Characterization of a simple glow discharge coupled to a time of flight mass spectrometer for in-depth profile analysis

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The capabilities of a simple glow discharge (GD) ion source coupled to an on-axis time-of-flight (TOF) mass spectrometer (MS) have been investigated in terms of several parameters of interest for in-depth profile quantification, including crater shapes, sample sputtering rates, sensitivity and mass resolving power. The GD ion source used here is a simple prototype constructed in our laboratory and designed to be easy and quickly exchanged with the inductively coupled plasma (ICP) source of a commercial ICP-(TOF)MS. Experiments showed that good crater shapes, with walls “perpendicular to” and base “parallel to” the sample's surface can be obtained for samples with different matrices, provided that a “sampler cone” in the interface between the GD chamber and the TOF is used; otherwise, an inhomogeneous sputtering in the sample is observed, ruining the crater shapes. The GD-(TOF)MS system developed offers a mass resolving power (full width at half maximum) of 2800 for 207Pb in the digital detection mode and 1700 in the analog mode. Furthermore, promising results have been obtained for possible use of polyatomic ions (formed between the argon plasma gas and a major element in the sample) as a quantitative indicator of the corresponding element. This approach could be very useful as an alternative strategy for the simultaneous determination of major, minor and trace elements, without saturation of the detector, by GD-(TOF)MS.