CH09 - Principles of MS Flashcards
1
Q
What is mass spetrometery?
A
The identification of molecules by determination of their molecular weight
2
Q
What are the capabilities of mass spectrometery?
A
Qualitative and quantitative composition or organic and inorganic analyses in complex mixtures; structures of wide variety of complex species; isotopic ratios of atoms in samples; structure and composition of solid surfaces
3
Q
How is a mass spectrum obtained?
A
- Molecule to gas phase
- Ionize
- Ions separated and detected
4
Q
What are the unique aspects of mass spectrometry as opposed to other types of spectroscopy?
A
Does not look at spectrum of photon energies, but mass/charge ratio; must be done in high vacuum environment; discriminates among molecular and atomic isotopes
5
Q
What units are used in Mass Spectrometry?
A
atomic mass unit (amu) also called daltons
6
Q
List the components of a mass spectrometer
A
Inlet systems, ion sources, mass analyzers, detectors, signal processors and vacuum systems
All components except for inlet systems are held in vacuum.
7
Q
inlet system
A
Introduces small amounts of sample
8
Q
ion sources
A
Convert sample to ions
9
Q
mass analyzers
A
disperses ions by m/x ratio; analogous to monochromatic in photon spectroscopies
10
Q
detectors
A
Converts ion beam into electrical signal
11
Q
vacuum systems
A
Must maintain high vacuum 10e-4 to 10e-8 torr
12
Q
What are the 3 ways to introduce samples into the ion source?
A
Gas expansion (molecular leak), direct insertion\exposure probe, chromatographic inlet
13
Q
What are the methods used to introduce gases in MS (name 2)?
A
- batch inlet; introduce through reservoir then leak the gas through a small aperture (gas expansion)
- chromatographic inlet (GC-MS)
14
Q
What are the methods used to introduce solids in MS? (name 2)
A
- Direct insertion probe; insertion probe with sample held on end
- Direct exposure probe; sample is dissolved to a solution, a drop of the solution is placed on a glass tip and the liquids are evaporated.
15
Q
What are the methods used to introduce liquids in MS? (name 2)
A
- Gas Expansion (molecular leak inlet) for volatile liquids
- Direct insertion probe
- chromatographic inlet (GC-MS)
16
Q
Electrospray Ionization?
A
- When a strong electric field is applied to a liquid passing through a metal capillary, the liquid becomes dispersed into a fine spray of positively or negatively charged droplets - an electrospray.
- The highly charged droplets shrink as the solvent evaporates until the droplets undergo a series of “explosions” due to increasing coulombic repulsion of the electrons as
their droplet surface density increases. - When the droplets become small enough, the analyte ions desorb from the droplets and enter the mass analyzer.
17
Q
Faraday cups advantages
A
- Absolute detector - reliable, can be used to calibrate other detectors
- Budget friendly
18
Q
electron multipliers
A
most common detector for MS; like a PMT without photocathode; each successive dynode held at higher voltage; can detect less than 10e-15 A currents
19
Q
Array Detectors
A
like multichannel array detectors; arrays of metallic electrodes are used, each acting as an individual electron multiplier detector causing an electron cascade; optical coupling by phosphorescent screen converting electrons to light
20
Q
Faraday cups disadvantages
A
- high impedence amplifier limits speed at which it can be scanned (long response)
- The Faraday cup detector has no gain associated with it (unlike dynode-based detectors) => limited
sensitivity of the measurement.
21
Q
Faraday cups principle
A
A metal or carbon cup that serves to capture
ions and store the charge. The resulting current of a few microamperes is measured and amplified.
22
Q
electron multipliers disadvantages
A
The number of secondary electrons released
depends on the type of incident primary particle, its angle and energy (ions with low kinetic energy emit a weak signal).
23
Q
What are the two key functions of ionization sources?
