Showing papers by "Environmental Molecular Sciences Laboratory published in 1997"

The impact of the resolution of the identity approximate integral method on modern ab initio algorithm development

Abstract: The computation of the two-electron four-center integrals over gaussian basis functions is a significant component of the overall work of many ab initio methods used today. Improvements in the computational efficiency of the base algorithms have provided significant impact. Somewhat overlooked are methods that provide approximations to these integrals and their implementation in application software. A partial review of approximate integral techniques focused on the resolution of the identity (RI) four-center, two-electron integral approximation is given. The past and current uses of the RI algorithms are presented along with possibilities for further exploitation of the technology.

Sensitivity-Enhanced Quadrupolar-Echo NMR of Half-Integer Quadrupolar Nuclei. Magnitudes and Relative Orientation of Chemical Shielding and Quadrupolar Coupling Tensors

Abstract: A novel approach to quadrupolar-echo (QE) NMR of half-integer quadrupolar nuclei in static powders is analyzed. By acquisition of the QE spectrum during a Carr−Purcell−Meiboom−Gill (CPMG) train of selective π pulses, the second-order quadrupolar line shape for the central transition is split into a comb of sidebands leading to a considerable increase in the sensitivity compared to a conventional QE spectrum. The applicability of the method for determination of magnitudes and relative orientation of chemical shielding and quadrupolar coupling tensors is examined. Through numerical simulation and iterative fitting of experimental 87Rb (RbClO4 and RbVO3) and 59Co spectra (Co(NH3)5 Cl3), it is demonstrated that the quadrupolar CPMG experiment represents a useful method for studying half-integer quadrupolar nuclei exhibiting large quadrupolar coupling combined with anisotropic chemical shielding interactions. Sensitivity enhancements by a factor of up to about 30 are observed for the samples studied.

Single-molecule spectral fluctuations at room temperature

Abstract: RECENT advances in near-field1and far-field2,3 fluorescence microscopy have made it possible to image single molecules and measure their emission3,4 and excitation5 spectra and fluorescence lifetimes3,6–8 at room temperature. These studies have revealed spectral shifts4 and intensity fluctuations6,7, the origins of which are not clear. Here we show that spontaneous fluctuations in the spectra of immobilized single dye molecules occur on two different timescales: hundreds of milliseconds and tens of seconds, indicating that these fluctuations have two distinct activation energies. In addition, we see photoinduced spectral fluctuations on repeated photoexcitation of single molecules. We suggest that all of these fluctuations can be understood as transitions between metastable minima in the molecular potential-energy surface.

A novel ion funnel for focusing ions at elevated pressure using electrospray ionization mass spectrometry

Abstract: The ability to effectively focus and transmit ions from relatively high pressure ion sources is a key factor that affects sensitivity and dynamic range in mass spectrometry. To improve upon the mass spectrometric sensitivity achievable with electrospray ionization sources a novel ion funnel interface has been developed and implemented with a triple quadrupole mass spectrometer. The ion funnel effectively consists of a series of ring electrodes of progressively smaller internal diamter to which RF and DC electric fields are co-applied. The electric fields create a pseudo-potential causing the collisionally damped ions to be more effectively focused and transmitted as a collimated ion beam. The ion funnel concept we describe is supported by results of SIMION simulations, ion current measurements and implementation with a mass spectrometer. Electrospray ionization mass spectra for an initial ion funnel configuration demonstrated over an order of magnitude increase in signal relative to that of the instrument operated in its standard (capillary inlet-skimmer) configuration under similar conditions. © 1997 John Wiley & Sons, Ltd.

The Molecular Volcano: Abrupt CC l 4 Desorption Driven by the Crystallization of Amorphous Solid Water

Abstract: The desorption kinetics of molecular beam deposited ultrathin films of CC${\mathrm{l}}_{4}$ and amorphous solid water (ASW) are studied. Overlayers of ASW impede CC${\mathrm{l}}_{4}$ desorption until the onset of crystallization, whereupon the CC${\mathrm{l}}_{4}$ desorbs abruptly. The abrupt desorption occurs through connected pathways that are formed in the water overlayer during the nucleation and growth of crystalline ice from ASW. The onset of the abrupt desorption corresponds to the threshold for dynamic percolation. As the crystallization proceeds, the number of connected pathways rapidly increases, giving rise to the episodic release of CC${\mathrm{l}}_{4}$.

