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Introduction to DART MS
Robert B. Cody
JEOL USA, Inc.
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Outline
•
•
•
•
Definition of terms
DART operating principle
TOF mass spectrometer overview
The information we obtain
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Definitions of MS terms and general concepts
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High Resolution Mass Spectrometry
• We will be using exact-mass measurements to
to confirm knowns and to determine elemental
compositions for unknowns
• Resolving power defines how well the mass
spectrometer can separate close peaks
(interferences)
• The elemental composition software gives us
other information for each candidate
composition (e.g. unsaturation)
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Resolving Power
R = M / DM
R = Resolving Power
M = m/z
DM = difference in mass that can be separated
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Resolving Power Defined as:
FWHM (Full width at half maximum)
R = M / DM
R = 5000
m/z 500
DM = Peak width at half-height = 0.1
0.1
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Resolving Power Defined as:
10% Valley Definition
R = M / DM
R = 500
m/z 500 and 501 can be
separated at a 10% Valley
DM = 1
500
501
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Examples for C36H74 (m/z 506.579)
R = 500 (10% valley)
Separate m/z 500 from 501
R = 5000 (10% valley)
Separate m/z 500 from 500.1
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Why the definition matters
R = 500 (10% valley)
R = 500 (FWHM)
R = 5000 (FWHM)
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Mass accuracy
• millimass units (0.001) or “mmu”
ppm = 106 * (DM / M)
• parts-per-million (ppm)
– “Resolution” (reciprocal of resolving power)
Note: ppm is a m/z – dependent value
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Unsaturation
(aka “rings and double bonds” aka “double bond equivalents”)
O
H3C
C6H6+.
D = 4.0
H+
CH3COOD = 1.5, subtract 0.5
CH3
C3H7O+.
D = 0.5, add 0.5
H3O+
D = -0.5, add 0.5
• Value is calculated from elemental composition
• Indicates total rings, double bonds, triple bonds
• Exact integer (e.g. “4.0”) or half-integer (“3.5”)
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Examples of Even-electron ions
and Odd-electron ions
• Even-electron ions (half integer unsaturation) :
Protonated molecule [M+H]+
Deprotonated molecule [M-H]Chloride adduct [M+Cl]Ammoniated molecule [M+NH4]+
Fragment F+
• Odd-electron ions (exact integer unsaturation) :
Molecular radical cation M+.
Molecular radical anion M-.
Fragment F +.
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On-line Resources
• DART Users’ Google Newsgroup
– http://groups.google.com/group/dart-mass-spectrometer-users?hl=en
• JEOL USA, Inc. Web Pages
– http://www.jeolusa.com
• IonSense Web Page
– http://www.ionsense.com
• Wikipedia article on DART
– http://en.wikipedia.org/wiki/DART_ion_source
• Proton affinities, ionization energies (NIST)
– http://webbook.nist.gov/chemistry/
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DART Basic Principles
See the JEOL News Article on the
AccuTOF-DART product page on
www.jeolusa.com
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DART:
“Direct Analysis in Real Time”
•
•
•
•
•
Operational in Jan. 2003
Patent filed in April 2003
Public disclosure, Jan. 2005
Commercial product introduced March 2005
First open-air, ambient ion source for MS
1. Cody, R. B.; Laramee, J. A. “Method for atmospheric pressure ionization”
US Patent Number 6,949,741 issued September 27, 2005.
2. Laramee, J. A.; Cody, R. B. “Method for Atmospheric Pressure Analyte Ionization”
US Patent Number 7,112,785 issued September 26, 2006.
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Prototype DART sources
Original prototype DART source (mid-2002)
Second DART prototype
(Early 2003)
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The Whole Package:AccuTOF-DART™
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Why DART?
• Fast and easy way to introduce samples
• Minimal sample preparation for most samples
• Can tolerate “dirty” or high-concentration
samples and without contamination
• Fast fingerprinting of materials
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Nothing comes without a price
• Chromatography/MS still has advantages over
DART in detection limits, selectivity and
sensitivity for certain samples
• Not useful for large biomolecules (no good for
DNA analysis, proteins)
• DART does not ionize metals, minerals, etc.
