Mass Spectrometry for Atmospheric Pressure Plasmas2021/02/11 · 3/57 J. Benedikt, MS for APP,...

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J. Benedikt, MS for APP, 11.02.2021

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Jan Benedikt, Tristan Winzer, Tobias Hahn

Institut für Experimentelle und Angewandte Physik, Kiel University

Gert Willems, Simon Große-Kreul, Simon Schneider, Dirk Ellerweg

Research Department Plasmas with Complex Interactions, Ruhr University Bochum

Mass Spectrometry for Atmospheric Pressure Plasmas

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Outline

Motivation

• The non-equilibrium atmospheric plasmas

Mass Spectrometry (MS)

Detection of reactive neutrals

• Molecular beam mass spectrometry (MBMS)

• Beam chopper with rotating skimmer

• Calibration issues in MBMS

• Threshold ionization MS (TIMS)

Detection of Ions

Analysis of Atmospheric Pressure Plasma jets

• Study of CO2 conversion

• Time-resolved ion measurements in plasma needle

Conclusions

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Non-equilibrium atmospheric pressure plasmas (APP)

6 t O3/day

Established applications of APP:Ozonizer, from 1857

Ozonia Ltd.

• O3 generation

• Surface activation: painting of plastics

• Excimer lamps in VUV und UV

• Air cleaning, particle precipitation

• Plasma medicine

• Plasmas with or in liquids

• Material synthesis:

thin film deposition,

nanoparticle generation

Emerging applications of APP:

Motivation:

CINOGY

Drexel

RUB

Cold with Tgas 20 000 K

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RUB

radicals ions + e- photons fields

Motivation:

APP in contact with tissues and liquids

free-standing or in aqueous solutions

Unique effect due to

combination of

several plasma

components

Radical effects

dominate

Plasma medicine, therapeutic plasma applications

PlasmaDerm® QuelleCINOGY,kINPen® MED

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Outline

Motivation








Detection of Ions




Conclusions

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Why mass spectrometry?

• Versatile diagnostic technique measuring mass (and energy) resolved ions at a detector

• Can be used to measure neutral species as well as ions, typically in their ground state

• Not limited e.g. by existence of reachable optical transitionsor quenching

• Measures densities at the wall - place of plasma treatment

• Can be absolutely calibrated (neutrals)

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Neutral mass spectrometry: signal calibration

electron impactionizer

quadrupole mass filter

detector(electron multiplier)

+

Si

ni,ionizer

ionizerielieionizeriii nEILmmTS ,)()()( =

mass filterAnd detector

ionizer detected species

e-

p

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Multiple stage differential pumping

Aligned orifices →

Molecular Beam (MB) formed

• Line-of-sight to ionizer

• Background pressure reduced

• Beam chopper for BG subtraction

Plasma Diagnostics with mass spectrometry: Molecular Beam Mass Spectrometry (MBMS)

iongeneration

extractionmass separation

and analysis

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Formation of molecular beam depends on Knudsen number K𝑛 =𝜆

𝑑

Kn > 1 Free molecular flow without collisions

Molecular Beam Sampling

• sheath non-collisional or weakly collisional for ions

→ directed movement

• non-collisional sampling in orifice and molecular

flow on MS side → no composition distortion

𝑛𝑏𝑒𝑎𝑚 𝑥 =1

4

𝑑

𝑥

2

𝑛0

+

Pla

sma

Große-Kreul et al., Eur. Phys. J. D 70 (2016) 103

Große-Kreul et al., Plasma Sources Sci. Technol. 24 (2015) 044008

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Kn 1 Free molecular flow without collisions

Molecular Beam Sampling

free jet

• sheath non-collisional or weakly collisional for ions

→ directed movement

• non-collisional sampling in orifice and molecular

flow on MS side → no composition distortion

• sheath and sampling collisional

→ pressure gradient in front of the orifice

• supersonic expansion behind the orifice

→ free jet

→ composition distortion

→ seeded molecular beam

Pla

sma

Große-Kreul et al., Eur. Phys. J. D 70 (2016) 103


𝑛𝑏𝑒𝑎𝑚 𝑥 =1

4

𝑑

𝑥

2

𝑛0

+

Pla

sma +

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• Radial diffusion in free-jet

• Mach-number focusing

Ionizer

p=1atmp

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• Radial diffusion in free-jet

• Mach-number focusing

Ionizer

p=1atmp

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MS calibration: effect of composition distortion

0 20 40 60 80 1000,0

0,2

0,4

0,6

0,8

1,0

norm

aliz

ed M

S s

ignal [a

.u.]

