Fractal morphology, imaging and mass spectrometry of single aerosol particles in flight.

The morphology of micrometre-size particulate matter is of critical importance in fields ranging from toxicology to climate science, yet these properties are surprisingly difficult to measure in the particles' native environment. Electron microscopy requires collection of particles on a substrate; visible light scattering provides insufficient resolution; and X-ray synchrotron studies have been limited to ensembles of particles. Here we demonstrate an in situ method for imaging individual sub-micrometre particles to nanometre resolution in their native environment, using intense, coherent X-ray pulses from the Linac Coherent Light Source free-electron laser. We introduced individual aerosol particles into the pulsed X-ray beam, which is sufficiently intense that diffraction from individual particles can be measured for morphological analysis. At the same time, ion fragments ejected from the beam were analysed using mass spectrometry, to determine the composition of single aerosol particles. Our results show the extent of internal dilation symmetry of individual soot particles subject to non-equilibrium aggregation, and the surprisingly large variability in their fractal dimensions. More broadly, our methods can be extended to resolve both static and dynamic morphology of general ensembles of disordered particles. Such general morphology has implications in topics such as solvent accessibilities in proteins, vibrational energy transfer by the hydrodynamic interaction of amino acids, and large-scale production of nanoscale structures by flame synthesis.

Fast determination of the relative elemental and organic carbon content of aerosol samples by on-line single-particle aerosol time-of-flight mass spectrometry.

Different particulate matter (PM) samples were investigated by on-line single-particle aerosol time-of-flight mass spectrometry (ATOFMS). The samples consist of soot particulates made by a diffusion flame soot generator (combustion aerosol standard, CAST), industrially produced soot material (printex), soot from a diesel passenger car as well as ambient particulates (urban dust (NIST) and road tunnel dust). Five different CAST soot particle samples were generated with different elemental carbon (EC) and organic carbon (OC) content. The samples were reaerosolized and on-line analyzed by ATOFMS, as well as precipitated on quartz filters for conventional EC/OC analysis. For each sample ca. 1000 ATOFMS single-particle mass spectra were recorded and averaged. A typical averaged soot ATOFMS mass spectrum shows characteristic carbon cluster peak progressions (Cn+) as well as hydrogen-poor carbon cluster peaks (CnH(1-3)+). These peaks are originated predominately from the elemental carbon (EC) content of the particles. Often additional peaks, which are not due to carbon clusters, are observed, which either are originated from organic compounds (OC-organic carbon), or from the non-carbonaceous inorganic content of the particles. By classification of the mass spectral peaks as elemental carbon (i.e., the carbon cluster progression peaks) or as peaks originated from organic compounds (i.e., molecular and fragment ions), the relative abundance of elemental (EC) and organic carbon (OC) can be determined. The dimensionless TC/EC values, i.e., the ratio of total carbon content (TC, TC = OC + EC) to elemental carbon (EC), were derived from the ATOFMS single-particle aerosol mass spectrometry data. The EC/TC values measured by ATOFMS were compared with the TC/EC values determined by the thermal standard techniques (thermooptical and thermocoulometric method). A good agreement between the EC/TC values obtained by on-line ATOFMS and the offline standard method was found.

A method for measuring vapor pressures of low-volatility organic aerosol compounds using a thermal desorption particle beam mass spectrometer.

A temperature-programmed thermal desorption method for measuring vapor pressures of low-volatility organic aerosol compounds has been developed. The technique employs a thermal desorption particle beam mass spectrometer we have recently developed for real-time composition analysis of organic aerosols. Particles are size selected using a differential mobility analyzer, sampled into a high-vacuum chamber as an aerodynamically focused beam, collected by impaction on a cryogenically cooled surface, slowly vaporized by resistive heating, and analyzed in a quadrupole mass spectrometer. A simple evaporation model developed from the kinetic theory of gases is used to calculate compound vapor pressures over the temperature range of evaporation. The data are fit to a Clausius-Clapeyron equation to obtain a relationship between vapor pressure and temperature and to determine the heat of vaporization. The technique has been evaluated using C13-C18 monocarboxylic and C6-C8 dicarboxylic acids, which have vapor pressures at 25 degrees C of approximately 10(-4) - 10(-6) Pa, but less volatile compounds can also be analyzed. The method is relatively simple and rapid and yields vapor pressures and heats of vaporization that are in good agreement with literature values. The technique will be used to generate a new database of vapor pressures for low-volatility atmospheric organic compounds.