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DART Schematic
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DART Ionization
Penning ionization
Sample ionized directly by energy
transfer from metastables (M*)
Proton transfer (positive ions)
1. He* ionizes atmospheric water
M*
DART Source
2. Ionized water clusters transfer
proton to sample
Electron capture (negative ions)
1. Penning electrons rapidly
thermalized
2. Oxygen captures electrons
3. O2- ionizes sample
MS API
Interface
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Penning Ionization
• Metastable atoms or molecules react with analytes
that posses ionization potentials less than the
metastable energy,
M* + S  S+. + M + electron
• The helium 23S state has 19.8 eV of internal energy
and lasts up to 8 minutes in vacuum.
– Most molecules have ionization energies much lower
than 19.8 eV
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Proton Transfer
He(23S) + H2O  H2O+• + He(11S) + electron
H2O+• + H2O  H3O+ + OH•
H3O+ + nH2O  [(H2O)n+1H]+
[(H2O)nH]+ + M  MH+ + nH2O
• Metastable atoms react with atmospheric water to
produce ionized water clusters
• Dominant reaction mechanism when helium carrier
used: He(23S) energy = 19.8 eV
• Huge reaction cross section: 100 A2
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Typical DART Low-Mass Background
Normal DART Parameters
[(H2O)2+H]+
100
Rel. Abund.
80
60
NH4+
40
H3O+
[(H2O)3+H]+
NO+
20
0
15
20
25
30
35
m/z
40
45
50
55
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Negative Ion Formation
• Electrons produced by direct or surface Penning
ionization are rapidly thermalized
• Thermal electrons react with atmospheric oxygen and
water to produce ionized clusters
• Oxygen/water cluster ions react with analyte molecules
to produce analyte ions
e-* + G  e- + G*
e- + O2  O2-.
O2-. + S  [S-H]- + OOH.
O2-. + S  S-. + O2
O2-. + S  [S+O2]-.* + G  [S+O2]-. + G*
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Typical DART Negative-Ion
Low-Mass Background
Rel. abundance
O2-
[H2O3]-
[H2O4][HCO3]-
20
40
60
[HCO4]80
100
m/z
Note the absence of nitrogen oxide ions that would
be produced by electrical discharge in air. NO2- and
NO3- are problematic for detection of nitro
explosives and reduce anion detection sensitivity
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95
OH
Example
HO
Ascorbic acid, C6H8O6
52
[M+H]+
HO
O
Rel. Abund.
177.0410
100
HO
Positive ions
9
136
139
142O]
145+ 148
[M+H-2H
2
127
130
133
(mainlib ) Asc orbic Ac id
50
151
154
157
160
163
[M+H-H2O]+
0
100
150
m/z
Rel. Abund.
O
100
[M-H]175.0232
Negative ions
50
0
100
150
m/z
Sampled directly from a melting point tube
166
169
172
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Notes on the AccuTOF Design and Operation
See the JEOL News Article on the
AccuTOF-LC product page on
www.jeolusa.com
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Types of mass spectrometers
• Scanning:
– magnetic sector, quadrupole and triple
quadrupole
• Trapped-ion:
– Fourier transform, 3D ion trap, Orbitrap
– linear trap (used in triple quadupole MS)
• Time-of-flight
• Hybrids
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DART can be fit on most mass
spectrometer types
DART signals can be transient, so,
• scanning mass specs work best with selected ion
monitoring or fast scanning
• Selected reaction monitoring on triple quadrupole
MS is good for target compound quantitation.
• Ion traps work, but are not a good choice for
quantitative analysis
• Time-of-flight is fastest MS for transient signals,
and gives high-resolution data for the entire mass
spectrum with no sensitivity loss.
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Time of flight principle
If everyone starts
at the same time
and has the same
kinetic energy,
lighter riders will
move faster
Heavy ions moving slowly
Light ions moving quickly
Detector
L’Alpe D’Huez
de
Spectrometrie
de Masse
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A more realistic TOF mass spectrometer
Ion source:
Short burst of ions
Flight tube
High voltage to accelerate ions
Kinetic Energy = qE = mv2/2
Ion detector
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What if ions that have the same mass have
slightly different energies?