Air content [%]

He + 1%Ne

1% of Ne:

relative MS signal

Atomic O calibrated with Ne

O3 calibrated with N2O

The change of the gas mixture has to be considered

D. Ellerweg et al., New J. Physics 12 (2010) 013021Corrigendum: G. Willems et al., New J. Phys. 19 (2019) 059501

air + 1%Ne

Factor 4 difference!

Calibration gas should be measured

under identical conditions as the

species of interesst

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xqs

)1/(1

1

−

C

Mdxqs

Low pressure(“vacuum”)→ no collisions

with back-ground gas

Preferred case for the MB sampling from high pressure

(

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Molecular Beam Sampling: free jet to background gas

Compressible Navier-Stokes equations for Ar

pBG [Pa] xMach disk [mm]

100 2.110 6.7

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Typical MBMS system configuration

• Three pumping stages• Beam-to-background ~1• Chopper mounted in the 3rd stage

Used e.g. for analysis of Combustion processes

But:With 100 mm diameter sampling orifice and effective pumping in 1st stage of 50 l/s:

pBG,1st ~ 10 Pa !

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Outline

Motivation








Detection of Ions




Conclusions

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20 Hz

MBMS with special chopper design

Rotating chopper (14 Hz) with an imbedded skimmer

Chopper openChopper closed

Chopper separates 1st and 2nd stage – it closes effectively the connection to the

atmosphere → low pressures are achieved

Allows formation of MB for a short time, which expands to a god vacuum region

→ very high beam-to-BG ratio can be achieved

50 Pa

< 0.1 Pa

1 bar

< 0.1 Pa

Benedikt et al., J. Phys. D: Appl. Phys. 45 (2012) 403001

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20 Hz

1 bar

100 mm hole

MSin

vacuum

MBMS with special chopper design

Rotating chopper (14 Hz) with an imbedded skimmer

Chopper openChopper closed

Chopper separates 1st and 2nd stage – it closes effectively the connection to the

atmosphere → low pressures are achieved

Allows formation of MB for a short time, which expands to a god vacuum region

→ very high beam-to-BG ratio can be achieved

50 Pa

< 0.1 Pa

1 bar

< 0.1 Pa

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Atomic oxygen in He/O2 microplasma jet

Benedikt et al.,

Rev. Sci. Instrum. 2009, 80, 055107

1 bar

100 mm hole

MSin

vacuum

1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5

0

2

4

6

8

10

12

14

16

18

20

O

counts

time [ms]

He

x 1/500

0,0 0,2 0,4 0,6 0,8 1,0 1,20

5

10

15

20

25

30

O c

ou

nt ra

te /1

00

0 r

evo

lutio

ns

O2 admixture [%]

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Outline

Motivation








Detection of Ions




Conclusions

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Important issues in signal calibration

Signal scaling with:Electron emission currentIonization cross sectionEffective ionizer lengthIon extraction efficiency

Expansion with com-position distortions:Acceleration into orifice Radial diffusionMach-number focusing

Entrance 2nd pumping

stage

reactive

gas

mixture

at 1 atm.

Electron impact ionizer

Energy analyzer m/z analyzer

detector

Sampling from1 atmosphere

Molecular beam

+

Ion source cage

electronsource

Signal scaling with:Mass-dependent ion transmission function

𝑛𝑖𝑚𝑖𝑥𝑡𝑢𝑟𝑒 𝑛𝑖

𝑖𝑜𝑛𝑖𝑧𝑒𝑟

𝜙𝑖𝑖𝑜𝑛𝑖𝑧𝑒𝑟 𝑆𝑖

𝑑𝑒𝑡𝑒𝑐𝑡𝑜𝑟

Background subtraction with beam chopper not

shown

G. Willems et al., New J. Phys. 19 (2019) 059501

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stage

reactive

gas

mixture

at 1 atm.



detector


Molecular beam

+

Ion source cage

electronsource







shown 1) Composition

distortion important


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He + 1%Ne

Ne





stage

reactive

gas

mixture

at 1 atm.



detector


Molecular beam

+

Ion source cage

electronsource







shown

→ Including gas temperature effects

1) Composition



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He + 1%Ne

Ne





stage

reactive

gas

mixture

at 1 atm.



detector


Molecular beam

+

Ion source cage

electronsource







shown



2) MS settings important

1) Composition



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He + 1%Ne

Ne





stage

reactive

gas

mixture

at 1 atm.



detector


Molecular beam

+

Ion source cage

electronsource







shown



2) MS settings important

3) Background subtraction important → next slide

1) Composition



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Benedikt et al., (2021) 10.1088/1361-6595/abe4bf

