Cyclohexane has been used as an image gas in field ion microscopy (FIM) and applied for imaging of tip surfaces at low field strengths. It is shown that at the onset of field ionization (FI) of cyclohexane on a freshly prepared Pt tip, chemisorbed cyclohexane is ionized, and bright spots appear in the field ion microscope due to enhanced FI of molecules supplied from the gas phase at these surface ions. At higher field strengths, the low partial pressure of water by which cyclohexane is typically contaminated leads to an oxidation of the surface with a complete disappearance of cyclohexane surface ions by field-induced reactions. During the oxidation process, mass spectrometry shows the enhanced formation of cyclohexyl ions, which is attributed to a field induced hydride ion transfer from cyclohexane to unoxidized parts of the Pt surface and to reactive adsorbates formed during the oxidation process. The FI behavior of cyclohexane on the oxidized surface at lower field strengths is determined by the high work function of the surface and a morphologically rough surface structure.

Capillary current pulsations appearing under conditions of electrospray mass spectrometry and their dependence on the disintegration of an H2O/MeOH solution at the end of the capillary have been investigated by combining measurements of the capillary current with optical microscopy of the disintegration of the liquid at the capillary tip. With increasing capillary potential two pulsating and one continuous axial spray mode could be distinguished. Pulsations with frequencies in the low kilohertz range are related to a transient emission of liquid via a cone jet and are caused by an imbalance between the emission rate of liquid and its supply rate to the apex of the cone. Low frequency pulsations of a sequence of emission pulses are due to an imbalance between the supply of liquid to the cone volume and its loss by a sequence of emission pulses. A mechanism of the disintegration of the jet into charged droplets is proposed. The dependence of the pulsations on the flow rate, electrolyte concentration, surface tension, and capillary diameter is shown. Mass spectrometry revealed that high ion signal intensities are obtained at higher capillary potentials in the continuous axial spray mode and at lower capillary potentials in the transition range between the two pulsating axial spray modes because of the formation of small highly charged droplets. In experiments with nanoelectrospray capillaries, current pulsations were observed only after the onset of a (corona) discharge.

Early in the development of electrohydrodynamic mass spectrometry (EH-MS), it was realized that ion source performance improved when the sample liquid was supplied discontinuously to the capillary emitter. Based on this realization, an ion source was designed that operates without an external sample supply system. Its essential part is a metal capillary that serves both as the liquid reservoir and the field anode and is mounted exchangeably on a pushrod. As the liquid flow is field induced, the flow rates are extremely low, in the range of 0.2 nl min(-1). Thus, in spite of high analyte concentrations needed, sample consumption is only a few picomoles per mass scan. Moreover, the emission of larger droplets is avoided, which contributes to the stability of the ion signal. As the capillary emitter is mounted on a standard emitter carrier known from field desorption (FD) MS, the new ion source is compatible with conventional FD ion sources. EH mass spectra of analytes could be obtained from substances whose solubility in glycerol was > 0.5 mol l(-1). The substances investigated with the new ion source include sugars, amino acids, peptides and an antibiotic. The general characteristics of the mass spectra are essentially the same as those known from other EH ion sources. Besides abundant solvent cluster ions, only cationated or protonated molecules of the analytes could be detected, frequently with one or more solvent molecules attached to the ions. Fragment ions were not observed.

Properties of benzene as the image gas in field ion microscopy (FIM) and its ionization behavior have been investigated. In FIM of Pt/Ir tips prepared by field evaporation large bright spots and dynamic phenomena with the appearance of ring structures are observed. These phenomena, which are not observed with cyclohexane as the image gas, are attributed to field ionization of molecules at the end of charged polymer chains grown by field induced cationic polymerization of benzene on the anode surface. Thermochemistry reveals that the cationic polymerization of benzene is only exothermic in a high electric field and depends on the formation of carbocations on the tip surface.

Field ion microscopy (FIM) with positive ions and negative ions (NFIM) is compared for polymer layers of two structures grown by field induced anionic polymerization of tetracyanoethylene (TCNE) on a metal tip. The positive and negative ion images of the polymer layer mainly consisting of linear polymer chains are complementary, i.e. positive ion emission is preferentially observed in dark areas of the corresponding negative ion images. For the layer with a crosslinked polymer structure the bright rings of negative ion images are not observed in positive ion images. The dissimilarities between NFIM with TCNE and FIM with benzene and TCNE are attributed to differences in the excess electron and electron hole conductivities of the polymer layers exposed to the image gases.

