Ch 13. Basics of Mass Sppy()ectrometry (I) : Principles...
Transcript of Ch 13. Basics of Mass Sppy()ectrometry (I) : Principles...
Ch 13. Basics of Mass Spectrometry (I)p y ( ): Principles & Ionization Sources
Why should you be interested in mass spectrometry (MS) ?
- to identify unknown compounds
- to quantify known materialsto quantify known materials
- to elucidate the structural and chemical properties
of moleculesof molecules
- detection of compounds at 10-12g, 10-15 mole for a
d f 100 d ltcompound of mass 100 dalton
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1. Where are MS used?
• Biotechnology: analysis of proteins, peptides, oligonucleotides
• Pharmaceutical Analysis: d di bi t i l h i t h ki tidrugs discovery, combinatorial chemistry, pharmokinetics, drug metabolism
• Clinical Examination:Clinical Examination:neonatal screening, haemoglobin analysis, drug testing
• Environmental Analysis :Environmental Analysis : water, food, air quality (PCBs etc)
• Geological Analysis : g yoil composition
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1.1 Instrumental Design of Mass Spectrometer
Ionization/ desorption Mass Sorting Detection
Analyzer Ion Detection
Source
+
DetectForm ions Sort Ions by Weight (m/z) Detect ions
Form ions
(charged molecules)
Sort Ions by Weight (m/z)
I l
10010010010075757575
Inlet
1330133013301330 1340134013401340 1350135013501350
50505050252525250000
• Solid• Liquid• Vapor
Data AnalysisSample IntroductionMethod to vaporize
sample
Mass Spectrum
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Turbo pumpsDiffusion PumpsRough pumps Rotary pumps
High Vacuum SystemHigh Vacuum System
Inlet Ionsource
Mass Filter
Detector DataSystemsource Filter System
HPLC Electrospray TOF MicrochannelHPLCSample plateGCSolid probeDCI
ElectrosprayMALDIFABEI
TOFQuadrupoleIon TrapMagnetic Sector
Microchannel Plate (MCP)
Electron Multiplier
DCI CI FT-ICR
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1.2 Vacuum TechnologyPressure Gauges
G P R T i l U
gy
Gauge Pressure Range Typical Use(torr)
Manometer 760-1 systems near atmThermocouple gauge 1 - 10-3 monitoring mechanical pumpsIonization gauge 10-3 - 10-11 high-vacuum systems
Vacuum Pumps
Pump Lowest Attainable Pressure Typical UsePump Lowest Attainable Pressure Typical UseMechanical pump 10-2 - 10-3 roughing or backing pumpDiffusion pump 10-6 vacuum linesTurbomolecular pump 10-9 high vacuum systemsTurbomolecular pump 10 9 high-vacuum systemsCryopump < 10-10 ultrahigh-vacuum sys.
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1 3 Reason for Vacuum in MS 10-6 to mid 10-5 Torr1.3 Reason for Vacuum in MS 10-6 to mid 10-5 Torr
• Increase sensitivity
• Avoid ion-molecule reactions
• Collision free ion trajectories
• Increase filament lifetime
• Avoid electric dischargeAvoid electric discharge
• Avoid background interference
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2 Sample Introduction Techniques2. Sample Introduction Techniques
• Direct Probe/Metal TargetsDirect Probe/Metal TargetsSample put onto the end of long probe and inserted into the MSSample spotted with matrix onto a metal plate
• Gas chromatography: EI, CISample must be volatile thermally stable- Sample must be volatile, thermally stable
• Liquid chromatography: FAB, APCI, ESI Widely used in pharmaceutical industryLC/MS applicable to thermally labile, high MW compoundsLC/MS suitable for proteins & peptidesO li t h iOn-line technique
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3. Ionization Methods3. Ionization Methods
I i iIonization Techniques
ElectronImpact(EI)
ChemicalIonization(CI)
ESIFast AtomBombardment(FAB)
MALDI
Soft IonizationIntact
Hard IonizationFragments
(EI) (CI) (FAB)
IntactFragments
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3.1 Electron Ionization (EI)3.1 Electron Ionization (EI)
M + e- (70 eV) M+ (5 eV) + 2e- (65 eV)
Excess E (65eV) in the molecule leads to some degree of fragmentationExcess E (65eV) in the molecule leads to some degree of fragmentation
M+ : molecular ions + fragment ions + neutral fragments
M + e- : M- (100 times less efficient)
•Energetic process. A heated filament emits electrons which are accelerated by a potential difference of usually 70eV into the sample chamber.
