Chemically-assigned classification of aerosol mass spectra.

An Algorithm for Discriminant Analysis of Mass Spectra--ADAMS--was created that classified aerosol mass spectra into dominant chemically-assigned classes, and grouped rare cases in an outlier class. ADAMS was trained with ambient particulate matter (PM) mass spectra, and then validated through classification tests on known spectra with random noise added, various standard chemicals, and salt-spiked polystyrene latex microspheres. The classification results showed that ADAMS gave a reasonable chemical description of the particle populations. In contrast to adaptive resonance theory (ART-2a) classification, ADAMS could be trained to be advantageously sensitive or insensitive to selected chemical markers. Application of ADAMS to Toronto ambient PM and diesel PM (NIST 2975) demonstrated that these samples could be well described, with a low proportion of the cases falling into the outlier class. Such an algorithm may find application for source-receptor modeling of aerosol mass spectra.

Mass spectrometry of N-linked oligosaccharides using atmospheric pressure infrared laser ionization from solution.

An atmospheric pressure (AP) infrared (IR) laser ionization technique, implemented on a quadrupole ion trap mass spectrometer, was used to analyze underivatized, N-linked oligosaccharides in solution. Experiments were conducted on an atmospheric pressure infrared ionization from solution (AP-IRIS) ion source which differed from previous AP IR matrix-assisted laser desorption/ionization (MALDI) interfaces in that the ion source operated in the absence of an extraction electric field with a higher power 2.94 microm IR laser. The general term 'IRIS' is used as the mechanism of ionization differs from that of MALDI, and is yet to be fully elucidated. The AP-IRIS ion source demonstrated femtomole-level sensitivity for branched oligosaccharides. AP-IRIS showed approximately 16 times improved sensitivity for oligomannose-6 and the core-fucosylated glycan M3N2F over optimal results obtainable on a AP UV-MALDI with a 2,4,6-trihydroxyacetophenone matrix. Comparison between IR and UV cases also showed less fragmentation in the IR spectrum for a glycan with a conserved trimannosyl core, core-substituted with fucose. A mixture of complex, high-mannose and sialylated glycans resulted in positive ion mass spectra with molecular ion peaks for each sugar. Tandem mass spectrometry of the sodiated molecular ions in a mixture of glycans revealed primarily glycosidic (B, Y) cleavages. The reported results show the practical utility of AP-IRIS while the ionization mechanism is still under investigation.

Aerosol laser ablation mass spectrometry of suspended powders from PM sources and its implications to receptor modeling.

Primary sources of particulate matter (PM) were analyzed by suspending powdered samples into an aerosol laser ablation mass spectrometer (LAMS). PM sources studied included vehicle exhaust particulates, dust from a non-ferrous smelter, cement powder, incinerator fly ash, two coal fly ash samples, and two soils. Marker peaks signified certain PM source sectors: construction particles could be distinguished by abundant Ca and Ca compounds, fuel combustion was marked by elemental carbon clusters, and nonferrous industrial particles showed inorganic As, Cu, Pb, Zn, and SOx. In addition to the distinction between particles from these different source sectors, mass spectral results also showed that for a single source, different particle types existed, and among different sources within a sector, similar spectra were present. The aerosol LAMS results show the difficulty in differentiating among separate fly ash sources as well as among different soil samples. A particle class balance receptor model that measures the amount of specific particle types rather than the amount of a chemical component is suggested as a means of source apportionment when particle spectra with overlapping source possibilities occur. The assumptions and limitations of receptor modeling aerosol LAMS data are also described. In particular, methods need to be developed to account for the contribution of secondary sources.

Atmospheric pressure MALDI with pulsed dynamic focusing for high-efficiency transmission of ions into a mass spectrometer.

