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Analytical Spectroscopy > AMS > Flashcards

AMS Flashcards

1
Q

steps in atomic mass spectrometic analysis

A
  1. atomization
    2 ionization
  2. separation of the ions
  3. Counting the number of ions
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2
Q

Atomization

A

conversion of sample to atoms

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3
Q

Ionization

A

conversion of atoms to a stream of ions (usually positive)

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4
Q

Separation of ions

A

separate based on their mass-to-charges (m/s)

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5
Q

Counting the number of ions

A
  • counting each types

- or measuring ion current produced when the ions formed from sample strike a suitable transducer

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6
Q

plot for mass spec

A

intensity v. m/s

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7
Q

atomic masses of isotopes are ____

A

discriminated

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8
Q

what is nominal mass

A

implies a while number precision in a mass measurement… if you get 17.3, your nominal mass is 17

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9
Q

what is the detector response

A

mass spectrum

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10
Q

two types of mass spectra

A
  1. Atomic mass spectra

2. Molecular mass spectra

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11
Q

components of mass spectrometer

A
  1. inlet
  2. ion source
  3. mass analyzer
  4. Vacuum pump
  5. ion transducer
  6. signal processor and readout
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12
Q

inet

A

introduction of sample to ion source

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13
Q

ion source

A

produces stream of ions which are accelerated to mass analyzer

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14
Q

mass analzer

A

separates ions base don m/z ratios

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15
Q

ion transducer

A

converts ion beam to electrical signal

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16
Q

signal processor

A

processes signals from transducer

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17
Q

readout

A

plot of signal v. m/z

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18
Q

Types of inlets

A
  1. nebulizers
  2. electrothermal vaporization
  3. Hydride generation
  4. chromatography (GC, LC, SFC)
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19
Q

vacuum pump

A
  • maintains low pressure in ion source, analyzer and ion transducer
  • no air should be present, rather O2 and N2
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20
Q

ion sources in AMS

A
  1. ICP
  2. DCP
  3. MIP
  4. Spark source (SS)
  5. Thermal ionization (TI)
  6. Glow discharge (GD)
  7. Laser Microbe (LM)
  8. Secondary ionization (SI)
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21
Q

Samples must be ___, ___, and ___ in AMS

A

vaporized
atomized
ionized

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22
Q

what dont we need in AMS

A

excitation

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23
Q

Types of mass analyzers

A
  1. magnetic sector analyzers
  2. double focusing analyzers
  3. QUADRUPOLE ANALYZERS
  4. Time of flight analyzers
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24
Q

Magnetic sector analyzer

A

ions are accelerated through a magnetic sector , where they are separated based on mass ratios

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25
Q

in magnetic sector analyzer, which is more positive slit A or B

A

A is more positive than B

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26
Q

Small molecules are (more or less) affected by magnetic field in magnetic sector

A

more

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27
Q

Large molecules are (more or less) affected by magnetic field in magnetic sector

A

less

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28
Q

disadvantage of magnetic sector analyzer

A

not a high enough resolution

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29
Q

what forces are ions subject to in magnetic sector analyzer

A 
magnetic field (Fm)
centripital force (Fc)
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30
Q

how can mass sprectra be obtained in magnetic sector analysis

A

by varying one of the following and keeping the other two constant:

  1. Magnetic field strength (B)
  2. Voltage difference between slits A and B (V)
  3. circulat path radius (r)
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31
Q

modern magnetic sector MS vary which variable

A

Magnetic field strength (B)

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32
Q

what does double focus analyzer employ

A

electrostatic and magnetic sector analysis

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33
Q

what does the electrostatic analyzer do in double focus

A

separates ions based on kinetic energies

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34
Q

do ions get separated by mass or kinetic energies first in double focus analysis

A
  1. kinetic energies

2. then mass

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35
Q

what is quadrupole mass analyzer composed of

A

4 parallel cylinder rods that act as electrodes

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36
Q

how are the cylindrical rods set up in quadrupole

A

+,-,+,-

- rods connected directly to a variable dc power source such that one pair is positive and one is negative

