (MS/MS), including deprotonation to form [Cat-H](+), one-electron reduction of the dication to form Cat(+.), demethylation of the paraquat cation to form [Cat-CH(3)](+), and loss of neutral salt to produce smaller clusters. The difference in acidity and reduction power between diquat and paraquat, evaluated by thermodynamical estimates, can rationalize the different fractional yields of even-electron ([Cat-H](+) and its clusters) and odd-electron (mostly Cat(+)) ions in ESI mass spectra of these pesticides. The [Cat+n. Solv](2+) doubly charged cluster ions, where n 相似文献   

Simultaneous Quantitation of Amino Acid Mixtures using Clustering Agents

A method that uses the abundances of large clusters formed in electrospray ionization to determine the solution-phase molar fractions of amino acids in multi-component mixtures is demonstrated. For solutions containing either four or 10 amino acids, the relative abundances of protonated molecules differed from their solution-phase molar fractions by up to 30-fold and 100-fold, respectively. For the four-component mixtures, the molar fractions determined from the abundances of larger clusters consisting of 19 or more molecules were within 25% of the solution-phase molar fractions, indicating that the abundances and compositions of these clusters reflect the relative concentrations of these amino acids in solution, and that ionization and detection biases are significantly reduced. Lower accuracy was obtained for the 10-component mixtures where values determined from the cluster abundances were typically within a factor of three of their solution molar fractions. The lower accuracy of this method with the more complex mixtures may be due to specific clustering effects owing to the heterogeneity as a result of significantly different physical properties of the components, or it may be the result of lower S/N for the more heterogeneous clusters and not including the low-abundance more highly heterogeneous clusters in this analysis. Although not as accurate as using traditional standards, this clustering method may find applications when suitable standards are not readily available.     

On the distributions of ion/neutral molecule clusters in electrospray and laser spray—A cluster division model for the electrospray process

The clustering of a medium-sized, involatile, neutral molecule, octyl beta-D-glucopyranoside (OG), with Na(+), Ca(2+), and Yb(3+) (M(z+)) ions in electrospray (ESI) was investigated using laser spray (LSI). Extensive distributions of [(M(z+))(i) (OG)(a)](n+)-clusters, extending beyond 50 kDa, were observed. The distributions were highly stable and reproducible and changed only marginally when concentrations of electrolyte or neutral compound were varied by orders of magnitude. Compared with ESI, laser spray yielded superior intensities, particularly of the larger clusters. The cluster distributions demonstrated a range of remarkable features. In particular, the Yb(3+)/OG cluster distribution was unusual. For example, no clusters with 35-52 or with 110-116 OG molecules were observed. The distribution pattern revealed that the clusters were formed as a result of cluster dissociations, such as [(Yb(3+))(3)(OG) ( approximately 110)W](9+) --> [(Yb(3+))(2)(OG)( approximately 90)W](6+) + [(Yb(3+))(1)(OG) ( approximately 20)W](3+), where W represents the water content at the time of dissociation. Based on this study, a cluster division model for electrospray of aqueous solutions of strongly solvated ions is proposed: the Rayleigh droplet disintegration process, which is well-established for the initial stages of electrospray, maintains its general character as it proceeds through a final regime of multiply charged cluster dissociations to the singly and multiply charged ions in mass spectrometry. In the dissociation of multiply charged clusters, the size of each daughter cluster is roughly proportional to the square of the cluster charge. Observed cluster distributions are consistent with a mixture of symmetric and asymmetric cluster dissociations.     

Neutron diffraction and simulation studies of CsNO3 and Cs2CO3 solutions

Neutron diffraction with isotopic substitution (NDIS) experiments and molecular dynamics (MD) simulations have been used to study the structuring in aqueous solution of two cesium salts, cesium carbonate, and cesium nitrate. As was previously found for guanidinium salts of carbonate, mesoscopic-scale clusters were seen to form in the Cs2CO3 solution both in the MD simulations and in the diffraction experiments. No such large scale ion clusters were found in the CsNO3 solutions in either the modeling or experiments. The results are dominated by the strength and geometry of the direct first-neighbor interactions, which explain the differences in the clustering behavior between the two solutions without need to refer to longer-range water-water structuring.     