Benchmark calculations with correlated molecular wave functions XII. Core correlation effects on the homonuclear diatomic molecules B2-F2

Abstract: Using systematic sequences of the newly developed correlation consistent core-valence basis sets from cc-pCVDZ through cc-pCV6Z, the spectroscopic constants of the homonuclear diatomic molecules containing first row atoms, B–F, are calculated both with and without inclusion of 1s correlation. Internally contracted multireference configuration interaction (IC-MRCI) and singles and doubles coupled cluster (CCSD) theory with a perturbational estimate of connected triple excitations, CCSD(T), have been investigated. By exploiting the convergence of the correlation consistent basis sets, complete basis set (CBS) limits have been estimated for total energies, dissociation energies, equilibrium geometries, and harmonic frequencies. Based on the estimated CBS limits the effects of 1s correlation on D e (kcal/mol), r e (A), and ω e (cm−1) are: +1.1, −0.0070, +10 for B2; +1.5, −0.0040, +13 for C2; +0.9, −0.0020, +9 for N2; +0.3, −0.0020, +6 for O2; and −0.1, −0.0015, +1 for F2.

Contribution of Many-Body Terms to the Energy for Small Water Clusters: A Comparison of ab Initio Calculations and Accurate Model Potentials

Abstract: We have tested the ability of two new model potentials constructed using intermolecular perturbation theory methods to reproduce ab initio results at a comparable level of theory. Several configurations of water trimer, tetramer, and pentamer are studied, and in addition to the contributions to the interaction energy, the potential energy surfaces are compared by optimizing the model potential geometries to local stationary points within a rigid-body framework. In general the agreement between the two methods is good, validating the model potentials as suitable candidates for providing starting geometries for further ab initio calculations and for the simulation of larger systems.

Complexity in the Decomposition of Formic Acid on the TiO2(110) Surface

Abstract: The decomposition of formic acid on the TiO2(110) surface was examined with temperature-programmed desorption (TPD), static secondary ion mass spectrometry (SSIMS), and high-resolution electron energy loss spectroscopy (HREELS) Formic acid decomposed upon adsorption on TiO2(110) at 110 K to formate and a proton The formate species was identified in HREELS by the symmetric O−C−O stretching mode at 1365 cm-1 and the C−H stretching mode at 2920 cm-1 The deposited acid proton was difficult to detect in HREELS but presumably formed a hydroxyl group at a bridging two-coordinate O2- site TPD indicated that the major formic acid decomposition products were CO and H2O, suggestive of a dehydration process However, a dehydration mechanism is unsuitable for describing the decomposition of formic acid on TiO2(110) because the CO and H2O products were formed from independent sources and by independent surface processes CO desorbed during decomposition of formate between 400 and 700 K, but the majority of the deso

Length and Base Composition of PCR-Amplified Nucleic Acids Using Mass Measurements from Electrospray Ionization Mass Spectrometry

Abstract: A generally applicable algorithm has been developed to allow base composition of polymerase chain reaction (PCR) products to be determined from mass spectrometrically measured molecular weights and the complementary nature of DNA. Mass measurements of arbitrary precision for single-stranded DNA species are compatible with an increasingly large number of possible base compositions as molecular weight increases. For example, the number of base compositions that are consistent with a molecular weight of 35 000 is ∼6000, based on a mass measurement precision of 0.01%. However, given the low misincorporation rate of standard DNA polymerases, mass measurement of both of the complementary single strands produced in the PCR reduces the number of possibilities to less than 100 at 0.01% mass precision, and base composition is uniquely defined at 0.001% mass precision. Taking into account the low misincorporation rate of standard DNA polymerases and the fact that the final PCR product also contains primers of known ...

