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MS TIP No. 4 - Selection of Vacuum Pump Oils for Lowest Mass Spec Background


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Author: John J. Manura

Affiliation: Scientific Instrument Services

Significant background peaks are present in the mass chromatogram especially when using the mass spectrometer to detect and identify samples in the sub nanogram levels. These peaks occur due to backstreaming of the hydrocarbons from the oils into the mass spectrometer source. In both the rough pumps and the diffusion pumps, oils are used which can contribute to this spectra background. Backgrounds in the spectra can occur due to byproducts in the oil itself or due to mass spectrometer samples which are collected and trapped in the pump oils as samples are analyzed. The high temperatures and continued 24 hour a day use of the oils can cause them to break down (commonly referred to as cracking). These lighter fractions can cause peaks in the mass spectrometer background. In order to minimize these background peaks the rough pump oils should be changed every three months and the diffusion pump oils every 2 years. When changed the oils should be replaced with the most highly refined highest molecular weight chain hydrocarbon fractions which can be safely utilized in the vacuum pumps.

About 5 years ago we replaced the Inland 19 Oils in our rough pumps with Invoil 21 which was more highly refined and produced better backgrounds. Although this oil was more expensive it produced lower vacuum pumping capacity and minimized mass spectrometer background. In the last two years Inland 45 has been developed which is even more highly refined and will produce the least amount of vacuum pump backstreaming. It can be used in all Alcatel, Edwards and Leybold vacuum rough pumps utilized on mass spectrometers to obtain the lowest mass spectrometer background and optimum vacuum pump performance. We now use it in all of our instruments.

For the diffusion pump Santovac 5 is still the best quality oil available. In the more than 30 years that Santovac 5 has been routinely used in mass spectrometers, no new oil has been developed which surpasses its low vacuum level pumping capacity as well as its low backstreaming. For diffusion pumps which currently use other oils, the user must check that Santovac 5 can be used in these pumps before replacing the oil. The boiling point of the diffusion pump oil must be matched to the heater cartridge wattage in the diffusion pump. We have safely replaced the diffusion pump oils in the H.P. 5971 with Santovac 5 without any change in the diffusion pump heaters. The only improvement to the use of this oil in diffusion pumps would be to replace the diffusion pumps with Turbo pumps. Turbo pumps will provide the lowest background levels.