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
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
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:
H 3 O+ + 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
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
Plot of H2S Peak Yield vs. H2S Concentration (H2s in Air)
(k is rate constant for the
reaction, and t is time to travel
through the drift tube)
500
0-50ppm H2S
calibration:
450
400
H2S Peak Yield (x50)
350
Raw data plot
300
Series1
250
Linear (Series1)
200
150
H2S proton affinity=
709kJ/mol
100
50
0
0
10
20
30
Calibrated H2S Concentration(ppm)
40
50
60
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
Soft Ionisation vs. Electron Impact Ionisation
Counts
5000
PTR-TOF-MS Spectrum of
5ppm Ethyl Butyrate
4000
Molecular Ion
at 117 m/z
3000
(molecular ion
+ proton)
2000
1000
Very little fragmentation
0
60
70
ce6.1, 15ml.min, ethylbuty, 2.0mbptr
80
90
100
110
Mass (m/z)
120
130
140
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
Analyte in
reactor in V/cm
Reagent gas in
GD Cathode
GD Anode
GD
SD
PTR
• 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 nonfragmentation of organic molecules
• If E/n too high, fragmentation of
molecule occurs
Transfer Optics
• If E/n too low, clustering of water
molecules occurs and can present a
problem for certain species
The PTR Reactor and the E/n Ratio
Counts
Tri-ethyl
phosphate,
protonated
mass 182
60000
130 Townsend
50000
40000
30000
20000
10000
Counts
60000
0
100
120
140
Mass (m/z)
160
180
50000
40000
30000
20000
Counts
10000
120000
0
260 Townsend
100000
100
80000
40000
0
100
120
140
Mass (m/z)
160
160
180
ethyl groups (28 mass units)
20000
TEP mass 182
140
Mass (m/z)
195 Townsend – loss of the
PO4 +
proton
60000
120
180
200
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?
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
TOF-MS: Accurate Mass Capability
30.00 m/z
NO+
31.99 m/z
O2+
Counts
33.04 m/z
CH3OH2+
36.04 m/z
(H2O)2+
1500
1000
500
0
29
30
31
32
33
Daltons
34
35
36
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
Continuous
Sample
TOF Source
Ions
Extracted
0
Volts
Source Off
-2kV
Extractor
pulses to
–400v
t
Source On
Detector
Light
Ions
Heavy
Ions
Ions of different masses within a single ‘cycle’
arrive at the detector at different times
according to the relation:
K.E. = mv2/2
Variant on ‘Wiley –
Maclaren’ source.
Compensation for ion
position within source
Ions with m/z = 1000 has flight time ~20-50s,
therefore ‘cycle time’ = 20-50s, so typical
pulsing frequency = 20-50 kHz
TOF MS: Orthogonal Pulsing TOF Source
Overview schematic of the instrument: TOF source to detector
TOF MS: Orthogonal Pulsing TOF Source
GD Source
2mbar
Overview schematic of
the instrument
PTR Reactor 1mbar
PTR Source
Transfer Optics
Transfer Optics
10-4
mbar
Deflectors
Reflectron
Extract
Pulser
Field Free Region
Blanker
Blanking
Pulser
TOF Source
Detector
PreAmplifier
Mass spectrometer
and detector
10-6 - 10-7 mbar
Mass Spectral Analysis: Applications to Food Analysis
University of Nottingham
Department of Food Science
Professor Andy Taylor
Dr. Rob Linforth
Annie 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.
Signal intensity
Breath-by-Breath Analysis: Release of Ethyl Butyrate
Idealised
appearance of breath
markers, such as
acetone, as
exhalation occurs
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
Breath-by-Breath Analysis At Different Data ‘Granularity’
Intensity
2500
Reconstruction of
Acetone signal with 1
second integration
time
2000
1500
1000
Breathing rate ~5/min
500
0
10
20
Signal for Mass 117
30
40
50
Time
10000
Reconstruction of
ethyl butyrate signal
with 1 second
integration time
8000
6000
4000
2000
0
0
1 second integration
10
20
30
Time in Seconds
40
50
Breath-by-Breath Analysis At Different Data ‘Granularity’
Signal for Mass 117
6000
Reconstruction of
ethyl butyrate signal
with 0.5 second
integration timefurther structure
emerging
5000
4000
3000
2000
1000
0
0
10
20
0.5 second integration
Signal for Mass 117
30
Time in Seconds
40
50
60
Reconstruction of
ethyl butyrate signal
with 0.25 second
integration timefurther structure
emerging still
3000
2000
1000
0
0
0.25 second integration
10
20
30
Time in Seconds
40
50
60
Breath-by-Breath Analysis At Different Data ‘Granularity’
Signal for Mass 117
Reconstruction of
ethyl butyrate signal
with 0.125 second
integration time
2000
1500
1000
500
0
0
10
20
0.125
second
Signal
for integration
Mass 117
30
Time in Seconds
40
50
60
1200
Reconstruction of
ethyl butyrate signal
with 0.0625 second
integration time: no
further structure
emerging
1000
800
600
400
200
0
0
0.0625 second integration
10
20
30
Time in Seconds
40
50
60
Breath-by-Breath Analysis: Different Person
Signal
1400
Reconstruction of
Acetone signal with
0.125 second
integration time
1200
1000
800
600
400
200
0
0
10
20
Mass 59
30
Time (seconds)
40
50
Note different
breathing rate ~10/min
for different person
Signal
Overlay of ethyl
butyrate signal with
acetone
2000
1500
1000
500
0
0
Mass 59 and 117
10
20
30
Time (seconds)
40
50
Full Mass Spectra From 125ms Time Slices
Counts/0.125s
4
34.50 –34.625
seconds
3
2
Acetone and
Ethyl butyrate
1
0
20
40
60
Counts/0.125s
4
80
Daltons
100
120
140
34.00 –34.125
seconds
3
2
1
0
20
40
60
80
Daltons
100
120
140
Mass Spectrum From Headspace of
Freshly Macerated Tomatoes
Counts
3 hexenol
4000
3 hexenal
3000
2000
1000
0
60
80
100
Daltons
120
140
3-Hexenol vs. Hexanal, Both C6H12O
In previous slide, masses 101 and 83 were identified as 3Hexenol, 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
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?
Counts
Counts
1000
1400
101
Hexanal
800
83
83
1200
3 Hexenol
1000
600
800
101
600
400
400
200
200
0
0
80
85
90
95
Daltons
100
105
80
85
90
95
100
Daltons
No ‘threshold phenomenon’ observed
Also, 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!
105
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