Chapter 8 - Vacuum systems Flashcards
Ions need to be guided on a specific pathway influenced only by
the imposition of electric, magnetic, and/or RF fields.
A vacuum is defined as less than
1 atm
1 atm = ____ torr
760
1 atm = _____ Pa
101325
1 atm = ______ psi
14.6959
High vacuum < ________
10-3 mbar
A high vacuum assist with ion movement and separation by
- Providing an adequate mean free path for analyte ions
- Providing collision free ion trajectories
- Reducing ion-molecular reactions
- Reducing background interference
The vacuum systems of most modern LC-MS systems consist of a
differentially pumped system.
A foreline pump establishes a
‘rough’ vacuum.
Instruments using an API interface will have ______ differentially pumped regions within them.
three or more
Mean free path
Average distance a molecule moves between collisions.
Mean free path estimate
L = 0.661/P
L is in cm
P is in Pa
A rotary pump consists of
vanes mounted to a rotor that rotates inside of a cavity.
Foreline pumps are also known as
roughing pumps.
Foreline pumps are employed to
assist the high vacuum pumps directly attached to the spectrometer.
Foreline pumps are ____ sealed rotary vane pumps.
oil
Foreline pump pumping capacity
0.1 Pa (10-3 torr)
Foreline pump at a speed of 1400 rpm capacity
300 L/min
The high vacuum created in the post skimmer regions of the mass spectrometer may be achieved using a
turbomolecular pump.
A turbomolecular pump consists of
rotating foils or blades that are angled to compress exiting molecules.
While spinning at around 60,000 rpm, turbomolecular pump capacity
200-500 L/s
The vapors exiting the turbomolecular pump will normally be entrained directly into the
foreline pump.
An alternative to turbomolecular pumps for maintaining high vacuum in LC-MS systems is the
oil-filled diffusion pump.
Diffusion pumps operate by
boiling a hydrocarbon oil and forcing the dense vapor stream through central jets angled downward.
Diffusion pump boilers usually operate between
280-300oC
Vapors exiting the pump in a diffusion boiler will often have ________ velocities and temperatures around ________.
supersonic (>750 m/s), 100oC
Diffusion pumps have capacities in the ______ order as turbomolecular pumps.
same
The vapor pressure created by the vaporized LC eluate and the level of vacuum attainable differs by approximately
two to three orders of magnitude.
Modern spectrometer systems use differentially pumped ion sources whose effective pumping speeds are around
300 to 700 L/s.
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.
300 and 2100 Pa L/s
Solutions to meet the low flow rates required to maintain vacuum levels:
- LC column can be miniaturized
- Column effluent can be split
- Cryogenic pumps can be used to significantly increase pumping capacity
- Preferentially removing the eluent vapor relative to the analyte species
The first vacuum region (nozzle-skimmer region) is separated from the LC interface by either a
transfer capillary or an orifice plate (nozzle).
Vapors are drawn through the orifice by a combination of
electrostatic attraction of charged species and the pressure difference between the atmospheric pressure interface and the vacuum of the nozzle-skimmer region.
The split ratio at the nozzle is usually in the ratio of
1:100
The pressure in the nozzle-skimmer region is approximately
10-3 atm
The nozzle skimmer region of the instrument functions as an ________ to perform __________.
expansion chamber, analyte enrichment
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.
lower momentum perpendicular to the axis of expansion
Lower mass particles diffuse to a _________ extent from the core of the expansion and _________ of heavier species will occur.
greater, enrichment
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.
3 L/min
When vapor species pass through the nozzle, the highest intensity of vapor species is situated
on and around the axis of the nozzle.
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.
increases, narrows, rapid cooling
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 ________.
non-directed, pumped away
The transition from directed to random motion will involve ___________ resulting in _________ that __________
- multiple collisions with vapor species and background gas molecules
- ‘shock-waves’
- scatter the ions and neutrals within the plasma of vaporized species.
The region inside the barrel shock wave is called the
zone of silence.
In the zone of silence, molecules will move with
equal speed and in the same direction.
The zone of silence terminates in a shock wave known as the
Mach Disk.
In the Mach Disk, molecules undergo
more frequent collisions and may completely lose the directed flow and travel perpendicularly to the nozzle axis.
The skimmer entrance is positioned so that the Mach disk is
in front of the sampling orifice.
Advantages of the Mach disk being in front of the sampling orifice.
- 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
Manufacturers often use a ring electrode to
keep ions focused onto the nozzle axis and stop them from diffusing from the expansion core.
A voltage applied to the ring will
reduce the spread of ions away from the axis resulting in an increase in instrument sensitivity (less noise).
Solvent clusters are preferentially sampled though the skimmer orifice into the post skimmer region due to their
low momentum.
On contact with hot metal surfaces, solvent clusters may decompose into
analyte ions and eluent molecules.
Manufacturers usually use the second expansion chamber to house ______ used to pre-focus the analyte beam.
ion optics
Gas ballasting serves two important purposes
- 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.
Gas ballasting should be performed on a _______ basis for approximately _________.
weekly, 30 minutes
Failure to gas ballast the rotary pump will lead to
shortened oil lifetime.
Gas ballasting should not be performed
during the operation of the instrument.
When the gas ballast valve is open, it is normal for the pump to
make more noise.
If a turbomolecular pump fails, it is usually
replaced.
Sudden ingress of air into vacuum systems can cause
significant damage to turbomolecular pumps.
_______ pumps require very little maintenance.
Oil diffusion
The most likely failure of an oil diffusion pump is the__________.
heating plate
Residual organic chemicals analyzed by the mass spectrometer are trapped in the
vacuum pump oil.
A _____ stage pump is highly recommended.
two
The first filter stage of a two-stage pump consists of
an oil mist filter to trap the heavy oils exiting the pump.
The second filter stage of a two-stage pump consists of
a charcoal trap designed to retain volatile organics.
The two elements to be maintained in an oil mist eliminator.
- 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
To maintain the correct level in pump oil,
- Check the oil level weekly
- Monitor the condition of the oil regularly
- Replace the oil when it gets darker or every 4 months
Change the rotary pump oil as follows:
- 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
Anti-suck back valves are used to
prevent pump sealing oil from back streaming into the analyzer.
Back streaming of hydrocarbons from the rotary vane pump can be prevented by
using a trap between the roughing pump and vacuum chamber.
Turbomolecular or diffusion pumps could switch off due to increased temperature if there is
- Poor foreline vacuum
- Failure of the water supply
- Leak in the mass analyzer and/or ion transfer regions
If a leak is suspected, the following might help locate it
- Suspect components that have recently been disturbed
- Ensure flange bolts and seal are tight
- Examine o-rings and gaskets
The Pirani gauge is a
roughing pressure vacuum gauge.
The Pirani gauge uses ________ of gases to measure pressure.
thermal conductivity
The Pirani gauge is useful in what range?
10-3-10 torr
Penning gauges are _______ vacuum gauges.
high
