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Questions and Answers
What is one issue that can reduce the efficiency of a pump?
What is one issue that can reduce the efficiency of a pump?
Why must a significant portion of boiling oil be allowed to pump along the backing arm?
Why must a significant portion of boiling oil be allowed to pump along the backing arm?
How often do pumps typically need to be checked for oil loss?
How often do pumps typically need to be checked for oil loss?
Which type of oil is very susceptible to oxidation when operated with a large air throughput?
Which type of oil is very susceptible to oxidation when operated with a large air throughput?
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What is a disadvantage of using silicone oils in diffusion pumps?
What is a disadvantage of using silicone oils in diffusion pumps?
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What is a critical characteristic of the pumping fluid that affects ultimate pressure in diffusion pumps?
What is a critical characteristic of the pumping fluid that affects ultimate pressure in diffusion pumps?
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Why is the use of mercury as a pump fluid limited in GC/MS systems?
Why is the use of mercury as a pump fluid limited in GC/MS systems?
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What factor can lead to reduced ultimate pressure in a diffusion pump?
What factor can lead to reduced ultimate pressure in a diffusion pump?
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What is a key reason for maintaining a very high vacuum in a mass spectrometer?
What is a key reason for maintaining a very high vacuum in a mass spectrometer?
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Which of the following is not a consequence of elevated pressure in a mass spectrometer?
Which of the following is not a consequence of elevated pressure in a mass spectrometer?
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What is one of the functions of rough pumping in vacuum systems?
What is one of the functions of rough pumping in vacuum systems?
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Why is it difficult to maintain pumping requirements in the chemical ionization mode?
Why is it difficult to maintain pumping requirements in the chemical ionization mode?
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What is considered the atmospheric pressure for scientific purposes?
What is considered the atmospheric pressure for scientific purposes?
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What problem arises from the presence of compounds in the mass spectrometer at high pressures?
What problem arises from the presence of compounds in the mass spectrometer at high pressures?
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What role do diverters play in the GC/MS interface?
What role do diverters play in the GC/MS interface?
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What does a higher conductance value indicate in a vacuum system?
What does a higher conductance value indicate in a vacuum system?
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What does the out-of-balance voltage, VB, primarily depend on?
What does the out-of-balance voltage, VB, primarily depend on?
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Which statement regarding auxiliary vacuum arrangements is true?
Which statement regarding auxiliary vacuum arrangements is true?
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Which vacuum gauge operates from approximately 1.3 X 10^4 Pa to 1.3 X 10^-1 Pa?
Which vacuum gauge operates from approximately 1.3 X 10^4 Pa to 1.3 X 10^-1 Pa?
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What is fore-vacuum commonly referred to as?
What is fore-vacuum commonly referred to as?
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Why is it necessary to rough-pump auxiliary inlets before connecting them to the main vacuum system?
Why is it necessary to rough-pump auxiliary inlets before connecting them to the main vacuum system?
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What is the meaning of 'mean free path' in the context of vacuum systems?
What is the meaning of 'mean free path' in the context of vacuum systems?
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What characteristic of thermocouple gauges is noted in the content?
What characteristic of thermocouple gauges is noted in the content?
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What component is often used in a thermocouple gauge to compensate for temperature variations?
What component is often used in a thermocouple gauge to compensate for temperature variations?
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What does 'differential pumping' refer to?
What does 'differential pumping' refer to?
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Which of the following best describes a virtual leak?
Which of the following best describes a virtual leak?
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Which technique is used for measuring vacuum pressures below 1.3 X 10^-1 Pa?
Which technique is used for measuring vacuum pressures below 1.3 X 10^-1 Pa?
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What role does the limiting circuit play in a thermocouple gauge?
What role does the limiting circuit play in a thermocouple gauge?
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What represents the region known as high vacuum (HV)?
What represents the region known as high vacuum (HV)?
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How is conductance defined in a pumping system?
How is conductance defined in a pumping system?
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What is the function of the thermocouple junction formed in the thermocouple gauge?
What is the function of the thermocouple junction formed in the thermocouple gauge?
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Which statement about buffer amplifiers in relation to out-of-balance voltage is correct?
Which statement about buffer amplifiers in relation to out-of-balance voltage is correct?
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What is the primary advantage of using metal seals in GC/MS vacuum systems?
What is the primary advantage of using metal seals in GC/MS vacuum systems?
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What is a characteristic method to join parts with spigot flanges?
What is a characteristic method to join parts with spigot flanges?
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What is a key disadvantage of using knife-edge flanges?
What is a key disadvantage of using knife-edge flanges?
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Which materials are commonly used in moving seals within vacuum systems?
Which materials are commonly used in moving seals within vacuum systems?
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What occurs when the flanges are bolted together with a spigot flange?
What occurs when the flanges are bolted together with a spigot flange?
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What is the typical fate of indium and gold rings used in spigot flanges after sealing?
What is the typical fate of indium and gold rings used in spigot flanges after sealing?
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What happens to the alignment when joining parts with O-rings and knife-edge flanges?
