Overview of AMS Instrument and Working Principles

Qi ZhangDepartment of Environmental Toxicology

University of California at Davis

Aerodyne/Nanjing University Chinese AMS/ACSM Clinic

Nanjing University, ChinaApril 20 -22, 2018

1

15 (10 AMS,5 ACSM)

17 (3 AMS,14 ACSM)

5 (2 AMS,3 ACSM)

20 (9 AMS,11 ACSM)

Prepared by Penglin Ye

AMS/ACSM in China

IonDetector

DataProcessor

Mass Spectrum

Ion Source

MassAnalyzer

Sample Inlet

How Does Mass Spectrometry Work?

Building blocks of a mass spectrometer

Display MS

High Vacuum3

Mass spectrometry is an analytical technique that measures the mass-to-charge ratio (m/z) and abundance of gas-phase ions generated from molecules.

IonDetector

DataProcessor

Mass Spectrum

Ion Source

MassAnalyzer

Sample Inlet

How Does Aerosol Mass Spectrometry Work?

High Vacuum4

AerosolSample

• Aerosol sampling inlet, P-ToF chamber, Vaporizer1. Aerosol sampling2. Particle sizing (AMS)3. Particle vaporization

Conceptual Schematic of an AMSAerosol Interface

AerosolInlet

ParticleSizing Vaporization

Mass Spectrometry

Ionization MassAnalysis

AerodynamicLens

Particle-ToFChem. Detect.

Impact on heatedSurface

(~ 600 oC)

e- Ionization(EI)

5

Quadrupole /ToF

• Analysis of particle ensemble• Detect non-refractory components

(evaporate rapidly at ~ 600 oC & vacuum)

critical orifice

nozzle

Orifices

Aerosol Inlet: Aerodynamic Lens

6

• Form particle beam• Concentrate aerosols

from gas-phase–by 107 vs. gas–Reduce gas-phase

interference• Particle transmission

efficiency: –PM1 lens:100% ~ 60 –

600 nm–PM2.5 lens

• Impart size-dependent velocity– To measure size by

particle time-of-flight

m d3

Fdrag d2 v

Finertia = m a a d-1

Fdrag

v v: relative velocity between the gas and the particle

Aerodynamic Forces on a Particle

Particle Sizing: Size-Dependent VelocityPressure difference b/w lens & sizing chamber Fdrag

size-dependent velocity

To DetectorAerodynamic Lens

Nozzle

Inertia Drag

2.4 Torr 10-3 Torr exit

Drag is also shape-dependent particle

morphology

DeCarlo P. et al. AS&T, 38, 1186-1205, 2004

Free-Molecular Regime

7

Jayne, J.T., Aerosol Sci. Technol., 33, 49-70, 2000.

lba

lg

p

cp v

DDvv

tLv

)/(1 *

Particle velocity

Particle diameter

Particle Sizing: Calibration

Empirical Power Law

• Analyze particles of known Da

• Determine vp for each size

• Plot & fit the data

8

Ensemble Composition

“Beam open”

Particle Beam Vaporizer

(~600oC)

ionization/detection

Determination of vp: Chopper + Chem. Detec.

Size Distribution

1500

1000

500

0

Ion

Sig

nal

8x10-36420Time of Flight (s)

Size distribution of m/z’sMass weighted size distribution

Low duty cycle signal cut by 25!

“Beam chopped”

Particle Beam

100

101

102

103

104

105

106

107

Ion

Rat

e (H

z)

28024020016012080400m/z (Daltons)

No size informationMuch higher S/N

Vaporizer(~600oC)

ionization/detection

particle

Ion

sign

al

9

AMS Detection Process (Vaporization + ionization)

Vaporizer

e-

Electron EmittingFilament

R+

Positive Ion Mass Spectrometry

Chemical Composition

FlashVaporization of

Non-Refractory (NR)components

600 CParticle

Time-Of-Flight

Particle Velocity

AerodynamicDiameter

Focused Particle Beam

Electron Impact Ionization

Universal, quantitative, and sensitive chemical analysisVaporization and analysis of most aerosol chemical constituents

- with primary exception of crustal oxides and elemental carbon. 10

Canagaratna, M. et al. Mass Spectrometry Reviews, 26, 185-222, 2007

• Hot filament (Tungsten or Rhenium) gives off electrons• e- accelerated by a potential difference• electron energy = 70 eV (NIST database, reference MS)• Interact with the gaseous molecules in their path• Hard ionization method extensive molecular

fragmentation

Electron Impact Ionization (EI)

molecule

e

e

e 70

55

0.1

4.9

frag1 frag2++

molecule

e

Fragmentation

11

Ionization Efficiency

present moleculesformed ions

IE

• Reproducible • Quantitative

• Nonselective ionization• Extensive fragmentation

Mass Analyzers: Analysis of Ions

• In the mass analyzer, gas-phase ions are subjected to a known electric (or magnetic) field.

