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Mass Spectrometry > Exam #3 > Flashcards

Exam #3 Flashcards

1
Q

Orbitrap

A
  • First new type of mass analyzer
  • Similar to QIT and FT-ICR
  • Based on orbital trapping
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2
Q

How does an orbitrap work?

A
  • An outer barrel-like electrode and a coaxial inner spindle-like electrode form an electrostatic field.
  • Voltage on the spindle is about 3500V and its polarity is opposite to the ions’ polarity
  • Diameter of spindle is 8 - 12 mm
  • Diameter of cavity is 20 - 30 mm
  • Ions are injected into the electrostatic field with an accelerating voltage between 1 - 2 kV; this frees the ions from external source to the mass analyzer
  • Ions are trapped due to their electrostatic forces to the inner electrode, which has a voltage of about 3.5 kV DC. They are balanced by centrifugal force.
  • Outer electrode is near ground (a few volts).
  • Ions cycle around the central electrode in rings and also move back and forth along the axis of the central electrode.
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3
Q

What is unique about orbitraps?

A
  • No magnetic field (unlike FT-ICR)

- No rf voltage or generators (unlike QIT)

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4
Q

The movement of the ions around the central electrode of an orbitrap are _______ and __________ movement.

A
  • coherent

- reproducible

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5
Q

To minimize ________ and ________, the pressure inside an orbitrap is extremely low.

A
  • collisions (which lower coherent motion)

- electrical arcing

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6
Q

The pressure in the orbitrap is ___________. Making it the ___________.

A

about 5 x 10^-10 Torr ; lowest pressure of mass analyzers

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7
Q

In relation to “k” of orbitraps, what is it and what term does it relate to?

A

curvature of the electrical field, does not change. Related to the geometry of the orbitrap

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8
Q

What is ω in orbitrap terminology? Independent of _________. Inversely proportional to the _______.

A

frequency of the harmonic oscillations ; ion velocity, v ; square root of m/z

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9
Q

ω = ?

A

ω = [z/(mk)]^1/2

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10
Q

m/z is proportional to ___________.

A

1/ (frequency)^2

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11
Q

What happens if ions are out of coherent motion in an orbitrap?

A
  • ions do not have the same m/z value

- ions would cancel each other out and an image will not be made

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12
Q

Longer free inductance decay (FID) means___________.

A

better mass accuracy, better resolution, and more intense signal.

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13
Q

Image current of an orbitrap is _________. This is a function of ________.

A

free inductance decay (FID) ; time

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14
Q

With orbitrap, the image current (in time) is converted to frequency via ___________. _______ is calculated from this.

A

Fourier Transform ; m/z

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15
Q

Orbitrap is capable of __________ and ____________. The dissociation event occurs _______ and the orbitrap detects the ______.

A

ion ejection and MS/MS experiments ; outside the orbitrap; products

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16
Q

Orbitrap most often acts as a _________.

A

mass detector

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17
Q

Orbitrap is usually sold with _____________ sources, but it is also available with _____________.

A

ESI/APCI ; MALDI

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18
Q

What is the benefit of using an orthogonal system such as ESI/LIT/C-TRP/Orbitrap?

A
  • Keeps neutrals from entering the orbitrap

- minimize pressure

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19
Q

__________ is the most common form of mass spectrometry.

A

Collision dissociation

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20
Q

Give the values for Orbitrap:

  • Mass Accuracy:
  • Resolution:
  • Dynamic Range:
  • Upper m/z range of:
A
  • Mass Accuracy: 1-2 ppm
  • Resolution: up to 200,000
  • Dynamic Range: about 5000-10,000 (relatively low)
  • Upper m/z range of: about 6000 m/z
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21
Q

_________ has a worse resolution in comparison to FT-ICR, but better the QIT.

A

Orbitrap

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22
Q

Advantages of Orbitrap

A
  • Small
  • Cheap ; cheap to buy and low operating costs
  • High resolution ( still lower than FT-ICR but better than the rest)
  • Sensitive
  • No danger of a strong magnet
  • Works well with ESI
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23
Q

MS/MS requires ________ to be controlled.

A

pressure

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24
Q 
MS spectrometers operate at low pressures to: 
1)
2)
3)
4)
A

1) Minimize ion/molecule collisions (would cause unplanned dissociations or reactions of ions or change ion path)
2) Prevent electrical arcing at kV potentials needed for some ion focusing/ ion acceleration
3) Reduce contamination of the ion source and mass analyzers . Reduces chemical noise
4) Prevent filament burnout due to the presence of O2.

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25
Q

Pressure is also important for _______ dissociations (collision-induced dissociations) and reactions (bimolecular reactions).

A

Planned

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26
Q

For bimolecular dissociations, the pressure of the neutral depends on its _________.

