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Instrumental Analysis > Ionization Sources in Mass Spec (6) > Flashcards

Ionization Sources in Mass Spec (6) Flashcards

1
Q

What are the two key functions of ionization sources?

A

produce and remove ions

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2
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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3
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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4
Q

What are the physical principles behind electron ionization?

A

gas phase hard source; 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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5
Q

What are the primary products in electron ionization?

A

single charged positive ions

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6
Q

What is the efficiency of electron ionization?

A

not very efficient; 1 in 10e6

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7
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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8
Q

What is the base peak?

A

largest abundance peak or the one with the highest response

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9
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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10
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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11
Q

What is the most common product in electron ionization?

A

(M+1)+

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12
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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13
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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14
Q

What is the physical principle behind chemical ionization?

A

gas phase soft source; based on gas phase ion molecule reactions. ions produced by bombardment of a reagent gas with electrons from a filament source

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15
Q

What are the most widely used reagent gases?

A

methane, isobutane, ammonia

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16
Q

What are the 3 main ionization reactions that occur in chemical ionization?

A

proton transfer, adduct formation, charge transfer

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17
Q

What is the most common ionization reaction in chemical ionization?

A

proton transfer

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18
Q

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

A

Reactions with high energy electrons

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19
Q

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

A

CH5+ and C2H5+

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20
Q

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

A

ions react through proton transfer to analyte molecules

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21
Q

What are the most widely used reagent ions when isobutane is the reagent gas?

A
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22
Q

Which types of molecules are useful to study with isobutane?

A

Polar

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23
Q

Which types of molecules are inefficient to study with isobutane?

A
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24
Q

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

A

Proton transfer

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25
Q

What are the primary reactions when ammonia is the reagent gas?

A

Ammonium ion and a radical

26
Q

What are the most widely used reagent ions when ammonia is the reagent gas?

A

most basic molecules, like amines

27
Q

What types of molecules are useful to study with ammonia?

A

polar molecules and hydrogen bonding molecules with no basic character

28
Q

What type of molecules are inefficient to study with ammonia?

A

alkanes, ethers, aromatics

29
Q

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

A

proton

30
Q

What is the major result with proton transfer?

A

(M + 1)+

31
Q

What is the major result with hydride transfer?

A

(M - 1)+

32
Q

How does chemical ionization differ from electron ionization?

A

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

33
Q

What are the special types of emitters used in field ionization?

A

fine tungsten wire, dendrites

34
Q

What is the sensitivity of field ionization?

A

10x less than that of EI

35
Q

How is the sample solution ionized in electrospray ionization?

A

resonance electron tunneling forming radical cation

36
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

37
Q

What atmospheric temperatures and pressures are needed for electrospray ionization?

A

288.15 K and 1 atm

38
Q

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

A

multiply charged that increases linearly with MW

39
Q

for what types of sample is electrospray ionization useful?

A

biomolecules like proteins and polymers with high MW

40
Q

Why can small analyzers having R = 1500 be used to analyze large molecules in electrospray ionization?

A

m/z values small enough to detect

41
Q

Why is electrospray ionization particularly useful for chromatographic interfaces?

A

Readily adaptable to direct sample introduction

42
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

43
Q

What are the disadvantages of electrospray ionization?

A

electrospray ionization procedure not identical for all compounds in a multi component mix; selective suppression or enhancement of peaks is observed; formation of ions is the result of an electrochemical process and of charge accumulation; electrospray current limited and sensitive to concentration rather than total amount of analyte

44
Q

how is field desorption different from field ionization?

A

it is applicable to other than gas phase samples

45
Q

What type of sample is best suited for field desorption?

A

high MW nonpolar species

46
Q

What is the major drawback of field desorption?

A

thermal degradation

47
Q

What type of ionization source is fast atom bombardment?

A

soft

48
Q

How are Ar atoms generated in fast atom bombardment?

A

Collision?

49
Q

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

A

liquid, glycerol mull matrix

50
Q

For what types of samples is fast atom bombardment suited?

A

high MW polar

51
Q

What is continuous flow fast atom bombardment?

A

flow solution continuously into probe

52
Q

What are the physical principles of matrix assisted laser desorption?

A

laser pulses produce gas ions

53
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

54
Q

What matrices are commonly used in matrix assisted laser desorption?

A

sinapinic acids, nicotinic acid

55
Q

What laser wavelengths are commonly used in matrix assisted laser desorption and why?

A

250 to 350 nm

56
Q

For what types of samples is matrix assisted laser desorption useful?

A

biopolymers in MW range of 1000 to 10,000 Da

57
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

58
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

59
Q

How does secondary ion mass spectrometry differ from other desorption techniques?

A

Determine both atomic and molecular surface compositions, high vacuum surface analysis, energetic ion beam, used secondary ions; complex spectra and destructive

60
Q

What is the primary use for secondary ion mass spectrometry?

A

determining atomic and molecular surface composition

Instrumental Analysis (45 decks)

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