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Instrumental Analysis Noa > CH09 - Principles of MS > Flashcards

CH09 - Principles of MS Flashcards

1
Q

What is mass spetrometery?

A

The identification of molecules by determination of their molecular weight

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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

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3
Q

How is a mass spectrum obtained?

A
  1. Molecule to gas phase
  2. Ionize
  3. Ions separated and detected
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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

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5
Q

What units are used in Mass Spectrometry?

A

atomic mass unit (amu) also called daltons

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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.

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7
Q

inlet system

A

Introduces small amounts of sample

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8
Q

ion sources

A

Convert sample to ions

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9
Q

mass analyzers

A

disperses ions by m/x ratio; analogous to monochromatic in photon spectroscopies

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10
Q

detectors

A

Converts ion beam into electrical signal

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11
Q

vacuum systems

A

Must maintain high vacuum 10e-4 to 10e-8 torr

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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

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13
Q

What are the methods used to introduce gases in MS (name 2)?

A
  1. batch inlet; introduce through reservoir then leak the gas through a small aperture (gas expansion)
  2. chromatographic inlet (GC-MS)
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14
Q

What are the methods used to introduce solids in MS? (name 2)

A
  1. Direct insertion probe; insertion probe with sample held on end
  2. 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.
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15
Q

What are the methods used to introduce liquids in MS? (name 2)

A
  1. Gas Expansion (molecular leak inlet) for volatile liquids
  2. Direct insertion probe
  3. chromatographic inlet (GC-MS)
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16
Q

Electrospray Ionization?

A
  1. 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.
  2. 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.
  3. When the droplets become small enough, the analyte ions desorb from the droplets and enter the mass analyzer.
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17
Q

Faraday cups advantages

A
  • Absolute detector - reliable, can be used to calibrate other detectors
  • Budget friendly
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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

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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

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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.
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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.

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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).

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23
Q

What are the two key functions of ionization sources?

A

produce and remove ions

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24
Q

Gas phase vs desorption

A

gas phase: sample volatilized then ionized
desorption: sample probe ionizes sample directly into gaseous ionic state

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25
Q

hard vs soft

A

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

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26
Q

What are the physical principles behind electron ionization?

A

electrons emitted from a heated filament then accelerated. electron path intersects gas sample at right angles. ionization occurs due to electrostatic repulsion

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27
Q

What are the primary products in electron ionization?

A

single charged positive ions

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28
Q

What is the efficiency of electron ionization?

A

not very efficient; 1 in 10e6

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29
Q

What is the molecular ion?

A

radical ion with the same MW as the molecule; ion peak that corresponds to the same MW as the parent

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30
Q

What is the base peak?

A

largest abundance peak or the one with the highest response

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31
Q

What are daughter ions?

A

large number of positive ions of varying masses less than that of the molecular ion

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32
Q

Why do peaks appear that are higher in m/z than the M+ peak?

A

isotope peaks and collisional product peaks

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33
Q

What is the most common product in electron ionization?

A

(M+1)+

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34
Q

What are the advantages of electron ionization sources?

A

convenient, produce large ion currents, good sensitivity, extensive fragmentation allowing for good compound ID

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35
Q

What are the disadvantages of electron ionization sources?

A

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

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36
Q

What is the physical principle behind chemical ionization?

A

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.

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37
Q

What are the most widely used reagent gases?

A

methane, isobutane, ammonia

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38
Q

What are the 2 main ionization reactions that occur in chemical ionization?

A
  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)+)
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39
Q

What is the most common ionization reaction in chemical ionization?

A

proton transfer

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40
Q

How are reagent ions produced when methane is the reagent gas?

A

Reactions with high energy electrons

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41
Q

What is the most widely used reagent ions when methane is used?

A

CH5+ and C2H5+

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42
Q

How are reagent ions produced when ammonia is the reagent gas?

A

Proton transfer

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43
Q

Which is the more common chemical ionization reaction, proton or hydride transfer?

A

proton

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44
Q

What is the major result with proton transfer?

A

(M + 1)+

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45
Q

What is the major result with hydride transfer?

A

(M - 1)+

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46
Q

How does chemical ionization differ from electron ionization?

A

less fragmentation; simpler spectra; stronger molecular ion peak; collisions with reagent gas remove excess energy, stabilize parent ion

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47
Q

What is the sensitivity of field ionization?

A

10x less than that of EI

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48
Q

How does the solution droplet ionization get transferred to the sample molecules?

A

High electric field accumulates drops. Charged spray passes through capillary where solvent is evaporated and ions convert to gas

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49
Q

What is the charge state of the molecules in electrospray ionization?

A

multiply charged that increases linearly with MW (e.g. Mn+), especially in biomolecules such as proteins.

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50
Q

for what types of sample is electrospray ionization useful?

