Residual Gas Analysis

Residual Gas Analysis

Residual Gas Analysis is a high vacuum, or ultra high vacuum technique, in which a mass spectrometer is used to detect leaks and outgassing. These devices can do the same job as a helium leak detector, and much more. The SRS RGA-100 is an example of an inexpensive RGA that does a good job.

Modern electronics and production techniques have dropped the prices and raised the performance of a number of scientific instruments which used to be prohibitive. One such instrument is the Residual Gas Analyzer. Once, these jewels cost twenty thousand dollars and more. Now, models such as the SRS RGA-100 sell for under $4000.

At a former job, we purchased several of this model, and had excellent results with them. The basic RGA-100 model, with a Faraday cup detector, measures partial pressures down to 1e-10 torr, about as low as most vacuum systems can hope for. For about $1100 extra, you can get the version with an electron multiplier detector capable of partial pressures down to the 1e-14 torr range. You've gotta be really good at building and operating vacuum systems to challenge the electron multiplier detector! The -100 model measures mass/charge ratios up to 100. Also available are units capable of going higher, the -200 and -300 units. We never saw any significant gases in our systems above about 54 AMU, but the higher ranges are available if you need them.

The SRS RGA series are compact quadrupole mass spectrometers, usually mounted on a 2.75-inch CF port, and controlled and monitored by an ordinary computer, running Windows. They work best at pressures of 1e-5 torr and lower, but will work at higher pressures, finally giving up and turning themselves off as they near the bottom of the Paschen curve, around 1e-3 torr. They're very protective of the filaments used to produce the ionization beam. I made several boo-boos which I feared would burn a filament, but each time the instrument reacted quickly and saved a filament (they cost something like $200 each) and the repair time. We did lose a filament once, but it took a major blowout to do it.

The units have proved generally reliable. We did have one develop an apparent filament fault, which turned out to be a simple connector problem. Occasionally, we had them lock up for no apparent reason. Typically, this was a software problem, one of Bill Gates' little surprises locking up the Windows software - the usual solution applies: reboot. The leak check function of the software is great, however the version we were using had a slight bug in the file storage/retrieval function, so that we were unable to retrieve prior leak test setups (all other functions worked well).

Why use an RGA instead of a helium leak detector? I've used helium leak detectors, too. I can think of several good reasons why not to, particularly at this price for an RGA. First, and foremost, leaks are only a small part of the problem in vacuum systems. Once a system has been thru its initial tribulations, it probably won't leak. Yet, every time you open the system to the air, it will take a long time to get the pressure back down. Most folks with no RGA will spend an awful lot of time wrenching on fittings (possibly over-tightening them), twisting valves to be sure they're seated (and mashing the seats), and dribbling alcohol on any seal they can't tighten, trying to find leaks that are not there. What is really happening is the system has picked up a nasty load of water vapor, and it is outgassing. With an RGA, you know that at a glance. Water vapor shows up as a triplet of peaks, at 18, 17, and 16 AMU. An air leak shows up at 28 and 32 AMU (nitrogen and oxygen), in a ratio of about 4:1 nitrogen to oxygen.

If you do have a leak, with a helium leak checker you have only one choice of test gas: helium. There's nothing wrong with helium. It is easy to get, as safe as a gas gets, and the dinky, low-mass atom slips thru the tiniest gaps easily. But in our case, we used deuterium gas in the system, the same mass as helium, and that meant our sensitivity to helium was poor. We preferred argon, also readily available, safe, and monoatomic. Surprisingly, the size of an argon molecule is about the same as helium: yes, it has more electrons, but the nucleus also has more charge, and the effective diameter is not much different from helium. It is slower getting thru a leak, by maybe a few milliseconds. The SRS RGAs use your computer's speaker to produce varying tones as levels of your leak-test gas change, allowing you to hunt for leaks without constantly looking at the computer screen, but they're a lot less annoying than the ear-piercing squeal of an antique Consolodated Vacuum helium leak checker.

Or, let's say you're out of helium. Do you, by chance, have a propane torch around? Give the suspected leak a wiff of propane -- not your best choice, but it works. Look for peaks starting at 44 AMU, with fragments galore below that.

OK, now the leaks are tight. Why won't the pressure drop? I know a system builder who once worked on a huge chamber in which one entire side was ion pumps. They sealed the beast up, pumped it down, and were appalled when it would not go below about 1e-5 torr. Suspecting water, they let it run ... days ... weeks ... at three months they gave up, the pressure down but not nearly what they'd designed it for. They opened it and found a brown paper bag containing a very dry sandwich. I suspect, in addition to a huge water load, an RGA would have quickly identified a huge range of peaks, including many hydrocarbon fragments, as the bologna and mayonaise outgassed. Cleaning solvents definitely show up readily, as does pump-oil contamination.

We developed a new working definition of the transition to ultra-high vacuum. If you slap enough turbopumps on a chamber, you can pull it down to the 1e-8 range, but if that's all you've done, the residual gas will probably still be 70% water vapor. Only after baking out did we get what we wanted, a true ultra-high-vacuum condition in which the primary gas was hydrogen. And it is great to watch the gas composition change during bakeout. You can easily see when the water is gone, or when the hydrocarbons have finally burned off.

And, of course, if you need a specific gas in your chamber, at a specific concentration, the RGA is perfect. And one of the many functions of the software is to provide pressure-versus-time plots of multiple masses. As I said, we wanted deuterium in the system. When we still were using an ion pump, we would introduce deuterium and assume that was what was in the chamber. Bad assumption! When we got the RGA we found that loading the ion pump with deuterium was making it regurgitate all sorts of stuff it had picked up before. Switching to a turbopump improved things, but then we discovered that the huge pressure rises when we turned on the e-beam were mostly hydrogen, making the gas we wanted a tiny fraction of what was present. The RGA showed us how many of our assumptions were wrong, but also pointed us in the right direction to fix the problems.

An RGA will give you quite a chemistry lesson. If there's more than one gas in the system, and anything in the system causes ionization, you'll probably get recombination products. This can be useful, as measuring the level of the recombination product can give you a pretty good idea of how much ionization occurred.

There is one good argument for having a leak checker instead of a system-mounted RGA. A leak checker has its own vacuum system. It can be rolled up to another system, even to something that can't be pumped down to high vacuum, and be used with a flow restriction to sniff gas from the thing being leak tested. I've used leak checkers that way. Given my choice, I'd yank the helium mass spec right out and install an RGA, and get three or four times the use out of the leak checker.

The only bad thing I can say about the SRS RGAs is that it is hard to get much useful help from SRS over the phone.

I don't sell the SRS RGA-100. I'm just a consultant, looking for work. If you think one of these little gems is just what you need, you'd like some help setting one up, and you live close to Northern Virginia, I'd be happy to come over and sell you a little of my time.