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.

Discrimination between bacterial spore types using time-of-flight mass spectrometry and matrix-free infrared laser desorption and ionization.

We demonstrate that molecular ions with mass-to-charge ratios (m/z) ranging from a few hundred to 19 050 can be desorbed from whole bacterial spores using infrared laser desorption and no chemical matrix. We have measured the mass of these ions using time-of-flight mass spectrometry and we observe that different ions are desorbed from spores of Bacillus cereus, Bacillus thuringiensis, Bacillus subtilis, and Bacillus niger. Our results raise the possibility of identifying microorganisms using mass spectrometry without conventional sample preparation techniques such as the addition of a matrix. We have measured the dependence of the ion yield from B. subtilis on desorption wavelength over the range 3.05-3.8 microm, and we observe the best results at 3.05 microm. We have also generated mass spectra from whole spores using 337-nm ultraviolet laser desorption, and we find that these spectra are inferior to spectra generated with infrared desorption. Since aerosol analysis is a natural application for matrix-free desorption, we have measured mass spectra from materials such as ragweed pollen and road dust that are likely to form a background to microbial aerosols. We find that these materials are readily differentiated from bacterial spores.

Energy-sensitive cryogenic detectors for high-mass biomolecule mass spectrometry.

Energy-sensitive calorimetric detectors that operate at low temperatures ("cryogenic detectors") have recently been applied for the first time as ion detectors in time-of-flight mass spectrometry. Compared to conventional, ionization-based detectors, which rely on secondary electron formation or the charge created in a semiconductor, cryogenic detectors measure low-energy solid state excitations created by a particle impact. This energy sensitivity of cryogenic detectors results in several potential advantages for TOF-MS. Cryogenic detectors are expected to have near 100% efficiency even for very large, slow-moving molecules, in contrast to microchannel plates whose efficiency drops considerably at large mass. Thus, cryogenic detectors could contribute to extending the mass range accessible by TOF-MS and help improving detection limits. In addition, the energy resolution provided by cryogenic detectors can be used for charge discrimination and studies of ion fragmentation, ion-detector interaction, and internal energies of large molecular ions. Cryogenic detectors could therefore prove to be a valuable diagnostic tool in TOF-MS. Here, we give a general introduction to the cryogenic detector types most applicable to TOF-MS including those types already used in several TOF-MS experiments. We review and compare the results of these experiments, discuss practical aspects of operating cryogenic detectors in TOF-MS systems, and describe potential near future improvements of cryogenic detectors for applications in mass spectrometry.

Mass determination of megadalton-DNA electrospray ions using charge detection mass spectrometry.

Charge detection mass spectrometry (CD-MS) has been used to determine the mass of double-stranded, circular DNA and single-stranded, circular DNA in the range of 2500 to 8000 base pairs (1.5-5.0 MDa). Simultaneous measurement of the charge and velocity of an electrostatically accelerated ion allows a mass determination of the ion, with instrument calibration determined independently of samples. Positive ion mass spectra of electrosprayed commercial DNA samples supplied in tris(hydroxymethyl)ethylenediaminetetraacetic acid buffer, diluted in 50 vol. % acetonitrile, were obtained without cleanup of the sample. A CD mass spectrum constructed from 3000 ion measurements takes 10 min to acquire and yields the DNA molecular mass directly (mass resolution = 6). The data collected represent progress toward a more automatable alternative to sizing of DNA by gel electrophoresis. In addition to the mass spectra, CD-MS generates charge versus mass plots, which provide another means to investigate the creation and fate of large electrospray ions.

Identification of denatured double-stranded DNA by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

We created double-stranded DNA (dsDNA) by annealing synthetic oligonucleotides and used matrix-assisted laser desorption/ionization time-of-flight mass spectrometry to measure the difference in molecular weight of the complementary strands after denaturation. Refined sample preparation and deliberate rectilinear construction at a linear mass spectrometer produced a mass resolution exceeding 1000 for single-stranded DNA. We present methodologies and demonstrate the potential for analysing dsDNA by determining the mass of the complementary strands.

Molecular weight determination of megadalton DNA electrospray ions using charge detection time-of-flight mass spectrometry.

We present results obtained with a novel mass spectrometer capable of determining the mass of multiply charged electrospray ions generated from samples of macromolecules in the megadalton (MDa) size range. The instrument utilizes a sensitive amplifier which can detect the charge on a single ion as it passes through a tube detector. A velocity measurement of an ion with known electrostatic energy provides the ion's mass-to-charge ratio. Simultaneous detection of the ion charge permits a mass assignment to be made for each ion. Electrospray ions of DNA and polymer molecules with masses greater than 1 x 10(6) Da and charge numbers (z) in excess of 425 e(-) are readily detected in this mass spectrometer. The weights of small particles were also measured. The on-axis single-ion detection configuration provides a duty cycle of nearly 100% and extends the practical application of electrospray mass spectrometry to the analysis of very large molecules with relatively inexpensive instrumentation.