A

concentration

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27
Q

_______ different gauges to measure pressure. No one gauge will fit all.

A

Many

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28
Q

Thermocouples (TC) is also known as _________. This is a specific type of TC gauge that uses a _________.

A
  • Pirani gauge

- Pt filament

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29
Q

Thermocouples:

  • Covers a pressure range of :
  • Used with ________
  • Junction between two dissimilar metals causes _________ when the metals are at different temperatures.
  • ____________ is fed to a filament.
  • Temperature of filament depends on _________.
  • Pressure changes lead to changes in the filament’s _____ and the _________.
A

-Rough pumps
- 10^-3 to 10 Torr
- thermal conductivity difference between the metals, which leads to current.
-Constant current
-thermal losses to the surrounding gases
temperature ; thermocouple voltage

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30
Q

Voltage of a thermocouple is related to

A

pressure

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31
Q

Penning Gauge

  • A type of __________
  • Intermediate pressure range of about ______.
  • A _______ voltage is applied to a central anode.
  • A ______ causes ______ to spiral around the anode until they hit a _______.
  • The _________ of the resulting current relates to pressure. A __________________ enables more electrons to reach the cathode, resulting in ________..
A
  • cold cathode gauge
  • 10^-3 to 10^-5 torr
  • DC
  • Magnetic field ; electrons ; cathode
  • Magnitude ; lower pressure ; a higher current
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32
Q

What is a benefit of the Penning gauge?

A
  • Very low maintenance

- Parts don’t wear out

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33
Q

Bayard-Alpert (B/A) Ion Gauge is the ____________.

A

most common ultra high vacuum gauge for surface analysis

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34
Q

B/A Ion Gauge

  • Electrons are emitted by a ______________ and travel to a _____, ionizing residual gas molecules along the way.
  • The ion current produced reaches the _________ and is measured.
  • Current depends on the _______ and on the _________, which must be kept constant.
A
  • hot filament ; grid
  • ion collector (anode)
  • pressure ; filament emission current
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35
Q

Benefits of B/A Gauge

A
  • Reliable and stable

- linear response over a wide pressure range (10^-5 to 10^-10 Torr)

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36
Q

With pressure gauges, different molecules give ___________ responses. Smaller molecules = _________ ; Larger molecules = __________

A
  • different
  • lower readings than true pressure (fewer ions being made)
  • higher readings than true pressure (more ions being made)
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37
Q

Most gauges are calibrated with _______ to obtain to pressure. True pressure equation is __________.

A

nitrogen ; P(true) = P(measured) * Correction Factor (for your gauge) / Rx

Note : Rx is the relative gauge sensitivity for the molecule of interest

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38
Q

What is a nude ion gauge and why might it be beneficial?

A
  • The ion gauge is not enclosed in glass

- Makes it more sensitive and accurate. Also allows the free flow of gas into the gauge head

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39
Q

With B/A gauge heads, the filament is made of and what does it do?

A

Tungsten ; current is applied to the filament. This causes EI to be conducted on the gas molecules in the vicinity of the gauge head.

40
Q

What is the purpose of the mesh of the B/A gauge head? What important note about magnets applies to this gaugehead?

A

creates an electric field for electrons to be trapped; doesn’t work well with magnets.

41
Q

Where is the pressure read on an ion gauge?

A

Gauge head

42
Q

What is Collision Theory? k=?

A

of collisions = (time in seconds)(pressure in Torr)(3.23 x 10^16 molecules cm^-3 Torr^-1)(k)

k= collision rate constant (cm^3 molecule^-1 sec^-1)

43
Q

What is polarizability?

A

Alpha symbol ; a coefficient related to the ability of a species to acquire a dipole moment in an electric field

Units: cm^3 or A

44
Q

Polarizability is a good approximation of a molecule’s ________.

A

volume

45
Q

What is “dipole moment”?

A

µD with unit: Debye (D); Bonds between two atoms of different electronegativities leave a negative charge (-Q) on the more electronegative atom and a positive charge (+Q) on the more electropositive atom. Distance, r, separates the charges.

Causes ion in the molecule to be more highly attracted

µ0 = Qr

46
Q

What is MS/MS (tandem mass spect)?

A

Ion generated in ion source, separated in the 1st mass analyzer, and do something with it in other mass analyzers.

47
Q

Single MS is used to _________________.

A

obtain a spectra of a neutral molecule

48
Q

Main areas (3) of application for Tandem mass spect

A

1) Qualitative and quantitative analysis (i.e. determining sequences of peptides) - about 90-95%
2) Ion chemistry (structures, reaction mechanisms, etc.)
3) Fundamentals (affinities, breakdown curves, etc.)

49
Q

Keys (3) to MS/MS

A

1) Two stages (in time or space) of mass analysis
2) Ion is surrogate analyte (means assume precursor ion or molecular ion)
3) Reaction ( e.g. dissociation) is separate from ionization

50
Q

For mixtures, MS/MS can take the place of chromatography and be used for ____________.