The dependence of the ion signal in electrohydrodynamic mass spectrometry (EH MS) on a number of experimental parameters has been investigated with the aim of finding the conditions of optimum signal intensity and stability, also with regard to the pulsations that were found recently in the ion signals in EH MS. Investigations of the influence of the electric field strength at the capillary tip, the liquid flow rate and the conductivity (electrolyte concentration) of the sample solution largely confirmed the results of previous investigations by Evans, Jr., and co-workers and Cook and co-workers. A reduction in the surface tension by the addition of a small amount of a surfactant and an increase in the liquid temperature from 20 to 50°C both led to an increase in the signal intensity and stability and to an increase in the pulse frequencies. Both reduction in surface tension and increase in temperature did not cause significant changes in the relative ion intensities. From the experimental results a mechanism of ion formation in EH MS is derived which is similar to that of field desorption (FD) MS. In both methods the ion emission can be regarded as a desolvation process in which ions are released from the tip of liquid protrusions by successive rupture of ion-solvent bonds; the energy required for the ion release is thus divided into several steps. The main difference between EH and FD MS is due to the comparatively low viscosity of the sample substance in EH MS which causes the specific properties of the EH ion emission: the occurrence of regular, low frequency pulsations in the ion signal, the preferential release of partially solvated ions and the strong influence of surface tension on the ion signals.

The initial stages of polymerization of tetracyanoethylene (TCNE) on a field cathode metal tip were investigated by negative field ion microscopy (NFIM). Field electron emission into the TCNE gas, leading to the formation of a deposit on the tip surface, was found to be a prerequisite for the growth of the polymer layer by anionic polymerization. In the first step after electron emission anionic polymerization of TCNE to long linear polymer chains occurs at low and even zero field strengths. The transition to the polymer structure which leads to the appearance of bright rings in the ion images is caused by a longer exposure of the polymer layer to NFI conditions, i.e., by anionic polymerization of TCNE molecules absorbed in the layer which changes the elastic properties of the layer.

Field-ion images of clean metal surfaces obtained by using organic image gases show bright spots rather than bright areas from surface regions of constant field strength. The origin of these spots has been investigated for cyclohexane and n-heptane as image gases and Pt/Ir tips prepared by field evaporation. It is shown that the spots arise from field ionization (FI) of molecules on surface deposits formed by electron-impact-induced fragmentation of adsorbed image-gas molecules. The impinging electrons are provided by FI of the image-gas molecules remote from the surface even at field strengths below those required for FI of the molecules adsorbed on the metal surface.

The growth of a polymer layer on the surface of a cathode tip has been observed for tetracyanoethylene (TCNE) and benzoquinones under conditions of negative ion formation by field ionization (NFI). Studies of the ion emission from the polymer layers by field ion microscopy showed the formation of negative ions at field strengths significantly below the onset of electron emission and revealed a decisive role of the polymer in lowering the field strength for NFI. This role of the polymer layer is discussed for TCNE and attributed to a loose structure of the TCNE layer with a large capacity for adsorption and absorption of molecules from the gas phase. A new mechanism of NFI is derived which explains the observed ion and electron emission phenomena. In this mechanism NFI is achieved by charge transfer reactions between absorbed molecules in the bulk and adsorbed molecules on the surface of the layer.

Cluster ion formation in thermospray ionization of ammonium salts with water as solvent is investigated and used to probe the state of the gas mixture at the end of the capillary vaporizer and effects of the gas expansion into the jet chamber. Experiments with ammonium nitrate reveal the formation of cluster ions by desolvation in the capillary and point to a nearly dry state of the gas mixture at the end of the capillary. A nearly dry state of the gas is also indicated by the particular conditions at which doubly charged cluster ions of ammonium chloride could be detected. For ammonium chloride the growth of cluster ions by recondensation of volatilization products in the jet chamber is observed. This effect reveals that the gas temperature remains significantly below the jet chamber temperature up to the sampling orifice even at higher jet chamber pressures.

