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- 19- What techniques or methods do you use to detect vacuum leaks in your mass spectrometer (This Page)
by: William Schutzer
Affiliation: Oregon State University
We use a HP 5890 GC with a J&W DB5-MS column interfaced to a 5971A MS running under HP ChemStation software. We make it a habit to run an air and water check once a week. When either values for mw 18 (water) or mw 32 (nitrogen/air) rise above 2%, we know we have a leak.
Almost always, the leak is due to a loose fitting where the column enters the MS. Simply tightening the fitting and then performing an air and water check in a couple of minutes usually solves the problem. However, the leak is not always there. Occasionally the MS will spring a leak elsewhere. To find the leak, we use dust removal aerosol; it is made up of chlorodifluoromethane, which produces a pronounced 52 mw peak.
Enter the Manual tune section and then select Edit MS param. Then, change the MS parameters to look for mw 52. Finally, choose repeated profile and spray the aerosol around the MS. The peak will show up almost instantaneously when you spray the aerosol at the leak point.
by: Richard A. Berger
Affiliation: Washington University Medical School
Testing for Leaks
I use various methods to test for leaks in a mass spectrometer, depending upon the severity and location of the leak. Of course, I first must determine if a leak really exists. Remember, vacuum measuring equipment can malfunction, tubes/contacts can get dirty, connections can become loose, etc.. If I verify that the vacuum measurements are not at fault, then I must determine whether the leak is real (atmosphere getting sucked into the mass spectrometer) or virtual (too much solvent/sample has been put into the instrument, outgassing of seals, etc.). No amount of leak testing will find the latter and the instrument simply must be cleaned and/or baked if a virtual leak is suspected. If atmosphere is leaking into the instrument and it can be operated, look for a Nitrogen to Oxygen (28 to 32) ratio of 4:1. (You will also see this ratio for some time after you have closed up the instrument and pumped it down but it will constantly deminish if there is no leak).
If you trust the vacuum measurement and know that you don't have a virtual leak, then its time to check for real leaks. Most real leaks occur at the last point(s) where the vacuum system was disassembled and reassembled. Check these points first. If the mass spectrometer can b e safely operated (i.e., the leak is not so large as to burn out the filament or multiplier) use the mass spec as a leak detector. Various gases can be sprayed on the flanges of the machine while monitoring the appropriate ion. I use Sulfur Hexafluoride (SF6) on our GC/MS instruments (m/e 127 in EI/PCI, m/e 146 in NCI) and Argon (m/e 40) on our isotope ratio machines. To detect very small leaks, especially Down stream of the source, apply the gas via a disposable plastic pipette tip to a limited area and wait to see if you get a response. Patience is the key to good testing. If the leak is too large to safely turn the mass spectrometer on, I use solvents with high vapor pressures such as Acetone (Caution: Flammable - don't use on hot parts) and watch for a deflection of the vacuum gauge(s). However gaskets of different material will respond differently to solvent. Metal gaskets such as Copper or Gold will show a positive gauge response while elastimer seals such as Viton® may swell and show a slow negative gauge response especially if the leak is small. The closer the leak is to the gauge the faster the response will be.
Leak testing can be a very frustrating task. Patience and perseverance are your allies. don't panic and tear the machine apart, replacing every gasket and seal. Such a shotgun approach often makes matters worse.
It is much easier to avoid leaks on mass spectrometers than to find them. There are several easy and relatively inexpensive things that you can do to prevent the majority of leaks.
1) Replace metal gaskets on a regular schedule when you disassemble and reassemble a flange. I generally reuse copper gaskets on Conflat type flanges only once. I leave the gaskets in position on one half of the flange. If they fall off or if the particular flange is difficult to get to, I replace the copper gasket. I replace gold gaskets every time I reseal a flange (if you run short you can sometime get a leak free seal by putting two used gold gaskets in the flange).
2) Apply a very light film of low vapor pressure lubricant such as Apiezon L to Viton® O rings before reassembling gaskets which use them. This also works well when attaching vacuum tubing to vacuum pumps. Don't lubricate O rings used in GC injection ports.
3) Keep your mating flange surfaces free of foreign matter and be careful not to nick the knife edges of Conflat type flanges.