Background subtraction

energy analyzer m/z analyzer

detector

reactive

gas

mixture

sampling2nd stage

samplingorifice

electron impact ionizer

molecularbeam

ion source cage

electron source

p0

p1 p2

pionizer

beamshutter

+

pump 1 pump 2


detector

reactive

gas

mixture

electron source

p0

p1 p2

pionizer

beamshutter

+

pump 1 pump 2

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Benedikt et al., (2021) 10.1088/1361-6595/abe4bf

Background subtraction


Krähling et al., Rev. Sci. Instrum. 83, 045114 (2012)


detector

reactive

gas

mixture

sampling2nd stage

samplingorifice

electron impact ionizer

molecularbeam

ion source cage

electron source

p0

p1 p2

pionizer

beamshutter

+

pump 1 pump 2


detector

reactive

gas

mixture

electron source

p0

p1 p2

pionizer

beamshutter

+

pump 1 pump 2

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Outline

Motivation








Detection of Ions




Conclusions

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Threshold Ionization Mass Spectrometry (TIMS)

(1) N + e → N+ + 2e Eel > 14,5 eV

(2) N2 + e→ N+ + N + 2e Eel > 24,3 eV

Energy Scan at mass 14

Dissociation of N2 has to be avoided.

(2):

(1) only:

(1) + (2):

Measurement of N atoms at mass 14

Measure N at electron energy in ionizer of 22 eV

Schneider et al., J. Phys. D 47 (2014) 505203

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Plasma analysis: N2 admixture – excited N2

11 12 13 14 15 16 17 18 19 20 21 22

1E-6

1E-5

1E-4

1E-3

0,01

0,1

1

MS

sig

na

l (a

.u.)

electron energy (eV)

mass 28 measured in

He/N2 gas

Mass 28 measured in He/N2 plasma

ionization of excited N2 molecules


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11 12 13 14 15 16 17 18

1E-5

1E-4

1E-3

0,01

0,1

1

MS

sig

na

l (a

.u.)

electron energy (eV)

mass 28 in He/N2 gas

scaled by 0.02

shifted by -2.2 eV (± 0.1eV)

N2 ground + N2 excited

→ IE at ~ 13.4 eV



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Excitation probably from

the 7th vibrational level…



Singlet delta oxygen

measurements:

Jiang et al., Plasma Sources

Sci. Technol. 29 (2020) 045023

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0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80.0

5.0x1013

1.0x1014

1.5x1014

2.0x1014

N d

ensity [cm

-3]

N2 Concentration [%]

N density

0.00

0.01

0.02

0.03

0.04

0.05

vibrationally excited N2

rela

tive

co

nce

ntr

atio

n o

f vib

ratio

na

lly e

xcite

d N

2 [

a.u

.]


Singlet delta oxygen

measurements:

Jiang et al., Plasma Sources

Sci. Technol. 29 (2020) 045023

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Outline

Motivation








Detection of Ions




Conclusions

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MS of ions: experimental setup: Schematic diagram

iongeneration

extraction andfocusing

massseparation and

analysis

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Results: Ion mass spectrum for He/N2 plasma1.4 slm He + 3.5 sccm N2, distance 2 mm

0 10 20 30 40 50 60 700

2

4

6

8

10

12

14

16

N+

4

N+

2

N+

3

NO+

sig

na

l (1

05 c

/s)

m/z

H2O

+

NO+ dominates due to its low ionization energy of 9.26 eV

→ not primary ions from plasma

mainly 2nd/3rd ion generation due to

fast ion-neutral CT collisions

+Q1

+Q2

Epot

I1I2

Neutral

(e.g. Ar, NO)

Ion

(e.g. He+)

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1 10

104

105

106

1 10

104

105

106

14:N+

42:N+3

19:H3O+

43:N4H+

28:N+2

54:?(H2O)3+

56:N+4

30:NO+

He flow 1.4 slm

N2 0.25%, Urms = 210V

He 5.0 + gas purifier + cold trap

MS

sig

nal [c

/s]

distance [mm]

29:N2H+

32:O+2

16:O+

17:HO+

He flow 1.4 slm

N2 0.214%, He 5.0

MS

sig

nal [c

/s]

18:H2O+

42:N+3

32:O+2

19:H3O+

57:N4H+

43:N3H+

28:N+2

56:N+4

30:NO+

12 13 14 15

16 17 18 19

20 21 22 23

24 25 26 27

28 29 30 31

32 33 34 35

36 37 38 39

40 41 42 43

44 45 46 47

48 49 50 51

52 53 54 55

56 57 58 59

60 61 62 63

64 65 66 67

68 69 70 71

72 73 74 75

76 77 78 79

80 81 82 83

84 85 86 87

88 89 90 91

92 93 94 95

96 97 98 99

100

29:N2H+

Ion masses:

Results: Ion mass spectrum for He/N2 plasma1.4 slm He + 3.5 sccm N2, distance variation

distance [mm]

0 10 20 30 40 50 60 700

2

4

6

8

10

12

14

16

N+

4

N+

2

N+

3

NO+

sig

na

l (1

05 c

/s)

m/z

H2O

+

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1 10

104

105

106

1 10

104

105

106

14:N+

42:N+3

19:H3O+

43:N4H+

28:N+2

54:?(H2O)3+

56:N+4

30:NO+

He flow 1.4 slm

N2 0.25%, Urms = 210V

He 5.0 + gas purifier + cold trap

MS

sig

nal [c

/s]

distance [mm]

29:N2H+

32:O+2

16:O+

17:HO+

He flow 1.4 slm

N2 0.214%, He 5.0

MS

sig

nal [c

/s]

18:H2O+

42:N+3

32:O+2

19:H3O+

57:N4H+

43:N3H+

28:N+2

56:N+4

30:NO+

12 13 14 15

16 17 18 19

20 21 22 23

24 25 26 27

28 29 30 31

32 33 34 35

36 37 38 39

40 41 42 43

44 45 46 47

48 49 50 51

52 53 54 55

56 57 58 59

60 61 62 63

64 65 66 67

68 69 70 71

72 73 74 75

76 77 78 79

80 81 82 83

84 85 86 87

88 89 90 91

92 93 94 95

96 97 98 99

100

29:N2H+

Ion masses:

distance [mm]

Effect of gas purity

on ions

0 10 20 30 40 50 60 700

2

4

6

8

10

12

14

16

N+

4

N+

2

N+

3

NO+

sig

na

l (1

05 c

/s)

m/z

H2O

+

Results: Ion mass spectrum for He/N2 plasma1.4 slm He + 3.5 sccm N2, distance variation

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MS of Ions: ion energyMass dependence of ion energy

-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.00.0

0.2

0.4

0.6

0.8

1.0

0 5 10 15 20 25 30 35 40 45 50 55 60-2.0

-1.5

-1.0

-0.5

0.0

no

rma

lize

d s

ign

al (a

.u.)

bessel box parameter 'energy' (V)

H2O

+ (18)

N+

2 (28)

N+

3 (42)

N+

4 (56)

peak position

linear fit

'en

erg

y' (

V)

mass (amu)

~ 0.8 eV

Velocity in the beam the same for all

→ mass-dependent terminal energy!

Important by relative

measurements of different

ions!

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MS of ions: ion trajectory simulationIon trajectory calculation for ion energy and ion denisty determination

-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.00.0

0.2

0.4

0.6

0.8

1.0

no

rma

lize

d tra

nsm

issio

n (

a.u

.)Bessel box parameter 'energy' (V)

experiment

simulation

Ion density in front of the sampling orifice can be reasonably estimated

DSMC Foam simulation

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Outline

Motivation








Detection of Ions




Conclusions

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Phase Resolved OES

He (l/min)+

Molecular gas (

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MS results example: neutrals in He/H2O plasma

Willems et al., J. Phys. D: Appl. Phys. 50 (2017) 335204

MSHe/0.6%O2

He/0.3%N2

He/H2O

O, O3

N

He/N2/O2NO, N2O

NO2, N, O

OH, H2O2 O2H • Many reactive and stable species

measurable

• Distance variation → recombinationchemistry

Schneider et al., J. Phys. D 47 (2014) 505203

Willems et al., J. Phys. D: Appl. Phys. 50 (2017) 335204

Ellerweg et al., New J. Physics 12 (2010) 013021Corrigendum: G. Willems et al., New J. Phys. 19 (2019) 059501

Douat et al., Plasma Sources Sci. Technol. 25 (2016) 025027

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MS result example: neutrals in He/O2 plasma

0 5 10 15 20 25 30 35 40 45 500

1x1015

2x1015

3x1015

4x1015

O (MBMS)

O3 (MBMS)

O (TALIF)

density [cm

-3]

distance from the jet nozzle [mm]

O

O3

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40

1x1015

2x1015

3x1015

4x1015

O (MBMS)

O (TALIF)

O3 (MBMS)

density [cm

-3]

O2 admixture [%]