Mechanism of charging and supercharging molecules in electrospray ionization

The origin of the extent of charging and the mechanism by which multiply charged ions are formed in electrospray ionization have been hotly debated for over a decade. Many factors can affect the number of charges on an analyte ion. Here, we investigate the extent of charging of poly(propyleneimine) dendrimers (generations 3.0 and 5.0), cytochrome c, poly(ethylene glycol)s, and 1,n-diaminoalkanes formed from solutions of different composition. We demonstrate that in the absence of other factors, the surface tension of the electrospray droplet late in the desolvation process is a significant factor in determining the overall analyte charge. For poly(ethylene glycol)s, 1,n-diaminoalkanes, and poly(propyleneimine) dendrimers electrosprayed from single-component solutions, there is a clear relationship between the analyte charge and the solvent surface tension. Addition of m-nitrobenzyl alcohol (m-NBA) into electrospray solutions increases the charging when the original solution has a lower surface tension than m-NBA, but the degree of charging decreases when this compound is added to water, which has a higher surface tension. Similarly, the charging of cytochrome c ions formed from acidified denaturing solutions generally increases with increasing surface tension of the least volatile solvent. For the dendrimers investigated, there is a strong correlation between the average charge state of the dendrimer and the Rayleigh limiting charge calculated for a droplet of the same size as the analyte molecule and with the surface tension of the electrospray solvent. A bimodal charge distribution is observed for larger dendrimers formed from water/m-NBA solutions, suggesting the presence of more than one conformation in solution. A similar correlation is found between the extent of charging for 1,n-diaminoalkanes and the calculated Rayleigh limiting charge. These results provide strong evidence that multiply charged organic ions are formed by the charged residue mechanism. A significantly smaller extent of charging for both dendrimers and 1,n-diaminoalkanes would be expected if the ion evaporation mechanism played a significant role.     

Efficient clustering of cyclic sulfonium salts applying liquid secondary ion mass spectrometry

Salt-like cluster ions of the ([cation](n)[anion](n-1))(+) type are commonly composed of mono-atomic, inorganic components. Clusters containing organic ions are also known, with nitrogen-centred cations being particularly prominent. However, sulphur-centred analogues, such as organic sulphonium salts, represent a notable exception. Fast atom bombardment and liquid secondary ion mass spectrometry of such compounds show, in general, a low tendency towards the formation of clusters. The present study reveals that sputtering of cyclic sulphonium salts leads to the efficient formation of cluster ions beyond previous observations. Cluster ions are characterised by isotopic pattern analysis and collision-induced dissociation.     

Electrospray ionization tandem mass spectrometric study of salt cluster ions: part 2--salts of polyatomic acid groups and of multivalent metals

Salt cluster ions formed from 0.05 M solutions of CaCl(2), CuCl(2) and Na(A)B (where A = 1 or 2 and B = CO(3)(2-), HCO(3)(-), H(2)PO(4)(-) and HPO(4)(2-)) were studied by electrospray ionization tandem mass spectrometry. The effects on salt cluster ions of droplet pH and of redox reactions induced by electrospray provide information on the electrospray process. CaCl(2) solution yielded salt cluster ions of the form (CaCl(2))(n)(CaCl)(x)(x+) and (CaCl(2))(n)(Cl)(y)(y-), where x, y = 1-3, in positive- and negative-ion modes, respectively. Upon collision induced dissociation (CID), singly charged CaCl(2) cluster ions fragmented, doubly charged cluster ions generated either singly or both singly and doubly charged fragment ions, depending on the cluster mass, and triply charged clusters fragmented predominantly by the loss of charged species. CuCl(2) solution yielded nine series of cluster ions of the form (CuCl(2))(n)(CuCl)(m) plus Cu(+), CuCl(+), or Cl(-). CuCl, the reductive product of CuCl(2), was observed as a neutral component of positively and negatively charged cluster ions. Free electrons were formed in a visible discharge that bridged the gap between the electrospray capillary and the sampling cone brought about the reduction of Cu(2+) to Cu(+). Upon CID, these cluster ions fragmented to lose CuCl(2), CuCl, Cl, and Cl(2). Na(2)CO(3) and NaHCO(3) solutions yielded cluster ions of the form (Na(2)CO(3))(n) plus Na(+) or NaCO(3)(-). Small numbers of NaHCO(3) molecules were found in some cluster ions obtained with the NaHCO(3) solution. For both Na(2)HPO(4) and NaH(2)PO(4) solutions, ions of the form (Na(2)HPO(4))(h), (NaH(2)PO(4))(i), (Na(3)PO(4))(j), (NaPO(3))(k) plus Na(+), PO(3)(-) or H(2)PO(4)(-) were observed. In addition, ions having one or two phosphoric acid (H(3)PO(4)) molecules were observed from the NaH(2)PO(4) solution while ions containing one sodium hydroxide (NaOH) molecule were observed from the Na(2)HPO(4) solution. The cluster ions observed from these four salts of polyatomic acid groups indicate that changes in pH occur in both directions during the electrospray process principally by solvent evaporation; the pH value of the acidic solution became lower and that of the basic solution higher.     