• Reflectron: make the more energetic ions
travel further
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Reflectron Time of flight mass analyzer
principle
1. Fast riders miss the turn
Lance
Me
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Reflectron Time of flight mass analyzer
principle
2. Fast riders turn around; have to travel further
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Reflectron TOF
3. Fast riders start to catch up
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Reflectron TOF
Focal point
4. Fast riders catch up, will eventually pass
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Time-of-flight math
• All ions fly with the same kinetic energy.
1
( M  mu )  v 2  q  e  V
2
M: mass of ion [u]
mu: Atom mass unit (1.6605 x 10-27 [kg/u])
v: flight speed of ion [m/s]
q: charge number of ion
e: unit electric charge (1.602 x 10-19 [C])
V: Accelerating voltage [V]
• Flight time is inversely proportional to the square root of the
mass/charge ratio.
L
q
tof 

M
V
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JMS-T100LC AccuTOFTM
Ion Source
Ion
Transportation
Detection
system
Analyser
To the data
collection
system
TMP2
RP
TMP1
RP
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AccuTOFTM Ion Source
Ion Source
Ion
Transportation
Detection
system
Analyser
To the data
collection
system
TMP2
RP
TMP1
RP
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Orthogonal ESI ion source and API interface
LC Eluent
Nebulizer Gas
Desolvating Chamber
Orifice2
Ion Guide
Desolvating
Gas
Ring Lens
Orifice1
RP
TMP
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Ion Source and Atmospheric Pressure
Ionization (API) Interface
• Orthogonal ESI
– Minimize contamination into API interface
• Simple API interface
– Robust, few parameters, minimal maintenance
• Off-axis skimmers and ring lens, bent ion guide
– Keep contamination out of high-vacuum region
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AccuTOFTM Ion Transport
Ion Source
Ion
Transportation
Detection
system
Analyser
To the data
collection
system
TMP2
RP
TMP1
RP
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Ion transport region
• Strong acceleration of ions only occurs in
high-vacuum region
– Minimize CID and scattering
• Quadrupole RF ion guide focuses ions to a
small spot size
– Spatial focus for good resolution
– “High-pass” filter (ions greater than given m/z)
• Multi-function focusing and steering lenses
– Beam should be perpendicular
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AccuTOFTM Analyzer
Ion Source
Ion
Transportation
Detection
system
Analyser
To the data
collection
system
TMP2
RP
TMP1
z
y
RP
x
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AccuTOFTM Analyzer
z
y (injection)
• Two-step acceleration
– Spatial focusing of ion beam
• Single reflectron
x (reflectron)
– Energy focusing of ion beam in the x-direction
– Minimize ion loss
• oa(Orthogonal-Acceleration)-TOF MS
– Kinetic energy spread in y-direction has no effect
on resolution
– The ions produced by the ESI ion source are
used efficiently.
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Flight cycle of oa-TOF MS
• 1. Introduction of ion
– Two kinds of ions are introduced at the same time.
Low mass ion
High mass ion
Mixture of both ions
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Flight cycle of oa-TOF MS
• 2. Turn on the pulser voltage
– Mixture of ions at the start of flight
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Flight cycle of oa-TOF MS
• 3. Turn off the pulser voltage
– continuing flight － mass separation
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Flight cycle of oa-TOF MS
• 4. Continuing flight
– New ions are introduced in the ion acceleration part.
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Flight cycle of oa-TOF MS
• 5. Low mass ion reaches detector
– The ion acceleration region is filled with the new ions.
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Flight cycle of oa-TOF MS
• 6. High mass ion reaches detector
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Flight cycle of oa-TOF MS
• 7. The detection of all ions is completed
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AccuTOFTM Detection
Ion Source
Ion
Transportation
system
Detection
system
Analyser
To the data
collection
system
TMP2
RP
TMP1
RP
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Detector
To impedance converter
in the atmosphere
in the vacuum
Anode
e-
①
②
＋
Dual MCP
Micro-channel plate (MCP)
 40mmφ
 Dual MCP
Anode
 Combined with high voltage capacitor
Patent pending
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MCP
•
•
•
•
Diameter：40mm
Thickness：0.6mm
I.D. of channel：10μm
Gap of each channel：12μm
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Data collection system for oa-TOF MS
- Requirements • High time resolution
– m/z 609, R=6,000 → Peak width: 3.5ns
• Continuous data collection
– High duty cycle
• Real-time accumulation of mass spectrum
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Data collection system for oa-TOF MS
• TDC
– Super-high speed
digital stop watch
– Measures the arrival
time of ions
– A premise is that there
are a few ions
• Each ion arrives
separately.
