ptr-tofms: quantitative detection of organic trace...

PTR-TOFMS: Quantitative Detection of Organic Trace Compounds in Gas-phase and Aerosol

M. Graus, M. Müller, A. Wisthaler, A. HanselUniversity of Innsbruck, Austria

PTR Drift Tube TOF-MS

Pulser Detector

Air Inlet

DRI - Reno, NV8th AMS Users’ Meeting Sept 29 – Oct 1 2007

H2O

H2O+

O+H+

OH+

H3O+

H3O+

H3O+

H3O+

VOC

VOC•H+e–

H3O+

H3O+

H3O+ + VOC → VOCH+ + H2O pseudo first-order kineticsk

[ ]tkOHi

VOCHi 1

)()( VOC

3+

+

=

PTR-MS How does it work?

H3O+ Proton transfer reaction when

PA(VOC)>PA(H2O)

VOC•H+

H3O+

H3O+ H3O+


— (CH3)2S

PTR-MS is sensitive to most hydrocarbons and hydrocarbon derivatives and some inorganic species.

Which Compounds can be monitored?

Exot

herm

ic P

TRs

Endothermic PTR

s

Reaction is fast and occurs at almost every collision.

Reaction does not occur at moderate collision energies.


Proton Transfer vs Electron Impact

Source magnets

methanol

EI-MS


EI-MS70eV sledge hammer

produces lots of peaks per compound that overlap

PTR-MSSoft ionization conserves

compound identity. MS is much easier to

deconvolution

PTR-MS is a soft ionization method.

Mixture analysis:Proton Transfer vs Electron Impact

DRI - Reno, NV8th AMS Users’ Meeting Sept 29 – Oct 1 2007Poor selectivity

Mostly non-dissociative ionization of VOC

Conserves compound identity

Absolute quantification

No or little humidity influence

Gas-phase and AerosolGas-phase analysis: No sample pre-treatment

Aerosol analysis: Thermal desorption into carrier/buffer gas

High time resolution

High sensitivity: 2σ-LOD tens of pptV @ 1 s

Excellent linearity and dynamic range: >106

PTR-MS Features

☺ Diverse detector


Improving Selectivity

Better Chemical

(Molecular) Analysis

PTR

Linear ion trap

LIT-MS

Q1 q2 Q3

PTR

Triple quadrupole

-3Q-MS

PTRTOF-MS

TOFWERKW-TOF

(high resolution)

(more / ” better”)

Organic Factors

Doug’s Statement

@ Perdue University together with Paul Shepson

@ Innsbruck University together with IONICON

@ Innsbruck University together with IONIMED

Inns

bruc

k Ef

fort

s



Better Chemical


PTR -X-MS

Gas-phase

Part

icle

Sa

mpl

er

Thermal Desorption

Particle-phase

Data Overkill

Classification(Factors)

Inns

bruc

k Ef

fort

sour main business

“just” do it!


Better Understanding

for the Atmosphere


Better Chemical


Classification(Factors)



Pulser Detector

Inlet

System Description: PTR-TOFMS

Time of Flight

Sign

al

Aquisition

Timing

Identification:

• Peak Separation

• Exact Mass Measurement

• Isotopic Ratio


Mass Resolving Power

Conventional PTR-MS (~Unity Resolution)

CH3CHOH+●H2O63,045 u

DMSH+

63,027 u

0%

20%

40%

60%

80%

100%

61 61.5 62 62.5 63 63.5 64 64.5 65Mass [u]

peak

hig

ht [%

]



0%

10%

20%

30%

40%

50%

60%

62,96 62,98 63 63,02 63,04 63,06 63,08 63,1 63,12Mass [u]

peak

hig

ht [%

]

High Resolution PTR-TOFMS (RFWHM = 5000)

CH3CHOH+●H2O63,045 u

DMSH+

63,027 u


RFWHM ≅ 4800



What mass resolving power is requiredto separate HC from oxygenated-HC ?