The Pb12(2-) and Pb10(2-) zintl ions and the M@Pb12(2-) and M@Pb10(2-) cluster series where M = Ni, Pd, Pt

Ethylenediamine (en) solutions of K4Pb9 react with toluene solutions of ML4 (M = Pt, Pd, L = PPh3; M = Ni, L2= COD) and 2,2,2-crypt to give M@Pb12(2-) cluster anions (M = Pt (1), Pd (2), Ni (3)) as the [K(2,2,2-crypt)]+ salts in low (Ni) to good (Pt) yields. The ions have near perfect Ih point symmetry and have been characterized by X-ray diffraction, 207Pb NMR and LDI-TOF mass spectrometry studies. For M = Ni, the primary product formed is the D4d Ni@Pb10(2-) cluster that has also been structurally characterized. The M@Pb10(2-) clusters (M = Pd, Pt) and the new Zintl ions closo-Pb10(2-) and closo-Pb12(2-) were formed in the gas phase but have not been detected in solution or the solid state. The structural trends of these series of clusters have been investigated through DFT calculations. The Ni@Pb10(2-) cluster is dynamic on the 207Pb NMR time scale at -45 degrees C and 104.7 MHz. The M@Pb12(2-) ions show unusually deshielded 207Pb NMR chemical shifts that presumably arise from sigma-aromatic effects associated with their high symmetries. In the solid state, the salts form superlattices of cations and anions (e.g. the AlB2 lattice of [K(2,2,2-crypt)](2)[Pt@Pb12]) and are prototypes for "assembled cluster materials".     

Field desorption mass spectrometry of quaternary ammonium salts: Cluster ion formation

The field desorption mass spectra of salts such as quaternary ammonium and carbenium salts with organic cations in addition to high cation intensities show signals for cluster ions composed of the salt cation + salt molecule, i.e. [C + nM]⁺, n = 1–5, thus allowing determination of the molecular weights of salts. In some cases cluster ions of the type [nM – 1]⁺ are detected. Conditions for the formation of cluster ions are discussed.     

Sucrose clusters exhibiting a magic number in dilute aqueous solutions

We report clustering of sucrose molecules in dilute aqueous solutions, based on measurements of the electrical mobility spectrum of singly charged airborne clusters produced by electrospraying the solution. The spectrum contains peaks with a smooth envelope, except for one peak which has about twice the amplitude of the envelope. The mobility at this peak is found to be somewhat higher than would be expected based on the rest of the sequence, suggesting a more compact structure for this cluster. This "magic" peak is prominent for basic, but not acidic, solutions. It is argued that these observations demonstrate the existence of clusters of many sizes in the solution prior to electrospraying, including the "magic" cluster. A simplified model is presented which reproduces the observed features.     