A
produce and remove ions
24
Q
Gas phase vs desorption
A
gas phase: sample volatilized then ionized
desorption: sample probe ionizes sample directly into gaseous ionic state
25
hard vs soft
hard: ionization imparts sufficient energy to rupture bonds, producing a significant number of fragment ions
soft: ionization not as energetic, resulting mass spectrum consists mostly of molecular ion and only a few other peaks
26
What are the physical principles behind electron ionization?
electrons emitted from a heated filament then accelerated. electron path intersects gas sample at right angles. ionization occurs due to electrostatic repulsion
27
What are the primary products in electron ionization?
single charged positive ions
28
What is the efficiency of electron ionization?
not very efficient; 1 in 10e6
29
What is the molecular ion?
radical ion with the same MW as the molecule; ion peak that corresponds to the same MW as the parent
30
What is the base peak?
largest abundance peak or the one with the highest response
31
What are daughter ions?
large number of positive ions of varying masses less than that of the molecular ion
32
Why do peaks appear that are higher in m/z than the M+ peak?
isotope peaks and collisional product peaks
33
What is the most common product in electron ionization?
(M+1)+
34
What are the advantages of electron ionization sources?
convenient, produce large ion currents, good sensitivity, extensive fragmentation allowing for good compound ID
35
What are the disadvantages of electron ionization sources?
extensive fragmentation can lead to disappearance of molecular ion peak, unable to establish MW, must volatilize sample so thermal degradation possible, only applicable when MW < 1000
36
What is the physical principle behind chemical ionization?
gas phase soft source;
1. large excess of reagent gas such as methane, isobutane, or ammonia is introduced into the ionization region.
2. The mixture of reagent gas and sample is subjected to electron bombardment.
3. Ionization of the sample molecules occurs
indirectly by collision with ionized reagent gas molecules and proton or hydride transfer.
37
What are the most widely used reagent gases?
methane, isobutane, ammonia
38
What are the 2 main ionization reactions that occur in chemical ionization?
1. proton transfer\Hydride transfer (e.g. M+CH5+→MH++CH4). forms (M+1)+ (for proton) and (M-1)+ (for hydride) peaks which enable analyte molecule identification.
2. adduct formation (e.g. (M+C2H5)+)
39
What is the most common ionization reaction in chemical ionization?
proton transfer
40
How are reagent ions produced when methane is the reagent gas?
Reactions with high energy electrons
41
What is the most widely used reagent ions when methane is used?
CH5+ and C2H5+
42
How are reagent ions produced when ammonia is the reagent gas?
Proton transfer
43
Which is the more common chemical ionization reaction, proton or hydride transfer?
proton
44
What is the major result with proton transfer?
(M + 1)+
45
What is the major result with hydride transfer?
(M - 1)+
46
How does chemical ionization differ from electron ionization?
less fragmentation; simpler spectra; stronger molecular ion peak; collisions with reagent gas remove excess energy, stabilize parent ion
47
What is the sensitivity of field ionization?
10x less than that of EI
48
How does the solution droplet ionization get transferred to the sample molecules?
High electric field accumulates drops. Charged spray passes through capillary where solvent is evaporated and ions convert to gas
49
What is the charge state of the molecules in electrospray ionization?
multiply charged that increases linearly with MW (e.g. Mn+), especially in biomolecules such as proteins.
50
for what types of sample is electrospray ionization useful?
biomolecules like proteins and polymers with high MW
51
Why can cheap analyzers be used to analyze large molecules in electrospray ionization?
range of m/z values small enough to detect with its narrow "dynamic range" (z>1 yields small m/z values). Also, high z ions have low velocities so their peaks resolve well, even with "cheap" analyzers.
52
What are the advantages of electrospray ionization?
takes place under atmospheric pressures and temperatures; important fo analyzing biomolecules such as proteins and polymers having MW > 10,000; readily adaptable to direct sample introduction; multiple charging allows use of moderate resolution analyzers (z=1 ions do not resolve well due to high velocity but z>1 ions are clearly detected, even with "cheap" analyzers)
53
What type of ionization source is fast atom bombardment?
soft
54
How are Ar atoms generated in fast atom bombardment ionization?
Collision between Ar+ ions from heated rod and Ar atoms. Ar+ collide and pick up charge from Ar atoms, ionizing them. The resulting ionized Ar+ atoms are slow and are filtered out of the system, leaving only fast Ar atoms to ionize the sample molecules.