• Ion counting
detection: signal is 0
or 1.
• Continuous Averager
– A signal from the
detector is converted
digital value by a highspeed ADC (Analog-toDigital Converter).
– Spectrum can be
accumulated
continuously in real time.
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TDC (Time-to-Digital Converter)
59us
Start Input
Time-to-Digital
Converter
High Voltage
Pulser
Amp
Discriminator
Stop Input
No. TOF [us]
1 29.4235
2 46.2890
....
Histogram
Memory
To Data System
No. of Ions
Detected
in a Cycle
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Simulation of spectrum accumulation by TDC
Output from Amplifier ： Cycle 1
20
mV
15
10
5
0
1
5
9
13 17 21 25 29 33 37 41 45
Histgram memory ： Cycle 1
No. of Ions
3
2
1
0
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46
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Simulation of spectrum accumulation by TDC
Output from Amplifier ： Cycle 2
20
mV
15
10
5
0
1
5
9
13 17 21 25 29 33 37 41 45
Histgram memory ： Cycle 2
No. of Ions
3
2
1
0
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46
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Simulation of spectrum accumulation by TDC
mV
Output from Amplifier ： Cycle 3
20
15
10
5
0
1
5
9 13 17 21 25 29 33 37 41 45
No. of Ions
Histgram memory ： Cycle 3
3
2
1
0
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46
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Simulation of spectrum accumulation by TDC
mV
Output from Amplifier ： Cycle 4
20
15
10
5
0
1
5
9 13 17 21 25 29 33 37 41 45
No. of Ions
Histgram memory ： Cycle 4
3
2
1
0
1
5
The ion which had
about two times
higher intensity
was detected.
9 13 17 21 25 29 33 37 41 45
It is counted
only once (not
twice) with TDC.
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Simulation of spectrum accumulation by TDC
Two ions detected
in succession!
Output from Amplifier ： Cycle 5
mV
20
15
10
45
41
37
33
29
25
21
17
13
9
5
1
5
0
The second ion
can't be counted
during dead time.
Histgram memory ： Cycle 5
2
1
45
41
37
33
29
25
21
17
13
9
5
0
1
No. of Ions
3
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Result of spectrum accumulation by TDC
37
41
45
37
41
45
33
29
25
21
17
13
9
5
1
25
20
15
10
5
0
Histgram memory ： Cycle 5
3
2
1
33
29
25
21
17
13
9
5
0
1
No. of Ions
• The ratio of the peak
intensity isn't correct.
• A high intense peak
shifts to low mass side.
No. of Ions
model spectrum :
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Continuous Averager
Continuous Averager
59us
Timing Control
Circuit
High Voltage
Pulser
Amp
ADC
(8bit)
Adder
Summing
Memory
To Data System
Intensity
15
28
....
No. of Data
Points on a
Spectrum
(up to 256K
points)
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Simulation of spectrum accumulation by
continuous averager
Output from Amplifier ： Cycle 1
20
mV
15
10
5
0
1
5
9
13 17 21 25 29 33 37 41 45
45
41
37
33
29
25
21
17
13
9
5
50
40
30
20
10
0
1
mV
Cycle 1
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Simulation of spectrum accumulation by
continuous averager
Output from Amplifier ： Cycle 2
20
mV
15
10
5
0
1
5
9
13 17 21 25 29 33 37 41 45
45
41
37
33
29
25
21
17
13
9
5
50
40
30
20
10
0
1
mV
Cycle 2
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Simulation of spectrum accumulation by
continuous averager
mV
Output from Amplifier ： Cycle 3
20
15
10
5
0
1
5
9 13 17 21 25 29 33 37 41 45
45
41
37
33
29
25
21
17
13
9
5
50
40
30
20
10
0
1
mV
Cycle 3
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Simulation of spectrum accumulation by
continuous averager
mV
Output from Amplifier ： Cycle 4
20
15
10
5
0
1
5
9 13 17 21 25 29 33 37 41 45
45
41
37
33
29
25
21
17
13
9
5
50
40
30
20
10
0
1
mV
Cycle 4
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Simulation of spectrum accumulation by
continuous averager
25
29
33
37
41
45
25
29
33
37
41
45
21
17
13
9
5
20
15
10
5
0
1
mV
Output from Amplifier ： Cycle 5
21
17
13
9
5
50
40
30
20
10
0
1
mV
Cycle 5
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Result of spectrum accumulation by
continuous averager
25
29
33
37
41
45
25
29
33
37
41
45
21
17
13
9
5
25
20
15
10
5
0
1
No. of Ions
model spectrum :
Cycle 5
21
17
13
9
5
50
40
30
20
10
0
1
mV
• The ratio of the peak
intensity is correct.