The effect of matrix evaporation on secondary ion formation in fast atom bombardment mass spectrometry has been investigated for samples dissolved in glycerol with and without the need of a co-solvent. Measurements of the evaporation rate of co-solvent-containing sample solutions exposed to vacuum show that supersaturation in the near surface layer is achieved by the loss of a significant fraction of the co-solvent in less than 1 min. Particle bombardment hampers the formation of a solid layer by precipitation of sample molecules. The physical state of viscous supersaturated solutions, amenable to molecular ion formation during continuous erosion of the layer, is considered as a metastable colloid state with precipitated microparticles. The time variation of the molecular ion intensity during erosion of a layer is mainly determined by the analyte concentration profile across the solution at the onset of sputtering and by the loss of matrix during sputtering which raises the arialyre concentration at the surface and worsens the sputtering and ionization conditions. For raffinose dissolved in glycerol molecular ions could be recorded under dynamic bombardment conditions even after a residence time of the solution in vacuum for more than 2 days.

The ion formation in both electrohydrodynamic (EH) and electrospray (ES) mass spectrometry (MS) is based on the electrohydrodynamic disintegration of sample solutions which are passed through a capillary biased at high potential. Vacuum is applied in EH and atmospheric pressure in ES MS. For glycerol applied as solvent in EH MS optical studies of its disintegration behavior revealed a change from axial spray modes to a rim emission mode in vacuum and a change from axial spray modes to a droplet ejection mode at atmospheric pressure conditions with increasing potential. EH MS investigations of the ion emission from only one or a few emission sites at the rim of the capillary showed a pulsed ion emission whose frequency increased with applied potential. The pulsed ion emission is attributed to an imbalance between the supply and loss of liquid at an emission site. By lowering the surface tension of glycerol with dodecyl sulfate sodium salt an increase of mass spectral ion intensity by more than one order of magnitude could be observed.

The surface topography of tungsten tips prepared for STM by electrochemical etching has been investigated by FIM at low imaging field strengths (< 10 V/nm) using cyclohexane as the image gas. The experiments revealed that protrusions of oxidized tungsten with low desorption field strengths (between 10 and 30 V/nm) are formed by the electrochemical etching process. These protrusions are considered to operate as electron donors and acceptors in STM.

The formation of negative ions by field ionization (NFI) is difficult even for molecules of high electron affinity because of a small difference in threshold field strength between ion formation by NFI and the onset of electron emission. For tetracyanoethylene and dichlorodicyanobenzoquinone we have studied the negative ion formation on a field cathode tip by field ion microscopy using the negative ions for imaging. The experiments reveal the growth of a conducting polymer layer under NFI conditions and the decisive role of this layer in the prevention of electron emission. The polymer layer has no significant effect on the field ion appearance energies of the molecular ions because the electron affinity obtained from measurements of the appearance energy of a number of molecules agrees quite well with those reported in the literature. The appearance energies of iodide and iodine cluster ions are also determined and interpreted by ion-forming field reactions.

In liquid SIMS (or fast atom bombardment mass spectrometry) a liquid sample solution is exposed to an incident keV particle beam and molecular ions are recorded during continuous erosion of the layer. We have investigated this mode of molecular ion formation under dynamic bombarding conditions regarding the effect of concentration, solid particle formation by cristallization of solutes from supersaturated solutions and the formation of a closed solid surface layer prior to the onset of particle bombardment. The experiments were performed with peptides and organic salts applying mass spectrometry and optical microscopy. They showed that molecular ions and/or cluster ions are obtained under dynamic bombarding conditions without accumulation of radiation damage from supersaturated solutions, even in the presence of small precipitated particles and after erosion of thin solid surface layers from the remaining solution. Furthermore it was found that the incident particle beam hampers crystallization of the solute from a supersaturated solution.

Different mechanisms of desolvation of ions by thermospray (TSP) vaporization of liquids are critically discussed. It is concluded that field effects cannot play a major role in the desolvation of ions and molecules. This conclusion is supported by results of experiments with dication salts and alkaline-earth metal salts. It was found that at low solute concentrations (⩽10−3 M) single ions and molecules are predominantly desolvated from small droplets by solvent evaporation. The application of a TSP electrolyte at high concentrations of about 0.1 M gives rise to the formation of salt clusters and cluster ions with and without analyte molecules attached, again by solvent evaporation from small droplets. Neutral and ionized molecules and fragments are released from the clusters by volatilization of the salt. The mean diameter of the small droplets is roughly estimated to be less than 10 nm for the applied indirectly heated vaporizer.