Juan Fernandez de la Mora

Secondary electrospray ionization (SESI) of ambient vapors for explosive detection at concentrations below parts per trillion

We determine the sensitivity of several commercial atmospheric pressure ionization mass spectrometers towards ambient vapors, ionized by contact with an electrospray of acidified or ammoniated solvent, a technique often referred to as secondary electrospray ionization (SESI). Although a record limit of detection of 0.2 × 10−12 atmospheres (0.2 ppt) is found for explosives such as PETN and 0.4 ppt for TNT (without preconcentration), this still implies the need for some 108–109 vapor molecules/s for positive identification of explosives. This extremely inefficient use of sample is partly due to low charging probability (∼10−4), finite ion transmission, and counting probability in the mass spectrometer (1/10 in quadrupoles), and a variable combination of duty cycle and background noise responsible typically for a 103 factor loss of useful signal.

On-line detection of human skin vapors.

Vapors released by the skin in the hand of one human subject are detected in real time by sampling them directly from the ambient gas surrounding the hand, ionizing them by secondary electrospray ionization (SESI, via contact with the charged cloud from an electrospray source), and analyzing them in a mass spectrometer with an atmospheric pressure source (API-MS). This gas-phase approach is complementary to alternative on-line surface ionization methods such as DESI and DART. A dominating peak of lactic acid and a complete series of saturated and singly unsaturated fatty acids (C12 to C18) are observed, in accordance with previous off-line studies by gas chromatography-mass spectrometry. Several other metabolites have been identified, including ketomonocarboxylic and hydroxymonocarboxylic acids.

Electrochemical processes in electrospray ionization mass spectrometry

Editorial Comment Last month we presented, as a Special Feature, a set of five articles that constituted a Commentary on the fundamentals and mechanism of electrospray ionization (ESI). These articles produced some lively discussion among the authors on the role of electrochemistry in ESI. Six authors participated in a detailed exchange of views on this topic, the final results of which constitute this months Special Feature. We particularly hope that younger scientists will find value in this months Special Feature, not only for the science that it teaches but also what it reveals about the processes by which scientific conclusions are drawn. To a degree, the contributions part the curtains on these processes and show science in action. We sincerely thank the contributors to this discussion. The give and take of intellectual debate is not always easy, and to a remarkable extent this set of authors has maintained good humor and friendships, even when disagreeing strongly on substance. Graham Cooks and Richard Caprioli Copyright 2000 John Wiley & Sons, Ltd.

PREPARATION OF ZnS NANOPARTICLES BY ELECTROSPRAY PYROLYSIS

Zinc sulfide particles 20–40 nm in diameter were prepared by electrically driven spray pyrolysis. Solutions of ethyl alcohol with zinc nitrate (Zn(NO3)2) and thiourea (SC(NH2)2) at concentrations from 0.0025 to 0.2 mol l-1 and electrical conductivities between 10-4 and 10-1 S m-1 were electrosprayed from steady cone-jets at flow rates from 0.05 to 0.25 ml h-1, with positive and negative polarity. The initially highly charged drops formed were neutralized by bipolar ions from a radioactive source to increase the overall transmission efficiency through a reactor furnace. This process was made particularly effective by the innovation of placing the ion source directly within the electrospray chamber. The diameters of the final ZnS particles were measured on-line by a differential mobility analyzer and a condensation nucleus counter. In spite of ambiguities in the flow rate of liquid through the cone–jet (associated to solvent evaporation from the meniscus), these diameters agree approximately with values expected from available scaling laws. Transmission electron micrographs also confirmed these results. Electrospray pyrolysis is hence able to generate non-agglomerated and spherical ZnS nanoparticles with geometrical standard deviation σg of about 1.3.

