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Chem 453 > Chapter 8 - Vacuum systems > Flashcards

Chapter 8 - Vacuum systems Flashcards

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1
Q

Ions need to be guided on a specific pathway influenced only by

A

the imposition of electric, magnetic, and/or RF fields.

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2
Q

A vacuum is defined as less than

A

1 atm

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3
Q

1 atm = ____ torr

A

760

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4
Q

1 atm = _____ Pa

A

101325

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5
Q

1 atm = ______ psi

A

14.6959

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6
Q

High vacuum < ________

A

10-3 mbar

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7
Q

A high vacuum assist with ion movement and separation by

A
  • Providing an adequate mean free path for analyte ions
  • Providing collision free ion trajectories
  • Reducing ion-molecular reactions
    • Reducing background interference
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8
Q

The vacuum systems of most modern LC-MS systems consist of a

A

differentially pumped system.

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9
Q

A foreline pump establishes a

A

‘rough’ vacuum.

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10
Q

Instruments using an API interface will have ______ differentially pumped regions within them.

A

three or more

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11
Q

Mean free path

A

Average distance a molecule moves between collisions.

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12
Q

Mean free path estimate

A

L = 0.661/P

L is in cm

P is in Pa

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13
Q

A rotary pump consists of

A

vanes mounted to a rotor that rotates inside of a cavity.

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14
Q

Foreline pumps are also known as

A

roughing pumps.

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15
Q

Foreline pumps are employed to

A

assist the high vacuum pumps directly attached to the spectrometer.

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16
Q

Foreline pumps are ____ sealed rotary vane pumps.

A

oil

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17
Q

Foreline pump pumping capacity

A

0.1 Pa (10-3 torr)

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18
Q

Foreline pump at a speed of 1400 rpm capacity

A

300 L/min

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19
Q

The high vacuum created in the post skimmer regions of the mass spectrometer may be achieved using a

A

turbomolecular pump.

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20
Q

A turbomolecular pump consists of

A

rotating foils or blades that are angled to compress exiting molecules.

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21
Q

While spinning at around 60,000 rpm, turbomolecular pump capacity

A

200-500 L/s

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22
Q

The vapors exiting the turbomolecular pump will normally be entrained directly into the

A

foreline pump.

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23
Q

An alternative to turbomolecular pumps for maintaining high vacuum in LC-MS systems is the

A

oil-filled diffusion pump.

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24
Q

Diffusion pumps operate by

A

boiling a hydrocarbon oil and forcing the dense vapor stream through central jets angled downward.

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25
Q

Diffusion pump boilers usually operate between

A

280-300oC

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26
Q

Vapors exiting the pump in a diffusion boiler will often have ________ velocities and temperatures around ________.

A

supersonic (>750 m/s), 100oC

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27
Q

Diffusion pumps have capacities in the ______ order as turbomolecular pumps.

A

same

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28
Q

The vapor pressure created by the vaporized LC eluate and the level of vacuum attainable differs by approximately

A

two to three orders of magnitude.

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29
Q

Modern spectrometer systems use differentially pumped ion sources whose effective pumping speeds are around

A

300 to 700 L/s.

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30
Q

A typical LC eluent at a flow rate of 1 mL/min results in a gas flow between ___________ depending on the molecular masses of the solvents used.

A

300 and 2100 Pa L/s

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31
Q

Solutions to meet the low flow rates required to maintain vacuum levels:

A
  1. LC column can be miniaturized
  2. Column effluent can be split
  3. Cryogenic pumps can be used to significantly increase pumping capacity
  4. Preferentially removing the eluent vapor relative to the analyte species
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32
Q

The first vacuum region (nozzle-skimmer region) is separated from the LC interface by either a

A

transfer capillary or an orifice plate (nozzle).

33
Q

Vapors are drawn through the orifice by a combination of

A

electrostatic attraction of charged species and the pressure difference between the atmospheric pressure interface and the vacuum of the nozzle-skimmer region.

34
Q

The split ratio at the nozzle is usually in the ratio of

A

1:100

35
Q

The pressure in the nozzle-skimmer region is approximately

A

10-3 atm

36
Q

The nozzle skimmer region of the instrument functions as an ________ to perform __________.

A

expansion chamber, analyte enrichment

37
Q

When the LC eluate vapor is sampled through the nozzle into the low-pressure region of the mass analyzer, higher mass particles show a _______________ than lower mass particles.

A

lower momentum perpendicular to the axis of expansion

38
Q

Lower mass particles diffuse to a _________ extent from the core of the expansion and _________ of heavier species will occur.

A

greater, enrichment

39
Q

A vapor flow of around ________ can be introduced from an atmospheric pressure interface into the nozzle-skimmer region maintained at 10-3 atm by means of a roughing pump operating with an effective pumping speed of 300 L/min.

A

3 L/min

40
Q

When vapor species pass through the nozzle, the highest intensity of vapor species is situated

A

on and around the axis of the nozzle.

41
Q

In jet expansion in the nozzle skimmer region, the velocity of the vapor borne species _____________, the difference between speeds between the various species ________ and ________ occurs.