A

improved specificity and detection limits, structural information

51
Q

For pure compounds, MS/MS is used for:

A

Optimization to achieve the most characteristic spectrum, structural information

52
Q

MS/MS is very _________. Meaning, a _______ ion or ions is chosen for MS/MS analysis. The _________ never sees the other ions b/c they are already kicked out. This greatly _________ the S/N in complex samples, especially for ________.

A

specific ; specific ion.

Detector ; increases ; biological molecules

53
Q

MIKES is the same thing as

A

MS/MS

54
Q

Mass spectrometers for “in space” include…… For adding additional MS stage would require ______ and ________.

A

Sectors, quads, hybrids ; additional pumps and electronics

55
Q

Mass spectrometers for “in time” include….. All mass analysis dissociation steps occur in ________. For adding additional MS stages would require ______.

A
  • FT-ICR, QIT
  • one region of space
  • software modifications
56
Q

Hybrid MS/MS techniques ________.

A

mix time MS with space MS

57
Q

For MS, what improves specificity? It also improves what?

A
  • MS/MS stages
  • high resolution
  • It improves lower detection limits by preferentially removing chemical noise (better S/N , poorer S)
58
Q

If the noise of a spectra is electronic (which means its constant), MS/MS or high resolution __________.

A

-Is not advisable ; cutting signal without reducing noise only reduces S/N. The signal would decrease.

59
Q

What are the 4 types of MS/MS scans?

A

1) Product-Ion Scan
2) Precursor-Ion Scan
3) Neutral-loss Scan
4) Selected Reaction Monitoring Scan (SRM ; aka “Multiple Reaction Monitoring” or MRM)

60
Q

Product-Ion Scan:

  • ________ and most ______ MS/MS scan. Performed with ____________.
  • Only type possible for MS/MS in ________ or _______.
A
  • Simplest , most common ; all mass analyzer

- time (FT-ICR, orbitrap, all types of QIT) or pulsed analyzers (TOF and all traps

61
Q

Any mass analyzer that can do ________ can do all 4 major MS/MS.

A

SIM

62
Q

How does the Product-Ion Scan work?

A

1) mass analyzer number one is set to pass only the precursor ions of a fixed m/z
2) ions dissociate in a reaction chamber
3) the second mass analyzer obtains (“scans”) the complete spectrum
4) Results in all product ions of a precursor ion can be determined.

63
Q

Uses for Product-Ion Scan (2)

A

1) Good for probing structures. Gives molecular formula, not the structure.
2) Frequently used with CI and all DI and spray ionization techniques (ionization gives MW but few fragments)

64
Q

Product-Ion Scan is not typical with _____________ because ________.

A

EI, because EI has abundant fragments

65
Q

What is the precursor ion?

A

The ion you select for dissociation

66
Q

What MS/MS sequence makes up about 50% of the mass spect. market?

A

QQQ

67
Q

For soft ionization techniques, Product-Ion MS/MS gives____________. The _______ does not matter.

A

structural information for quasi-molecular ions obtained from spray ionization, CI, and DI techniques. Ion source

68
Q

Product-Ion MS/MS with ESI is used for __________.

A

-giving a mass spectrum with ions from a mixture of peptides. Gives the product-ion spectrum with excellent S/N and diagnostic fragment ions.

69
Q

Product-Ion MS/MS can also be used to distinguish between

A

isomeric ions

70
Q

Precursor-Ion Scan is the _______ type of MS scan.

A

second major

71
Q

Precursor-Ion Scan

  • First mass analyzer passes __________.
  • Second mass analyzer is fixed to pass the m/z of __________.
  • Correlation between the time ______________ and the m/z of the ________ must be passing through the mass analyzer at the same time needed to make this product.
  • Results in __________.
  • Used for ________.
A
  • all ions
  • the product ion of interest
  • that the specified product ion is observed ; precursor ion
  • All precursor ions of a specific product ion can be determined.
  • mIXTURE CHARACTERIZATION
72
Q

Precursor-Ion Scan only works when the mass analyzers are ________, which means only for _____________________.

A

scanning ; linear Q and sectors (B or E)

73
Q

Precursor-Ion Scan MS/MS requires knowledge of _______ in order to choose an ion (m2+) that is characteristic of a class of compounds.

A

fragmentation patterns

74
Q

Neutral-loss Scan

  • Can only happen with _____ and _____ because it requires a _______ mass analyzer.
  • _________ and ________ are both scanned.
  • “_________” between the scans is set so that the neutral loss (m3) is fixed.
  • Used mostly for _______.
A
  • sectors and quadrupoles ; scanning
  • Analyzer 1 and analyzer 2
  • “Stagger”
  • Mixture analysis
75
Q

A good way to determine specific types of compounds in mixtures.