• Electric (or magnetic) forces sort ions according to their m/z ( mass spectrum)

• Types used in AMS– Quadrupoles– Time-of-flight (TOF)

• m/z range, mass resolution

12

•Quadrupole consists of four parallel rods

•Typical length:10’s cm

•Precise dimensions and spacing

•Rods connected diagonally in pairs

• Voltages (DC + AC) applied to rods define time-varying fields between rods and determine the m/z that is transmitted. (Mass Filter)

• One m/z at a time, unit mass resolution (UMR)• Simple, rugged, lightweight

Quadrupole Mass Analyzers

13

Detector

V

Source, S Drift Region, D

E = V/S E = 0

mqEsD

vDt

mqEsv

mvqEs

qEsqVU

aDD

aD

Da

aa

2

221 2

Ions accelerated by strong field, E, within short source region, S.

Drift times recorded across long, field-free drift region, D.

vD depends on starting position of ion –ideally all ions start from same plane.

Drawing adapted from p20 of Cotter reference:

a

Cotter, “Time-of-flight Mass Spectrometery: Instrumentation and Applications in Biological Research,” ACS, 1997.

Time-of-Flight Mass Analyzers

14

m/z tD2

Ion detectors: To collect and convert ions into an electrical signal, which can be easily measured

Types of Ion Detectors• Electron multiplier• Micro-channel plate (array transducer)

Mass Spectrometry Detectors

• Ions impact the wall starts a cascade of e-

• e- propagates through the channel Very large amplification!

15http://www.chm.bris.ac.uk/ms/theory/detection.html

e-

High Resolution Time‐of‐Flight AMS (HR‐ToF‐AMS)

16Jayne et al., AST, 2000; DeCarlo et al., Anal. Chem. 2006

AMS Schematic

MS Signatures for Aerosol Species Identificationcolor coded to match spectra

Water H2O H2O+ , HO+ , O+ 18, 17, 16Ammonium NH3 NH3

+, NH2+, NH+ 17, 16, 15

Nitrate HNO3 HNO3+, NO2

+, NO+ 63, 46, 30

Sulfate H2SO4 H2SO4+, HSO3

+, SO3+ 98, 81, 80SO2

+, SO+ 64, 48Organic CnHmOy H2O+, CO+, CO2

+ 18, 28, 44(Oxygenated) C2H3O+, HCO2

+, CxHyOZ+ 43, 45, ...

Organic CnHm CxHy+ 27,29,41,43,55,57,69,71...

(hydrocarbon)

Group Molecule/Species Ion Fragments Mass Fragments

e-

e-

e-

e-

e-

e-

Standard electron impact ionization @ 70 eVEasy to quantify: ca. NIST MS libraryEasy to separate inorganic and organic componentsSpeciation of organic composition is challenging

17Canagaratna, M. et al. Mass Spectrometry Reviews, 26, 185-222, 2007

Principle of EI Mass Spectrometry and Quantification

Mass Loading A (MWA/IEA) Ion Signalai

EI Ionization: A + e- A+ ai+

m/z

msmix = ci ∙ msimsi – mass spectrum of sample ici – concentration of sample i

2 ∙ msa + 2 ∙ msb + 1 ∙ msc

ca = 2cb = 2cc = 1

Mass Loading A (MWA/IEA) Ion Signalai

EI Ionization: A + e- A+ ai+

Principle of EI Mass Spectrometry and Quantification

1st

• Fragmentation pattern of inorganic species (e.g., SO4

2-, NO3-, water, NH4

+, Cl+) & gas molecules (e.g., N2, O2, CO2, H2O)

• Isotopic ratios of elements

Continuous mass spectrum

Stick mass spectrum(UMR or HR)

Aerosol Sampling

AMS/ACSM

Size resolved MS size distributions

Ensemble MS

MS of Organics

-MS of Air

MS of Sulfate

MS of Nitrate

MS of Ammonium

MS of Chloride

MS of Water

An ensemble AMS/ACSM mass spectrum is the linear superposition of the mass spectra of individual components.

Jayne et al., AST, 2000Jimenez et al., JGR, 2003 Allan et al., J. Aero. Sci., 2004.