What happens to the alignment when joining parts with O-rings and knife-edge flanges?
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What is the composition of the soft metal typically used in spigot flanges for vacuum seals?
What is the composition of the soft metal typically used in spigot flanges for vacuum seals?
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What is the primary reason for using multiple seals in a high vacuum connection?
What is the primary reason for using multiple seals in a high vacuum connection?
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What is a common issue associated with Teflon seals used in vacuum applications?
What is a common issue associated with Teflon seals used in vacuum applications?
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What should be done before opening diffusion pump isolation valves?
What should be done before opening diffusion pump isolation valves?
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What is a potential consequence of applying too much lubricating fluid in a vacuum system?
What is a potential consequence of applying too much lubricating fluid in a vacuum system?
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Which valve allows the operator to vent the manifold region to atmosphere without shutting down the pump?
Which valve allows the operator to vent the manifold region to atmosphere without shutting down the pump?
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What are vacuum interlocks primarily used for in a vacuum system?
What are vacuum interlocks primarily used for in a vacuum system?
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What can occur if the valve between the manifold and a rotary pump is not closed before opening the isolation valves?
What can occur if the valve between the manifold and a rotary pump is not closed before opening the isolation valves?
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Which service fluid is suitable for lubricating Viton seals in a vacuum system?
Which service fluid is suitable for lubricating Viton seals in a vacuum system?
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Study Notes
Vacuum Systems
- GC/MS instruments require sophisticated pumping and vacuum systems for high-vacuum operation in the mass spectrometer.
- High voltage breakdown can occur in the multiplier, source, or analyzer if the pressure is too high.
- Oxygen buildup from residual air and leaks leads to filament burnout in the ion source and ion gauges.
- A long mean free path for molecules in the system is necessary to prevent ion-molecule collisions.
- Electron current regulation in the ion source becomes harder as pressure increases.
- Ion-molecule reactions alter fragmentation patterns; this is sometimes used advantageously in chemical ionization (CI).
- High background pressures from compounds in the mass spectrometer generate interfering mass spectra.
- Increased pressure leads to contamination in ion-source components (slits, rods, and multipliers), requiring more downtime for cleaning.
- Instruments need rough pumping prior to high-vacuum pumping, especially after venting to the atmosphere (e.g., during source cleaning).
- Rough pumping is needed to clear reagent and carrier gas lines to avoid cross-contamination.
- Most GC/MS interfaces have a sample-enrichment device for electron impact work that requires a pump to remove excess carrier gas.
- Diverters in GC/MS interfaces prevent solvent fronts and unwanted GC eluent compounds from entering the mass spectrometer.
- Auxiliary inlets (probe and batch) require rough pumping before connecting to the main vacuum system.
- Vacuum systems are not completely separate; multiple pumps and systems are often shared for tasks.
Vacuum Terms
- Atmospheric Pressure: A constant 1.013 x 105 Pa (760 torr).
- Conductance: The inverse of pumping resistance in a vacuum system, higher conductance means faster pumping. Conductiances of elements add up. Pumping system diameter and straightness directly affect conductance.
- Differential Pumping: Independent pumping of two vacuum regions separated by a restriction.
- Fore Vacuum: A lower vacuum level (typically 1.3 x 102 to 1.3 x 10-1 Pa or 1 to 10-3 torr) usually produced by a rotary pump. (also called backing pressure when used before a diffusion pump)
- High Vacuum (HV): A pressure region at or below 10-3 torr.
- Leaks and Virtual Leaks: All vacuum systems leak; virtual leaks are trapped inside the system and may leak out later.
- Mean Free Path: The distance a molecule travels before colliding with another molecule.
- Partial Pressure: The pressure contributed by one component in a gas mixture.
- Pascal: A unit of pressure, equaling 1 newton per square meter (N/m2).
- Pumping Speed: The volume of gas pumped away per unit time at a given pressure.
- Rough Vacuum: The vacuum usually produced by a rotary pump, 1 to 10-3 torr (1.3 x 102 to 1.3 x 10-1 Pa).
- Torr: A unit of pressure, equal to 1 mm of mercury (1 mm Hg).
- Vacuum: Any pressure below atmospheric pressure.
- Vanous levels of vacuum: Very High Vacuum (VHV), Ultrahigh vacuum (UHV)
Vacuum Pressure Units and Typical System Parameters
- The standard pressure measurement unit in the SI system is the Pascal (Pa).
- Typically rotary pumps reach pressures around 1 Pa, while diffusion pumps work in the millipascal range.
- At 1.3 x 10-6 Pa (10-8 torr), the approximate limit for most GC/MS systems, the pressure isn't sufficient to support a column of mercury even one atom high.
- Despite very low pressures, there will still be about 3.8 x 1010 molecular impacts per second per square millimeter of the vacuum chamber at the pressure limit.
- A higher mean free path(5km) is calculated in this extremely low pressure region.
