PTR-TOF-MS: A New Instrument For Real-Time Analysis Of Multi-Component

Systems: Applications To Food Analysis

Rob Linforth, Annie Blisset, Andy Taylor Dept. of Food Science

University of Nottingham

Caroline Lamont-Smith, Steve Mullock & Fraser Reich

Kore Technology Ltd. Ely, Cambs

Talk: Outline

• Overview of the Proton Transfer Reactor method

• Why a PTR-TOF-MS?• The Instrument• Some Example Data• Summary

First a confession:

This talk is heavy on the instrument description, its properties and possibilities of the instrument, because the speaker is one of the team that designed the instrument.

PTR: Proton Transfer Reactions

Essentially, PTR is a subset of Chemical Ionisation. It was defined and refined by Werner Lindinger in the 1990’s, and resulted in the first commercially available instrument (Ionicon)

• Aim is to achieve ‘Soft’ ionisation so as not to fragment the molecule(s) of interest: less fragments = less mass spectral ‘noise’.

• In positive ion mode, most frequently used reaction is:

H3O+ + R H2O + RH+

• For this protonation reaction to occur, the proton affinity of R must be greater than that of water

• The ionisation probability is almost the same as the collision cross-section, so the aim is to induce sufficient collisions to ensure efficient analyte ionisation

PTR: Proton Transfer Reactions With H3O+

Compound PA (kJ/mol)

Oxygen 421

Nitrogen 493

Carbon Dioxide 541

Sulphur Dioxide 672

Water 691

Hydrogen Sulphide 709

Benzene 750

Methanol 754

Toluene 784

Naphthalene 803

Ethyl Acetate 836

Tri ethyl phosphate 909

Compounds above water in the proton affinity table will not be ionised, whereas compounds below will

The good news is that most VOCs have higher proton affinities than water and will receive a proton from H3O+

Generally, larger molecules have larger proton affinities

Linearity of Response

Plot of H2S Peak Yield vs. H2S Concentration (H2s in Air)

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Calibrated H2S Concentration(ppm)

H2S

Peak Y

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(x50)

Series1

Linear (Series1)

Provided that [RH+]<< [H3O+], the H3O+ signal does not change with analyte concentration, and the detected analyte intensity is linear with concentration:

[RH+] = [H3O+]0(1-e-k(R)t) [H3O+]0 [R] kt

0-50ppm H2S calibration:

Raw data plot

(k is rate constant for the reaction, and t is time to travel through the drift tube)

H2S proton affinity= 709kJ/mol

Soft Ionisation vs. Electron Impact Ionisation

70eV EI spectrum of Ethyl Butyrate

Molecular Ion appears at mass 116 m/z

Significant fragmentation to lower masses

Soft Ionisation vs. Electron Impact Ionisation

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Mass (m/z)ce6.1, 15ml.min, ethylbuty, 2.0mbptr

PTR-TOF-MS Spectrum of 5ppm Ethyl Butyrate

Molecular Ion at 117 m/z

(molecular ion + proton)

Very little fragmentation

Charge Transfer In The PTR Reactor

• Need to induce sufficient collisions: H3O+ and analyte mixed in a reactor tube (drift cell)

• Voltage gradient across reactor

• Pressure ~ 1 mbar, approximately 2000 collisions down 10cm length reactor

• Aim is to have dilute analyte, so that analyte molecules do not collide with anything except H3O+ . This keeps the ionisation scheme and calibration simple, unlike in Ion Mobility Spectrometry (complex inter-analyte charge transfer reactions)

The PTR Reactor and the E/n Ratio

• E is the voltage gradient down the reactor in V/cm

• n is the gas density in molecules / cc (1mbar in reactor, typically)

• E/n in Townsend; 1 Townsend = 10-17 cm2 / Vs

• Lindinger et al. (1998) showed that 120-140 T is ideal for non-fragmentation of organic molecules

• If E/n too high, fragmentation of molecule occurs

• If E/n too low, clustering of water molecules occurs and can present a problem for certain species

Reagent gas in

Analyte in

GD

SD

PTR

GD Cathode

GD Anode

Transfer Optics

The PTR Reactor and the E/n Ratio

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Mass (m/z)TEP mass 182

Tri-ethyl phosphate, protonated mass 182

130 Townsend

195 Townsend – loss of the ethyl groups (28 mass units)

260 Townsend

PO4 + proton

Why Use A TOF-MS?

• Parallel detection means no analytical price to pay for monitoring many species, unlike a sequential analyser such as a quadrupole or magnetic sector

• TOF Cycle frequency typically 20-50 kHz = high data rate

• Full mass spectra to several hundred masses with 4-5 orders dynamic range in one second

• Possibility of ‘real time’ data analysis with < 100ms resolution, multiple species and sensible counting statistics

• Ability to interrogate a data set ‘retrospectively’: intensity of any species as a function of time, mass spectra for a variety of ‘time slices’

• Parallel detection and full mass spectra more suitable to software data reduction techniques, e.g. principle component analysis

Why Use A TOF-MS?

This mass spectral plot, acquired with a PTR-Quad-MS (a sequential scanning device) took 40 seconds

A full mass spectrum to several hundred m/z can be acquired in less than a second with a PTR-TOF-MS

Mass spectrum from breath (U. Nijmegan)

TOF-MS: Accurate Mass Capability

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Daltons

30.00 m/zNO+

31.99 m/zO2

+

33.04 m/zCH3OH2

+ 36.04 m/z(H2O)2

+

Downside Of A TOF-MS?