AMS Spectral Analysis

1st

• Fragmentation pattern of inorganic species (e.g., SO4

2-, NO3-, water, NH4

+, Cl+) & gas molecules (e.g., N2, O2, CO2, H2O)

• Isotopic ratios of elements

Continuous mass spectrum

Stick mass spectrum(unit m/z resolution)

Aerosol Sampling

Ensemble MS

MS of Organics

-MS of Air

MS of Sulfate

MS of Nitrate

MS of Ammonium

MS of Chloride

MS of Water

The concentration of a species in question is calculated by summing signals in its partial MS

E.g., Corg (g m-3) = m/ziorg

Jayne et al., AST, 2000Jimenez et al., JGR, 2003 Allan et al., J. Aero. Sci., 2004.

An ensemble AMS/ACSM mass spectrum is the linear superposition of the mass spectra of individual components.

AMS/ACSM

Size resolved MS size distributions

AMS Schematic and Data

• Real‐time measurement (sec – min resolution )• Non‐refractory submicron PM (NR‐PM1 )• NR species: evaporate rapidly at ~ 600 oC and vacuum (10‐8 torr)

Jayne et al., AST, 2000; DeCarlo et al., Anal. Chem. 2006

2. Size‐resolved composition

1. Quantitative composition

3. 0

2. 5

2. 0

1. 5

1. 0

0. 5

0. 0

Nitr

ate

Equi

vale

nt M

ass

Con

cent

ratio

n (µ

g m

-3)

141201 0080604 020m /z (D alt ons )

Am m on ium 4 .8 ug/m 3Nit ra te 5 .8Su lpha te 9 .4Organ ics 13.4Ch loride 0.15

M ex ic o C i ty 2/200 2

SO+SO2

+

S+

SO3+

HSO3+

H2SO4+

43 44

57

3. Elemental comp. (CnHmNpOzSw)

m/z 43

High Resolution Time‐of‐Flight AMS (HR‐ToF‐AMS)

C2H3O+

CHNO+C2H5N+

C3H7+CH3N2

+

~ 600 oC 

Transmission: 30 ~ 1500 nm

22

(mass weighted)

Important Considerations for AMS Mass Concentration Quantification

• Ionization efficiency and relative ionization efficiencies (Qi Chen et al.)

• Collection efficiency (Weiwei Hu et al.)• Aerodynamic Lens: PM1 vs. PM2.5

• Vaporizer: Standard vs. Capture• Dependence on particle phase state…

• Sensitivity• Detection limit: 3 × σ of 1-min particle-free data

• Signal intensity and noises• IE/AB ratio• Data analysis issues: resolution, m/z accuracy, isobaric ions

(frag table), peak shape …

ReferencesAllan, J.D., Delia, A.E., Coe, H., Bower, K.N., Alfarra, M.R., Jimenez, J.L., Middlebrook, A.M., Drewnick, F., Onasch, T.B., Canagaratna, M.R., Jayne, J.T., Worsnop, D.R., 2004. A generalised method for the extraction of chemically resolved mass spectra from Aerodyne aerosol mass spectrometer data. Journal of Aerosol Science 35, 909-922

Canagaratna, M., Jayne, J., Jimenez, J.L., Allan, J.A., Alfarra, R., Zhang, Q., Onasch, T., Drewnick, F., Coe, H., Middlebrook, A., Delia, A., Williams, L., Trimborn, A., Northway, M., DeCarlo, P., Kolb, C., Davidovits, P., Worsnop, D., 2007. Chemical and Microphysical Characterization of Ambient Aerosols with the Aerodyne Aerosol Mass Spectrometer. Mass Spectrometry Reviews 26, 185-222, DOI:110.1002/mas.20115.

DeCarlo, P.F., Kimmel, J.R., Trimborn, A., Jayne, J., Aiken, A.C., Gonin, M., Fuhrer, K., Horvath, T., Worsnop, D.R., Jimenez, J.L., 2006. A Field-Deployable High-Resolution Time-of-Flight Aerosol Mass Spectrometer. Analytical Chemistry.

DeCarlo, P., Slowik, J.G., Worsnop, D.R., Davidovits, P., Jimenez, J.L., 2004. Particle morphology and density characterization by combined mobility and aerodynamic diameter measurements. Part 1: Theory. Aerosol Science & Technology 38, 1185-1205.

Jayne, J.T., Leard, D.C., Zhang, X., Davidovits, P., Smith, K.A., Kolb, C.E., Worsnop, D.R., 2000. Development of an aerosol mass spectrometer for size and composition analysis of submicron particles.Aerosol Science and Technology 33, 49-70.

Jimenez, J.L., Jayne, J.T., Shi, Q., Kolb, C.E., Worsnop, D.R., Yourshaw, I., Seinfeld, J.H., Flagan, R.C., Zhang, X., Smith, K.A., Morris, J.W., Davidovits, P., 2003. Ambient aerosol sampling with an Aerosol Mass Spectrometer. J. Geophys. Res. 108, 13.