小中大Calibration using the C1s peak is indeed the most common way of peak-calibration before determining the Chemical state in XPS spectra.
But calibrating what you think is "a known peak" as C-C peak to 284.8eV meaning = You believe or have made an ASSUMPTION, that this particular peak is in the Chemical state form of C-C. The judgment from your auditor is challenging that from the nature of your sample the peak you calibrated to 284.8eV may NOT be purely in C-C Chemical state because he/she is expecting that with sample nature it is something more than C-C (meaning its energy would not fall at exactly 284.8eV). So the Auditor is simply challenging the ASSUMPTION you made is not correct.
If you do not want to do like the auditor suggested to introduce standard thin-layer of pure metal so be able to use its peak for calibration, what you should do is that to show and prove your final result and ASSUMPTION is "reasonable" instead of simply saying "All other papers did the same way and got expected result".
My suggestion is that I am guessing your sample mainly just have some O1s and C1s signal. If there are other signals, then your final result should have Chemical state of all elements. To prove a correct Chemical state judgment, one will need to do a complete data interpretation including:
(1) Determine all presented elements
(2) Do curve fitting for all presented elements and assign Chemical state
(3) With all elements assigned with Chemical state, one should do a Quantification of all presented elements in each Chem-state.
If the final quantification between Chem-state is matching to the assigned Chem-state formula, then you can show this is a good proof to the auditor your result is correct.
For example, if C1s peak after curve fitting have 2's Oxide-related Chem-state as CO, CO2 state and corresponding quantification are 10% and 20% > then respectively in the O1s peak there should be corresponding 10% and 40% (because CO2=1C+2O), so O1s concentration should be roughly 50%.
If the quantification result between ALL elements in final assigned Chem-state forms can match roughly your judgment, then you can say to the auditor your result is correct.