4) Where compression type fittings (Swagelok, Parker, etc.) are used, beware of overtightening the fittings. PTFE ferrules can be substituted for metal ferrules on fittings which will not be heated above 250 degrees C. When using PTFE seals in fittings which are repeatedly heated and cooled, the PTFE will flow and the fitting will need to be retightened.
5) Avoid pure Vespel and Graphite/Vespel ferrules. You only get one chance at making a seal with pure Vespel - retightening will cause it to crack. Graphite/Vespel is more forgiving, but cracking can still occur depending on the ratio of the two components.
6) When I pump an instrument down from atmospheric pressure, I test the reassembled flanges at several points in the pump-down process:
A) Note whether the mechanical pump stops gurgling in the usual amount of time or whether it is laboring;
B) Before switching on the diffusion/turbo pump, use a high vapor pressure solvent on the reassembled flange and note the mechanical pump vacuum gauge response;
C) After switching on the ion gauge (but before switching on the mass spec) test again with solvent, watching the ion gauge; and,
D) After the instrument is stabilized, test with Sulfur Hexafluoride or Argon.
by: Dennis Beauchamp
Affiliation: Roy F. Weston Analytics
Detecting Leaks in Your
A leak in a GCMS system can be a frustrating problem. But should not be hard to find if you use a systematic approach to finding it.
Indications of a Leak
Higher than normal ultimate vacuum. If your system has an Ion Gauge you should note what it normally reads so that in the case of a leak you will recognize the change.
Higher than normal foreline or ion source pressure.
Depending on system type these can usually be read either from a front panel gauge or through a software program. Again knowing what these are when everything is OK will help when there is a problem.
Higher than normal Background.
Usually a relatively tight system will show very little contamination. A foreline pump problem might show some hydrocarbon background which are peaks 14 amu apart. A diffusion pump problem might show some peaks around m/z 446 along with other masses between m/z 77 and 355.
Poor high end sensitivity and decrease in overall sensitivity.
Usually a slight air leak will effect high mass sensitivity. A problem like poor DFTPP tuning with low m/z 442 (below 40%). Also a leak could cause a fall off in overall sensitivity. An increase in multiplier voltage would be necessary to make up for the loss of sensitivity.
Finding and Fixing a Leak
Sometimes a leak will occur after injector maintenance has been performed. Cooling heated zones will cause fittings to leak even after they are reheated. Anywhere there is a PTFE ferrule is a good place to check for leaks. Usually PTFE ferrules are used on separators and places where a constant temperature is maintained. Finnigan 5100 interface ovens are usually leak prone after heating and cooling have taken place.
Look at the air/water spectrum between m/z 15 and 50. On a leak tight capillary system manuals say m/z 28 (nitrogen) should be 3 times smaller than m/z 18 (water) but for all practical purposes when 18 and 28 are equal, with the cal gas off, it usually means a pretty tight system. The masses that really mean trouble are m/z 40 (argon) and m/z 44 (CO2) when these are fairly visible it indicates a leak.
Now while looking at masses between m/z 35 and m/z 75, spray the new environmentally safe dust off (chlorodifluoromethane) and look for peaks to appear at m/z 51 and 67. Spray short bursts of this at fittings in the interface oven, around the separator and in the GC allowing time for it to show up on the oscilloscope or monitor. Tightening or replacing the fittings or ferrules should fix the leak.
by: Richard Milberg
Affiliation: University of Illinois
The easiest way to leak check vacuum systems is to have a helium leak detector such as our Veeco MS-20 or any of the other brands and models. A portable one such as the MS-20 can be wheeled to any of the instruments and attached. It takes 6 minutes for the diff pump to warm-up and pump the detector down.
The leak detector is attached to the portion of the vacuum system under test by replacing the mechanical backing pump (whether it is backing a turbo or diff pump doesn't matter) of that section with the leak detector. This is done under vacuum since most of the pumps in the Lab have Edwards Speedy valves between the backing pumps and the high vacuum pumps.
Many leaks, especially at the detector end of the instrument can only be found quickly this way. A small vacuum leak at a conventional SEM will cause rapid and irreversible loss of gain. Small leaks around pumps below the analyzer are difficult to find without a leak detector.
A leak detector with a sniffer probe can also be used to look for leaks in GC plumbing. Vacuum leaks between the end of the GC column and the ion source can usually be found by spraying argon and looking at the 40 peak on the real time display.
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