• There is no shift of the
ion peak.
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Specifications
• Mass resolution： 6,000
– FWHM, Reserpine m/z 609
• Sensitivity： Reserpine 10pg S/N>10
– LC-ESI ［Flow rate: 0.2mL/min］
– Mass chromatogram of m/z 609, RMS
• Mass accuracy： 5ppm RMS
– With internal reference
– (Typically better than that!)
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Only 3 analyzer parameters are critical
Detection
for routine DART analysis
system
Ion Source
1
Ion
Transportation
Analyser
To the data
collection
system
2
TMP2
3
RP
TMP1
RP
1: Orifice 1
2: “Peaks voltage”
3. Multiplier V
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The 3 important parameters
• 1: Orifice 1: Typically 20V
– Increase O1 to increase fragmentation
• 2: “Peaks voltage” (RF ion guide voltage)
– Divide by 10 to estimate lowest detected m/z
• 3. Multiplier V: Typically 2200V to 2600V
– Increase multiplier to increase signal (and
noise)
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Information from the TOF mass
spectrum
• Exact mass + isotope peaks: elemental
composition
• Fragmentation: distinguish isomers
• “Fingerprint” pattern: material ID
• Ion abundance: quantitative analysis
• Other experiments: H/D exchange,
derivatization, etc.
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Example: DART mass spectrum of a leaf
What is this?
304.154
100
290.174
Rel. Abund.
80
60
40
20
0
100
150
200
m/z
250
300
350
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We can treat this as an unknown
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Elemental compositions
Measured
Exact Mass
Constraints
Candidate
compositions
Isotope pattern matching
Ranked compositions
Elemental Composition Program
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We have a composition. Now what?
m/z 304.1548 is C17H22NO4
Cocaine
Scopolamine
Fragments at99m/z 138, 156
N
138
C8H12NO+
Fragments at m/z 182, 82
O
N
O
H
O
O
O
56
OH
156
C8H14NO2+
O
H
O
182
C10H16NO2+
13
204
214
224
202
212
222
232
242
252
262
272
234 ) 8-Azabic
244
264
(mainlib
yc254
lo[3.2.1]oc
tane-2-c274
arboxylic284
ac id , 3-(benzoyloxy)-8-methyl-, methyl ester, [1R
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API interface change potentials to
LCinduce
Eluent fragmentation
Nebulizer Gas
Desolvating Chamber
Orifice2
Ion Guide
Desolvating
Gas
Ring Lens
Orifice1
Control fragmentation
with Orifice 1 and
Ring Lens potentials
RP
TMP
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Fragment spectrum increase cone
voltage from 20 V to 60 V
Atropine
100
Rel. Abund.
80
60
40
20
290.174
Scopolamine
C8H12NO+
C8H14NO2+
Scopolamine
156.099
138.089
304.154
C8H14N+
0
100
150
200
m/z
250
300
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For comparison,
m/z 305.1548 fragments from a dollar bill
Cocaine
C10H16NO2+
182.118
100
C17H22NO4+
Rel. Abund.
80
60
C 5H 8N +
40
82.065
20
0
100
150
200
m/z
250
300
350
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or…we can search for candidates
from a list of target compounds.
Components in a
smokeless powder
SearchFromList Program
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Whew…!
Confused?
It’ll make more sense when you see it in
the lab.