R(CH4)n vs RO(CH4)n-1 ∆m = m(CH4) - m(16O) ≅ 0.03639 u

010002000300040005000600070008000

0 50 100 150 200 250

m/z (amu)

requ

ired

Mas

s R

esol

utio

n


Mass Resolution


Mass Resolution

C10H8.H+

C8H16O.H+


Condensable Organic Matter – Aerosol Precursors

Allen Goldstein et al. (2007) ES&T

Not only size matters, but…

… the degree of oxidation.


e.g. counting the oxygen atoms

# oxygen atoms

12

34

Identifying atomic composition by exact mass and natural isotope abundance…


Smog Chamber Study (@ PSI – LAC, Villigen, CH)

1,3,5-TMB photo-oxidation150ppbV TMB + ~10ppbV HONO+ 400pptV SO2 seed aerosol+ ~55% rH

Average Spectrum141st – 150th minute after light-on

Measured m/z: 73.0296 ± σ =0.0007

Deviation: - 8.6ppmC3H4O2.H+

m/z: 232.0779 ± σ =0.0006

Dev.: - 18.1ppmC9H13NO6.H+

*

*

PTR-TOFMS Application

Gas Phase Analysis


Analysis of Organic Aerosol

Smog Chamber Study (@ PSI – LAC, Villigen, CH)

SOA sample on Teflon filter (141st-150th min)

Basic thermal desorption unit Coupled to HR PTR-TOFMS

Temperature steps give additional information on volatility


Desorption and System Temperature

too low?


Conversation of compound identity / degree of oxidation

*) Bahreini et al. (2005) ES&T downloaded form AMS Spectral Database. Ulbrich, I.M., and Jimenez, J.L. URL: http://cires.colorado.edu/jimenez-group/AMSsd/


SOA from photo-oxidation of 1,3-dimethylbenzene q-AMS; 650°C; 70eV *)

SOA from photo-oxidation of 1,3,5-trimethylbenzene

PTR-TOFMS; 125°C

? ? ?

More info about the super highly oxidized fraction(?) OOA1 OOA0

Summary PSI Smog Chamber measurements High Resolution PTR-TOFMS

• Gas-phase products of TMB photo-oxidation with high time and mass resolution in a smog chamber successfully monitored

• Thermal desorption from simultaneously collected filter samples. First characterisation of SOA by PTR-TOFMS (proof of principle – lot of improvement potential)

• Poor performance of this very basic thermal desorption for very heavy and sticky compounds

• Overwhelming amount of data collected

Performance Summary:High Resolution PTR-TOFMS

• Allows “easy” gas-phase monitoring• Particle phase with considerable effort

• Mass resolution of ~5500 in V-mode–Separation of isobaric ion species (e.g. DMS – Acetaldehyde Water Cluster; “count oxygens”)– Identification of elemental composition

• Detects all ions in one instant • Excellent linearity• Absolute quantification


Instrument

Two Units:Electronic Unit:86cm x 60cm x 95cm

60 kgPTR-TOF Unit86cm x 80cm x 120cm

70 kg

130 kg total


High Resolution TOF Challenges

Temperature stabilization to avoid mass scale drift and distortionIon scattering reduce signal loss and abundance sensitivityImprove coupling of soft ionisation source and TOFImprove mass resolutionImprove time resolution to get more points per peak (multiple peaks)

Pre-correction of TOF data (saturation and dead time distortion; mainly TDC)Automating peak detection/fit and mass scale correctionAutomating identification of elemental composition (exact mass, isotope ratio, ionization info)Use identified peaks for mass re-scaling to expand calibrated mass range

HardwareHardware SoftwareSoftware

Acknowledgement:

IONICONAlfons Jordan, Gernot Hanel, Stefan Haidacher, Ralf Schottkowsky

TOFWERKKatrin Fuhrer, Marc Gonin, Christian Tanner

PSI – Villigen (CH)

Josef Dommen, Axel Metzger, Urs Baltensperger

Financial support:

•University of Innsbruck Infrastructure Programme•Bridge Programme - FFG•Introp - ESF


Pulser Detector

Air Inlet

PTR-TOFMS: Quantitative Detection of Organic Trace Compounds in Gas-phase and Aerosol

Martin Graus, Markus Müller, Armin Hansel, Armin Wisthaler

Ion Molecule Reactions and Environmental Physics

Institute ofIon Physics and Applied Physics

ptr-tofms: quantitative detection of organic trace...

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