•Ionization of the sample occurs by removal of an electron from the molecule thus
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o at o o t e sa p e occu s by e ova o a e ect o o t e o ecu e t usgenerating a positively charged ion with one unpaired electron.
1) Schematics of EI1) Schematics of EI
N M M+ 2N M + e- M+. + 2e-
Fragmentation
M+. A+ B+
A+
M+.
B+
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2) Characteristics of EI 2) Characteristics of EI
Molecular ionMolecular ion eABeAB 2
))
Fragment ionFragment ion eBAeAB 2• Standard ionization methods in MS• Standard ionization methods in MS
• Vaporized sample bombards with high energy electrons (~ 70 eV)
• “Hard” ionization method leads to significant fragmentationg g
• Ionization is efficient but non-selective
• Widely used technique when coupled to GC
• Suitable for volatile organic compounds
eg. hydrocarbons, oils, flavors, fragrances
• Not really coupled to LC today• Not really coupled to LC today
• Produces M+. radical cation giving molecular weight
• Produces abundant fragment ions
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g
• Library searchable spectra
EI Spectra
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3.2 Chemical Ionization (CI)
• Positive Ion Chemical Ionization (PCI or CI)Positive Ion Chemical Ionization (PCI or CI)
• Negative Ion Chemical Ionization (NCI,NICI or ECNI)
1. Instrumentation for CI
2. Selection of Reagent Gas
3. Analytical Application y pp
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1) EI – CI Comparison) p
EI CISource pressure
10-6 torr 0.2 –2 torr
Mean free path
-200 mm -2 x 10-4 mm
High Energy process Low Energy process
( th l )(possess thermal energy)
Odd E- ions formed Even E- ions formed
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2) Advantages and Disadvantages of CI2) Advantages and Disadvantages of CI
Advantages of Chemical ionization:g1. Large (M+1)+ ion identifies molecular weight (M)
2. Sensitivity is enhanced by
- simple fragmentation (fewer peaks of higher abundance)
- direct GC/MS interface
3. CI spectra complement EI spectra
Disadvantages of Chemical ionization :Disadvantages of Chemical ionization :1. Simple fragmentation gives little structural information
2 E i t i ti2. Easy ion source contamination
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3) What happens in the CI source ? ) pp
•Reactant ions are formed via EI/CI:
CH4 + e- CH4+ + 2e-
CH4 + e- CH3+ + H• + 2e-CH4 e CH3 H 2e
CH4 + e- CH2+ + H2 + 2e-
CH4+ + CH4 CH5
+ + CH3 •
CH3+ + CH4 C2H5
+ + H23 4 2 5 2
CH2+ + 2 CH4 C3H5
+ + 2H2 +H•
At 1 t th j i th tAt 1 torr, the major ions are those at
17amu (CH5+), 29amu (C2H5
+), 41amu (C3H5+ )
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S l I i ti•Sample Ionization:
1. Proton transfer: higher proton affinity than that of reagent
CH5+ + RH CH4 + RH2
+ (M+H)+
C H + + RH C H + RH + (M+H)+C2H5+ + RH C2H4 + RH2
+ (M+H)+
2. Alkyl addition:
C2H5+ + RH RH • C2H5
+ (M+29)+
C3H5+ + RH RH • C3H5
+ (M+41)+C3H5 + RH RH C3H5 (M+41)
3. Hydride abstraction : lower PA molecules (M-1)+
CH5+ + RH CH4 + H2 + R+
C2H5+ + RH C2H6 + R+
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2 5 2 6
• CI Mass spectra of reagent gas ?
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3.3 Fast Atom Bombardment (FAB) Ion Source( )
Comatrix MatrixCesium ion beam
P
MH+
[ or MX ]+
Proton
X : Na, K, Li[ or MX ] X : Na, K, Li8000Vor 1KV Ion desorbed from matrix
Advantage Soft Ionization(MH+, MNa+...): little fragmentationgEasy adaptation of HRMS: accurate mass measurementMatrix can be useful as ref. ionSample and Matrix
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1) Schematics of FAB Ion Source
FAB gun
6~10 keV
Primary atom or ion beam
Probe Secondary ions
Through Vacuum Lock
Sample/Matrix To mass analyzerVacuum Lock
Xe Xe+ Xe+ Xe0ionization acceleration neutralization
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Xe Xe Xe XeSlow atoms Slow ions Fast ions Fast atoms
2) Mechanism of ion formation )
•Impact of a high-energy atom or ionformation of a high-temperature, high density
gas in the cavity that is formed at the point of impact
•Generation of additional ionsd l t di l d it dsecondary electrons, radicals, and exited
neutral species
Mass Spectrom. Rev., 5, 191 (1986)
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4) Matrix of FAB Ionization
Successful ionization by FAB is deeply dependent on the matrix selection for the analysis.