55
In what form is the sample usually introduced for fast atom bombardment?
liquid, glycerol mull matrix (inert solvent)
56
For what types of samples is fast atom bombardment suited?
high MW polar
57
What is continuous flow fast atom bombardment?
flow solution continuously into probe
58
What are the physical principles of matrix assisted laser desorption?
laser pulses produce gas ions
59
What is the mechanism involved in matrix assisted laser desorption?
laser ablates material from surface, creates microplasm of ions and neutral molecules; vaporizes and ionizes sample; sample mixed in alcohol solution with matrix specifically chosen to absorb UV radiation
60
What are the advantages of matrix assisted laser desorption?
very little fragmentation of analyte ion occurs; very large analyte parent ions can be desorbed; especially useful for polymers and biomolecules
61
What are the disadvantages of matrix assisted laser desorption?
Mechanism of desorption process not completely understood; ionization reactions not fully understood; most widely accepted explanation for ion formation is gas phase proton transfer in the expanding plume with photoionized matrix molecules; sample prep somewhat empirical
62
What is the primary use for secondary ion mass spectrometry?
determining atomic and molecular surface composition
63
What are mass separation methods based on?
the movement if ions with different KE, momentum, or a combination; inherent diffusion rates; motion of charged particles
64
What are the main performance characteristics of a mass analyzer?
separate ionized masses based on m/z to be sent to detector
65
single focusing
ions are separated once. can be done in several ways:
1. accelerates ions with an electric field and separates them with a magnetic field (magnetic sector)
2. time of flight
66
double focusing
ions separated twice, once with electric field once with magnetic. high resolution
67
quadrupole focusing
mass filters
68
ion trap
ions confined by electric/magnetic fields for extended periods of time
69
ICR analyzers
aka Fourier transform, uses principles of ion cyclotron resonance
70
What are the physical principles behind magnetic sector MS?
permanent magnet to steer ionized beam from MS source in circular path. ions of different mass scanned by varying field strength of magnet
71
How are ions sorted by mass in a magnetic sector mass analyzer?
heavier ions travel through sector at lower velocities
v=√(2zV/m)
72
What are the advantages of a magnetic sector MS?
widely used, relatively simple
73
What are the physical principles behind a time of flight MS?
accelerate ions in an electrostatic field to a constant KE, accelerated ions are then released into a field free drift tube. ion velocity depends on mass only, so ions can be sorted by mass according to the time they take to travel through the tube.
74
How are ions sorted in a time of flight MS?
ions of different masses have different velocities but the same KE; sorted temporally (in time) with lighter mass reaching detector first and heavier later
75
What are the advantages of a time of flight MS?
Fast analysis and therefore
1. can avg many mass spectra for sensitivity (increased S/N); usefu
2. for detection of transient species with short lifetimes;
3. suitable for use in a fast GC analysis
Additionaly it is simple; inexpensive
76
What is a reflectron time of flight MS?
variation on a time of flight, focuses arrival times for m/z (reflectron plays a role similar to that of the electric sector in a double sector analyzer)
77
What are the advantages of the reflectron design?
improve resolution;
_Why and how_: sometimes, fragments with the same m/z value obtain different velocities (for reasons similar to those in magnetic\electric sector). This results in broad peaks for given m/z values and poor resolution. A reflectron design "focuses" arrival times of m/z-identical fragments by allowing faster fragments to traverse a longer distance.
78
What is a double focusing analyzer?
the use of two types of mass analyzers in series
79
What are the physical principles behind double focusing MS?
ion beam passes through an electric sector (electrostatic analyzer), which limits KE of ions reaching magnetic sector
80
What is the function of the electric sector analyzer?
elect narrower range of KE than conventionally feasible
81
What is the function of the magnetic sector?
provide directional focusing (elect only fragments with m/z that match B, V and r)
82
What are the advantages and disadvantages of a double focusing analyzer?
advantage: much higher resolution
disadvantage: ions are lost in the electric sector, resulting in a weaker signal.
