final project

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Introduction
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Aerosol Mass Spectrometry (AMS)for atmospheric
particle research

• Tianyi Chen

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Contents
Introduction
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Measurement approach
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Characterization of particles
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Example application
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1 Introduction
Why do we care about atmospheric
particles?

• Tiny clumps of liquid or solid material with
  aerodynamic diameters between 3 and 1000 nm
• Retention time ranges from minutes to weeks

http//www.pnl.gov/atmospheric/research/aci/images
/aerosol_clouds.jpg
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ery/peggyscove1.jpg
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ue/aerial_photo_haze-dsc04565-640.jpg
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Scientific challenge

• Wide range Three modes
• Nucleation mode
• Accumulation mode
• Coarse mode
• Characterization of physical and chemical
  properties
• Atmospheric scientist sizechemistry
• Health scientist surface areasurface
  composition

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MS to meet the challenge
Major Types of MS
Use thermal vaporization followed by various
ionization techniques
Use laser to vaporize and ionize individual
atmospheric particles
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What is mass spectrometer ?

• A mass spectrometer determines the massto-charge
  ratio (m/z) of gas-phase ions by subjecting them
  to known electric or magnetic fields and
  analyzing their resultant motion.
• Analysis by MS does not require
• Chemical modification of the analyte
• Any unique or specific chemical properties
• In theory, MS is capable of measuring any
  gas-phase molecule that carries a charge

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Differential Pump
Why?
See Aerosol Tech P21
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Where are we?
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One way of ionization--Electron Ionization

• Hot filament giving off electrons
• Accelerated by a potential difference towards the
  anode
• Interact with the gaseous molecules in their path
  (Do not impact them)

70 eV
What characteristic of the electron can we change
to try to improve the results of ionization?
http//www.colorado.edu/chemistry/chem5181/Lecture
s07/2007_MS8_Ionization_I.pdf
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Advantages and disadvantages of EI

• For molecular weight info, chemical
  ionization (CI) is used. So, EI and CI are
  always used together.

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Other ways of ionization

• Chemical Ionization (CI)
• Electrospray (ESI) / Nanospray
• Desorption Techniques
• Fast Atom Bombardment (FAB)
• Matrix-Assisted Laser Desorption/Ionization
  (MALDI)
• DESI / DART
• Ionization for Elemental Analysis
• Thermal Ionization Source
• Spark Source
• Glow Discharge
• Inductively-Coupled Plasma (ICP)

Q why are so many ionization techniques used in
MS?
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Ionization goals
No sigle method can achieve all the goals
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Where are we?
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One of Mass analyzer--Quadrupoles

• Continuous introduction of ions into analyzer
• Transmit only specific m/z value to detector
• m/z determination based on band-pass filtering
• Based on time-vary, RF fields

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Quadrupoles

• a is proportional to U/m
• q is proportional to V/m

Narrow mass range
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Quadrupoles m/z scanning
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Features of Quadrupole

• Maximum m/z 4,000
• Resolution 3,000
• Quadrupoles are low resolution instruments
• Usually operated at Unit Mass Resolution
• Small, lightweight
• Easy to couple with chromatography
• Can be used in MS/MS mode

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Other Mass analyzers

• Time-of-Flight
• Magnetic sector
• Ion traps
• FTICR

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Comparison of different analyzer
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Where are we?
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Detectors

• High amplification
• Fast time response
• Low noise
• High collection efficiency
• Low cost
• Narrow distribution of responses
• Same response for all masses
• Large dynamic range
• Long term stability
• Long life
• Mounted outside of the vacuum if possible

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2 Measurement approach
Focusing of beams
Detection scheme
Particle sizing
Instrument overview
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Instrument overview I
A
http//cires.colorado.edu/jimenez/ams.htmlAerodyn
e-AMS
Thermally vaporized and EI 1E-5 pa 600oC

• Chopper position
• Close no beam through
• Open 100 transmission
• Chopped 1-4 duty cycle

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Instrument overview II
B

• Three types of AMS (only different in detector)
  
• Q-AMS TOF-AMS HR-TOF-AMS
• Modes of operation
• Mass spectrum (MS) mode
• Chopper in open position
• ensemble averaged MS of sample (1300m/z)
• Particle Time of Flight (PTOF) mode
• Steps through 15-20 preset ion fragment masses
• Jump mass spec (JMS) mode for high R
• 10 to 20 m/z selected and qudrupole tuned by
  jumping between them instead of scanning the
  rest.