Matrix requirements
- Dissolve the sample to be analyzed, (usually 1 mg/100 mL)
- Facilitate in the ionization of the sample
- Dissolve the sample to be analyzed, (usually 1 mg/100 mL)
- Facilitate in the ionization of the sample
Matrix requirements
- low volatility
- Not undergo a chemical reaction with the sample.
- Constantly replenish the surface with new sample
- low volatility
- Not undergo a chemical reaction with the sample.
- Constantly replenish the surface with new sampley p p
- Minimize sample damage from the high-energy particle beam
- Reduce damage to the sample by absorbing the impact of the primary beam
- Prolongs the sample ion current by constantly replenishing the upper layer with the fresh sample
y p p
- Minimize sample damage from the high-energy particle beam
- Reduce damage to the sample by absorbing the impact of the primary beam
- Prolongs the sample ion current by constantly replenishing the upper layer with the fresh sample
proton donor(+ve mode)proton donor(+ve mode)Functions
Prolongs the sample ion current by constantly replenishing the upper layer with the fresh sample
- Reduces the binding energy of the sample molecules
Prolongs the sample ion current by constantly replenishing the upper layer with the fresh sample
- Reduces the binding energy of the sample molecules
- proton donor(+ve mode)
- proton acceptor(-ve mode)
- solvent
- sample reservoir
- proton donor(+ve mode)
- proton acceptor(-ve mode)
- solvent
- sample reservoir
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- sample reservoir
- reagent
- sample reservoir
- reagent
Matrix Selection of FAB MS
MatrixMol.
Formula M/Z Characteristics
Glycerol C3H8O3 92 0473 •Best choice for polar compoundsGlycerol C3H8O3 92.0473 •Best choice for polar compounds
1-thioglycerol C3H8O2S 108.0245 •More volatile than glycerol, evaporates quickly
3-nitrobenzyl alcohol (NBA) C7H7NO3 153.0416
•Best choice for less polar compounds and many
organometallics3 nitrobenzyl alcohol (NBA) C7H7NO3 153.0416 organometallics
2-nitrophenyl octyl ether
(NPOE)C14H21NO3 251.1521
•Only FAB matrix with no reactive hydrogen
Triethanolamine C6H15NO3 149.1052•Good matrix for negative-ion FAB
•Enhances [M-H]- formation
magic bullet C4H10S2O2 154.0122
dioctyl phthalate (DOP) C H O 390 54
•Widespread contaminant in solvents, gives characteristic 149
peak in EI mass spectra dioctyl phthalate (DOP) C24H38O4 390.54
•Use care to avoid contaminating the mass spectrometer
[bis(2-ethylhexyl) phthalate] 384
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sulfuric acid H2SO4 97.967
•Good for some inorganics and organometallics (e.g. copper
phthalocyanine)
•Corrosive, use care
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• Applications of FAB MSpp
• Synthetic Peptides
• Recombinant DNA Proteins and Glycoproteins
• Carbohydrates
• Drug Metabolites (including intact congugates)
• Detergents (anionic, cationic, non-ionic, amphoteric)Detergents (anionic, cationic, non ionic, amphoteric)
• Biocides
• Petrochemicals• Petrochemicals
• Oil Additives
Oi i C i• Oil Field Chemicals
• Phospholipids
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3.4 Matrix Assisted Laser Desorption Ionization (MALDI)
Comatrix MatrixLaser
P
MH+
[ or MX ]+
Proton
X : Na, K, Li[ or MX ] X : Na, K, Li
Advantage High Sensitivity Soft Ionization(MH+, MNa+...)( , )Mixture AnalysisLow CostNo Contamination
S l & i
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Sample & matrix
• Selection of Matrix in MALDI
Sinapinic Acid Proteins >10kDa
a-Cyano-4-Hydroxy- Peptides<10kDaa Cyano 4 Hydroxycinnamic acid
Peptides 10kDa
2,5 Dihydroxybenzoicacid
Neutral Carbohydrates,Synthetic Polymersacid Synthetic Polymers
“Super DHB” Proteins,
Glycosylated proteins
acid
Oligonucleotides
HABA Proteins,
3-Hydroxypicolinic
Oligosaccharides
,
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• Advantage
– High sensitivity, mass range
– Simple structure
– Easy operation & maintenance, low cost
– Fast acquisition and automation
• Disadvantage
– Accuracy resolution [~10 000]Accuracy, resolution [~10,000]
– delayed extraction, reflectron
– Different calibration for different mass range & experimental g pcondition (laser, matrix)
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MALDI/TOF
4 I l t d b l t i l fi ld t th
1. Sample is mixed with matrix& dried on target
5. Ions strike thedetector at4. Ions are accelerated by an electrical field to the
same kinetic energy, and they drift (or fly) down afield free flight tube where they are separated inspace.