Computer simulation of ion cluster speciation in concentrated aqueous solutions at ambient conditions

Dynamic simulations are used to investigate ion cluster formation in unsaturated aqueous NaCl at 25 degrees C. Statistical, structural, and dynamic properties are reported. An effort is made to identify general behaviors that are expected to hold beyond the limitations of the force field. Above approximately 1 M, clusters with more than ten ions begin to form after approximately 10-20 ns of simulation time, but no evidence of irreversible ion aggregation is observed. Cluster survival times are estimated, showing that the kinetics become increasingly complex as salt is added, leading to multiple decay rates. Cluster dipole moment distributions show characteristic peaks that reflect the preferred conformations of clusters in solution. These are modulated by electrostatic and liquid-structure forces and are described in detail for clusters of up to five ions. For a given size and charge, the cluster morphology is independent of salt concentration. Below approximately 2 M, clusters affect the structure of water in their first hydration shells, so dipole moments parallel to the cluster macrodipoles are induced. These effects show a weak dependence with concentration below approximately 2 M, but vanish in the 2-3 M range. A possible connection with the structural transition recently suggested by NMR data in concentrated electrolytes is discussed. The effects of electrostatics on cluster speciation and morphology are discussed based on results from a set of simulations carried out with the ionic charges removed.     

Charge carrier field emission determines the number of charges on native state proteins in electrospray ionization

Although multiple charging in electrospray ionization (ESI) is essential to protein mass spectrometry, the underlying mechanism of multiple charging has not been explicated. Here, we present a new theory to describe ESI of native-state proteins and predict the number of excess charges on proteins in ESI. The theory proposes that proteins are ionized as charged residues in ESI, as they retain residual excess charges after solvent evaporation and do not desorb from charged ESI droplets. However, their charge state is not determined by the Rayleigh limit of a droplet of similar size to the protein; rather, their final charge state is determined by the electric field-induced emission of small charged solute ions and clusters from protein-containing ESI droplets. This theory predicts that the number of charges on a protein in ESI should be directly proportional to the square of the gas-phase protein diameter and to E*, the critical electric field strength at which ion emission from droplets occurs. This critical field strength is determined by the properties of the excess charge carriers (i.e., the solute) in droplets. Charge-state measurements of native-state proteins with molecular masses in the 5-76 kDa range in ammonium acetate and triethylammonium bicarbonate are in excellent agreement with theoretical predictions and strongly support the mechanism of protein ESI proposed here.     

Electrospray ionization tandem mass spectrometric study of salt cluster ions. Part 1--investigations of alkali metal chloride and sodium salt cluster ions

Salt cluster ions of alkali metal chlorides ACl (A = Li(+), Na(+), K(+), Rb(+) and Cs(+)) and sodium salts NaB (B = I(-), HCOO(-), CH(3)COO(-), NO(2)(-), and NO(3)(-)), formed by electrospray ionization, were studied systematically by mass spectrometry. The influences on the total positive ion and negative ion currents of variation of solvent, solution concentration, desolvation temperature, solution flow-rate, capillary voltage and cone voltage were investigated. Only cone voltage was found to influence dramatically the distribution of salt cluster ions in the mass spectra observed. Under conditions of normal cone voltage of approximately 70 V, cluster ions having magic numbers of molecules are detected with high relative signal intensity. Under conditions of low cone voltage of approximately 10 V, the distribution of cluster ions detected is characterized by a relatively low average mass/charge ratio due to the presence of multiply charged cluster ions; in addition, there is a marked reduction in cluster ions having a magic number of molecules. Product ion mass spectra obtained by tandem mass spectrometry of cluster ions are characterized by a base peak having a magic number of molecules that is less than and closest to the number of molecules in the precursor ion. Structures have been proposed for some dications and some quadruply charged ions. At pH 3 and 11, the mass spectra of NaCl clusters show the presence of mixed clusters of NaCl with HCl and NaOH, respectively. The effects of ionic radius on 20 distributions of cluster ions for 10 salts were investigated; however, the fine structure of these effects is not readily discerned.     

