High-Vacuum Pumps in Mass Spectrometers

............................. MASS SPECTROMETRY FORUM .............................Figure 2. Components of a turbomolecularpump.ecules. Repeated collisions move theresidual gas molecules toward the regionof higher pressure at the bottom of thediffusion pump. At the ejector jet, thepressure (300–500 mTorr) is such thatthe rough pump attached to the forelinecan capture and exhaust the residual gasmolecules to the atmosphere.The working fluid used in diffusionpumps is a low-vapor-pressure hydrocarbonfluid such as Octoil, Convoil, or Santovac5 (all proprietary product names),or a silicone oil fluid such as DC 702, DC704, or DC 705 (also proprietary). Therequisite characteristics of the workingfluid are an appropriate vapor pressureand chemical inertness. The ultimate vacuumpressure attainable with a diffusionpump is directly related to the vapor pressureof the working fluid; the lower thevapor pressure, the lower the ultimatevacuum. Of course, a lower vapor pressuremeans that a higher boiler temperatureis needed to evaporate the fluid andachieve a pumping action.The chemical inertness is important ongeneral principles, but also especiallyduring the vent and pumpdown cyclesequences, and especially if the propersequences are not followed (5).Hydrocarbon-based oils will crack and oxidizeto tar if exposed to air when hot.The resultant tar coating the boiler of adiffusion pump and clogging the annularjets is difficult to remove. An abused silicone-basedoil will crystallize in theboiler; the crystals are striking in appearance,but the residue is often impossibleto remove, and the entire pump may needto be replaced.Backstreaming is a term that refers tothe migration of working fluid out of thediffusion pump into the main vacuumchamber. Backstreaming occurs whenthe operating pressure of the diffusionpump is too high, and the net downwardmotion of the working fluid moleculescannot be retained. The gas molecule motionsbecome randomized, and the workingfluid molecules diffuse back into themain vacuum chamber, coating the relativelycool metal surfaces that they eventuallyencounter. A vacuum systemdisassembled for inspection will often exhibita thin, greasy-looking film on interiormetal surfaces; this is the result ofbackstreaming of a hydrocarbon-basedworking fluid. Backstreaming occurs to asmall degree even in normal operationbut is minimized by the use of cold capsor cold traps.The aftermath of a vacuum accident —the unintentional or too-rapid venting of a“hot” system — will be a thorough dispersalof working fluid throughout the massspectrometer, and therefore the need fora lengthy, involved, and complete cleaningand parts replacement, in addition toa rejuvenation of the diffusion pump itself.The proper vent cycle includes acooldown for the diffusion pump oil beforeventing (30–60 min). A warm-uptime is also required during the pumpdowncycle for the system to reach itsfull pumping speed. The ability ofturbomolecular-pumped systems to cyclefrom atmosphere to working vacuum in ashorter time is an advantage in some situations,but the initial cost of a diffusionpumpedsystem is lower. Costs of electricityand cooling water required fordiffusion pump operation also have to befigured into a cost-based decision betweenthe two approaches to achievinghigh vacuum.The turbomolecular pump must alsoimpose a net directional motion to theresidual gas molecules in the vacuum system.The turbomolecular pump does soby repeated collisions of the residual gasmolecules with rotating blades of a turbinerotor (Figure 2). The blades of therotor spin at speeds of 20,000–50,000rpm; the edge speed of the rotors approachesthe velocities of the residual gasmolecules themselves. When a collisionoccurs, a direction is imparted to the motionof the residual gas molecules towarda region of higher pressure and towardthe pump exhaust (again, a rough pumpoperating on the foreline).There are several stages of spinningrotors, with the angles of the rotor bladeschanging along the profile of the pumpfrom inlet to exhaust, as shown in Figure2. Residual gas-phase molecules aremoved toward the exhaust and then areremoved from the foreline by the roughpump. Higher molecular mass gas moleculesare pumped more efficiently by theturbomolecular pump than lower molecularmass molecules; therefore, there isvery little backstreaming of lubrication oilmolecules into the main vacuum chamber.The turbomolecular pump has thecharacteristic of being a clean pumpingsystem with no backstreaming.Clearly there is no heater to warm upor cool down in a turbomolecular pump.A system so pumped can be vented to atmosphereand then pumped down morequickly than can a diffusion-pumped system(minus the use of isolation gatevalves in the latter, which decrease the effectivepumping speed). Diffusion pumpsmust be mounted at the bottom of a vacuumchamber, and in a vertical orientationso that gravity can return the condensedworking fluid to the boiler. Aturbomolecular pump can be mounted inany orientation relative to the vacuumchamber. There is residual vibration froma turbomolecular pump, so the mountingsystem often includes an integral vibrationisolation device. Finally, although thepumping speed is not as high as with diffusionpumps, the smaller size of the turbomolecularpump is often an advantagein today’s smaller mass spectrometers,and there is no need for a cooling waterconnection.Mass spectrometrists seem to pay lessattention to vacuum-related issues todaythan in the past. High vacuum has becomeeasier to attain, pumps are more reliable,and pumping systems are moreautomated. In vacuum, as with so muchelse, the modern mass spectrometrist exhibitsthe habit of deferring or ignoringmaintenance. And, as with much else,there is an ultimate cost to be paid for sodoing. In a mass spectrometer, the cost isdecreased performance in terms of lowersensitivity or lower resolution; mass spectrathat deviate from those measured andarchived under more ideal conditions;and shorter lifetimes for replaceableitems such as electron multipliers, ionizationgauges, and source filaments. Therough pump has the final task of exhaust-16 SPECTROSCOPY 16(5) MAY 2001 www.spectroscopyonline.com