Flight tube
g
20 - 30 kV
different times,depending on themass to chargeratio of the ion.
High vacuum2. Target is introduced into highvacuum of MS
g
High voltage
•Pulsed laser
3. Sample spot is irradiated with laser,desorbing ions into the gas phase and startingthe clock measuring the time of flight.
Time
6. A data system controls all instrument parameters,acquires the signal vs. time, and permits dataprocessing
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processing.
Reflector for TOFReflector
Higher resolutionHigher mass accuracyPSD(Post Source Decay) First Detector
Reflector(Ion mirror)
PSD(Post Source Decay)Second Detector
Ion Gate
Laser
The reflector increases the overall path length for an ion and it corrects for minor variation in the energy spread of ions of the same mass Both effects improve
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variation in the energy spread of ions of the same mass. Both effects improve resolution.
• Applications of MALDI/TOF-MSApplications of MALDI/TOF MS
Molecular biology, biochemistry
– Biopolymer CharacterizationBiopolymer Characterization
• Protein & peptides [proteomics]– Sequence Analysis
• DNA, RNA [genomics]– In- Source Decay
– Exonuclease Sequencing– Exonuclease Sequencing
• Carbohydrate
• Lipid
Pharmaceutical Industry
– Combinatorial Chemistry
Ch i l I dChemical Industry
– Synthetic Polymer Characterization
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3.5 Atmospheric Pressure Chemical Ionization (APCI)
S l I l t N lSample Inlet Nozzle(Lower Voltage)Pressure = 760 torr
MH+ MH+ MH+S l t Fl F HPLC
N2
MH+
MH+ MH MHSolvent Flow From HPLC
MH+
Heat applied to metal sheath
Corona discharge needle
Heat
to metal sheath needle
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APCI
Simple and efficient device for the LC/MS analysis of compounds
Needle : 3 6kV potential creates a corona dischargeNeedle : 3-6kV potential creates a corona discharge
Ionization : by CISoft ionization which generates pseudo molecular ions
MH+ (M NH )+• MH+, (M+NH4)+ etc.
• (M-H)-, (M+CH3COO)-, (M+Cl)- etc
Generates singly charged ions, in general
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Suitable for less polar compounds compared to ESI
Same degradation may occur in case of labile compounds
• Characteristics of APCI
• Both positive and negative ions can preferentially be formed in the
APCI source using a corona dischargeAPCI source using a corona discharge
• Ion – molecule reaction
• Positive ion mode
- the creation of reactant positive ions in ambient air
(the proton hydrates, H3O+[H2O]n)
- the major primary ions N2+, O2
+, H2O+ and NO+ are formed by electron
impact of corona-created electrons on the major neutral components in air
- Proton transfer reaction H3O+[H2O]n + T TH+ (H2O)m + (n-m+1)H2O
if the gas basicity (proton affinity) of T is greater than that of water
TH+ (H2O)m TH+ by stripped off in the gas curtain and declustering lens region
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lens region
I i i d• In negative ion mode,- electron created by the corona are rapidly thermalized (lose excess energy) th h lli i ith t l d t d b l t ti i h Othrough collision with neutrals and captured by electronegative species such as O2
to form O2- and O-.
