Analytical methodology for characterization of atmospheric nanoparticles
Date
2017
Authors
Journal Title
Journal ISSN
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Publisher
University of Delaware
Abstract
The goal of this dissertation is the development of novel analytical methods to
aid in the both the detection and characterization of atmospherically relevant, airborne
nanoparticles. This field of study is of particular importance to further the
community’s level of understanding with regard to the formation of new particles
from gas phase precursors, a process referred to as New Particle Formation (NPF).
During such an event, small molecular clusters are formed that grow rapidly from 1nm
to greater than 50nm, where they can begin to impact the formation of cloud droplets.
In this dissertation, newly developed methods are described and current methods are
more fully characterized with the goal of alleviating the lack of comprehensive
experimental measurements regarding airborne nanoparticles relevant to NPF. ☐ All of the methods described in this dissertation involve the use of mass
spectrometry as it is particularly well suited to the analysis of complex mixtures. In
order to employ mass spectrometry, however, the analyte of interest must first be
ionized so that it can be manipulated with electromagnetic fields inside the instrument.
The online methods discussed in this work describe methods of ionization which
provide characterization with time resolution anywhere from the nearly instantaneous
analysis of a single particle to the measurement of a sampling of particles over the
course of tens of seconds. ☐ To that end, two newly developed online techniques are described which seek
to use the ionization mechanisms of currently available methods to ionize airborne
nanoparticles without the need for sample collection. The first method introduces the
aerosol to a conventional plume of solvent droplets from an electrospray ionization
setup (Ambient Electrospray Ionization, AESI). Both gas and condensed phase
components are extracted into the spray droplets where charge is transferred to the
analyte molecules. As the droplets desolvate and undergo electrified fission, analyte
molecules are released into the gas phase for mass analysis. Experimenting with the
composition of the solvent spray supported this as the primary ionization mechanism,
although with gas phase analytes a minor degree of gas phase charge transfer was also
observed. ☐ Coupled to a triple quadrupole linear-ion-trap mass spectrometer (QTRAP),
AESI provides molecular ions with little to no fragmentation, which can then be
detected and characterized. Specifically, full-scan mass spectra were successfully
obtained for laboratory generated aerosols of cesium iodide and glycine, as well as
flow-tube reactor generated secondary organic aerosol from the oxidation of α-pinene.
Additionally, tandem mass spectrometry successfully identified the structure of a well
characterized oxidation product from the flow-tube oxidation. Similarly, ions from
both dimethylamine and dimethylnapthylamine vapors were successfully detected.
With the use of an internal sample introduced into the solvent spray, AESI was shown
to be quantitative with dimethylamine vapor at concentrations of 8ppb to 1ppm. ☐ The second technique utilizes the principles of inlet ionization to first convert the individual nanoparticles into micron scale aqueous droplets prior to entrance into a
heated inlet interface (Droplet Assisted Inlet Ionization, DAII). The vacuum and
temperature gradient in the narrow inlet tube cause rapid formation of micro-bubbles
which induce charge separation as the droplets rapidly desolvate, thus producing gas
phase, molecular ions from the sampled aerosol particles. DAII successfully produced
ions from aerosols composed of polypropylene glycol, angiotensin II, bovine serum
albumin, and p-methoxybenzyl pyridinium chloride at concentrations relevant to the
ambient environment (~0.001μg/m3). Unlike many online techniques, this sensitivity
is sufficient to perform analysis on size selected, monodisperse aerosols in order to
relate particle size to composition during nanoparticle growth processes. A well
characterized thermometer ion was analyzed in order to determine the operating
conditions of the setup (i.e. temperature, nanoparticle water content) which optimize
sensitivity while reducing fragmentation. ☐ These techniques provide information over the span of 10-20 seconds,
allowing for bulk characteristics of an aerosol sample to be determined in real time.
While this is a vast improvement over offline collection methods, the ~1Lpm aerosol
flowrate corresponds to several hundred thousand particles entering the source during
that sampling time. In ambient conditions, these nanoparticles could have different
origins and components, the differentiation of which provides useful insight into the
atmospheric environment as a whole. As such, other techniques seek to obtain
characterization on a particle-by-particle basis. One such method provides
quantitative analysis of relative elemental composition of single nanoparticles by
quantitatively converting an entire particle into multiply charged atomic ions (Laser
Induced Plasma Ionization LIPI). For the most part, this method has been utilized
with a specific sample set: ~20nm particles composed mainly of low atomic number
non-metals. ☐ Here, an extended range of elemental components were analyzed to investigate
the effect of composition on plasma formation and subsequent measurement of atomic
ions. LIPI was found to be quantitative for a large number of components, however
some problematic elements were identified. A quenching effect was observed for
some of these elements, leading to a reduction in the relative signal contribution from
multiply charged atomic ions compared to those which were singly charged.
Comparisons of the melting points of these elements revealed that certain
compositions have particularly high cohesive energy thresholds, leading to a
quenching effect and loss of quantitation. Despite this limitation, the presence or
absence of all elements examined was correctly confirmed at the mole-percent level.
Additionally, increasing particle diameter was shown to shift the charge distribution of
atomic ions toward a +1 state. The resulting loss of quantitation was resolved by two
methods which serve to ensure that the charge state distributions of the sample and
calibrant were sufficiently similar. Use of these methods provides quantitative relative
elemental composition for a myriad of new compositions, independent of particle
diameter. ☐ The final method developed in this work seeks to improve upon offline
collection and analysis capabilities to drastically reduce the required sample time and
achieve the necessary temporal resolution. This allows for the sample to be brought
back to the lab from a field site and analyze on stationary instrumentation which can
provide complementary analyses (high resolution, chromatography, etc). There are
two main parts to achieving this goal. First, in order to reduce the extraction volume
and sample handling steps required for an offline collection, a Nano-Aerosol Sampler
focuses the nanoparticles onto a micro-collection well for deposition. This allows the
sample to be retrieved in as little as 1μL of solvent as opposed to ~1mL or more using
conventional filter methods. 1ng to 1μg samples were collected containing an aerosol
standard (tricarballylic acid, TCA, mixed with ammonium sulfate) as well as several
10ng depositions of ambient aerosol sampled from directly outside the laboratory.
These samples were successfully extracted into low-volume HPLC vials and sealed
until analysis. This process reduces the required collection time down to less than an
hour, a vast improvement over conventional filter collection times between ~12 and 24
hours. ☐ Second, a method is described using nano-flow liquid chromatography to
provide a method of introduction into the nano-electrospray interface of a high
resolution, high accuracy Orbitrap mass spectrometer. This provides removal of
interferents (namely ammonium sulfate) which are ubiquitous to many types of
atmospheric aerosols, a concentration effect prior to mass analysis, as well as
separation of components with respect to time to reduce the complexity of the mass
spectrum. Using a pre-concentrator column and specifically designed LC program,
the ammonium sulfate interferent was screened out to waste before the system
switched into nano-flow conditions and back flowed the sample onto the analytical
column for chromatographic separation. The eluate of the analytical column was
flowed into the nano-electrospray interface of the Orbitrap mass spectrometer. In all
cases, 100% of the 1μL extracted sample was injected onto the LC system. In this
way, the method successfully removed the ammonium sulfate from the aerosol
standard while retaining the TCA for analysis. Additionally, in the ambient samples,
differences in compositions between the collections were successfully identified
without any observed interference from the ammonium sulfate.