Submitted by anthias on
Most GC-MS vacuum systems comprise of a two-stage pumping arrangement - an initial low-vacuum, high capacity stage consisting of a rotary oil pump or diaphragm pump; and a second high stage, low capacity vacuum pump such as a diffusion or turbo-molecular pump. The first stage or low-vacuum pump requires maintenance to ensure optimum pumping performance. Two aspects that are often overlooked are the use of the ballast valve on the vacuum pump and replacement of the pump oil at the correct intervals.
In normal use, all moisture, chemical vapours and solvent fumes introduced into the GC-MS instrument will end up through the vacuum system, dissolved in the vacuum pump oil or in the oil-mist filter. For GC-MS this will be limited to solvents and chemicals associated with the injected samples, whereas in LC-MS systems the solvent contribution is much larger as a result of the mobile phase. As the contribution of volatiles builds up in the oil, the pumping efficiency of the oil is reduced. The ballast valve allows the ingress of air from the atmosphere into the vacuum pump, purging volatiles from the oil and out through the exhaust port (where they are trapped on the adsorbent trap). Whilst this is happening, the vacuum capacity of the system will be limited to approx 10-2 mmHg in place of the normal 10-3 mmHg operating limit.
For general GC-MS use, ballasting the low vacuum pump for around 15 minutes each week should be adequate to ensure optimum pumping efficiency, however this will be dependant on the instrument use. The manufacturers’ manual should be consulted for more specific information.
Even with adequate ballasting of the pump, the pump oil will degrade, indicated by a slow increase in colouration. New, fresh oil is clear and colourless; with use this darkens, slowly turning pale straw-coloured, at which point it should be emptied from the pump and replaced with fresh oil. If the oil is dark yellow or brown immediate action is required. For most GC-MS instruments in general use, this will occur over a period of approximately 6-12 months.
Changing the pump oil usually requires the following simple steps:
- Be aware that any compounds introduced into the analytical system will have been concentrated in the oil being drained, it should be considered as hazardous waste and handled as such. Use suitable PPE for this operation and if possible replace the oil in a fumehood.
- Either vent the GC-MS system or isolate it from the pump undergoing the oil change, as recommended by the instrument manufacturer, venting is usually required.
- Turn off the pump. The oil will be hot, so allow to cool.
- Locate the drain/sump nut on the rotary pump and position above a suitable collection vessel. Unscrew and remove the sump nut to allow the oil to drain from the pump (NB the oil will be warm/hot and contain concentrated sample, so take care).
- Replace the sump nut and refill with fresh pump oil to just the minimum fill level. Flush the pump with fresh oil by running it for 1-2 mins, then allow the oil to settle before removing the sump nut and emptying this rinse-oil from the pump. The pump has now been thoroughly emptied and rinsed out of “old” oil.
- Replace the sump nut and refill the pump with fresh oil to 75% of the full mark or between the minimum and maximum lines.
- Change the oil-mist filter attached to the pump outlet.
- Run the pump for 5-10 minutes and check if the oil level settles after running, if necessary turn off and top-up the oil level.
- Reconnect the pump to the instrument and pump down.
Remember to dispose of the oil as Hazardous Waste.
Retain the equipment used for oil draining for this purpose; it will be impossible to clean for any other use.
To learn more about GC & GC-MS maintenance, attend Day 5 of our Complete GC & GC-MS course and Module 11 of our Virtual Classroom Complete GC & GC-MS course.
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