In bulk analysis, no matter what the mode of operation, it is common practice to restrict the range of compositions so that qM and Ri do not vary significantly. They are therefore omitted from the calibration function, which then simplifies to
where the restricted composition range means we rarely need third order.
Calibration Curves for Cast Iron, Ratioed to Total Light (Fi) using RF-GD-OES
Courtesy: Claude Blain, France
To improve the precision it is common to use ratios and there are four common ways to do this:
The elemental intensity is divided by a reference intensity Ij, usually the intensity of the major element or the total light (as shown above) or argon intensity
There is no satisfactory theory to describe total light or argon intensity, so their use can only be justified empirically, i.e. they are seen to improve precision in certain cases.
The use of the major element intensity is more problematic. The intensity depends on the stability of the plasma (which is the main reason we are using it) but it also depends on the product cj.qM. Therefore it will change if either its concentration or the sputtering rate changes from sample to sample. It is therefore most useful when there is no significant change in its concentration. But it does have the advantage that it will correct for changes in qM.
Hence the Normal method, ratioed to the major element, is recommended when there is no significant change in the major element composition. It should then improve precision by adjusting for small changes in the plasma during measurement and for changes in sputtering rate. An example is shown for C 156 in low alloy steel:
The Ratio method, sometimes called Virtual method, is used principally when there are significant changes in concentration of the major element. Both the intensities and concentrations are ratioed
An example is shown for the C 156 nm line in stainless steel. The sputtering rate of stainless steel samples can vary significantly due to the changing content of chromium and Nickel.
The equation above can be derived readily from the accepted Calibration Function provided:
In other words, the reference intensity must be first order and with negligible background signal. This assumption then leads to
where we note the background term is no longer constant. This has major consequences at low concentrations.
If the reference signal is not first order or has significant background then the use of the ratio or virtual method will lead to systematic errors in calibration and analysis.
This method is similar to the Normal method except that in analysis concentrations are normalised to 100%. This has the advantage of further correcting for changes in sputtering rate from sample to sample (to understand why see the discussion on quantitative depth profiling). The 100% method can therefore extend the range of samples that can be analysed by the Normal method. But it has the disadvantage that all major and minor elements must be included or the normalisation will give strange results.
- Th.Nelis, M.Aeberhard, R.Payling, J.Michler, P.Chapon; Relative calibration mode for compositional depth profling in GD-OES; J. Anal. At . Spectrom., 2004 , 19 , 1354 – 1360, DOI: 10.1039/B406187j
Thomas Nelis, Richard Payling, Royal Society of Chemistry, Cambridge (2003), ‘Practical Guide to Glow Discharge Optical Emission Spectroscopy’