A

biomolecules like proteins and polymers with high MW

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51
Q

Why can cheap analyzers be used to analyze large molecules in electrospray ionization?

A

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.

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52
Q

What are the advantages of electrospray ionization?

A

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)

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53
Q

What type of ionization source is fast atom bombardment?

A

soft

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54
Q

How are Ar atoms generated in fast atom bombardment ionization?

A

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.

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55
Q

In what form is the sample usually introduced for fast atom bombardment?

A

liquid, glycerol mull matrix (inert solvent)

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56
Q

For what types of samples is fast atom bombardment suited?

A

high MW polar

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57
Q

What is continuous flow fast atom bombardment?

A

flow solution continuously into probe

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58
Q

What are the physical principles of matrix assisted laser desorption?

A

laser pulses produce gas ions

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59
Q

What is the mechanism involved in matrix assisted laser desorption?

A

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

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60
Q

What are the advantages of matrix assisted laser desorption?

A

very little fragmentation of analyte ion occurs; very large analyte parent ions can be desorbed; especially useful for polymers and biomolecules

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61
Q

What are the disadvantages of matrix assisted laser desorption?

A

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
Q

What is the primary use for secondary ion mass spectrometry?

A

determining atomic and molecular surface composition

63
Q

What are mass separation methods based on?

A

the movement if ions with different KE, momentum, or a combination; inherent diffusion rates; motion of charged particles

64
Q

What are the main performance characteristics of a mass analyzer?

A

separate ionized masses based on m/z to be sent to detector

65
Q

single focusing

A

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
Q

double focusing

A

ions separated twice, once with electric field once with magnetic. high resolution

67
Q

quadrupole focusing

A

mass filters

68
Q

ion trap

A

ions confined by electric/magnetic fields for extended periods of time

69
Q

ICR analyzers

A

aka Fourier transform, uses principles of ion cyclotron resonance

70
Q

What are the physical principles behind magnetic sector MS?

A

permanent magnet to steer ionized beam from MS source in circular path. ions of different mass scanned by varying field strength of magnet

71
Q

How are ions sorted by mass in a magnetic sector mass analyzer?

A

heavier ions travel through sector at lower velocities
v=√(2zV/m)

72
Q

What are the advantages of a magnetic sector MS?

A

widely used, relatively simple

73
Q

What are the physical principles behind a time of flight MS?

A

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
Q

How are ions sorted in a time of flight MS?

A

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
Q

What are the advantages of a time of flight MS?

A

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
Q

What is a reflectron time of flight MS?

A

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
Q

What are the advantages of the reflectron design?

A

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
Q

What is a double focusing analyzer?

A

the use of two types of mass analyzers in series

79
Q

What are the physical principles behind double focusing MS?

A

ion beam passes through an electric sector (electrostatic analyzer), which limits KE of ions reaching magnetic sector

80
Q

What is the function of the electric sector analyzer?

A

elect narrower range of KE than conventionally feasible

81
Q

What is the function of the magnetic sector?

A

provide directional focusing (elect only fragments with m/z that match B, V and r)

82
Q

What are the advantages and disadvantages of a double focusing analyzer?

A

advantage: much higher resolution
disadvantage: ions are lost in the electric sector, resulting in a weaker signal.

83
Q

What are the physical principles behind quadrupole mass analyzers?

A

transmits only ions within a narrow range of m/z. all others neutralized and carried away

84
Q

What is the layout of the quadrupole electrodes?

A

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
Q

How do the DC and AC voltages interact with ions of different masses?

A

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
Q

What determines resolution in a quadrupole?

A

ratio of AC/DC potential

87
Q

What are the advantages of a quadrupole?

A

compact; rugged; less expensive; low scan time; can be used as chromatography detectors

88
Q

What is a tandem MS?

A

quadrupole mass analyzers placed in a series

89
Q

Function of first quadrupole in tandem?

A

output largely molecular ions (parents); serves to separate parent ions by mass selection

90
Q

What is an ion trap?

A

device in which ions can be formed and confined for extended periods by electric and/or magnetic fields

91
Q

How does an ion trap work?

A

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
Q

What are the advantages of an ion trap?

A

rugged, less costly, used as GC detectors

93
Q

Why use fourier transforms in mass spectrometry?

A

improved speed, S/N, sensitivity, and resolution

94
Q

What is the physical principle behind fourier transform mass spectrometry?

A

ion cyclotron resonance: motion of gas ion in magnetic field becomes circular in a plane perpendicular to the field direction

95
Q

What is an image current?

A

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
Q

Function of second quadrupole in tandem?

A

introduce collision gas to fragment parent ions selected by first quadrupole

97
Q

Function of third quadrupole in tandem?

A

allows mass selective detection of daughter ion fragments

98
Q

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 ______.

A

Charge, Mass, Mass

99
Q

Ions of _____ travel in circles with _____; this is the basis of the separation by m/z.

