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Chem 453 > Chapter 7 - Mass spectrometry > Flashcards

Chapter 7 - Mass spectrometry Flashcards

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1
Q

In magnetic sector mass analyzers, the radius, r, depends on

A

the momentum, mv, of the ion.

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2
Q

The magnetic sector is a ______ analyzer rather than a _______ analyzer.

A

momentum, direct mass

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3
Q

For magnetic sector, m/z =

A

B2r2e/2Vs

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4
Q

Vs =

A

Kinetic energy of ions accelerated by a potential difference

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5
Q

In magnetic sector, if r is fixed and Vs is kept constant, for a given value of B, only the ions

A

with the corresponding m/z go through the analyzer.

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6
Q

In a magnetic sector, changing the value of B as a function of time allows

A

successive observations of ions and various values of m/z.

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7
Q

The magnetic sector can be used as a mass analyzer provided that

A

the kinetic energy of the ions is constant.

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8
Q

Magnetic sector is _______ focusing.

A

direction

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9
Q

An optimized magnetic sector alone can provide resolutions of resolving power

A

2000-7000

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10
Q

The intensity of the magnetic field in magnetic sector mass analyzers is usually between

A

0.4 and 0.6 Tesla (T)

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11
Q

The bigger the magnetic field in magnetic sector,

A

the heavier the ion mass analyzed.

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12
Q

________ limits the resolution of magnetic sector.

A

Energy dispersion

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13
Q

Magnetic sectors produce an energy dispersion when

A

a beam of ions with different kinetic energies issues from the source.

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14
Q

In order to carry out single ion monitoring, the magnetic field strength of the magnetic sector may be

A

‘tuned’ to the required m/z value and only ions of the specified value will be allowed to pass through the magnet, all other ions will be lost to the walls of the analyzer.

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15
Q

A minor of magnetic field strength tuning is related to

A

the quadratic dependence of m/z on the value of B, resulting in high mass peaks appearing closer together.

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16
Q

At high mass, a small change in b will result in

A

a much larger shift in the mass calibration.

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17
Q

The electrostatic analyzer (ESA) produces a radial electric field between

A

two oppositely charged plates extending over the ESA angle.

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18
Q

In ESA, an ion travelling in a direction perpendicular to the electrostatic field will follow

A

a circular path of radius R that fulfills the condition of equilibrium of centripetal force Fc and electric force Fe.

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19
Q

The electrostatic analyzer is a _______ analyzer if E is kept constant.

A

kinetic energy

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20
Q

Electrostatic analyzers are _______ focusing.

A

direction

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21
Q

Electric and magnetic sectors with the same energy dispersion oriented perfectly are called

A

double focusing.

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22
Q

The double focusing instrument is designed in such a way that ions of

A

different energies (but of the same mass) converge at the detector.

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23
Q

In double focusing forward geometry

A

the ESA is placed before the magnetic sector.

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24
Q

In double focusing revers geometry

A

the magnetic sector is placed before the ESA.

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25
Q

The scan rate of a magnetic sector instrument is limited by

A

the reluctance (or inertia) of an electromagnet, limiting the scan rate.

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26
Q

Laminated magnets permit

A

more rapid changes in the magnetic field such that the mass range may be scanned at rates approaching 0.1 s/decade mass (70-700, 50-500 m/z)

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27
Q

Problems with hysteresis (path X-Y not the same as Y-X) of the magnet are minimized by

A

discharging the magnet completely between scans, adding a settling time between scan cycles, and only scanning unidirectional for data collection.

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28
Q

In double focusing, the ion should experience a __________ field strength during its transit through the magnetic field.

A

homogenous

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29
Q

If scan speeds are too high in double focusing,

A

the field strength may change during the transit of the ion through the analyzer, causing the radius of deflection curvature to change.

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30
Q

In TOF, ions are subjected to an accelerating electric field that is typically ______ eV.

A

3000

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31
Q

In TOF, an ion of m/z 100 will take ________ to travel the distance d as an ion of m/z 25.

A

twice as long

32
Q

In TOF, the arrival time at the detector between two ions with unit mass difference becomes smaller as

A

the mass of the ions increases, so the ions are increasingly more difficult to differentiate.

33
Q

2 problems with regarding resolution when using TOF

A
  1. difference in flight times between ions.
    1. distribution of arrival times at the detector due to a distribution of kinetic energies, even for ions of the same m/z.
34
Q

A reflectron is a series of

A

electrostatic lenses which create a homogeneous electrostatic field at the end of the flight path of ions.

35
Q

In the reflectron, isobaric ions can bunch together to

A

reduce the distribution of flight times and greatly enhance the resolving power of the instrument.

36
Q

The reflectron is often referred to as an

A

ion mirror.