83
What are the physical principles behind quadrupole mass analyzers?
transmits only ions within a narrow range of m/z. all others neutralized and carried away
84
What is the layout of the quadrupole electrodes?
4 parallel metal rods serving as electrodes held at a DC voltage and modulated with RF frequencies. Ions are introduced into the space between the rods.
85
How do the DC and AC voltages interact with ions of different masses?
if m/z is correct, ion will travel all the way to the detector; if m/z is not correct, ion will collide and neutralize to not be detected;
86
What determines resolution in a quadrupole?
ratio of AC/DC potential
87
What are the advantages of a quadrupole?
compact; rugged; less expensive; low scan time; can be used as chromatography detectors
88
What is a tandem MS?
quadrupole mass analyzers placed in a series
89
Function of first quadrupole in tandem?
output largely molecular ions (parents); serves to separate parent ions by mass selection
90
What is an ion trap?
device in which ions can be formed and confined for extended periods by electric and/or magnetic fields
91
How does an ion trap work?
analyte ions admitted to cell through grid in upper end cap; RF voltage applied to ring electrode confining ions in trap; variable RF voltages applied to ring electrode destabilizes lighter ions that are then swept from cell, passing through lower end cap to detector; mass spectrum obtained by increasing RF amplitude, destabilizing ions of increasing mass
92
What are the advantages of an ion trap?
rugged, less costly, used as GC detectors
93
Why use fourier transforms in mass spectrometry?
improved speed, S/N, sensitivity, and resolution
94
What is the physical principle behind fourier transform mass spectrometry?
ion cyclotron resonance: motion of gas ion in magnetic field becomes circular in a plane perpendicular to the field direction
95
What is an image current?
result of coherent motion of resonant ions, experimentally observed to decrease with time; induced by circular motion of charged ions interacting with detector plates in ICR cell
96
Function of second quadrupole in tandem?
introduce collision gas to fragment parent ions selected by first quadrupole
97
Function of third quadrupole in tandem?
allows mass selective detection of daughter ion fragments
98
the **kinetic energy** of an ion accelerated through a voltage V depends only on \_\_\_\_\_\_ of the ion and the voltage, not on the \_\_\_\_\_\_.
However the velocity of the ion depends on its \_\_\_\_\_\_.
Charge, Mass, Mass
99
Ions of \_\_\_\_\_ travel in circles with \_\_\_\_\_; this is the basis of the separation by m/z.
different m/z, different radii
100
By varying \_\_\_\_ or \_\_\_\_, we can select which m/z will pass through the system.
V (ion acceleration voltage); B (magnetic field in magnetic sector analyzer)
101
The resolving power of a mass spectrometer is defined as its ability to \_\_\_\_\_ and is numerically given by \_\_\_\_\_.
separate ions of two
different m/z values; resolving power=M/ΔM
102
Formula for calculating resolution in ppm?
resolution [ppm] =ΔM/M⋅106
## Footnote
resolution is the inverse of resolving power.
103
Units used in MS to express atomic or molecular masses; defined relative to the mass of the carbon isotope 12/6 , so 1/12 the mass of one neutral 12/6 C atom
Atomic Mass Unit (amu) / Dalton (Da).
104
Refers to the rounded, whole-number precision of an amu measurement
nominal mass
105
Naturally occurring mass of an element in nature
Chemical Atomic Mass / Average Atomic Weight (A).
106
Common abscissa units used for plotting mass spectra; obtained by dividing the atomic or molecular mass of an ion (m) by the number of charges the ion carries
mass-to-charge ratio
107
Method of introducing gaseous or liquid samples directly into MS ionization chamber without the need for preliminary separation stages; usually accomplished by direct injection of small gaseous or liquid volumes into ionization chamber with continuous vacuum; heated inlets are sometimes used to volatilize the sample.
Batch Inlet Sample Introduction
108
Method of introducing solid samples into the MS ionization chamber; direct insertion probe is used where the sample is held onto the end of the probe; probe and sample are introduced into the MS and vacuum then introduced; inlet can be heated to help volatilize the sample.
