Agilent Single Quad 5975 GCMS Vacuum Overview Video
Introduction
In this video we will go over the main types of pumps used with the mass spectrometer. We will also go over identifying different types of vacuum problems and how to troubleshoot them.
The reason for a vacuum system is to create a mean free path in the mass spectrometer that is devoid of any molecules that could impede creation, transmission and detection of the ions. The M. S incorporates a foreline pump capable of reaching 10 to minus 3 torr, and a turbo or diffusion pump capable of reaching down to the 10 to the minus 6 torr.
Types of Vacuum and pumps
In vacuum terminology, a high vacuum is less than around 10 to the minus 3 torr, a very high vacuum is less than 10 to the minus 6 torr, and the ultra high vacuum is less than 10 to the minus 8 torr. Typical mass spectrometer manifold values are within the range of 10 to the minus 4 torr to 10 to the mnus 6 torr.
There are three main types of turbo pumps used on the 5975 single quad. The diffusion pump,, the standard turbo,,and the performance turbo. There are two main types of rotary pumps used on the 5975 quad. These are the dual stage oil pump and the oil free diaphragm pump.
Diffusion Pump Operation
The diffusion pump is shown here. Oil is heated in the bottom of the pump where it vaporizes and moves upwards in the central cavity. It then is sprayed downward through 3 nozzles at high velocity. This moves gas molecules in a downward direction where they are eventually removed by the rotary pump. The oil is cooled by the sides of the pump and re-condenses. The oil re-enters the heater and process repeats. The cooling is done by air and the roughing pump is measured by an ion gauge tube.
The diffusion pump is two staged air cooled. It uses a poly phenyl ether fluid called Sant O vac 5P. The pump fluid can be viewed through a site glass for hot and cold levels. The too hot, too cold switches determines when the pump is up to temperature or overheats.
Two switches control the operation of the heater. One is the too cold switch which is open when the temperature is less than 140 degrees C. The other is a too hot switch that opens when when the temperature is greater than 365 degrees C. The system goes to a ready state when both switches are closed.
A Site glass shows the fluid level in the pump. It should be between the two indicators shown.
Here is a diagram of the diffusion pump assembly. A clamp and O ring secure it to the bottom of the manifold. A pirani gauge or foreline gauge is connected to the outlet side of the pump.
It is important to verify that the fluid level is correct and the oil is clear. Oil can be replaced as part of the preventative maintenance procedures. Also, the cooling fins should be cleaned if necessary.
Roughing Pump Operation
The roughing pump operates by drawing in gas from the turbo exit, then isolating and compressing it. The gas leaves through the exhaust port of the pump.
The Gas ballast is a valve used to remove condensation during pumping. It is positioned between the pump stages here. It can be opened by turning the red knob on the side of the pump shown here. Also shown is a suck back valve to prevent back streaming during power failures.
To replace the oil on a foreline pump, remove the drain plug and collect the oil into a waste vessel. Then re-install the drain plug and remove the oil fill cap to add oil. Make sure the oil is at the correct level as viewed through the site glass.
Typical types of problems encountered with the rotary pumps are that they won't turn on or have poor pumping efficiency. Also, a saturated trap and incorrect oil levels can cause the pump to not work correctly. Other problems commonly encountered are leaking oil and defective suck back valves..
Dry Pumps
A dry foreline pump can also be purchased with the single quad. The benefits are that it uses no oil and has no maintenance. The pump can work with the CI option.
Other benefits of the pump is that it is quiet and easier to install. Cons of the pump are that it is more expensive then the rotary pump and can not be used with the diffusion systems.
The dry pump can not be serviced and the repair policy is to replace when problems arise. There is no preventative maintenance procedures for the dry pump.
Turbo Pumps
The turbo pumps used on the 5975 are the standard turbo and the performance turbo. There specs are shown here.
The turbo works by spinning a set of rotating blades such that they deflect gas downwards toward the pump outlet, creating a region of low pressure on the upper part of the pump.
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The turbo pump is maintanance free and should give around 20,000 hours of operation. The pump is replaced on failure.
In rare cases, turbo pumps that fail catastrophically can sometimes shatter and disperse debris throughout the manifold.
System Startup
To start up a diffusion pump, close the vent valve and side plate. Turn on the M. S. When the foreline pressure gets below 300 milli torr, the diffusion pump turns on. When the too cold switch closes, the pump is ready. If the foreline pump pressure goes above 400 milli torr, the diffusion pump heater is turned off.
To start up a turbo system, close the vent valve and side plate. Turn on the M. S. When the turbo reaches 100 percent speed the M. S goes into a ready state. If the turbo does not get to 80 percent within 8 minutes, both pumps are turned off
To vent the M. S., go to the chemstation or mass hunter vacuum and control view and click on the vent button. THe M. S can also be vented from the front panel as well. The system will slowly spin down and heater zones of source and quad will turn off. The transferline heater should also be turned off. When the temperature is below 100 degrees C, the vent valve can be opened. To not open the vent valve if the system is hot.
Common Problems in Vacuum system
Potential leak areas in the M. S include the side plate O ring, the vent valve O ring, and column fittings. The most likely spots for leaks are the parts the customer opens to do maintenance.
In the case of a power failure the anti suck back valve closes to prevent back streaming of oil into the M. S.
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