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37
Q

with dc, what is applied to each quadrupole rod

A
  • variable radio frequency

- ac voltages

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38
Q

what influence is int he xz plane of quadrupole

A

positive rods

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39
Q

in xz plane of quadrupole, if no ac current is applied, what is the result

A
  • big and small ions go through

- no fltering

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40
Q

in xz plane of quadrupole, if ac current is applied, what is the result

A

-big ions will make it through, small ions will not make it through

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41
Q

why do big ions make it and small ions not make it through xz plane of quadrupole, if ac current is applied

A
  • big ions take a lot of time to respond to directional changes so they pass the filter
  • small ions respond strongly to direction change so they end up hitting the rod and are neutralized and do not pass the filter
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42
Q

what influence is int he yz plane of quadrupole

A

negative rods

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43
Q

in yz plane of quadrupole, if no ac current is applied, what is the result

A

No ions will pass

44
Q

in yz plane of quadrupole, if ac current is applied, what is the result

A

-small ions make it through, big ions will not make it through

45
Q

why do small ions make it and big ions not make it through in yz plane of quadrupole, if ac current is applied

A

the ac voltage offsets the dc effect so the small ions make it, while the big ions get attracted to the negative rods

46
Q

in the both planes, why is there a narrow window of m/z that pass

A

bc ions must be heavy enough to not be eliminated by high mass filter and light enough to not be removed by low mass filter

47
Q

for a mass spectrum to be obtained on this device, what must be

A
  1. ions accelerated into space between rods by potential difference of 5-10 V
  2. AC and DC voltages are increased simultaneously while maintaining a constant ratio
  3. All ions except those in a limited m/z range strike rod and converted to neutral atoms
  4. only ions int he narrow m/z range reach the transducer
48
Q

what else is quadrupole called

A

mass filter

49
Q

how can selectivity be increased using quadrupoles

A

by placing multiple quadrupoles in series, the selectivity can be increased

50
Q

how are ions formed during time of flight mass analyzers

A

formed during pulse laser impact

51
Q

what happens in drift tube of time of flight mass analyzer

A

ions spread due to their different velocities

52
Q

what happens at ion detector of time of flight mass analyzer

A

ions reach detector one by one

53
Q

order of ion arrival in time of flight mass analyzers

A

smaller ions move quicker so they arrive first

larger ions move slower so their arrive last

54
Q

downfall of time of flight mass analyzer

A

it must have a fast respinse

55
Q

what are ions accelerated by in time of flight mass analysis

A

electric field pulse

56
Q

what are typical flight times

A

1-50 µs

57
Q

what gives you a better resolution in time of flight mass analysis

A

a longer drift tube

58
Q

advantages of time of flight mass analysis

A
  • simple
  • ruggedness
  • ease of accessibility of ion source
  • virtually unlimited mass range
59
Q

disadvantages of time of flight mass analysis

A
  • limited resolution
  • limited sensitivity
  • requires fast electronics due to ion arrival at transducer are only fractions of seconds appart
60
Q

AMS transudcers

A
  1. electron multiplies
  2. Faraday cup
  3. Array transducer
61
Q

In discrete electron multiplier, what does ion beam strike

A

Cu/Be oxide cathode with enough energy to create a burst of electrons

62
Q

After the Cu/Be oxide electrode, what happens to the electrons in discrete electron multiplier

A

they bounce between multiple dynode surfaces to create several more electrons

63
Q

In 20 discrete electron multiplier dynode, how many electrons are produced for 1 electron

A

10,000,000 electrons

64
Q

What is the most sensitive and common transducer in AMS

A

continuous- dynode electron multiplier

65
Q

what is the continuous- dynode electron multiplier composed of and what’s its shape

A

Horn shaped glass doped with Pb

66
Q

What electron gain can be seen in continuous- dynode electron multiplier

A

100000-100000000 electrons

67
Q

what does the ion beam hit to produce electrons then continue to hit

A

restive conductive surface

68
Q

In Faraday cup what do ions strike

A

the collector electrode,surrounded by a cage that prevents the escape of reflected ions and ejected secondary electrons

69
Q

In Faraday cup, what are the collector electrode and cage attached to

A

ground through large resistor

70
Q

In faraday cup, how are positive ions neutralized

A

by electrons flowing from ground through the resistor

71
Q

In faraday cup, how is the voltage drop amplified

A

by a high-impedance amplifier

72
Q

In Faraday cup, what is the transducer response independent of

A
  1. Energy of ion
  2. mass of ion
  3. chemical nature of ion
73
Q

advantages of Faraday cup

A
  • inexpensive

- mechanically and electronically simple

74
Q

disadvantages of faradays cup

A
  • need high impedance amplifier bc that limits speed of spectral scanning
  • provides no internal amplification
75
Q

