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Introduction Conclusions

Methods

Valerie Derpmann1; Hannah Sonderfeld2; Iustinian Bejan1; Hendrik Kersten1; Jörg Kleffmann1; Ralf Koppmann2; Thorsten Benter1

1Physical & Theoretical Chemistry Institute for Pure and Applied Mass Spectrometry

2 Physics DepartmentWuppertal, Germany

Product study of the photo-oxidation of3-methyl-2-nitrophenolMixing ratio: ~260 ppbVReaction time: ~ 22 min

Experimental Setup

Acknowledgement

Literature1) Mayhew, C.A.; Sulzer, P.; Petersson, F.; Haidacher, S.; Jordan, A.; Märk, L.;

Watts, P.; Märk, T. D. Applications of proton transfer reaction time-of-flight mass spectrometry for the sensitive and rapid real-time detection of solid high explosives, Int. J. Mass Spectrom. 2010, 289, 58-63.

2) Kersten, H.; Development of an Atmospheric Pressure Ionization source for in situ monitoring of degradation products of atmospherically relevant volatile organic compounds, Dissertation, University of Wuppertal 2011.

3) Song, L; Wellman, A. D.; Yao, H.; Bartmess, J. E. Negative Ion-Atmospheric Pressure Photoionization: Electron Capture, Dissociative Electron Capture, Proton Transfer and Anion Attachement, J. Am. Soc. Mass Spectorm. 2007, 18, 1789-1798.

4) Ewing, R.G.; Atkinson, D.A.; Eiceman, G.A.; Ewing, G. J. A critical review of ion mobiliy spectrometry for detection of explosives and explosive related compounds, Talanta 2001, 54, 515-529.

5) Bejan, I.; Abd El Aal, Y.; Barnes, I.; Benter, T.; Bohn, B.; Wiesen, P.; Kleffmann, J. The photolysis of ortho-nitrophenols: a new gas phase source of HONO, Phys. Chem. Chem. Phys. 2006, 8, 2028-2035.

6) Harrison, M.A.J.; Barra, B.; Borghesi, D.; Vione, D.; Arsene, C.; Olariu, R.I. Nitrated phenols in the atmosphere: a review, Atmos. Environ. 2005, 39, 231-248.

Figure 1) Schematic of the PAPI ion source; ionization occurs inside the transfer capillary

• All three ionization methods were capable of detecting nitro-aromatic compounds efficiently

• Signal pattern (amount of fragmentation/ion transformation processes) is different:PAPI→ Generally only one signal for each

compound is observed→ [M-H]- for compounds with acidic proton,

otherwise [M]-

→ O2- adduct/dimer formation in rare cases

PTR→ [M+H]+ formation→ Fragmentation observed, particularly for

hydroxy-nitro-species

DA-APPI→ Main signals are M+/[M+H]+ in positive

mode; M-/[M-H]- in negative mode → Very complex mass spectra due to the

use of a dopant

• PTR-TOF-MS shows a very good linearity over a wide concentration range. Linearity ranges of FT-IR (high conc.), PTR-TOF-MS (large conc. range), and PAPI (low conc. range) is of benefit for atmospheric chemistry studies

• The estimated detection limits of PAPI are in the pptV-range

• The low background noise level of PAPI mass spectra allows for detection of minor product signals

• Optimum performance is obtained through parallel PTR and PAPI MS measurements, resulting in virtually orthogonal structural product information (proton affinity/gas phase basicity vs. electron affinity/gas phase acidity

Comparison of PAPI, PTR and DA-APPI Mass Spectra

Figure 2) Schematic of the PTR-MS ion source

Highly Efficient Ionization of Nitro-aromatic Compounds using Photoelectron Induced Atmospheric Pressure Ionization (PAPI)

PAPI

PAPI MS: Bruker Daltonics esquire6000

quadrupole ion trapIon Source: Quartz transfer capillary; internally

partially coated with silverRadiation Source: PenRay low pressure Mercury

UV lamp (254 nm)PTR-MS

MS and Ion Source: Ionicon PTR-HRTOF-MS

DA-APPIMS: Bruker Daltonics esquire6000

quadrupole ion trapIon Source: Home built VUV spark discharge

lamp operated with Ar. Mounted directly onto a transfer capillary

Dopant: Toluene

Flow Tube SystemReaction Chamber: 1.2 l glass tubeRadiation Source: 6 lamps (280-550 nm); mounted

around the reaction tube Concentration measurement: FT-IR (Nicolet NEXUS) with long path

absorption White optics (32.8 m)

PTR APPI on capillaryLinearity of PAPI and PTR

Financial support is gratefully acknowledged:• VD: Graduate Student Research stipend, University of

Wuppertal• VD: German Academic Exchange Service (DAAD)• German Research Foundation (DFG) within project

POXSA (BE 2124/4-1) and KL1392/2-1• Bruker Daltonics GmbH, Bremen, Germany

Analyte:3-Methyl-2-NitrophenolFor mass spectra see section“Photolysis Study”

Limits of Detection (3σ):PTR-MS: < 250 ppt (30 s)PAPI: < 1 ppb (<1s) Figure 4) Concentration dependency of the signal intensity

for PAPI-MS and PTR-MS

2-Nitrophenol

Molar Mass: 139 g/mol

2-Nitro-Isopropyl-Benzene

Molar Mass: 165 g/mol

Figure 5) PAPI Extracted Ion Chromatograms of reactant and product mass signals

Figure 6) PAPI mass spectrum after photo-oxidation started. Insert: Spectrum before photo-oxidation started.