Origin of porous silicon photoluminescence: Evidence for a surface bound oxyhydride-like emitter

Abstract: Time-dependent excitation spectroscopy coupled with quantum chemical calculations is used to demonstrate that the photoluminescence (PL) resulting from the ultraviolet optical pumping of an etched porous silicon (PS) surface results from a silicon oxyhydride-like fluorophor bound to the PS surface. The time-dependent PL, in both aqueous (${\mathrm{H}\mathrm{F}/\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{H}\mathrm{F}/\mathrm{C}\mathrm{H}}_{3}{\mathrm{O}\mathrm{H}/\mathrm{H}}_{2}\mathrm{O}$) and nonaqueous [MeCN/HF (anhydrous)] etching media, has been monitored both in situ, during the etching cycle and before the PS sample is removed from the etching solution, and ex situ, after removal of the PS sample from the etching solution. The early appearance in time of the PS luminescence is consistent with the formation of a surface bound emitter created on a time scale $(l~10\mathrm{s})$ much shorter than that needed for pore formation. Laser excitation spectra (PLE) over the wavelength range extending from 193 to 400 nm produce an almost identical time-dependent PL emission feature between 550 and 700 nm. Influenced strongly by the chemical composition of the etch solution, an intermediate ``green'' emitter can be excited with select laser pumping wavelengths and observed to transform to the final ``orange-red'' luminescent product. In conjunction with experiments whose focus has been to compare the time-dependent PL after ArF (193 nm) and ${\mathrm{N}}_{2}$ (337 nm) laser excitation (PLE), the data suggest the pumping of an excited-state manifold for a molecule-like species followed by rapid relaxation via nonradiative transitions down the manifold and the subsequent emission of radiation at much longer wavelength. Detailed quantum chemical modeling supports this interpretation and suggests a correlation to changes in the bonding associated with electronic transitions that involve silanone-like ground electronic singlet states and their low-lying triplet excitons. Especially important are those changes involving SiO related bonds. A substantial shift in the excited-state manifold, relative to the ground state, correlates with the character of the observed PL spectra as the excitation to a manifold of states greatly shifted from the ground electronic state produces a considerable redshift of the PL spectrum $(\ensuremath{\sim}600--800\mathrm{nm})$ compared to the known peak wavelength of the PLE (excitation) spectrum at 350 nm. The combination of quantum chemical modeling and time-dependent spectroscopic studies also suggests that the multiexponential PL decay commonly observed as a function of increasing wavelength (550--750 nm) after excitation at 355 nm results primarily from nonradiative cascade. The optical detection of magnetic resonance (ODMR) spectrum obtained for PS and associated with a triplet exciton is assigned to an oxyhydride-like emitter possessing silicon-oxygen and silicon-hydroxide fluorophors similar to the much more complex annealed siloxene. Calculated infrared spectra are correlated with experimentally observed features and are consistent with a surface-based oxyhydride-like emitting fluorophor. A recent analysis that associates the linewidth of the triplet ODMR spectrum with an inhomogeneous distribution of quantum confined crystallites is shown to be in error. We demonstrate that the correct extension of the arguments used in this analysis provides clear evidence for the existence of a common radiative center associated with a molecule-like species bound to the surface of the PS framework. The results obtained in this study are thus not consistent with quantum confinement and suggest a surface bound emitter as the source of the PS photoluminescence.

Room-Temperature Fluorescence Imaging and Spectroscopy of Single Molecules by Two-Photon Excitation.

Abstract: We report fluorescence imaging of single dye molecules on a glass substrate by two-photon excitation with femtosecond pulses from a mode-locked Ti:sapphire laser. The single-molecule images exhibit...

Chemical Bonding between Cu and OxygenCopper Oxides vs O2 Complexes: A Study of CuOx (x = 0−6) Species by Anion Photoelectron Spectroscopy

Abstract: An extensive photoelectron spectroscopic study on the CuOx- (x = 0−6) species is presented. The photoelectron spectra of these species are obtained at four detachment photon energies: 2.33, 3.49, 4.66, and 6.42 eV. The spectra of the copper atom are included to show the dependence of the detachment cross sections on the photon energies. An intense two-electron transition to the 2P excited state of Cu is also observed in the 6.42 eV spectrum of Cu-. For CuO-, we observe an excited state of the anion, as well as photodetachment transitions to charge transfer excited states of CuO (Cu2+O2-). Six transitions are observed for CuO2- at 6.42 eV, revealing all six valence molecular orbitals of the linear OCuO molecule. CuO3- is observed to undergo photodissociation at 3.49 eV to give an internally hot CuO- plus O2. It is shown to have an OCuO2 type of structure, and its electronic structure can be viewed to be due to that of CuO perturbed by an O2. For CuO4-, two isomers are observed. One of them undergoes photo...

