Untitled Quiz

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.