C
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Focusing of beams

• Beam gets wider for non-spherical particles,
  which needs correction.

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Particle sizing I

• Velocity is determined by flight time between
  chopper and vaporizer surface
• Particle arrival is indicated by burst of ion
  signal at selected m/z after a particle
  size-dependent delay from the opening of the
  chopper
• Velocity converted to vacuum aerodynamic
  diameters (using relation determined by
  polystyrene latex size standards)

signalK mass size distribution is mass-weighted
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Particle sizing II
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Particle sizing III
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Detection scheme
To carry out ionization without plasma chemical
charge transfer matrix effects which can create
significant difficulty for quantitative analyses
of combined laser vaporization/ionization mass
spectral data, one method is to separate
vaporization and ionization

• WHY EI?
• linear and reproducible technique
• universal ionization method

particles are flash vaporized on a resistively
heated porous tungsten surface and the vapor is
ionized by EI
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3 Characterization of properties
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Determination of chemical composition

• Most parent molecules yield AMS spectra similar
  to NIST
• Some are different but the difference from NIST
  spectra is repeatable and constant
• Aliphatic organic molecules (same fragmentation
  different intensity)
• Di- and poly-carboxylic acids (Thermally induced
  decarboxylation and dehydration with significant
  signals at m/z 18 (H2O), 28 (CO), and 44 (CO2))

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Quantification of chemicals

• Csmass concentration of some chemical species
• MWNO3molecular weight of NO3
• RIEsrelative ionization efficiency
• (Usually 1.4 for organic molecules and 1.1, 1.15,
  and 3.56 for NO3, SO4, and NH4 moieties)
• Qvolumetric flow rate
• NA---Avogadros number
• Is- detection ion rate
• CEs---collection efficiency (EsEbEL)
• IENO3--- ionization efficiency for NO3

Esshape-related collection losses at vaporizer
from inefficient focusing on non-spherical
solid Eb--- due to bouncing of less sticky
Particle before vaporization EL--- loss in inlet
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Instrument Intercomparisons
InorganicAMS vs PILS/IC
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Instrument Intercomparisons
OrganicAMS vs OC/EC
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Aerosol Size Distributions
internally mixed
sulfate (m/z 48, 64) nitrate (m/z30) oxygenated
organics (m/z 44) hydrocarbon-like organics (m/z
57)
Small particles have more hydrocarbon-like
organics

• dvaaerodynamic diameter in vacuum
• dvevolume equivalent diameter

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Aerosol Morphology

• The non-spherical
• particle has the
• same dve as the
• small sphere

• Three particles of
• identical material
• Density
• the non-spherical
• particle has the
• same dve as the
• larger sphere

dva lt dve
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Aerosol Density

• AMS and DMA sizing measurements of the same
  particles can be combined together to obtain
  particle effective densities which are a
  function of both the true density and the shape
  of the particle

fractal dimension of a particle using
themassmobility relationship
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4 Example applications of the AMS
A

• Lab studies

SOA from a-pinene and 1,3,5-TMB
Polymerization can be detected by other methods,
but not AMS because of thermal decomposition
In ambient its higher due to more aging (more
COOH groups)
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Source characterization
B
chasing a bus
more similar
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Field study
C
Ambient air (urban vs rural)
strong m/z 57 peak and the prominent CH2 ion
series characteristic of hydrocarbon species
More small particles Less small particles
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Hope you enjoy it
Thank You !