• Main drawback is analysis of a continuous analyte stream: When the TOF cycle has started, no further ions can be injected without resulting in multiple overlapping spectra

• ‘Duty cycle’ describes the % of the analyte stream that is sampled

• In an orthogonal TOF (analyte stream enters TOF source at 90°), the duty cycle can reach 10%: when the Kore TOF source pulses, it ‘empties out’ ~5-10 microseconds worth of ions. Usage is therefore 5µs in 50µs, I.e. 10%

• Currently we are observing up to 500kc/s of Hydronium at the detector, approximately half that reported by the PTR-Quad (single ion mode).

• Prospect of improving duty cycle using ‘Hadamard’ methods – high frequency random pulsing of source with mathematical deconvolution of spectra - still not really proven as a workable solution

TOF MS: Orthogonal Pulsing TOF Source

ContinuousSample

DetectorIonsExtracted

Light IonsHeavy

Ions

Ions of different masses within a single ‘cycle’ arrive at the detector at different times according to the relation:

K.E. = mv2/2

Ions with m/z = 1000 has flight time ~20-50s, therefore ‘cycle time’ = 20-50s, so typical pulsing frequency = 20-50 kHz

Volts Source Off

t

Source On

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-2kV

Extractor pulses to –400v

TOF Source

Variant on ‘Wiley – Maclaren’ source. Compensation for ion position within source

TOF MS: Orthogonal Pulsing TOF Source

Overview schematic of the instrument: TOF source to detector

Reflectron

Blanker

Blanking

Field Free Region

Detector

Pre-

DeflectorsTOF Source

Transfer Optics

PTR Source

Extract

Pulser

Pulser

Amplifier

TOF MS: Orthogonal Pulsing TOF Source

Overview schematic of the instrument

GD Source2mbar

PTR Reactor 1mbar

Transfer Optics 10-4 mbar

Mass spectrometer and detector 10-6 - 10-7 mbar

Mass Spectral Analysis: Applications to Food Analysis

University of NottinghamDepartment of Food Science

Professor Andy TaylorDr. Rob LinforthAnnie Blisset

• “Breath-by-breath” Research

• Main technique in lab: APCI

• Wanted to add PTR-TOF-MS

• Instrument recently delivered; has both PTR and APCI functionality

Data acquired at Nottingham by one of us (FR) with Rob Linforth and Annie Blisset

Breath-by-Breath Analysis Of Juicy Fruit Gum

The principle compound released during chewing of Juicy Fruit gum is ethyl butyrate, molecular mass 116

• Soft tube inserted lightly into the nostril of subject

• Breathing normally, the mass spectrometer begins acquiring data.

• Most of the exhaled breath passes out into open space, but a side-mounted capillary pipe samples the breath into the proton transfer reactor.

Breath-by-Breath Analysis: Release of Ethyl Butyrate

Idealised appearance of breath markers, such as acetone, as exhalation occurs

Release of flavours during mastication

Sig

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Time

• First data set: take ‘raw data set’ (all ion flight times recorded as a function of elapsed experiment time)

• Integrate the ethyl butyrate protonated ion (mass 117) every second

Breath-by-Breath Analysis At Different Data ‘Granularity’

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Signal for Mass 117

Time in Seconds1 second integration

Reconstruction of ethyl butyrate signal with 1 second integration time

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Intensity

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Reconstruction of Acetone signal with 1 second integration time

Breathing rate ~5/min

Breath-by-Breath Analysis At Different Data ‘Granularity’

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Signal for Mass 117

Time in Seconds0.25 second integration

Reconstruction of ethyl butyrate signal with 0.25 second integration time- further structure emerging still

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Signal for Mass 117

Time in Seconds0.5 second integration

Reconstruction of ethyl butyrate signal with 0.5 second integration time- further structure emerging

Breath-by-Breath Analysis At Different Data ‘Granularity’

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Signal for Mass 117

Time in Seconds0.125 second integration

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Signal for Mass 117

Time in Seconds0.0625 second integration

Reconstruction of ethyl butyrate signal with 0.125 second integration time

Reconstruction of ethyl butyrate signal with 0.0625 second integration time: no further structure emerging

Breath-by-Breath Analysis: Different Person

Reconstruction of Acetone signal with 0.125 second integration time

Note different breathing rate ~10/min for different person

Overlay of ethyl butyrate signal with acetone

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Signal

Time (seconds)Mass 59

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Signal

Time (seconds)Mass 59 and 117

Full Mass Spectra From 125ms Time Slices

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34.00 –34.125 seconds

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Logs

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dat

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Acetone and Ethyl butyrate

Mass Spectrum From Headspace of Freshly Macerated Tomatoes

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3 hexenol 3 hexenal

3-Hexenol vs. Hexanal, Both C6H12O

Hexanal: protonated masses at 101, 83, 55

3-Hexenol: protonated masses at 101, 83, 59 and 55

In previous slide, masses 101 and 83 were identified as 3-Hexenol, but in truth there is the possibility that 101 and 83 can be due to Hexanal.

3-Hexenol vs. Hexanal, Both C6H12O

Is it possible to find an E/n value that will give a mass 101 for one compound but not the other?

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Hexanal 3 Hexenol83

83101

101

No ‘threshold phenomenon’ observedAlso, no E/n found for 3-Hexenol at which intensity of 101>83

Possibility of modulating PTR voltage and using software tools for compound differentiation? More work required, clearly!

Summary

• PTR-MS instruments based on quadrupole mass spectrometers are now widely used in studies of environmental and atmospheric chemistry as well as food and medical applications

• A TOF-MS increases the possibilities of real-time analysis down to <100ms “data granularity”, with full mass spectral acquisition in each time slice

• A TOF-MS permits any ion chromatogram to be re-constructed from the data set

• A TOF-MS can be operated at higher mass resolutions than a quadrupole mass spectrometer. Even at relatively low mass resolution the mass accuracy is better than 30 millimass units

• Greater mass range capability, with no discrimination against high masses up to several hundred Daltons