The mass spectrometer method involves pressurizing the test object with a helium mixture and placing it in a snug-fitting vacuum chamber. The air is then evacuated from the chamber, creating a pressure gradient between the internal volume of the part and the vacuum. The helium molecules move out of the part through any porosity, holes and cracks. A mass spectrometer then samples the air inside the chamber and finds individual atoms of helium. This is the most sensitive test presently available. These testers are capable of detecting a leak of R600a refrigerant as small as 0.0028 ounce per year. In use, the helium is usually a disposable item adding cost, although recovery systems are now available. Electronic parts often are tested by placing them in a chamber and pressurizing with helium, then placed in the vacuum chamber of the mass spectrometer to see if helium is drawn out. This method also will find porosity.

The initial cost of a snug-fitting chamber is high, requiring precise machining and extremely tight seals, limiting the practical part size. A basic operation limitation is that a large leak—more than 4 sccm—will saturate the vacuum container with helium. This requires several hours of flushing to lower the helium background to a workable level. A pre-test can be employed to weed out the gross leakers. As with dunk testing, a 19-second inspection has proven troublesome and is slowly giving way to pressure decay testing, with or without permanent recording. Even a condenser 16- to 18-inches high, and 24-inches long can be easily tested to 3 sccm in 12 to 15 seconds at 200 psi. Many air conditioner condensers (radiators) are quite large, but they still must not leak for 8 to 10 years. For these applications, a wand testing system has been developed which, while not as accurate as placing the part in a vacuum chamber, is superior to any other kind of test, and substantially less expensive than the vacuum chamber system.