Direct Probe INlet Sample Introduction
109
Method of introducing gaseous samples into the MS ionization chamber by interfacing the output of a gas chromatograph to the MS ionization chamber
gas chromatographic inlet sample introduction
110
Method of introducing liquid samples into the MS ionization chamber by interfacing the output of a liquid chromatograph to the MS ionization chamber
liquid chromatographic inlet sample introduction
111
Relatively simple MS detector; uses an aligned or tilted collector electrode that is connected to ground through a resistor; voltage drop is amplified using high impedance amplifier
Faraday Cup
112
Most common MS detector; analogous to a photomultiplier tube; ions strike cathode, emitting multiple electrons; each secondary electron strikes a series of intermediate dynodes held at successively higher voltages
electron multiplier
113
MS source in which the sample is first volatilized into the gas phase, then ionized.
Gas-Phase Ionization Source
114
MS source in which the sample is not first volatilized; rather, the sample probe is used to ionize the sample directly from a liquid or solid state into the gaseous ionic state.
Desorption Ionization Source
115
Highly energetic MS source that imparts large energies to the analyte molecule, resulting in bond cleavage and extensive fragmentation; molecular ion peak may be reduced or absent.
Hard Ionization Source
116
Less energetic MS source that produces simpler spectra with relatively little fragmentation; molecular ion peak predominates.
Soft Ionization Source
117
Most common MS ionization method; hard, gas phase source that ionizes molecules due to electrostatic repulsion; primary products are singly charged positive ions; not an efficient ionization process
Electron Ionization Source
118
radical ion that corresponds to the same molecular weight as the parent molecule
Molecular Ion
119
The largest abundance peak, or the one with the highest response, in the mass spectrum.
base peak
120
Large numbers of positive ion peaks that have m/z values less than that of the molecular ion
daughter ions
121
Peaks in the mass spectrum that occur at m/z values greater than that of the molecular ion; these peaks are attributable to ions having the same chemical formula as the molecular ion, but with different isotopic compositions.
Isotope Peaks
122
Peaks in the mass spectrum due to ion-molecule collisions; most common is the peak that gives the protonated molecular ion due to hydrogen ion exchange.
Collision Product Peaks
123
Second most common MS ionization method; gas phase and soft source; ionization process based on gas phase ion-molecule reactions; most commonly used reagent gases are methane, isobutane, and ammonia; main ionization reactions occur in CI through proton transfer (most common), adduct formation, or charge transfer; CI is much gentler ionization source than EI; less fragmentation seen, simpler spectra, much stronger molecular ion peak.
Chemical Ionization Source
124
type of gas phase solvation mechanism used by polar molecules in chemical ionization, in which ions can result from association of analyte molecule M with reagent gas RH+, resulting in (R + M)+, with protonated molecular ion MH+, resulting in (2M + H)+, or with a fragment ion F+, resulting in (F + M)+.
Adduct Formation
125
Chemical ionization mechanism that uses gases with high ionization potential, e.g. rare gases (Xe), N2 or CO as the reagent gas; reaction occurs by charge transfer; less commonly used ionization method.
Charge Transfer
126
Most common mechanism in chemical ionization; gas phase acid-base reaction with reagent ion RH+ (acid) and analyte molecule MH (base) resulting in R and MH2+; results in peak (M + 1)+.
Proton Transfer
127
Gas phase acid-base reaction mechanism seen in chemical ionization with reagent ion RH+ (base) and analyte molecule MH (acid) resulting in M+ and RH2; results in peak (M – 1)+.
Hydride Transfer
128
Gas phase, soft ionization method in which the ions are formed under the influence of a large electric field; special fine tungsten wires with carbon dendrites are used as electron emitters; results in very little fragmentation, mostly forms molecular ions
Field Ionization Source
129
Ionization method in which sample solution is pumped at atmospheric pressure through stainless steel capillary needle at a rate of 1-10 μL min-1; needle is maintained at 3-6 kV with respect to surrounding electrode (electric fields of ~10e6 V m-1); high electric field results in charge accumulation in droplet spray of molecules; as solution droplets become smaller as a consequence of solvent evaporation, charge density becomes greater and desorption of ions into ambient gas phase occurs; ions formed are multiply charged so that their m/z values are small enough to detect with analyzers such as quadrupoles; little fragmentation occurs, application to large biomolecules.