Cation-ether complexes in the gas Phase: Bond dissociation energies of Na+(dimethyl ether)x, x = 1-4; Na+(1,2-dimethoxyethane)x, x = 1 and 2; and Na+(12-crown-4)

Abstract: Bond dissociation energies of Na+[O(CH3)2]x, x = 1−4; Na+[(CH2OCH3)2]x, x = 1 and 2; and Na+[c-(C2H4O)4] are reported The bond dissociation energies are determined experimentally by analysis of the thresholds for collision-induced dissociation of the cation−ether complexes by xenon measured using guided ion beam mass spectrometry In all cases, the primary and lowest energy dissociation channel observed experimentally is endothermic loss of one ligand molecule The cross section thresholds are interpreted to yield 0 and 298 K bond dissociation energies after accounting for the effects of multiple ion−molecule collisions, internal energy of the complexes, and unimolecular decay rates Trends in the bond dissociation energies determined by experiment and recent theoretical ab initio calculations are in good agreement Our best experimental values, which have an average uncertainty of ±7 kJ/mol, are lower than the theoretical values by 7 ± 5 kJ/mol per metal−oxygen interaction These values are compared wit

Single-Molecule Kinetics of Interfacial Electron Transfer

Abstract: Measurements of single-molecule chemical reaction kinetics are demonstrated for interfacial electron transfer from excited cresyl violet molecules to the conduction band of indium tin oxide (ITO) or energetically accessible surface electronic states under ambient conditions by using a far-field fluorescence microscope. In this system, each single molecule exhibits a single-exponential electron transfer kinetics. A wide distribution of sitespecific electron transfer rates is observed for many single cresyl violet molecules examined. This work reveals that the physical origin of multiexponential kinetics of electron transfer in this system is the inhomogeneity of molecular interactions on the semiconductor surface of ITO. We illustrate that the singlemolecule experiments provide detailed information not obtainable from experiments conducted on large ensembles of molecules. Single-molecule kinetics is particularly useful in understanding multiexponential behavior of chemical reactions in heterogeneous systems.

Ab Initio Study of M+:18-Crown-6 Microsolvation

Abstract: The effect of aqueous microsolvation on the relative binding affinities of 18-crown-6 for the alkali metal cations (Li+−Cs+) was studied using second-order perturbation theory and polarized basis sets augmented with diffuse functions to minimize basis set superposition error. A cation exchange reaction involving the replacement of potassium in a cation/crown complex with a different alkali cation contained within a cation/water cluster served as the basis for modeling binding preferences in liquid water. Up through four crown ether waters of solvation (six for Li+ and K+) were considered, in conjunction with cation/water complexes including as many as nine waters. The principal impact of the added waters on the K+ ↔ M+ exchange reaction was to sharply reduce the spread in binding enthalpies among the different elements, narrowing the discrepancy between the theoretical gas phase cluster results and experimental findings obtained in aqueous solutions.

The CO molecule: the role of basis set and correlation treatment in the calculation of molecular properties

Abstract: Although Hartree-Fock wave functions can provide a semi-quantitative description of the electronic structure of molecules, accurate predictions cannot be made without explicit inclusion of the effects of electron correlation. In correlated calculations, the accuracy of the wave function is determined by two expansions: the many-electron expansion in terms of molecular orbitals that defines the form of the wave function and the basis set used to expand the one-electron molecular orbitals. Thus, to assess the accuracy of a given wave function (correlation method), it is necessary to examine the dependence of a given property on the basis set. In this work, systematic sequences of correlation consistent basis sets ranging in size from double- to sextuple-zeta (cc- pVnZ ) have been employed together with several commonly used electron correlation methods, e.g., MPn ( n = 2 − 4), CCSD, CCSD(T), and MRCI, to calculate the spectroscopic constants and selected molecular properties of the carbon monoxide molecule. The computed spectroscopic constants show excellent convergence toward the complete basis set (CBS) limit, and the intrinsic errors of each correlation method have been assessed and compared. The effects of correlating the ls-like core electrons have also been determined using a sequence of core-valence cc- pCVnZ basis sets with the CCSD(T) and ACPF methods. A number of other properties have also been calculated for each correlation method as a function of the correlation consistent basis set: the dipole moment, quadrupole moment, dipole polarizability, and the first and second hyperpolarizabilities. For these calculations, results using the aug-cc- pVnZ basis sets are compared with those obtained using basis sets incorporating another complete shell of diffuse functions, d-aug-cc- pVnZ . In each case well-behaved convergence toward the CBS limit is observed for each theoretical method. For both the calculated spectroscopic constants and molecular properties, comparisons are made to previous calculations and the available experimental data.