- super oxide (O2-) and its hydrates (O2
-[H2O]n) and cluster (O2- [O2]n)p ( 2 ) y ( 2 2 n) ( 2 2 n)
Ion – Molecule Reaction
• Charge transfer
R+ + T T+ + R
R- + T T- + R
• Proton transfer
RH+ + T TH+ + R
R- + TH T- + RH
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3.6 Electrospray Ionization (ESI)3.6 Electrospray Ionization (ESI)
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1) Characteristics of ESISoft ionization, which generates pseudo molecular ions
• MH+, (M+NH4)+, (M+Na)+, (M+K)+ etc.MH , (M+NH4) , (M+Na) , (M+K) etc.
• (M-H)-, (M+CH3COO)-, (M+Cl)- etc.
Applicable for wide range of compounds with relatively high sensitivity
• Up to 150 kDa in case of proteins
• Middle to High polar compounds
• Thermally labile compounds
• Non covalently binding complex
Generates multiply charged ions of biopolymers
• (M+nH)n+ / (M-nH)n-
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•ESI Mass Spectra
1picomole Myoglobin
MW: 16 939 4 amMW: 16,939.4 amu
LC/MS analysis m/z 1230
Heme dimer
Second series protein + Heme
Synthetic DNA
MW 4260 7MW: 4260.7 amu
Negative ion mode
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Ch 14. Basics of Mass Spectrometry II: m/z analysis & MS spectra
• All analyzers sort ions based on their mass-to-charge ratios (m/z)
Magnetic Sector
TOFTOF
1970s/80s
1990s
Q-TOF
1950s/60s1980s/90s
1990s-2000s-
Ion TrapsQuadrupoles
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1. Analyzer
• Ability to separate ionsy p
• High accuracy, High sensitivity
• High mass range
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g g
• Structural information
1.1 Time of Flight (TOF)g
• TOF commonly used with MALDI
• Different velocity
• Reflectron improves Resolution• Reduce KE distribution of ions
y
• KE = 1/2mv2 v = (2KE/m)1/2
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• Reduce KE distribution of ions• Expense of sensitivity• Limited mass range
Assumed with Ions moving with the same initial E /Assumed with Ions moving with the same initial E zm /ions produced by pulse
Potential energy = z V
= neV
n: number of chargese: charge of electron
neV
Kinetic energy = ½ mv2m: ion massv: velocity
1
2
2
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1mvzV
22/
Vt2)/(
2
1 tLm 2
/L
zm
2/ tzm / tzm TOF with MALDI: widespread applications in protein, DNA fragment
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monoisotopic peptide MW<20 ppm (+ 0.2 Da for 1000Da)
1.2 Magnetic Sector
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Ions formed continuously are accelerated toward detectoryby electrical potential V
Under magnetic forcecentripetal force : Bzv B: magnetic field
r
mvBzv
2
centrifugal force :
z: charge of ionv: ion velocity
r
mv 2 Brzm
r
/r v
kinetic E of moving ion at magnet =2mvg g
potential energy = zV
2
m
zVv
2
rBzm /
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under const. V, scan B
only specific m/z passes
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Vzm
2/ – only specific m/z passes
1.3 Quadrupole
most widely used with GC – organic compd.
opposing poles are connected to RF & dc generators
RF/DC = Constant
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-ouput of RF field,energy delivered as sinusoidal wave /c
: RF frequencyf d V V U : amplitude-ouput of dc generator, +V, -V
Resulting two variables
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8
mr
zUa U: amplitude
V: applied dc voltage
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4
mr
zVq
mr
V: applied dc voltager: radius of circle tangent to the inner
surface of the quadrupoemr
a, q have no physical meaning, solution of d.eqn.
If r are constant and a/q= by instrumentU2If r, are constant, and a/q= by instrumentV
8/
Uzm or
4V
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22/
arzm or 22qr
4/
Vzm
22/
qrzm
m/z : varies linearly with Veasy to control instrumentally
to vary rapidly over a scan with little or no lag timebetween scans
Mass Selective Detector (MSD) in GC/MSMass Selective Detector (MSD) in GC/MS
dc generator: 0~200 Vdc generator: 0 200 VRF generator: 0~1200 Vupper limit : 800~1000 in GC/MSpp
~2000 in LC/MS
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Selective Ion Monitoring (SIM)Selective Ion Monitoring (SIM)
: programmed to produce only discrete U or V.
