STUDY OF A PULSED GLOW-DISCHARGE ION-SOURCE FOR TIME-OF-FLIGHT MASS-SPECTROMETRY

This paper describes a new type of glow discharge (GD) ion source coup led 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 1 80 Pa for the discharge pressure, 3 A for the transient discharge curr ent, 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 sig nificantly higher than that for the d.c.-GD. Typical mass spectra of b rass and nickel samples were studied and are discussed. (C) 1997 Elsev ier Science B.V.