SSIMS quantification

It should be emphasized that the determination of surface concentrations and coverages is only one aspect of quantification. SSIMS can also be used successfully for the determination of molecular weight distributions and average molecular weights in polymers, for the determination of diffusion constants (vertical and horizontal diffusion) and for the determination of surface coverages by evaluation of ion images. Some examples will be shown in the next section. 

The sputtering yield S can be measured from elemental and multielemental standards under various operational conditions and can therefore be known to reasonable accuracy for the material being analyzed by judicious interptolation between standards. However, the ionization probability a cannot be measured easily, and attempting to measure it for a set of standards and then trying to interpolate would be of no use, because it is very matrix and concentration dependent and can vary very rapidly. In fact, it represents the principal obstacle in achieving proper quantification in SSIMS. 

In general. SSIMS has not been used for quantification of surface composition because of all the uncertainties described above. Its application has been more qualitative in nature, with emphasis on its advantages of high surface specificity, very high sensitivity for certain elements, and multiplicity of chemical information. Increasingly, it... 

Although treated here as one of the major surface analytical techniques, ISS, like SSIMS. is used much less extensively than XPS or AES. Again, like SSIMS, it is often used in conjunction with other techniques and is valuable as a complement to the others for its two advantages of high surface specificity and ease of quantification,... 

Modern SSIM spectrometers are equipped with data acquisition/processing stations, and the programs allow control of the spectrometer during acquisition and standard spectrum interpretation and quantification (mass scale calibration, peak locating. 

The situation becomes even more complex if mixtures of different materials exist at the surface. Fortunately in most cases the spectra of mixtures can be explained by superposition of the spectra of the individual compounds. Unfortunately such a superposition cannot necessarily be used for quantification since the matrix effect in SSIMS has to be taken into account, but is useful for qualitative identification i.e., the peaks present in the spectra of the individual compounds appear also in the spectrum of the mixture. Mixed ions, i.e.. those formed by reaction between the individual compounds, are rarely observed, Spectra libraries can therefore be used successfully to identify the individual compounds,... 

Is quantification of SSIMS therefore completely impossible or unreliable. Fortunately, three areas of application exist where quantification is indeed possible. Quantification here is used in the sense that the change in the number of detected secondary ions does mirror truly the change in surface concentration. 

Although the use of internal standards can be a very powerful approach for absolute quantification, there are several limitations to its use in SSIMS. 

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