Ion formation from charged droplets: Roles of geometry,energy, and time

The formation of ions from the charged droplets produced in the several spray ionization techniques is viewed as an activated rate process involving field-assisted desorption, in accord with the ideas first set forth by Iribame and Thomson. The novel features of the present treatment are particularly relevant to the unique ability of electrospray ionization to transform large molecules in solution to free ions in the gas phase, with extensive multiple charging. These new features stem mainly from the realization that the spacing of charges on a desorbed ion must relate to the spacing of charges on the surface of the droplet whence it came. The consequences of this “rule” can account for the existence of maxima and minima in the number of charges on the ions of a particular species as well as the nature of the distribution of ions among the intervening charge states. They also explain the dependence of charge state on the configuration in solution of the parent molecule of the desorbed ion. In addition, they provide insight into the sequence in time at which ions in the various charge states leave an evaporating droplet.     

Doubly-charged positive ions following loss of oxygen dianion under conditions of field desorption mass spectrometry

A number of cyclic aromatic anhydrides have been found to exhibit doubly-charged positive ions in their field desorption mass spectrum which correspond to the formal loss of oxygen dianion followed by clustering with a neutral molecule. Admixtures of two anhydrides, which both exhibit this phenomenon, produce heterogeneous doubly-charged cluster ions. Admixture with anhydrides which do not show the effect, produces only homogeneous clusters at reduced intensity.     

A New Method to Investigate the Clustering Phenomena in Supercritical Fluids

EstimationoflocaldensityofsolventaboutthesoluteSolvatochromicbehaviorsofspectroscopicprobesarewidelyusedtoestimatethesol-ventstrengthofsupercriticalfluids(SCF,).i-3lnthiswork,thesolvatochrondcshiftofthen-n*transitionbandforacetone(O.o37mo1.L-')insupercrihcal(SC)CO2wasde-terminedbyUVspectroscopytostUdythesolvationeffect.TheMcRae-BaylissexpressionbasedonthedielectriccontinUUInmodelgivestherelahonshipbetWeenso1vatochromicshiftandpo1arizabilityofnonPolarsolvents4-5asfollows:wherevisthewave…     

Electrical charges in nonaqueous solutions I: acetone-water mixtures as model polar solvents

To establish the mechanism of charging of solute entities in nonaqueous solvents, the number density of charges must be measured. Conductivity measurements alone are not always sufficient. Here we explore the use of total internal reflection microscopy (TIRM) to determine the Debye length and ionic strength by measuring the potential energy profile between a charged microscopic sphere and a charged plate separated by a solution of 0.1-2 mol.m-3 KOH dissolved in a mixture of acetone with 10-20% v/v water as a model nonaqueous solvent. KOH behaves as a weak electrolyte in these solvents. The dissociation constant was determined to be 1.99 mol.m-3 in the 15% v/v solution by both conductivity and TIRM experiments. Conductivity is much preferred for determining the concentration of charge in solutions like this in which the mechanism of charging is simple dissociation of ion pairs. But in nonpolar solvents, in which the mechanism might be proton transfer between two neutral micelles, TIRM can yield the number density of charges while conductivity alone cannot.     

Unified molecular picture of the surfaces of aqueous acid, base, and salt solutions

The molecular structure of the interfacial regions of aqueous electrolytes is poorly understood, despite its crucial importance in many biological, technological, and atmospheric processes. A long-term controversy pertains between the standard picture of an ion-free surface layer and the strongly ion specific behavior indicating in many cases significant propensities of simple inorganic ions for the interface. Here, we present a unified and consistent view of the structure of the air/solution interface of aqueous electrolytes containing monovalent inorganic ions. Molecular dynamics calculations show that in salt solutions and bases the positively charged ions, such as alkali cations, are repelled from the interface, whereas the anions, such as halides or hydroxide, exhibit a varying surface propensity, correlated primarily with the ion polarizability and size. The behavior of acids is different due to a significant propensity of hydronium cations for the air/solution interface. Therefore, both cations and anions exhibit enhanced concentrations at the surface and, consequently, these acids (unlike bases and salts) reduce the surface tension of water. The results of the simulations are supported by surface selective nonlinear vibrational spectroscopy, which reveals among other things that the hydronium cations are present at the air/solution interface. The ion specific propensities for the air/solution interface have important implications for a whole range of heterogeneous physical and chemical processes, including atmospheric chemistry of aerosols, corrosion processes, and bubble coalescence.