Types of array transducers

A
  1. Electrooptical ion detector(EOID)

2. microchannel plate

76
Q

what does the ion beam hit in EIOD and what results

A

microchannel plate to produce cascade of electrons

77
Q

what to electrons hit in EIOD and what results

A

phosphorescent screen

radiation results in photons

78
Q

what do the photons hit in EIOD

A

the transducer to convert optical signal to electrical signal

79
Q

How is amplification carried out in microchannel plate

A

-particles strike inner surface of tiny channels taht act as individual electron multipliers to produce about 1000 electrons for every incoming particle

80
Q

What is resolving power

A

ability for mass spectrometer to separate two peaks of similar mass

81
Q

most common mass spec technique

A

ICPMS

82
Q

other kinds of mass spec techniques

A
  • DCPMS
  • MIPMS
  • SSMS
  • TIMS
  • GDMS
  • LMMS
  • SIMS
83
Q

characteristics of ICPMS

A
  • low LOD (0.1-10 ppb)
  • High selectivity
  • good precision (2-4%)
  • good accuracy
  • 90% of periodic table can be analyzed bc large mass range
  • 1 m/z resolution
  • Dynamic range of 10^6
  • approx. 10s analysis time
  • various sample introduction method
84
Q

why would one use laser ablasion for ICPMS

A

so your sample doesnt have to be dissolved

85
Q

Spectroscopic interferences

A
  1. isobaric interference
  2. poly-atomic ion interference
  3. Oxide and hydroxide species interference
  4. matrix effects
86
Q

Isobaric interference

A

two elements with isotopes differing by < 1 mass unit

87
Q

Isobaric interference correction

A

another element isotope is used

88
Q

Why are isobaric interferences not that bad

A

bc theyre predictable and correction can be done with appropriate software

89
Q

Poly-atomic interference

A

occurs when poly-atomic species form from interactions between species in the plasma and the species in the matrix or atmosphere

90
Q

poly-atomic interference corection

A
  • blank measurement

- using different analyte isotope

91
Q

Oxide and hydroxide species interference

A

MO+ or MOH+ formed from the analyte, matrix components, solvent, and plasma gasses

92
Q

Oxide and hydroxide species interference correction

A

adjusting the following variables may reduce the interference:

  1. injection flow rate
  2. radio-frequency power
  3. sample skimmer spacing
  4. sample orifice size
  5. plasma gas composition
  6. oxygen elimination
  7. solvent removal
93
Q

Matrix effects

A

important when concentration of species in the matrix is too large which can cause the the reduction of analyte signal

94
Q

matrix effect corrections

A
  1. use more dilute solutions
  2. altering sample introduction procedure (such as LC)
  3. LLE or SPE to separate out offending species
  4. Use appropriate internal standard’
95
Q

what can icpms be used for

A
  1. qualitative
  2. semi quantitative
  3. quantitative
  4. iosotope ratio measurements
96
Q

qualitative and semi-qualitative analysis

A

Perform analysis to determine what’s in sample with rough concentration estimate

97
Q

most widely used quantitative method of ICPMS anaysis

A

multiple standard calibration

98
Q

what plots are generally used for ICPMS and why

A

log-log plots bc dynamic range is so large

99
Q

Isotope ratio measurement

A

the measurement of isotope ratios is important in science and medicin

100
Q

archaeology application of icpms

A

carbon dating to determine the age of artifacts and various deposits

101
Q

medicinal application of icpms

A

using an isotope to see how long it takes a drug to circulate the body and where it ends up

102
Q

advantages of ICP-MS over ICP-OES

A
  • detection limits are better
  • simple spectra, easy to interpret
  • ability to measure isotope ratios
103
Q

disadvantages of ICP-MS over ICP-OES

A
  • much more expensive
  • instrument drift
  • isotopic interferences
104
Q

ICP-MS involves atomization and ___

A

ionization

105
Q

ICP-OES involves atomization and ____

A

excitiation

106
Q

order of detection limits

A

ICPMS

Analytical Spectroscopy (3 decks)

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