Evidence for Molecular Translational Diffusion during the Crystallization of Amorphous Solid Water

Abstract: Molecular diffusion in amorphous solid water is studied by measuring the extent of isotope exchange and mixing in vapor-deposited ultrathin films using temperature-programmed desorption. Our results indicate there is long-range translational diffusion that occurs concomitantly with the amorphous to crystalline ice phase transition. The apparent diffusivity estimated from these data is a factor of 106±1 greater than an Arrhenius extrapolation for diffusion in crystalline ice. This finding suggests that the amorphous material exhibits liquid-like translational diffusion prior to crystallization at temperatures near 155 K.

Si 3 O y ( y = 1 – 6 ) Clusters: Models for Oxidation of Silicon Surfaces and Defect Sites in Bulk Oxide Materials

Abstract: We studied the structure and bonding of a series of silicon oxide clusters, ${\mathrm{Si}}_{3}{\mathrm{O}}_{y}$ $(y\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1--6)$, using anion photoelectron spectroscopy and ab initio calculations. For $y\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1--3$ the clusters represent the sequential oxidation of ${\mathrm{Si}}_{3}$, and provide structural models for the oxidation of silicon surfaces. For $y\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}4--6$, the clusters contain a central Si in a tetrahedral bonding environment, suggesting the onset of the bulklike structure. Evidence is presented that suggests the ${\mathrm{Si}}_{3}{\mathrm{O}}_{4}$ cluster $({D}_{2d})$ may provide a structural model for oxygen-deficient defect sites in bulk ${\mathrm{SiO}}_{2}$ materials.

Cation−Ether Complexes in the Gas Phase: Bond Dissociation Energies of M+(dimethyl ether)x, x = 1−3, M+(1,2-dimethoxyethane)x, x = 1 and 2, and M+(12-crown-4) Where M = Rb and Cs

Abstract: Bond dissociation energies of M+[O(CH3)2]x, x = 1−3; M+[(CH2OCH3)2]x, x = 1 and 2; and M+[c-(C2H4O)4], where M = Rb and Cs are reported. The bond dissociation energies (BDEs) are determined experimentally by analysis of the thresholds for collision-induced dissociation of the cation-ether complexes by xenon (measured using guided ion beam mass spectrometry). In all cases, the primary dissociation channel observed experimentally is endothermic loss of one ligand molecule. The cross section thresholds are interpreted to yield 0 and 298 K BDEs after accounting for the effects of multiple ion−molecule collisions, internal energy of the complexes, and unimolecular decay rates. The experimentally determined BDEs for the monodentate ligand complexes are in good agreement with conventional ideas of electrostatic ligation of gas-phase ions and with recent ab initio calculations by Feller et al. (average discrepancy of 5 ± 6 kJ/mol). The experimentally determined BDEs for the multidentate ligand complexes do not ag...

A Solid Acid Catalyst at the Threshold of Superacid Strength: NMR, Calorimetry, and Density Functional Theory Studies of Silica-Supported Aluminum Chloride

Abstract: Solid state NMR, calorimetry, and density functional theory (DFT) all provide a consistent interpretation of the acidity of the solid acid catalyst (SG)nAlCl2, which is prepared by reacting aluminum chloride with conditioned silica gel. These studies firmly establish that the acid sites are Bronsted in nature and that their strength is significantly greater than those in zeolites. Proton NMR results, including experiments exploiting 1H−27Al dipolar couplings, demonstrate that the Bronsted acid sites have an isotropic 1H chemical shift of 5.7 ppm and a concentration of 0.58 mmol/g. The strongest sites on this solid acid, present at 0.03 mmol/g, have −ΔHav values of 52 kcal/mol for reaction with pyridine. A value of 44 kcal/mol is maintained for incremental addition of pyridine up to 0.1 mmol/g. In comparison, −ΔHav for the strongest sites in zeolite HZSM-5 is only 42 kcal/mol. 15N magic angle spinning (MAS) NMR studies of adsorbed pyridine and 31P MAS NMR of trimethylphosphine confirm the Bronsted nature o...