Vacuum Components
- A GC/MS vacuum system consists of various units, including high- and low-vacuum pumps, pipework, baffles, valves, gauges, sensors, and gas controllers, (as well as unseen components like pump oils and seals).
Rotary Pumps
- Several types of rotary pumps exist; they all compress a large volume of low-pressure gas to a smaller one via rotation.
- The most common type for GC/MS use has a cylindrical rotor that rotates within a cylindrical chamber with spring-loaded blades; there is often oil to help with seals and lubrication.
- Gas is drawn in at a port where the volume is highest and expelled via a smaller port where the volume is lowest. There is a flap valve to prevent backflow.
- Oil immersion assists in sealing clearances between the rotor blade and the chamber..
- Oil used in the pumps needs to have high operating temperatures and low vapor pressure to operate well.
- Condensation can occur in the lower pumps, causing problems. Gas ballasting helps overcome condensation.
- Typical operation speed of a rotary vacuum pump is 40-80,000 RPM, requiring carefully balanced and lubricated bearings.
- Pumps with different sizes and rotational speeds exist.
- The limiting pressure for a rotary pump depends largely on its compression ratio and the oil vapor pressure; good oils enable 1.3 x 10-2Pa (10-4 torr)
Diffusion Pumps
- A stack of concentric jets is a key part of a diffusion pump.
- The oil at the base of the chamber is heated strongly, causing intense boiling and generation of vapor that flows upward through the jets.
- Angled jets force gas downwards so vapor produces a cloud moving rapidly toward the base of the pump; this creates a high differential pressure(up to five orders of magnitude).
- The oil vapor is condensed at the bottom and repeats the cycle.
- Diffusion pumps can generate ultimate vacuums of approximately 1.3 x 10-8 Pa (10-8 torr).
- The performance is affected by gas molecules entering the pump and condensing in the oil.
- The design of diffusion pumps incorporates a series of baffles that slowly cool the vapor and separate the impurity to avoid these problems.
- Diffusion pumps require backing from another pump to function at the required pressure range.
- Fluid choice in diffusion pumps depends on pumping requirements; low vapor-pressure fluids are crucial.
Turbomolecular Pumps
- Rotors in a turbomolecular pump collide with gas molecules, transferring energy in a specific direction.
- The mean free path is higher in one direction with these pumps (unlike diffusion pumps where it is dependent on the vapor).
- This directional force leads to efficient gas pumping through the rotor/stator.
- Pressure increases as gas is pushed through the pumps, and if the pressure increases too much the mean free path that is needed to keep the process going will become too short.
- Turbomolecular pumps need another type of pump, typically a rotary mechanical pump, to be used with them to prevent this.
- Turbomolecular rotors/stators are carefully balanced to prevent vibration during high-speed operation. High rotational speeds(40,000 or even 80,000 RPM) are normal in these pumps. Cooling of the rotor bearings, especially in environments with hot and/or moving components, are usually important. (oil issues can occur in high speed rotating systems)
- The need for a separate cooling system or lubrication systems for bearings.
Vacuum Gauges
- Pirani Gauges: Used for pressures from 1333 Pa (10 torr) down to 0.13 Pa (10-3 torr).
- They use resistance elements in a Wheatstone bridge circuit that change based on temperature of the surrounding gas (the pressure).
- Resistance measurements depend on temperature in direct relationship to the surrounding pressure; compensation is necessary to account for temperature variations.
- Thermocouple Gauges: Used for pressures from 100 torr to 1.3 x 10-1 Pa (10-3 torr),
- Two dissimilar wires form a junction that heats; a temperature difference is correlated to pressure (more nonlinear than Pirani gauges).
- Ion Gauges: Used to measure very low pressures (below ~ 1.3x10-2Pa or ~10-1 torr).
- Measures the pressure based on ion current from the electron collision, with the pressure determining the quantity of gas.
- External circuits regulate filament heating current and hence the temperature.
- The ion gauge creates ions, and the measured currents determine the gas pressure.
Vacuum Seals
- Mass spectrometer vacuum chambers consist of components (not a single enclosure) joined with seals. Vacuum seals are essential to prevent gas flow through junctions; this includes the absence of significant hydrocarbon outgassing (in conjunction with low vapor pressure and soft enough material).
- Demountable Seals: Used in repairable system sections, using "rubber" O-rings is common, especially with medium- to high-vacuum applications
- Moving Seals: Used in systems with moving parts or with great vacuum requirements, including both Teflon and Viton seals.
Valves
- Valves in GC/MS systems are used to control gas flow(On/Off with specific flow rates).
- Pressure differentials are important factors in these valves (the pressures across the valve).
- Valves are used to isolate and/or direct gas flow into the vacuum system (and prevent contamination). Valves may also allow the system to be vented to the atmosphere.
Other Pumping Arrangements
- Some GC/MS instruments place the mass spectrometer inside the diffusion pump, making the unit more compact.
- Problems arise from passing the chromatograph transfer line through the hot oil in the pump base.
- Other designs, for example, keep the mass spectrometer outside and utilize external pumps as a design solution.
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