Figure 7) PTR-TOF mass spectrum after photo-oxidation started. Insert: Spectrum before photo-oxidation started.

hνOH•

Several nitro-aromatic compounds were analyzed.Two examples are shown, representing: • OH-containing species• Alkylated nitro-

compounds

PAPI

Inte

nsity

[a.u

.]

m/z

PAPI

Inte

nsity

[a.u

.]

M-

165

m/z

DA-APPI – negative mode[M-H]-

138

170

89

m/z

m/z

DA-APPI – positive mode DA-APPI – positive mode

DA-APPI – negative mode

[M+H]+

166

5177 89

121156

183200 243

PTR-TOF-MSPTR-TOF-MS

[M-H]-

138

[M+H]+

140

195

38 212

M-

165In

tens

ity[a

.u.]

Inte

nsity

[a.u

.]

m/z

m/z

Inte

nsity

[a.u

.]

Inte

nsity

[a.u

.]

Inte

nsity

[a.u

.]

Inte

nsity

[a.u

.]

m/z

40 60 80 100 120 140 160 180 200 Time [min]0.0

0.5

1.0

1.5

2.0

4x10Intens.

Lamps onLamps off Lamps off

Reactant: 152 m/zProducts: 137 m/z

167 m/z

128

m/z

Inte

nsity

[a.u

.] 50 100 150 200 250 3000

1

2

3

4

5

m/z

Inte

nsity

[a.u

.]

Analyte SpectrumPhoto-oxidation Spectrum [M-H]-

152

[M+O2]-

185 [M+M-H]-

305

137 167

Analyte SpectrumPhoto-oxidation Spectrum

137 g/mol 167 g/mol

Figure 3) Schematic of the VUV spark discharge lamp mounted onto the transfer capillary

Nitro-aromatic compounds (NACs) are important todifferent research fields, e.g., the detection of explo-sives or in atmospheric chemistry. NACs are ubiquitousin the urban atmosphere, being well known for theirphytotoxic and mutagenic properties. Widely usedanalytical approaches for NAC measurements are IonMobility Spectrometry (IMS) and Mass Spectrometry,equipped with various ionization methods, e.g., PTR(Proton Transfer Reaction) and DA-APPI (DopantAssisted Atmospheric Pressure Photo Ionization). Thesemethods are not selective towards NACs. Severalapproaches are known to increase the selectivity ofthese methods, but are not yet widely used. Most ofthem increase the complexity of the setup and/or thereaction chain necessary for detection.In this work we present a comparison between threeionization methods for the detection of several NACs.These are PTR, a modified APPI design and a newlydeveloped Photoelectron Induced Atmospheric Pres-sure Ionization (PAPI) stage.

PTR-TOF-MSPTR-TOF-MS has gained importance in recent years(e.g., atmospheric chemistry, clinical studies). In PTR-MS ionization takes place inside a reaction drift tubeby proton transfer. The reagent gas is protonatedwater, which is generated from water vapor flowingthrough a hollow cathode discharge. Only positivelycharged reagent ions are released to the drift tube.

DA-APPIThe VUV light for photoionization is provided by ahome built spark discharge lamp mounted directly onthe transfer capillary. This setup has some benefits overthe usual setup, e.g., a reduced reaction time for iontransformation processes and the availability of VUVlight with wavelengths below 105 nm (the LiF-cutoff)for ionization due to the windowless lamp design.See also Session MP01; Poster #017The VUV-light produces positive ions and electrons.The latter are rapidly captured to yield negative ions.Therefore, mass spectra are obtained in both the posi-tive and negative modes. Frequently a dopant (e.g.,toluene) is used to increase the concentration ofcharge carrying species (ions as well as electrons), butthis approach renders the recorded mass spectramuch more complex.

PAPIThe interaction of UV-light with metal surfaces (e.g.,silver) produces photo-electrons. In air they are swiftlycaptured by oxygen in less than microseconds. O2

-

can then react with the analyte by charge or protontransfer, depending on the electron affinity and gasphase acidity, respectively. Ionization can take placedirectly inside the ion transfer capillary. Majoradvantages of this approach are:

• exclusively negative ions are generated• soft ionization with very little fragmentation• high selectivity leading to high S/N-ratios• short reaction times (< 1ms)

See also Session MP01; Poster #004

m/z

Photo-oxidation Study

[M+H]+

166

106

[M+H]+

140

78

Inte

nsity

[a.u

.]

m/z

138 168

[M+H-H2O]+

122

m/z

Inte

nsity

[a.u

.]

[M+H]+

154

[M+H-H2O]+

136

92

Quartztransfer capillary

Mercury UV-lamp(λ = 254 nm)

Silver coatingAnalyte

gas stream

Transfer capillary

HV

Gas in/out

Cannulas(blunted/bent)

Aperture(0.1 mm2)

Dischargechannel(0.8 mm)

Glasscover

92

Analyte gas stream

Watervaporinlet

Hollowcathode

Drift tube(approx. 3 mbar)

To massanalyzer

92 126

149

183

154

188 293

Mixing ratio (FT-IR) [ppmV]

Inte

nsity

[nor

ma

lized

coun

ts/s

]

C7H7NO2???

Analyte gas

stream

PTR154 m/z

PAPI

Synth. air

Analyte gas stream

Synth. air PTR-TOF-MS

PAPI

Lamps

Photo reactor(glass tube)