O

O3

Used for treatment of:

biomolecules bacteria, spores liquids solids

GAPDH b. subtilisCu → CuO

+ H2O

Phenol oxidation

D. Ellerweg et al., New J. Physics 12 (2010) 013021

Corrigendum: G. Willems et al., New J. Phys. 19

(2019) 059501

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MS result example: neutrals in He/CO2 plasma

overall reaction: 2CO2 → 2CO + O2

A. Bogaerts et al., Faraday Discuss. 183 (2015) 217

CO2 + e → CO2(v) → CO + O

CO2(v) + O → CO + O2

Plasma energetically more efficient than a thermal process (50% efficiency)

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MS results: neutrals in He/CO2 plasma

He/CO2

0 2000 4000 6000 8000 100000.0

5.0x1014

1.0x1015

1.5x1015

2.0x1015

2.5x1015

density [cm

-3]

CO2 in He [ppm]

0.0

0.5

1.0

1.5

2.0

O2 absorb

ed p

ow

er

[W]

CO

power


The CO/O2 ratio much higher than two?!

Willems et al., Plasma Phys. Control. Fusion 62 (2020) 034005

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MS results: neutrals in He/CO2 plasma


Additional Oxygen species: O atoms and O3

0 2000 4000 6000 8000 100001012

1013

1014

1015

1016

O2

O

density [cm

-3]

CO2 in He [ppm]

CO

O3

𝑛𝑂 > 𝑛𝑂2 !

→ vibrational excitation ineffective in this type of plasma (below detection limit)

He/CO2


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m-APPJ in He/CO2 gas mixture

• Max energy efficiency ~ 14%

• Max conversion efficiency ~ 5 %

→ confirms low efficiency

0 2000 4000 6000 8000 100001012

1013

1014

1015

1016

O2

O

de

nsity [cm

-3]

CO2 in He [ppm]

CO

O3


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Positive ions


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Positive ions


„primary“ ions and secondary ion clusters

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Outline

Motivation








Detection of Ions




Conclusions

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8 kHz plasma needle

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Ion mass spectra in 8 kHz plasma needle

0 10 20 30 40 50 60 70 80 90 100

0.01

0.1

1

10

sig

nal (1

05 c

/s)

mass [amu]

neg ions

pos ions

8 kHz pulsed Plasma needle

N2+ and O2

+ dominant

He+, He2+(N2+), N+, O+

ions detected in the

streamer head

O- the lightest

negative ion

Mass 48 (O3-) dominant

Many negative clusters

present

He+

He2+

(N2+)

N+

O+

N2+

O2+

O2-

O-

H2O+

OH-

O3-

(H2O)O3-

CO3OH-

(H2O)2O3-

CO3-

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Mass dependent delay due to

mass-dependent drift time in

the mass spectrometer

0 20 40 60 80 100 1200

20

40

60

80

100

120

140

negative ions

tim

e (

µs)

mass (amu)

mass dependence of mean ion drift time

positive ions

102

103

104

105

106

0 20 40 60 80 100 120 140 160102

103

104

105

106

sig

nal (c

/s) 4 8

14

18

28

32

37

pos. ions

neg. ions

sig

nal (c

/s)

time (µs)

16

32

48

60

66

77

109

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102

103

104

105

106

0 20 40 60 80 100 120 140 160102

103

104

105

106

sig

nal (c

/s) 4 8

14

18

28

32

37

pos. ions

neg. ions

sig

nal (c

/s)

time (µs)

16

32

48

60

66

77

109

-30 -20 -10 0 10 20 30 40 50

0

2000

4000

6000

8000

10000

12000

14000

16000

O2-

sig

na

l [c

ps]

time [ms]

O2+

-30 -20 -10 0 10 20 30 40 50

0

2000

4000

6000

8000

10000

12000

14000

16000

O-

sig

na

l [c

ps]

time [ms]

O+

• broader then the current pulse

• complex temporal behavior

Validation of plasma chemistry models?

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Acknowledgements:

Plasmadecon and Placid

DFG PROJECT PACKAGES

ConclusionsMass Spectrometry at one atmosphere

• TI MBMS allows absolutely calibrated measurements

of reactive and stable species densities:

→ N, O, OH, O2H, NOx→ H2O2, O3, O2, N2, CO2, CO, HMDSO…

• Relative measurement of vibrationally excited species: N2(v), 1O2

• Ion measurements: mainly ions resulting from CT reactions, clusters

MS analysis of Non-equilibrium APPs

• APPs are effective source of reactive species

• Study of plasma-chemical processes for solar fuels → CO2 conversion

• Time-resolved ion measurements provide insights in plasma dynamics

and ion chemistry

1 bar

100 mm hole

MSin

vacuum

Mass Spectrometry for Atmospheric Pressure Plasmas2021/02/11 · 3/57 J. Benedikt, MS for APP,...

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