Chemical analysis of diesel engine nanoparticles using a nano-DMA/thermal desorption particle beam mass spectrometer.

Diesel engines are known to emit high number concentrations of nanoparticles (diameter < 50 nm), but the physical and chemical mechanisms by which they form are not understood. Information on chemical composition is lacking because the small size, low mass concentration, and potential for contamination of samples obtained by standard techniques make nanoparticles difficult to analyze. A nano-differential mobility analyzer was used to size-select nanoparticles (mass median diameter approximately 25-60 nm) from diesel engine exhaust for subsequent chemical analysis by thermal desorption particle beam mass spectrometry. Mass spectra were used to identify and quantify nanoparticle components, and compound molecular weights and vapor pressures were estimated from calibrated desorption temperatures. Branched alkanes and alkyl-substituted cycloalkanes from unburned fuel and/or lubricating oil appear to contribute most of the diesel nanoparticle mass. The volatility of the organic fraction of the aerosol increases as the engine load decreases and as particle size increases. Sulfuric acid was also detected at estimated concentrations of a few percent of the total nanoparticle mass. The results are consistent with a mechanism of nanoparticle formation involving nucleation of sulfuric acid and water, followed by particle growth by condensation of organic species.

On-line pyrolysis as a limitless reduction source for high-precision isotopic analysis of organic-derived hydrogen.

On-line pyrolysis is described as an alternative to chemical means for reducing organic compounds to hydrogen gas for hydrogen isotopic analysis by continuous-flow isotope ratio mass spectrometry. An empty ceramic tube, held at temperatures above 1000 degree C, is used in-line to pyrolyze organic compounds separated by gas chromatography and is followed by a hydrogen-selective filter and isotope ratio mass spectrometer for hydrogen purification and isotopic measurement. Precision of light gas D/H isotopic measurement over several-fold signal intensity is shown to average SD < 2% (delta D), with corrections for ion source nonlinearities. It is demonstrated that isotopic measurement using pyrolysis is indefinitely stable, as opposed to the gradual loss of isotopic accuracy using on-line chemical reactors due to degradation of the reducing capacity of reactor metals. Pyrolytic conversion to hydrogen becomes more efficient with increasing temperature, although quantitative conversion was not achieved at the highest temperature tested (1150 degree C) in our system. In addition, pyrolysis efficiency varies with compound type; therefore, the technique requires a separate calibration for each compound of interest. This approach shows promise as a component of a robust and low-maintenance system.

High-precision continuous-flow isotope ratio mass spectrometry.

Although high-precision isotope determinations are routine in many areas of natural science, the instrument principles for their measurements have remained remarkably unchanged for four decades. The introduction of continuous-flow techniques to isotope ratio mass spectrometry (IRMS) instrumentation has precipitated a rapid expansion in capabilities for high-precision measurement of C, N, O, S, and H isotopes in the 1990s. Elemental analyzers, based on the flash combustion of solid organic samples, are interfaced to IRMS to facilitate routine C and N isotopic analysis of unprocessed samples. Gas/liquid equilibrators have automated O and H isotopic analysis of water in untreated aqueous fluids as complex as urine. Automated cryogenic concentrators permit analysis at part-per-million concentrations in environmental samples. Capillary gas chromatography interfaced to IRMS via on-line microchemistry facilitates compound-specific isotope analysis (CSIA) for purified organic analytes of 1 nmol of C, N, or O. GC-based CSIA for hydrogen and liquid chromatography-based interfaces to IRMS have both been demonstrated, and continuing progress promises to bring these advances to routine use. Automated position-specific isotope analysis (PSIA) using noncatalytic pyrolysis has been shown to produce fragments without appreciable carbon scrambling or major isotopic fractionation, and shows great promise for intramolecular isotope ratio analysis. Finally, IRMS notation and useful elementary isotopic relationships derived from the fundamental mass balance equation are presented.

High-precision D/H measurement from organic mixtures by gas chromatography continuous-flow isotope ratio mass spectrometry using a palladium filter.

