Introduction%20to%20DART%20MS

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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
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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
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Examples for C36H74 (m/z 506.579)
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Why the definition matters
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)
CH3COO- D 1.5, subtract 0.5
H3O D -0.5, add 0.5
C3H7O. D 0.5, add 0.5
C6H6. D 4.0

• 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 ionsand Odd-electron
ions

• Even-electron ions (half integer unsaturation)
• Protonated molecule MH
• Deprotonated molecule M-H-
• Chloride adduct MCl-
• Ammoniated molecule MNH4
• 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-spectrome
  ter-users?hlen
• 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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DARTDirect 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 PackageAccuTOF-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 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
M
DART Source
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)n1H (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
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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 ? SO2-. G ? SO2-. G

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Typical DART Negative-IonLow-Mass Background
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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Example
Ascorbic acid, C6H8O6
Sampled directly from a melting point tube
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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
Detector
Light ions moving quickly
If everyone starts at the same time and has the
same kinetic energy, lighter riders will move
faster
Heavy ions moving slowly
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A more realistic TOF mass spectrometer
Ion source Short burst of ions
Ion detector
Flight tube
High voltage to accelerate ions
Kinetic Energy qE mv2/2
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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.
• Flight time is inversely proportional to the
  square root of the mass/charge ratio.

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
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JMS-T100LC AccuTOFTM
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AccuTOFTM Ion Source
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Orthogonal ESI ion source and API interface
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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
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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
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AccuTOFTM Analyzer

• Two-step acceleration
• Spatial focusing of ion beam
• Single 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 system
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Detector

• Micro-channel plate (MCP)
• 40mmf
• Dual MCP
• Anode
• Combined with high voltage capacitor
  Patent pending

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MCP

• Diameter40mm
• Thickness0.6mm
• I.D. of channel10µm
• Gap of each channel12µm

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Data collection system for oa-TOF MS
- Requirements -

• High time resolution
• m/z 609, R6,000 ? 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

• Continuous Averager
• A signal from the detector is converted digital
  value by a high-speed ADC (Analog-to-Digital
  Converter).
• Spectrum can be accumulated continuously in real
  time.

• 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.

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TDC (Time-to-Digital Converter)
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Simulation of spectrum accumulation by TDC
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Simulation of spectrum accumulation by TDC
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Simulation of spectrum accumulation by TDC
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Simulation of spectrum accumulation by TDC
The ion which had about two times higher
intensity was detected.
It is counted only once (not twice) with TDC.
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Simulation of spectrum accumulation by TDC
Two ions detected in succession!
The second ion can't be counted during dead time.
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Result of spectrum accumulation by TDC

• The ratio of the peak intensity isn't correct.
• A high intense peak shifts to low mass side.

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Continuous Averager
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Simulation of spectrum accumulation by continuous
averager
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Simulation of spectrum accumulation by continuous
averager
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Simulation of spectrum accumulation by continuous
averager
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Simulation of spectrum accumulation by continuous
averager
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Simulation of spectrum accumulation by continuous
averager
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Result of spectrum accumulation by continuous
averager

• 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/Ngt10
• 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 for
routine DART analysis
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
290.174
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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 at m/z 182, 82
Fragments at m/z 138, 156
138
C8H12NO
156
182
C8H14NO2
C10H16NO2
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API interface change potentials to induce
fragmentation
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Fragment spectrum increase cone voltage from 20 V
to 60 V
Atropine
Scopolamine
290.174
C8H12NO
C8H14NO2
Scopolamine
156.099
138.089
304.154
C8H14N
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For comparison, m/z 305.1548 fragments from a
dollar bill
Cocaine
C10H16NO2
C17H22NO4
C5H8N
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orwe can search for candidates from a list of
target compounds.
Components in a smokeless powder
SearchFromList Program
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Whew!Confused? Itll make more sense when
you see it in the lab.