Electrospray Ionization (ESI)
130
Desorption technique in which condensed-phase sample is bombarded with energetic (keV) Ar0 atoms; sample is typically in form of glycerol mull; used for high molecular weight (10 kDa) polar samples, especially good for biomolecules and polymers.
Fast Atom Bombardment (FAB)
131
Desorption technique in which sample is mixed into a UV- absorbing organic matrix and applied to the surface of a metallic probe; probe surface is exposed to UV (250 – 350 nm) excimer laser pulses; matrix absorbs laser pulses causing rapid heating and sublimation of matrix along with ionization of analyte; ion is directed into TOF MS for analysis; spectrum is recorded between laser pulses; little fragmentation occurs, molecular ion peak predominates; very useful for large (>100 kDa) polymers and biomolecules.
Matrix Assisted Laser Desorption (MALDI)
132
High vacuum surface analysis technique in which surface is bombarded with high energy (5 – 20 keV) ions (e.g. Ar+); impact of primary Ar+ ions hitting surface causes surface layer of atoms to be stripped or sputtered off; these secondary ions are then directed into a mass spectrometer; very surface sensitive – samples only the first few atomic or molecular layers.
Secondary Ion Mass Spectrometry (SIMS)
133
The ability of a mass analyzer to yield distinct signals for two ions with a small m/z difference.
Mass Resolution, R
134
Single-focusing mass analyzer that uses a permanent magnet or electromagnet to steer the beam of ionized molecules from the MS source; magnet causes ion beam to travel in a circular path of 60° / 90° /180°; ions of different mass are scanned by varying the field strength of the magnet; ions are separated spatially, i.e. sorted in space.
Magnetic Sector Mass Analyzer
135
The product of the mass and velocity of a molecule
Momentum
136
The circular path that ions travel in the magnetic field of a magnetic sector MS instrument; most common values are 60°, 90°, or 180°
degree of deflection
137
Single-focusing mass analyzer that uses a pulsed ionization source (e.g. a laser) to generate ions and accelerates them in an electrostatic field to constant kinetic energy; after leaving source, ions enter a field-free drift tube; since all ions entering the drift tube have the same kinetic energy, their velocities vary inversely with their masses; lighter masses reach detector first, heavier masses later; ions are sorted temporally, i.e. sorted in time.
Time-of-Flight Mass Analyzer
138
Electric- and magnetic-field free region of length L in a time-of-flight mass spectrometer; section of TOF MS in which ion separation takes place
Drift Tube
139
Type of TOF MS that incorporates a static or time-dependent electric field as an ion mirror to reverse the direction of travel of the ions entering the TOF drift tube; substantially diminishes the spread of flight times of all ions with the same m/z caused by the spread in kinetic energy of these ions, thereby increasing resolution.
Reflectron Time-of-Flight Mass Analyzer
140
Refers to the use of two types of mass analyzers in series – electrostatic analyzer (electric sector) followed by a magnetic analyzer – that takes into account both the directional and kinetic energy distributions of the ions to increase resolution; ion beam passes first through electric sector (electrostatic analyzer), which limits the kinetic energy of ions reaching the magnetic sector analyzer to a small range; magnetic sector analyzer then provides a homogeneous B-field to directionally focus a given mass by momentum & radius of curvature onto the detector; resolution can be up to R ~ 10e5 in some instruments.
Double-Focusing Mass Analyzer
141
consists of 4 parallel metal rods that serve as electrodes; these 4 rods are held at a DC voltage, and modulated with AC RF frequencies; At any given set of AC/DC operating conditions, it will transmit only ions within a narrow range of m/z; all other ions are neutralized and carried away as uncharged molecules; mass is tuned by tuning the AC RF frequency; varying the electrical signals to the quadrupole makes possible the variation of the range of m/z values transmitted, therefore spectral scanning is possible.