The atmospheric pressure (AP) matrix-assisted laser desorption/ionization (MALDI) technique described to date has proven to be a convenient and rapid method for soft ionization of biomolecules. However, this technique, like other AP ionization methods, has so far suffered from a low efficiency in transmitting ions from atmospheric pressure into the vacuum of the mass spectrometer (MS). In this work, a novel technique we termed pulsed dynamic focusing, or PDF, which improves the ion transmission efficiency and sensitivity of AP-MALDI by over an order of magnitude, is described. Pulsed dynamic focusing operates on the basis of pulsing a high-voltage extraction field to zero, when ions are just outside of the MS entrance, to allow the intake gas flow of the MS to effectively entrain the ions into the MS. Results from application of the PDF technique to an AP-MALDI ion trap MS demonstrated that in comparison to static AP-MALDI operation (1). up to 2.1 times more ions from a given laser shot could be transferred into the MS, (2). applying higher voltages in combination with the switching scheme yielded up to 1.6-times-higher ion intensities, and (3). a 3-times-larger laser spot area could be utilized. The combination of these factors produced an enhancement in throughput and sensitivity, as measured by the ions detected per unit time, of over 12 times for a digest sample of bovine serum albumin. In addition, the PDF technique proved to make AP-MALDI less sensitive to laser positioning, creating a more robust ion source in comparison to static AP-MALDI.

Atmospheric pressure MALDI-fourier transform mass spectrometry.

The coupling of atmospheric pressure matrix-assisted laser desorption/ionization (AP MALDI) with Fourier transform mass spectrometry (FTMS) is described, and its significance for the high-resolution analysis of complex peptide mixtures is demonstrated. High kinetic energy and extensive metastable decay characteristic of ions generated by vacuum MALDI have been known to constitute a possible obstacle to high-resolution analysis by FTMS. Since the initial coupling of laser desorption techniques with FTMS was realized two decades ago, several different solutions have been proposed to control the energy of the ions and fulfill the promise of high sensitivity and high resolution offered by this analytical method. Initial results obtained on quadrupole time-of-flight and ion trap analyzers have shown that ions generated by MALDI at atmospheric pressure are intrinsically less energetic than those provided by vacuum MALDI. Our report indicates that this characteristic is particularly beneficial for FTMS applications in which a sharp reduction of metastable decay can make larger ion currents available for detection and possible tandem experiments. In our hands, AP MALDI-FTMS has enabled the analysis of complex peptide mixtures with resolution and accuracy comparable to those obtained by analogous electrospray ionization-FTMS experiments, with no evidence of either metastable decomposition or significant formation of matrix adducts. Analysis of a trypsin digest of bovine serum albumin provided signal-to-noise ratios and limits of detection similar to those obtained by ion trap analyzers, but with unmatched resolution and accuracy. AP MALDI has been shown to provide stable precursor ions in amounts that allowed for informative tandem experiments. Finally, the potential of AP MALDI-FTMS for the high-resolution screening of complex mixtures was demonstrated by the analysis of isobaric peptides differing in mass by less than 0.04 Da.

On-line analysis of urban particulate matter focusing on elevated wintertime aerosol concentrations.

A 10-day winter sampling campaign was conducted in downtown Toronto for particulate matter (PM) air pollution in the fine (<2.5 microm) size range. An aerosol laser ablation mass spectrometer (LAMS), a tapered-element oscillating microbalance (TEOM), and an aerodynamic particle sizer (APS) were operated in parallel to characterize the PM on-line. In this study, the LAMS observed differences in the chemical composition between three separate episodes with higher PM2.5 mass and APS counts. LAMS results showed that in one instance of elevated PM, organic amines were present in the particulates. Temporal analyses of this episode revealed chemical transformations as the amines, characterized by m/z peaks 58(C3HeN)+, 86(C5H2N)+, and nitrates, increased in number concentration while Ca and hydrocarbon particle classes concurrently decreased. On another day, sulfates were found to have increased significantly. The third event was only 4 h in duration and exhibited an increase in the number of submicron-sized K/hydrocarbons and sulfate-containing particles. In this last event, the hydrocarbons and a K to Fe ratio enrichment indicated there was likely a contribution from a combustion source. This work offers some of the first insights into single particle size and chemistry in a cold winter climate.