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PTR-TOF-MS: A New Instrument For Real-Time Analysis Of Multi-Component Systems: Applications To Food Analysis. Caroline Lamont-Smith, Steve Mullock & Fraser Reich Kore Technology Ltd. Ely, Cambs. Rob Linforth, Annie Blisset, Andy Taylor Dept. of Food Science University of Nottingham.

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PTR-TOF-MS: A New Instrument For Real-Time Analysis Of Multi-Component Systems: Applications To Food Analysis


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slide1

PTR-TOF-MS: A New Instrument For Real-Time Analysis Of Multi-Component Systems: Applications To Food Analysis

Caroline Lamont-Smith, Steve Mullock & Fraser Reich Kore Technology Ltd. Ely, Cambs

Rob Linforth, Annie Blisset, Andy Taylor Dept. of Food ScienceUniversity of Nottingham

slide2

Talk: Outline

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.

  • Overview of the Proton Transfer Reactor method
  • Why a PTR-TOF-MS?
  • The Instrument
  • Some Example Data
  • Summary
slide3

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
slide4

PTR: Proton Transfer Reactions With H3O+

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

slide5

Linearity of Response

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

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

0-50ppm H2S calibration:

Raw data plot

H2S proton affinity= 709kJ/mol

slide6

Soft Ionisation vs. Electron Impact Ionisation

Significant fragmentation to lower masses

Molecular Ion appears at mass 116 m/z

70eV EI spectrum of Ethyl Butyrate

slide7

Soft Ionisation vs. Electron Impact Ionisation

PTR-TOF-MS Spectrum of 5ppm Ethyl Butyrate

Molecular Ion at 117 m/z

(molecular ion + proton)

Very little fragmentation

slide8

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)
slide9

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
slide10

The PTR Reactor and the E/n Ratio

Tri-ethyl phosphate, protonated mass 182

130 Townsend

260 Townsend

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

PO4 + proton

slide11

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
slide12

Why Use A TOF-MS?

Mass spectrum from breath (U. Nijmegan)

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

slide13

TOF-MS: Accurate Mass Capability

33.04 m/zCH3OH2+

30.00 m/zNO+

36.04 m/z(H2O)2+

31.99 m/zO2+

slide14

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
slide15

TOF MS: Orthogonal Pulsing TOF Source

ContinuousSample

TOF Source

Ions

Extracted

Detector

Light Ions

Heavy Ions

0

Volts

Source Off

-2kV

t

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 ~20-50s, therefore ‘cycle time’ = 20-50s, so typical pulsing frequency = 20-50 kHz

Extractor pulses to –400v

Source On

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

slide16

TOF MS: Orthogonal Pulsing TOF Source

Overview schematic of the instrument: TOF source to detector

slide17

TOF MS: Orthogonal Pulsing TOF Source

GD Source2mbar

Overview schematic of the instrument

PTR Reactor 1mbar

Transfer Optics 10-4 mbar

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

slide18

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

slide19

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.
slide20

Breath-by-Breath Analysis: Release of Ethyl Butyrate

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

Signal intensity

Time

Release of flavours during mastication

  • 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
slide21

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

Reconstruction of Acetone signal with 1 second integration time

Breathing rate ~5/min

Reconstruction of ethyl butyrate signal with 1 second integration time

slide22

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

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

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

slide23

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

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

slide24

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

slide25

Full Mass Spectra From 125ms Time Slices

34.50 –34.625 seconds

Acetone and Ethyl butyrate

Logscale data

34.00 –34.125 seconds

slide27

3-Hexenol vs. Hexanal, Both C6H12O

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.

Hexanal: protonated masses at 101, 83, 55

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

slide28

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?

83

101

Hexanal

3 Hexenol

83

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!

slide29

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