A

different m/z, different radii

100
Q

By varying ____ or ____, we can select which m/z will pass through the system.

A

V (ion acceleration voltage); B (magnetic field in magnetic sector analyzer)

101
Q

The resolving power of a mass spectrometer is defined as its ability to _____ and is numerically given by _____.

A

separate ions of two
different m/z values; resolving power=M/ΔM

102
Q

Formula for calculating resolution in ppm?

A

resolution [ppm] =ΔM/M⋅106

resolution is the inverse of resolving power.

103
Q

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

A

Atomic Mass Unit (amu) / Dalton (Da).

104
Q

Refers to the rounded, whole-number precision of an amu measurement

A

nominal mass

105
Q

Naturally occurring mass of an element in nature

A

Chemical Atomic Mass / Average Atomic Weight (A).

106
Q

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

A

mass-to-charge ratio

107
Q

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.

A

Batch Inlet Sample Introduction

108
Q

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.

A

Direct Probe INlet Sample Introduction

109
Q

Method of introducing gaseous samples into the MS ionization chamber by interfacing the output of a gas chromatograph to the MS ionization chamber

A

gas chromatographic inlet sample introduction

110
Q

Method of introducing liquid samples into the MS ionization chamber by interfacing the output of a liquid chromatograph to the MS ionization chamber

A

liquid chromatographic inlet sample introduction

111
Q

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

A

Faraday Cup

112
Q

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

A

electron multiplier

113
Q

MS source in which the sample is first volatilized into the gas phase, then ionized.

A

Gas-Phase Ionization Source

114
Q

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.

A

Desorption Ionization Source

115
Q

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.

A

Hard Ionization Source

116
Q

Less energetic MS source that produces simpler spectra with relatively little fragmentation; molecular ion peak predominates.

A

Soft Ionization Source

117
Q

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

A

Electron Ionization Source

118
Q

radical ion that corresponds to the same molecular weight as the parent molecule

A

Molecular Ion

119
Q

The largest abundance peak, or the one with the highest response, in the mass spectrum.

A

base peak

120
Q

Large numbers of positive ion peaks that have m/z values less than that of the molecular ion

A

daughter ions

121
Q

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.

A

Isotope Peaks

122
Q

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.

A

Collision Product Peaks

123
Q

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.

A

Chemical Ionization Source

124
Q

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)+.

A

Adduct Formation

125
Q

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.

A

Charge Transfer

126
Q

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)+.

A

Proton Transfer

127
Q

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)+.

A

Hydride Transfer

128
Q

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

A

Field Ionization Source

129
Q

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.

A

Electrospray Ionization (ESI)

130
Q

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.

A

Fast Atom Bombardment (FAB)

131
Q

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.

A

Matrix Assisted Laser Desorption (MALDI)

132
Q

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.

A

Secondary Ion Mass Spectrometry (SIMS)

133
Q

The ability of a mass analyzer to yield distinct signals for two ions with a small m/z difference.

A

Mass Resolution, R

134
Q

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.

A

Magnetic Sector Mass Analyzer

135
Q

The product of the mass and velocity of a molecule

A

Momentum

136
Q

The circular path that ions travel in the magnetic field of a magnetic sector MS instrument; most common values are 60°, 90°, or 180°

A

degree of deflection

137
Q

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.

A

Time-of-Flight Mass Analyzer

138
Q

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

A

Drift Tube

139
Q

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.

A

Reflectron Time-of-Flight Mass Analyzer

140
Q

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.

A

Double-Focusing Mass Analyzer

141
Q

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.

A

Quadrupole Mass Analyzer / Mass Filter

142
Q

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.

A

Tandem Mass Spectrometer

143
Q

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

A

Ion Trap Mass Spectrometer

144
Q

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.

A

Fourier Transform Mass Spectrometer

145
Q

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

A

Ion Cyclotron Resonance (ICR)

146
Q

Angular frequency of the motion of gaseous ions in a magnetic field perpendicular to the field direction; depends inversely on the m/z value

A

Cyclotron Resonance Frequency ωc

147
Q

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.

A

Image Current

148
Q

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.

A

Trapped Ion Analyzer Cell (ICR instrument)

149
Q

Is MALDI (matrix assisted laser desorption) a hard or soft ionization method?

A

Soft (large molecular peak)

Additionally, MALDI usually creates z=1 ions.

150
Q

What is a DeESI (Desorption Electrospray Ionization) source?

A
  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.

Does not require presence of analyte in vacuum\special conditions. Used in airports to detect explosives in passenger luggage.

151
Q

What is a DART (Direct Analysis in Real Time) Ionization source?

A
  • 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
Q

In CI, a diagnostic peak to molecular ion are…

A

M+1, M-1 peaks (in CI, ions are formed by proton or hydride transfer)

Instrumental Analysis Noa (7 decks)

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