- only specific m/z traverse the analyzer & all others rejected
advantage: enhanced selectivity for detecting low conc. compounds
User selects only few ions (~3): abundant and characteristic structure by increasing y gdwell time (at specific U or V) fraction increases (nanogram to picogram level)
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1.2 Ion Trap (quadrupole ion trap)
RFz0
ac or dc supplied
0
r0
supplied
- Applicable to ESI- Tandem mass spectrometry
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p y
ac or dc supplied
RFz0
r0ac or dc supplied
RFz0
r0
suppliedsupplied
- Ions are trapped in concentric, three dimensional orbits (according to m/z) around the center of the ion trap(according to m/z) around the center of the ion trap.
8zU22
0
4
8
zV
mr
zUaz V: dc voltage to end caps
U: amplitude of RF voltage to ring
220
4
mr
zVqz z0: distance
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- For any given value of q Uzm4
/ For any given value of qz Uqr
zmz
220
/
when instrument scans from low to high Uwhen instrument scans from low to high U, ion motion with higher m/z oscillates larger and largerin z-direction until they finally escape the trapped ion cloudy y p ppand exit to detector.
-more sensitive than transmission quadrupole -mass range : up to m/z 70,000
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General Comparison of Mass Analyzers
Mass range limited to about 3000 m/zi i A
Ease of switching between positive/negative ionS i
Range m/z 3000i 2000
Quadrupole
DisadvantagesAdvantagesmass range
and resolutionMass analyzer
Poor adaptability to MALDISmall sizeRelatively low costSmall sizeMedium resolution
Resolution 2000Q p
Si l d i l tR / 2000 Li it d f t i lI Simple design, low costWell-suited for tandem mass spectrometry(MSn , n≤4)Easy for positive/negative ions
Range m/z 2000Resolution 1500
Not tolerant of high pressureCapable of high resolutionRange m/z 20,000Magnetic sector
Limited mass range of current commercial versions;however, progress is being made in their development
Ion trap
Low resolutionDiffi lt f d t ti t l t
Highest mass rangeV f t d
Range m/z ∞R l ti 350
Time-of-flight(TOF)
g pExpensiveInstrumentation is massiveRelatively slow scanning
p gCapable of exact massMedium mass rangeCan be very reliable,manufacturer dependent
g ,Resolution 10,000
Magnetic sector
Good resolving power has limited m/z rangeLower sensitivity than TOF
Good resolutionVery fast scan speed
Range m/z ∞Resolution 1500
Time-of-flight f t
Difficulty of adaptation to electrospray Very fast scan speedSimple design,low costEase of adaptation to MALDI
Resolution 350
High vacuum (<10-7 Torr) requiredSuperconducting magnet required, expensive
High resolutionWell-suited for tandom mass spectrometry(MSn , n≤4)
Range m/z 10,000Resolution 30,000
Fourier transform-mass spectrometry
yy pSimple design,low costrefrectron
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pInstrumentation massive
( , ≤ )(FT-MS)
2. Other Analysis: Tandem Mass Spectrometry(MS/MS)(MS/MS)
Fragmentation induced by ion-molecule collision (CID)
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Tandem Mass SpectrometryTandem Mass Spectrometry
MS 1 MS 2
Collision CellP1
P2
Tandem MS
MS-1 MS-2Hegas
2
P3
P4
P5
ES Source D t t
Select for a particular ion
HPLC
F1 F2 F3 F4 F5
ES Source Detector
Input: peptides from enzymatic digest
particular ion(peptide)
Output: fragments fromdaughter ionsy g daughter ions
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CID (Collision Induced Dissociation) Patternof a Tryptic Peptide
b series b1
114.1b2
261.2b3
348.2b4
476.3b5
575.3b6
632.3[LFSQVGK+H]+
of a Tryptic Peptide
L F S Q V G Kions
665.4y
518.3 431.3 303.2 204.1 147.1
[LFSQVGK+H]+
=778.4 Da
y series ionsy6 y5 y4 y3 y2 y1
b
K G V Q S FCID
b2
y6
y5spectrum
b4b5
y6
y
b3b6
y4y3y2
y1
58m/z
3. Detectors
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3.1 Faraday Cup
• Create a current
• Induce several 2nd electron
• Small amplification of signal
• Relatively insensitive
• Simple in design
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• Simple in design
3.2 Electron Multiplier
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3.3 Photomultiplier
-Ion initially strike a dynode resulting in the emission of e- strike-Ion initially strike a dynode, resulting in the emission of e strike
a phosphorus screen, release photons
-PM tube is sealed in vacuum
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-Lifetime is 5-year or greater