Study of a pulsed glow discharge ion source for time-of-flight mass spectrometry
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This paper describes a new type of glow discharge (GD) ion source coupled to a time-of-flight mass spectrometer (TOFMS). The GD is operated in the microsecond pulse (mu s-pulse) mode. The operational parameters of the mu s-pulse GD were optimized against the ion signals, giving 180 Pa for the discharge pressure, 3 A for the transient discharge current, 1.75 kHz for the discharge frequency and 2 mu s for the discharge pulse duration. Experimental results show that the discharge current in the mu s-pulse mode can be one order of magnitude higher than that obtained in the d.c. mode. The structure of the interface between the mu s-pulse GD and the mass spectrometer was found to be critical, and a Macer disc must be applied in front of the sampling orifice in order to shield the sampling plate from the anode of the GD to achieve both a good vacuum and the best sputtering. A transient sputtering rate of 24.4 mu g s(-1) mm(-2) was reached in the mu s-pulse mode and was significantly higher than that for the d.c.-GD. Typical mass spectra of brass and nickel samples were studied and are discussed. (C) 1997 Elsevier Science B.V.