Energies of dipole-bound anionic states

Abstract: Dipole-bound anionic states of CH CN, C H , and HF were studied using highly 33 2 2 correlated electronic structure methods and extended one-electron basis sets. The electron detachment energies were calculated using the coupled cluster method with single, double, and noniterative triple excitations. Geometrical relaxation of the molecular framework upon electron attachment and the difference in the harmonic zero-point vibrational energies between the neutral and the dipole-bound anionic species were calculated at the MP2 level of theory. We demonstrate that the dispersion interaction between the loosely bound electron and the electrons of the neutral molecule is an important component of the electron binding energy, comparable in magnitude to the electrostatic electron)dipole stabilization. The geometrical relaxation upon electron attachment and the change in the zero-point vibrational energy is important for the weakly bound HF dimer. The predicted values of the vertical electron detachment energies for the dipole bound states of CH CN and C H of 112 and 188 cm y1 , 33 2 respectively, are in excellent agreement with the recent experimental results of 93 and y1 . y

Cation−Ether Complexes in the Gas Phase: Bond Dissociation Energies of K+(dimethyl ether)x, x = 1−4; K+(1,2-dimethoxyethane)x, x = 1 and 2; and K+(12-crown-4)

Abstract: Bond dissociation energies (BDEs) of K+[O(CH3)2]x, x = 1−4; K+[(CH2OCH3)2]x, x = 1 and 2; and K+[c-(C2H4O)4] are reported. The BDEs are determined experimentally by analysis of the thresholds for c...

Hybrid Organic/Inorganic Copolymers with Strongly Hydrogen-Bond Acidic Properties for Acoustic Wave and Optical Sensors

Abstract: Hybrid organic/inorganic polymers have been prepared incorporating fluoroalkyl-substituted bisphenol groups linked using oligosiloxane spacers. These hydrogen-bond acidic materials have glass-to-rubber transition temperatures below room temperature and are excellent sorbents for basic vapors. The physical properties such as viscosity and refractive index can be tuned by varying the length of the oligosiloxane spacers and the molecular weight. In addition, the materials are easily cross-linked to yield solid elastomers. The potential use of these materials for chemical sensing has been demonstrated by applying them to surface acoustic wave devices as thin films and detecting the hydrogen-bond basic vapor dimethyl methylphosphonate with high sensitivity. It has also been demonstrated that one of these materials with suitable viscosity and refractive index can be used to clad silica optical fibers; the cladding was applied to freshly drawn fiber using a fiber drawing tower. These fibers have potential as eva...

Improving the Microdialysis Procedure for Electrospray Ionization Mass Spectrometry of Biological Samples

Abstract: Significant advances in the area of microdialysis which allowed more effective handling of small volumes (microliters) of samples, more efficient desalting and enhanced mass spectrometric detection sensitivity are described. The previously reported on-line coupling of microdialysis with electrospray ionization (ESI) mass spectrometry has been found to be highly effective; however, direct coupling requires relatively high sample flow rates (∽2 μl min-1) to obtain a stable ESI current compared with the flow rates of newer ESI sources (e.g. ‘microspray,’ 10–100 nl min-1). To circumvent this major limitation imposed by the dimensions of currently available materials, the microdialysis procedure was modified to an off-line mode in order to avoid excessive sample consumption. A more than tenfold decrease in sample consumption was achieved using the off-line modevsthe on-line mode, which resulted in a similar quality spectrum. In addition, several other aspects of the microdialysis approach were altered to improve its performance further: (i) an increase in dialysis temperature was found to increase the desalting efficiency greatly and therefore improve the spectrum quality; (ii) the addition of piperidine and imidazole to the dialysis buffer solution resulted in a reduction of charge states and a further increase in detection sensitivity for DNA and (iii) use of low concentrations (0–2.5 mM NH4OAc) of dialysis buffer shifted the DNA negative ions to higher charge states and produced a nearly tenfold increase in detection sensitivity and a slightly decreased desalting efficiency. Protocols for desalting different samples using microdialysis are discussed. © 1997 by John Wiley & Sons, Ltd.