A glow discharge tube for emission spectral analysis is disclosed. The tube includes an anode cavity which is connected on one side to a carrier gas source and is connected on the other side to a first vacuum pump. The anode cavity is in communication, by way of a restrictor, with a cathode cavity which is connected to a second vacuum pump. The second pump produces a vacuum that is greater than that produced by the first pump. To reduce the margin of error in the accuracy of spectral-analytical tests, the gas pressure in the main cavity is maintained at a constant value by an electronic control which regulates the amount of carrier gas being fed to the anode cavity.
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