Cleaning of Mass Spectrometer Ion Sources by Electropolishing Gerald L. Peele" and David A. Brent Wellcome Research Laboratories, Burroughs Wellcome Co., Research Triangle Park, N.C. 27709
Cleaning of ion sources is a routine procedure in all mass spectrometry laboratories. This is most often accomplished by abrading the surface of the metal plates with wet 600 grit silicon carbide paper followed by thorough solvent rinsing. While this procedure removes deposits on flat surfaces quite well, it is considerably less effective on irregular ones. Another important consideration is that all of the metal surfaces are covered with fine scratches after only one cleaning. These scratches are made deeper and more irregular by repeated cleaning, thereby degrading the homogeneity of the electrical field around the ion source components. Also the scratches accelerate contamination resulting in electrical leakage paths. The end result is an ion source in which the sensitivity and stability are reduced. Seeking a less abrasive cleaning method, several laboratories have turned to electropolishing sources after chemical pretreatment (1). When we sought to set up such a system in our laboratory, we found the electropolishing literature to be limited with no references to methods for electropolishing mass spectrometer ion sources. We are presenting our experiences as an aid to those who might wish to adopt a similar procedure. We have successfully employed an electropolishing technique to clean and polish the metal components in the ion sources, both E1 and FD, for a Varian MAT CH5-DF and a Varian MAT 731 double focusing mass spectrometers. A standard battery jar, 16 X 12 inches, was used for the bath. A Yg-inch copper rod, bent into a 14-inch diameter circle, was suspended inside the top of the jar. Six lead cathodes, 3 X 6 X 1h inches, were suspended from the copper rod with large copper alligator clips. The bath was filled to within 1inch of the top of the cathodes with Electropolish No. 4 (Ashland Chemical Co.). Power was supplied by a 60-ampere plating power supply (Swest, Inc.). Clips made from brass test tube clamps were used to suspend the work pieces in the bath from a %-inch copper rod. Quartz immersion heaters with variacs for temperature control and a variable speed stirrer completed the apparatus. Electropolishing could be referred to as electroplating in reverse. Since metal is removed from the surface of the work pieces during polishing, they must be made anodic. The amount of metal that is removed should be held to the absolute minimum that will yield a clean, polished surface on the
ion source parts. We found that 316 stainless steel parts polish best at a bath temperature of 70-80 OC and 10-20 amperes current for 1-3 minutes. This would normally remove less than 0.0005 inch of metal. Similar results were obtained on Varian source parts, a nickel-chromium alloy, with the bath temperature at 21-24 "C and 2-6 amperes current for 5-15 minutes. The current is varied according to the size and thickness of the work piece, larger pieces requiring higher current. Bath temperatures that are too high can produce an etched, rather than polished, surface. The polishing bath used here is suitable for stainless steel and nickel-chromium alloys only. Solutions for electropolishing other metals are easily prepared from chemicals available in most research stockrooms (2). Our experience has been that the Varian ion source parts can usually be electropolished with no pretreatment. In areas of heavy dirt buildup, a light cleaning with abrasives prior to polishing is advisable. After polishing, the parts are rinsed thoroughly under running water and dipped for 1-2 minutes in chromic acid cleaning solution to remove any film left on the surfaces. The pieces are rinsed again in water, boiled in distilled water for 10-15 minutes, and rinsed in acetone followed by methanol. Electropolishing is a simple, effective, and inexpensive technique. We have applied it to E1 and FD ion sources for our Varian mass spectrometers. Excellent sensitivity is obtained with no background contamination. After the initial polishing to remove all scratches, ion source parts can be cleaned and polished with no scratching and essentially no metal removed (