The mobility-volume relationship below 3.0 nm examined by tandem mobility-mass measurement

The validity of the Stokes-Millikan equation is examined in light of mass and mobility measurements of clusters of the ionic liquid 1-ethyl-3-methyl-imidazolium tetrafluoroborate (EMI-BF4) in ambient air. The mobility diameter dZ based on the measured mobility and the Stokes-Millikan law is compared with the volume diameter dv , which generalizes the mass diameter for binary substances such as salts. dv is based on the sum of anion and cation volumes in the cluster corrected for the void fraction of the bulk ionic liquid. For dv > 1.5 nm, d Z is within 1.4% of dv + 0.3 nm. For smaller clusters 3.84 and 14.3% deviations are observed at dv = 1.21 nm and 0.68 nm, respectively. These differences are smaller than expected due to a cancellation of competing effects. The increasing difference seen for dv < 1.5 nm is due primarily to the interaction between the cluster and the dipole it induces in the gas molecules. Other potential sources of disagreement are non-globular cluster geometries, and departures of the cluster void fraction from the bulk value. These two effects are examined via molecular dynamics simulations, which confirm that the volume diameter concept is accurate for EMI-BF4 nanodrops with dv as small as 1.6 nm.

Tandem Differential Mobility Analysis-Mass Spectrometry Reveals Partial Gas-Phase Collapse of the GroEL Complex

A parallel-plate differential mobility analyzer and a time-of-flight mass spectrometer (DMA-MS) are used in series to measure true mobility in dry atmospheric pressure air for mass-resolved electrosprayed GroEL tetradecamers (14-mers; ~800 kDa). Narrow mobility peaks are found (2.6-2.9% fwhm); hence, precise mobilities can be obtained for these ions without collisional activation, just following their generation by electrospray ionization. In contrast to previous studies, two conformers are found with mobilities (Z) differing by ~5% at charge state z ~ 79. By extrapolating to small z, a common mobility/charge ratio Z(0)/z = 0.0117 cm(2) V(-1) s(-1) is found for both conformers. When interpreted as if the GroEL ion surface were smooth and the gas molecule-protein collisions were perfectly elastic and specular, this mobility yields an experimental collision cross section, Ω, 11% smaller than in an earlier measurement, and close to the cross section, A(C,crystal), expected for the crystal structure (determined by a geometric approximation). However, the similarity between Ω and A(C,crystal) does not imply a coincidence between the native and gas-phase structures. The nonideal nature of protein-gas molecule collisions introduces a drag enhancement factor, ξ = 1.36, with which the true cross section A(C) is related to Ω via A(C) = Ω/ξ. Therefore, A(C) for GroEL 14-mer ions determined by DMA measurements is 0.69A(C,crystal). The factor 1.36 used here is based on the experimental Stokes-Millikan equation, as well as on prior and new numerical modeling accounting for multiple scattering events via exact hard-sphere scattering calculations. Therefore, we conclude that the gas-phase structure of the GroEL complex as electrosprayed is substantially more compact than the corresponding X-ray crystal structure.

Relation between Electrical Mobility, Mass, and Size for Nanodrops 1–6.5 nm in Diameter in Air

A large number of data on mobility and mass have been newly obtained or reanalyzed for clusters of a diversity of materials, with the aim of determining the relation between electrical mobility (Z) and mass diameter d m = (6m/ π ρ ) 1/3 (m is the particle mass and ρ the bulk density of the material forming the cluster) for nanoparticles with d m ranging from 1 nm to 6.5 nm. The clusters were generated by electrospraying solutions of ionic liquids, tetra-alkyl ammonium salts, cyclodextrin, bradykinin, etc., in acetonitrile, ethanol, water, or formamide. Their electrical mobilities Z in air were measured directly by a differential mobility analyzer (DMA) of high resolution. Their masses m were determined either directly via mass spectrometry, or assigned indirectly by first distinguishing singly (z = 1) and doubly (z = 2) charged clusters, and then identifying monomers, dimers, … n-mers, etc., from their ordering in the mobility spectrum. Provided that d m > 1.3 nm, data of the form d m vs. [z(1+m g /m) 1/2 /Z)] 1/2 fall in a single curve for nanodrops of ionic liquids (ILs) for which ρ is known (m g is the mass of the molecules of suspending gas). Using an effective particle diameter d p = d m + d g and a gas molecule diameter d g = 0.300 nm, this curve is also in excellent agreement with the Stokes-Millikan law for spheres. Particles of solid materials fit similarly well the same Stokes-Millikan law when their (unknown) bulk density is assigned appropriately.