A

Precursor-Ion Scan and Neutral-Loss scan

76
Q

Selected Reaction Monitoring (SRM) Scan

  • ________ mass analyzers are fixed. They must be _______.
  • A ________ is monitored.
  • SRM MS/MS gains _______ and increased _____ by reducing chemical noise over MS alone.
  • Most commonly used for ________.
  • Most often used with _________, where specific analytes are monitored during their retention time window,
A
  • Both ; Q or sector
  • specific
  • specificity, S/N
  • Quantitation, esp. for EPA
  • LC-MS and GC-MS
77
Q

Retention time is ________ in SRM.

A

an additional conformation of the identity of a compound being quantified.

78
Q

As the number of mass analyzers increase, the number of __________ increases.

A

(MS)^n

79
Q

Precursor-Ion Resolution relates to the ______ with which the final mass analysis can be performed. Aka ________.

A

accuracy ; Back End Rsolution

80
Q

Back End Resolution

A

Depends on last mass analyzer

81
Q

MS/MS in Space

A
  • 2 or more mass analyzers with reaction regions between analyzers
  • Uses depend on the properties of the mass analyzers
82
Q

Reaction region is

A

field free regions; do not have electric or magnetic fields.

83
Q

MS/MS in Space analyzers include:
1)
2)
3)

A

1) QQQ
2) Sector MS/MS Configurations
3) Hybrids

84
Q

QQQ

A
  • Major non-hybrid MS/MS in space instrument today
  • 1st and 3rd Q are operated in normal manner w/ combo of rf and dc voltages
  • Collision quad (center quad) is operated in rf-only mode. Used as reaction region
  • Rf frequency and voltage are set in a manner that passes all ions (serves as ion beam guide.
  • Collisional dissociation occurs only at low energies.
  • Quads do not use high accelerating voltages
85
Q

Benefits of QQQ

A

-Rapid scanning
-Small size
-Simple Operation
=Easily couple to LC
-(relatively) low cost
-Can perform all types of MS/MS scans (advantage over QIT, Q-TOF, FT-ICR, orbitrap, etc.)

86
Q

What was the major motivation for the formation of QQQ?

A
  • Desire to incr. product ion (back end) resolution compared to the BE
  • With sector MS/MS, there was such a wide velocity spread and peaks were broad, low resolution. Not used today
87
Q

Downside of QQQs

A
  • Less flexible in terms of reactions that can be performed

- low precursor ion transmission

88
Q

Sector MS/MS

A
  • A variety of different combinations
  • Higher front end resolution than with QQQ
  • When coupled with EB as the last analyzer, high back end resolution is possible
  • Combinations of E, B, or V can be scanned (Linked scans)
  • W/ collisional dissociation, the high V results in high energy collisions.
89
Q

Hybrids

A
  • Instruments with a combo of two diff. types of mass analyzers
  • Often a rxn region allowing high energy collisions (sector) and low energy collisions (quads) are both present, giving maximum flexibility in dissociation energy range.
90
Q

For sector/quad hybrids ________ are placed first, allowing higher _______________. Sectors are also placed first because the ion beam of the Q is ___________.

A

sectors; resolution precursor ion selection; unfocused

91
Q

Sectors’ can’t take _________ionization techniques.

A

Spray

92
Q

Advantages of having TOF as final analyzer over Q

A
  • All m/z of product ions can be analyzed within a few hundred microseconds
  • Transmission efficiency is much higher than with Q ( Limits of detection is about 100 times better than QQQ)
  • Not as discrimitive as Q. Every ion at the start of the TOF makes it to the end, no matter the m/z value
  • If reflectron-TOF is used, QTOF has a higher resolving power (50,000) than a QQQ (1000)
93
Q

Disadvantage of using TOF as final analyzer over Q

A
  • Highly priced considering their capabilities

- Any system with TOF, CAN ONLY PERFOM PRODUCT ION SCANS. Can’t do neutral loss scans.

94
Q

What does orthogonal accelerator mean with Q-TOFs?

A
  • Most common setup for QTOFs
  • The ion gate’s pulsed voltages are only set for ions to make the 90 degree turn at the start time for the TOF analysis.
  • Narrower start time and incr. resolution. ( All ions started at the same time = enter the TOF to give you better resolution.)
  • Keeps neutrals from the ESI source from entering the TOF and raising its pressure
95
Q

Features of Q-TOF

A
  • Medium front end resolution at Q1 (meaning you could select precursor ion within 1 m/z unit)
  • Moderately high back end resolution at TOF (about 50,000)
  • High transmission of MS/MS product ions through TOF
  • Ultrahigh sensitivity.
  • In normal operation, Q-TOFs can only perfom product-ion MS/MS scans

Mass Spectrometry (6 decks)

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