For some years we have known that gaseous contamination of the argon plasma gas can have deleterious, or at least interesting, effects on the plasma.(1) Very recently, excitement has grown over the effect of small amounts of hydrogen contamination.(2-5) Hydrogen is now known to alter emission yields, making some lines more intense (eg, O I 130.1 nm) and some less intense (eg, Fe I 371.9 nm). It also increases background signals by creating molecular bands (hydrides and molecular hydrogen) particularly in the region 220 nm to 440 nm.(3) The problem is that it doesn't matter whether the hydrogen comes from hydrogen deliberately added to the plasma gas (something we could avoid) or from hydrogen sputtered from the sample being analysed.
The question then arises as to how best to correct for the effect of hydrogen, or any other gas, in the plasma gas. If we assume that the effect is proportional to the measured hydrogen signal, then it is a relatively straightforward matter to include both additive and multiplicative corrections.
The effect of hydrogen on the background signal can be treated by an additive correction proportional to the hydrogen signal. Such an additive correction can then be included like any other spectral interference, see Quantification.
The effect of hydrogen on emission yields can be treated as a multiplicative correction, ie one effect of hydrogen is to change the slope of the calibration curves. Initially we thought these changes to emission yield could be treated in the same way as other multiplicative parameters, such as pressure or DC bias voltage, see Emission Yield. However, these methods assume the range of values is relatively small (eg 500-1000 Pa for pressure or 500-800 V for DC bias voltage) but this is not the case for hydrogen since the hydrogen content in samples can vary from near 0% in many materials to perhaps 10% (by mass) in some polymeric materials. We therefore need to know more about the real function relating changes in emission yield and hydrogen before we can proceed.
Fortunately Hodoroaba et al. have measured the variation in intensity of a variety of emission lines as a function of hydrogen content in the plasma gas.(4)
Their data was used to construct the following two graphs.(6) For Si 288.1, the intensities were normalised to the value at zero hydrogen. Similar graphs were obtained for N 149.26, C 156.14, and S 180.73. For Mn 403.4, the intensities were first inverted and then normalised to the value at zero hydrogen. Similar graphs were obtained for Ti 337.27, Mo 386.41, Al 396.15, Cr 425.43, and Fe 249.3.
These graphs suggest that some line intensities (non-metals?) increase nearly linearly with hydrogen and others (metals?) decrease such that their inverse intensity increases nearly linearly with hydrogen.
These graphs can be represented by either of the following two equations
where Ri is the inverse relative emission yield for element i, IH is the measured hydrogen signal and hi is a fitted parameter.
Authors: R Payling, Surface Analytical, Australia and M Aeberhard, EMPA, Switzerland
(1) W Fischer, in R Payling, D G Jones and A Bengtson (Eds), Glow Discharge Optical Emission Spectrometry, John Wiley, Chichester (1997), pp 403-9.
(2) A Bengtson and S Hägström, Proc. 5th Internal. Conf. on Prog. in Anal. Chem. in Steel and Metals Industries, European Communities, Luxembourg (1999) pp 47-54.
(3) V-D Hodoroaba, V Hoffmann, E B M Steers, K Wetzig, J. Anal. Atom. Spectrom. 15 (2000) 951.
(4) V-D Hodoroaba, V Hoffmann, E B M Steers, K Wetzig, J. Anal. Atom. Spectrom. 15 (2000) B0023671 (on web).
(5) A Bogaerts and R Gijbels, J. Anal. Atom. Spectrom. 15 (2000) on web.
(6) R Payling, M Aeberhard and D Delfosse, J. Anal. Atom. Spectrosc. 16 (2001) 50. Available on the web athttp://www.rsc.org/ej/gd/2000/b007543o.pdf
First published on the web: 1 June 2000 and extended on 5 March 2001.
Author: Richard Payling