A Simple Turbulent Mixing CNC for Charged Particle Detection Down to 1.2 nm

A simple modification of the turbulent mixing condensation nucleus counter (CNC) of Okuyama et al. enables it to grow positively charged particles/ions with diameters smaller than 1.2 nm (1.7 nm mobility diameter) into visible drops. The design is simpler than its predecessor of Gamero and Fernández de la Mora and has lower inlet losses, though the activation efficiency curve for monodisperse particles exhibits two steps, leading to an inferior resolution when the CNC is used as a particle sizer. Successful activation of such small objects by dibutyl phthalate vapor condensation is highly dependent on achieving good thermal control in the sections of the CNC prior to and including the mixing point.

Tandem ion mobility-mass spectrometry (IMS-MS) study of ion evaporation from ionic liquid-acetonitrile nanodrops

Ion evaporation is an essential step in the formation of charged ions from electrosprays, yet many aspects of the process are poorly understood. The ion evaporation kinetics of the 1-ethyl-3-methyl-imidazolium+ (EMI+) based ionic liquids (ILs) EMI-BF4, EMI-bis(perfluoroethylsulfonyl)imide, EMI-bis(trifluoromethylsulfonyl)imide and EMI-tris(trifluoromethylsulfonyl)methide (EMI-Methide) are studied by tandem ion mobility-mass spectrometry (IMS-MS) of IL nanodrop residues from positive and negative electrosprays of IL-acetonitrile solutions. Two-dimensional (2D) IMS-MS spectra are obtained using a differential mobility analyzer (DMA) coupled to a commercial quadrupole-time-of-flight mass spectrometer. Nanodrops of different charge states (z=1,2,...,10,...) are separated into distinct bands in 2D DMA-MS spectra, allowing for determination of both nanodrop size (radius) and charge. With the exception of negatively charged EMI-BF4, all clusters observed are charged below the Rayleigh limit of both the ILs and acetonitrile, showing that the charge loss mechanism is ion evaporation. Solvation energies, DeltaG, of evaporating ions from acetonitrile are inferred from radius and charge state data. With the exclusion of EMI-BF4 in negative mode (DeltaG>1.84 eV), all are in the 1.54-1.65 eV range, considerably lower than previously reported for tetra-alkyl ammonium salts in formamide. Measured size distributions of EMI-Methide nanodrops agree with those predicted by ion evaporation theory, though with narrower widths observed for doubly and singly charged nanodrops.

Performance Evaluation of an Improved Particle Size Magnifier (PSM) for Single Nanoparticle Detection

Several modifications of the particle size magnifier (PSM) developed by Okuyama et al. have been introduced recently for detection of particles at diameters of 1 nm and below. However, their evaluation has been incomplete. Here we provide the first direct measurements of counting efficiencies near unity below 2 nm. We use the modified PSM described by Sgro and Fernández de la Mora, which separates thermally the PSMs original vapor generator from the water-cooled growth chamber by means of a narrow and short T where turbulent mixing with the aerosol takes place. The counting efficiency is seen to depend greatly on the aerosol flow, the amount of vapor, and temperature. With ethylene glycol vapor, under optimal conditions, the counting efficiency is 100% down to 1.6 nm (actual diameter of 1.2 nm), and negative particles are more easily activated than positive particles. The improved PSM is applied to the measurement of gold nanoparticle size distributions, and the results show it is a powerful aerosol detector for nanoparticles.