A

increases, narrows, rapid cooling

42
Q

In barrel shockwave, as vapor species more appreciable distance away from the nozzle, the gas molecules move in a _________ fashion, the lighter molecules that deviate most from the nozzle axis will be ________.

A

non-directed, pumped away

43
Q

The transition from directed to random motion will involve ___________ resulting in _________ that __________

A
  • multiple collisions with vapor species and background gas molecules
  • ‘shock-waves’
  • scatter the ions and neutrals within the plasma of vaporized species.
44
Q

The region inside the barrel shock wave is called the

A

zone of silence.

45
Q

In the zone of silence, molecules will move with

A

equal speed and in the same direction.

46
Q

The zone of silence terminates in a shock wave known as the

A

Mach Disk.

47
Q

In the Mach Disk, molecules undergo

A

more frequent collisions and may completely lose the directed flow and travel perpendicularly to the nozzle axis.

48
Q

The skimmer entrance is positioned so that the Mach disk is

A

in front of the sampling orifice.

49
Q

Advantages of the Mach disk being in front of the sampling orifice.

A
  • Intermolecular collisions in the Mach disk and associated shock waves leads to an increase in gas temperature
  • Applying a voltage to the skimmer allows ions to be sampled in preference to neutral species
    • Altering the skimmer voltage can give rise to highly accelerated ions that can cause fragmentation
50
Q

Manufacturers often use a ring electrode to

A

keep ions focused onto the nozzle axis and stop them from diffusing from the expansion core.

51
Q

A voltage applied to the ring will

A

reduce the spread of ions away from the axis resulting in an increase in instrument sensitivity (less noise).

52
Q

Solvent clusters are preferentially sampled though the skimmer orifice into the post skimmer region due to their

A

low momentum.

53
Q

On contact with hot metal surfaces, solvent clusters may decompose into

A

analyte ions and eluent molecules.

54
Q

Manufacturers usually use the second expansion chamber to house ______ used to pre-focus the analyte beam.

A

ion optics

55
Q

Gas ballasting serves two important purposes

A
  • Purging the pump oil to remove dissolved contaminants.
  • Oil mist expelled from the rotary pump that is trapped in the oil mist filter is returned to the pump during gas ballasting.
56
Q

Gas ballasting should be performed on a _______ basis for approximately _________.

A

weekly, 30 minutes

57
Q

Failure to gas ballast the rotary pump will lead to

A

shortened oil lifetime.

58
Q

Gas ballasting should not be performed

A

during the operation of the instrument.

59
Q

When the gas ballast valve is open, it is normal for the pump to

A

make more noise.

60
Q

If a turbomolecular pump fails, it is usually

A

replaced.

61
Q

Sudden ingress of air into vacuum systems can cause

A

significant damage to turbomolecular pumps.

62
Q

_______ pumps require very little maintenance.

A

Oil diffusion

63
Q

The most likely failure of an oil diffusion pump is the__________.

A

heating plate

64
Q

Residual organic chemicals analyzed by the mass spectrometer are trapped in the

A

vacuum pump oil.

65
Q

A _____ stage pump is highly recommended.

A

two

66
Q

The first filter stage of a two-stage pump consists of

A

an oil mist filter to trap the heavy oils exiting the pump.

67
Q

The second filter stage of a two-stage pump consists of

A

a charcoal trap designed to retain volatile organics.

68
Q

The two elements to be maintained in an oil mist eliminator.

A
  • Change the odor element (charcoal trap) monthly or when the pump emits oily odor
  • Change the mist element every time the rotary pump oil is changed
69
Q

To maintain the correct level in pump oil,

A
  • Check the oil level weekly
  • Monitor the condition of the oil regularly
  • Replace the oil when it gets darker or every 4 months
70
Q

Change the rotary pump oil as follows:

A
  • Gas ballast for 30 minutes
  • Vent and shut down the instrument
  • Drain when still warm
  • Drain oil through the drain hole
  • Flush the pump and refill
  • Gas ballast for 30 minutes
71
Q

Anti-suck back valves are used to

A

prevent pump sealing oil from back streaming into the analyzer.

72
Q

Back streaming of hydrocarbons from the rotary vane pump can be prevented by

A

using a trap between the roughing pump and vacuum chamber.

73
Q

Turbomolecular or diffusion pumps could switch off due to increased temperature if there is

A
  • Poor foreline vacuum
  • Failure of the water supply
  • Leak in the mass analyzer and/or ion transfer regions
74
Q

If a leak is suspected, the following might help locate it

A
  • Suspect components that have recently been disturbed
  • Ensure flange bolts and seal are tight
  • Examine o-rings and gaskets
75
Q

The Pirani gauge is a

A

roughing pressure vacuum gauge.

76
Q

The Pirani gauge uses ________ of gases to measure pressure.

A

thermal conductivity

77
Q

The Pirani gauge is useful in what range?

A

10-3-10 torr

78
Q

Penning gauges are _______ vacuum gauges.

A

high

Chem 453 (11 decks)

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