37
Q

There is no theoretical upper mass limitation in

A

linear TOF analysis.

38
Q

In TOF, it becomes increasingly difficult to discriminate between arrival times when m/z becomes

A

large.

39
Q

In TOF, discrimination between unit masses becomes difficult over

A

3000 m/z

40
Q

In TOF, at 50,000 m/z the mass overlap may be as much as

A

50 mass units.

41
Q

The transmittance of a linear TOF analyzer approaches ______ that leads to very high sensitivity.

A

90%

42
Q

In TOF, ion losses are solely caused by

A

collisional scattering due to residual gas or by poor spatial focusing of the ion source.

43
Q

The mass calibration of the TOF is easy with only

A

two reference points.

44
Q

The disadvantage of using reflectron technology is

A

a slight decrease in the sensitivity of the analyzer due to ion loss and dispersion of the ion beam (accentuated at high mass).

45
Q

The recording and storage of TOF data can be problematic as

A

the data is produced at extremely high frequency.

46
Q

A TOF instrument with a 2 m flight tube and ion energy of 2 keV may produce a complete mass spectrum from

A

m/z 1 to m/z 800 in 90 microseconds.

47
Q

The direct coupling of API sources with TOF analyzers can be problematic because

A

API techniques yield a continuous ion beam, while TOF analyzer operates on a pulsed process.

48
Q

Ways to overcome the difference between continuous API sources and pulsed TOF analyzer

A
  1. Direct the ion beams past a slit placed at the ion source entrance.
    1. Collect ions in a storage device perpendicular to the incident ion beam.
49
Q

Practical constraints of pulsing electrode in orthogonally accelerated TOF

A
  1. A practical upper pulse frequency is around 30 kHz
    1. Slow pulsing speeds can limit sensitivity.
50
Q

Quadrupole ion rods need mechanical accuracy in the _____ range in both the machining of the rods and their spatial positioning.

A

micron

51
Q

In a quadrupole, the desired cross section of the rods is

A

hyperbolic.

52
Q

Quadrupoles range in length from

A

50-250 mm.

53
Q

Quadrupole diameters range from

A

5-15 mm

54
Q

Typical quadrupole tunnel radii (r0) is

A

5 mm.

55
Q

Each quadrupole rod is paired with

A

its diametric opposite.

56
Q

In a quadrupole, when x and y are less than the radius, the ion will have _____ trajectory.

A

resonant, non-collisional, or stable

57
Q

For any quadrupole, ___ and ____ are fixed.

A

ro and omega

58
Q

For any quadrupole, the variable parameters are

A

voltages (U and V)

59
Q

The DC voltage (U) and RF voltage (V) are altered according to a linear relationship which is often referred to as the

A

SCAN line.

60
Q

The slope of the SCAN function is often referred to as the

A

quadrupole GAIN.

61
Q

The intersection of the scan line with the U axis is the

A

quadrupole OFFSET.

62
Q

To ensure that ions of only one selected m/z value are transmitted, the parameters U and V must be chosen so that

A

the line representing U/V passes close to the apex of region A but still lies within the stable region.

63
Q

If the slope of the SCAN line is increased so that it misses the apex of the stability region,

A

no ions of that m/z value will be transmitted.

64
Q

To transmit ions of subsequently higher unit mass, the voltages U and V are ________, while the ratio U/V is _________.

A

increased, fixed

65
Q

Increasing the mass GAIN (the slope of the scan line) by increasing the voltages U and V in constant proportion will

A

lead to an increase in spectral resolution but decrease sensitivity.

66
Q

Changes in mass gain affect higher mass ions to a

A

greater extent.

67
Q

When increasing the mass quadrupole DC offset voltage, the resolution will_______, but the sensitivity will________.

A

increase, decrease

68
Q

Altering the DC offset voltage affects ions of all masses

A

to the same extent.

69
Q

If the DC component of the applied voltage is removed, the scan line will lie

A

horizontally along the U/V plot abscissa. Acts as RF only ion bridge.

70
Q

The efficiency of transporting and focusing ions is proportional to

A

V2

71
Q

As the RF voltage, V, increases, the transmission of ions with high masses can be increased by a

A

more efficient focalization.

72
Q

Quadrupole, Hexapole, and Octapole focusing power.

A

Quadrupole > Hexapole > Octapole

73
Q

Quadrupole, Hexapole, and Octapole mass range.

A

Quadrupole < Hexapole < Octapole

Narrow ⇒ Wide

74
Q

As the number of poles increases, the focusing power of the ion guide

A

decreases.

75
Q

As the number of poles increases, the mass range for simultaneous transmission of ions of the ion guide becomes

A

wider.

Chem 453 (11 decks)

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