Quadrupole Mass Analyzer / Mass Filter
142
usually 3 quadrupole mass analyzers placed in series; first quadrupole : output is largely molecular ions; serves to separate parent ions by mass selection; second quadrupole : introduce collision gas here (e.g. N2 or He) to fragment parent ions selected by first quadrupole; third quadrupole : allows mass selective detection of daughter ion fragments, provides a spectrum rich in structure.
Tandem Mass Spectrometer
143
Device in which ions can be formed and confined for extended periods by electric and/or magnetic fields; formed from central doughnut-shaped ring electrode and pair of end-cap electrodes; ions confined between electrodes of a particular shape that resembles operation of a quadrupole; analyte ions admitted to cell through grid in upper end cap; RF voltage applied to ring electrode: confines ions in trap; variable RF voltages applied to ring electrode: destabilizes lighter ions; lighter ions swept from cell, pass through lower end cap into detector; mass spectrum obtained by increasing RF amplitude, which destabilizes ions of increasing mass
Ion Trap Mass Spectrometer
144
Determines m/z of ions based on the cyclotron resonance frequency of ions in a fixed magnetic field; uses trapped ion analyzer cell; gaseous molecules in cell are ionized by pulsed electron beam from source filament; ions held in place by 1 - 5 V potential applied to trap plate; ions are accelerated by RF-frequency pulse applied to transmitter plate; after RF pulse, image current is detected as FID; time domain FID is then Fourier transformed into mass domain spectrum; very high resolution MS technique that can determine masses with high accuracy.
Fourier Transform Mass Spectrometer
145
Phenomena related to the motion of gaseous ions in a magnetic field; path of these ions becomes circular in a plane perpendicular to the B field direction
Ion Cyclotron Resonance (ICR)
146
Angular frequency of the motion of gaseous ions in a magnetic field perpendicular to the field direction; depends inversely on the m/z value
Cyclotron Resonance Frequency ωc
147
Result of the coherent motion of all resonant ions with a particular cyclotron resonant frequency ; induced by circular motion of charged ions interacting with detector plates in ICR cell; coherent character of circulating ions eventually lost due to collisions; decay provides time domain signal detected by detector plates in ICR cell.
Image Current
148
FT-MS ICR cell which is conceptually similar to an ion trap; ion source, analyzer and detector are all incorporated into the same cell; gaseous molecules in the cell are ionized by pulsed electron beam from source filament; ions held in place by 1 – 5 V potential applied to trap plates; ions accelerated by RF pulse applied to transmitter plate; ions circulate for extended periods (circular motion due to magnetic field passing through trap); after RF pulse, image current detected by detector plates.
Trapped Ion Analyzer Cell (ICR instrument)
149
Is MALDI (matrix assisted laser desorption) a hard or soft ionization method?
Soft (large molecular peak)
## Footnote
Additionally, MALDI usually creates z=1 ions.
150
What is a DeESI (Desorption Electrospray Ionization) source?
1. Under ambient atmospheric conditions, an **electrically charged** cloud of solvent droplets is directed at a small spot on a sample surface, only a few mm distant.
2. The charged mist is pulled to the surface by a voltage of opposite polarity applied to it.
3. The droplet charge is transferred to analytes on the surface, and the resultant ions travel into an atmospheric pressure interface similar to that in ESI.
## Footnote
Does not require presence of analyte in vacuum\special conditions. Used in airports to detect explosives in passenger luggage.
151
What is a DART (Direct Analysis in Real Time) Ionization source?
- Operates in atmospheric pressure (like DeESI source).
1. A carrier gas (He or N2) is ionized in a plasma.
2. The ionized carrier gas passes through a heated chamber, where it **recombines** with an electron, but forms a relatively stable excited atom M*.
3. M* then ionizes the analyte. The ionized analyte is then drawn into MS chamber.
- analyte ions are pulled out at an angle by a charged lens into the MS analyzer
stage inlet, while neutral contaminants is directed to
a trapping region and pumped away.
152
In CI, a diagnostic peak to molecular ion are...
M+1, M-1 peaks (in CI, ions are formed by proton or hydride transfer)