Continuous-flow high-precision determination of D/H ratios from an organic mixture is described using gas chromatography coupled to a Pd filter system as an interface for isotope ratio mass spectrometry. A gas chromatograph and combustion and reduction furnaces are connected to a Pd filter via a postcolumn head pressure makeup gas to increase chromatographic sensitivity. This interface is evaluated using benzene as an internal standard in a mixture of ethylbenzene and cyclohexanone in hexane with analyte quantities of < 3 ng (< 300 pg of H). A calibration curve is constructed using four benzene samples over a range of -48 to 372/1000 (delta DSMOW), resulting in an average benzene D/H precision of SD < 5/1000 (delta DSMOW) and deviations of < 4/1000 from the calibration curve. Ethylbenzene and cyclohexanone of a single D enrichment are analyzed as unknowns in three sample mixtures with varying D-enriched benzene and result in precisions of SD < 5/1000. No apparent memory is observed between peaks of differently enriched analytes within the same chromatogram. All results are corrected for ion source nonlinearities characteristic of hydrogen analysis, using the internal peakwise correction algorithm, described previously. A small dependence of isotope ratio on palladium membrane temperature is demonstrated over a range of 4 degrees C; therefore, with tighter control of palladium temperature, precision can probably be improved. The data indicate that this system is useful for rapid continuous-flow IRMS analysis of D/H ratios from organics in complex mixtures characteristic of geological and biological samples.

Correction of ion source nonlinearities over a wide signal range in continuous-flow isotope ratio mass spectrometry of water-derived hydrogen.

Ion source nonlinearities are characterized over a wide range of signal intensities characteristic of complex mixtures, and correction schemes are proposed and evaluated for high-precision determinations of D/H ratios from water via an on-line reduction system facilitating continuous-flow isotope ratio mass spectrometry. Hydrogen isotope ratios are shown to be sensitive to analyte pressure in the IRMS ion source with or without carrier gas admitted with analyte, indicating that analyte level must be taken into account for isotope ratio calculation. Two experimentally simple "peakwise" correction schemes, in which hydrogen isotope ratios are corrected after peak identification and ratio calculation, are compared to the method routinely applied to static dual-inlet IRMS measurements. It is demonstrated that traditional linear correction applied to continuous-flow peaks is adequate over small signal ranges, about m/z 2 +/- 0.5 V; however, a second order correction is required for acceptable accuracy and precision over larger ranges. In addition, tests of the peakwise algorithms were made using a set of liquid water samples with delta D Tap Water over the range of 39-407/1000 with uncorrected data with precisions of SD-(delta D Tap Water < 34/1000 and accuracy within 11/1000. Peakwise correction using a linear calibration model resulted in substantial improvements in precision (SD < 10/1000) and accuracy (< 4/1000). Peakwise-corrected data, calibrated using a second-order regression to account for unmatched detector response, are still further improved to accuracy within 2/1000 from the calibration curve. The peakwise correction schemes are advantageous because of experimental simplicity when applied to peaks of the same or similar shapes. This study shows that ion source non-linearities in hydrogen analysis require correction for optimal analytical performance and can successfully be handled using straightforward procedures over the wide signal range required for chromatographic analysis.

High-precision D/H measurement from hydrogen gas and water by continuous-flow isotope ratio mass spectrometry.

Two instrumental approaches are described for continuous-flow high-precision determinations of D/H ratios from hydrogen gas or via on-line reduction of water. In the first system, Ar is used as a carrier gas, with a Ni reduction furnace and a water trap to remove minor levels of unreduced water that are a potential source of memory effects. Precisions of SD < 10/1000 (delta DSMOW) over a 600/1000 range from -55 to +532/1000 are obtained for liquid water (0.4 microL). Linearity is excellent over 4 orders of magnitude of D concentration in tap water (r2 > 0.9999), although precision degrades at enrichments delta DSMOW > 5000/1000. In the second system, a heated Pd metal foil functions as a filter to admit purified hydrogen into the mass spectrometer. Hydrogen gas injections are made into flowing Ar and are directed to the Pd filter (approximately 330 degrees C) which passes hydrogen isotopes only while diverting the carrier flow to waste. Precisions of these measurements are SD < 6/1000 over the D enrichment range -213 to 340/1000, with excellent linearity (r2 > 0.9999) and accuracy (< 2/1000). Similar precision is obtained using the on-line reduction apparatus and a water trap prior to the Pd filter with injections of 0.4 microL of liquid water, with acceptable linearity (r2 > 0.999) over 3 orders of magnitude of D concentration. Neither system shows any sign of memory effects when water is analyzed. The data indicate that either one of these systems is a useful means for continuous-flow IRMS of D/H isotope ratio determinations.