As you know Avantage will automatically produce the at% concentration for you (see screen shot attached). If you want, for example the O/C ratio, you just take the calculated at% and divide accordingly - in the atatched example, O/C = 29.31/70.69 = 0.41

These values are corrected for transmision, escape depth and elemental sensitivity so are the ratios you want

David J. Morgan Thank you so much for your input, much appreciated

It's an approximation using the sensitivity factors, there are some impressions in the background and in the fitting. There are more accuracy methodologies that you can consult at https://rdataa.com/

regards