The stability is the easiest part: Derivatise your samples (the pure standards, a mixture of the cmpounds you are interested in, or your sample) and measure it again and again over time. Prepare multiple aliquots instead injecting the same sample multiple time. Then compare the intensities and also the different derivatisation products, over some hours the ratio may go more towards the completely derivatised amino acid. Acids are quite stable. This may also provide information about the efficiency and completeness: If the total yield increases your compounds are stable and you did not reached 100% efficiancy after a short time. If your total yield decreases the stability is low but efficiency might be high enough. In the end some compounds might be better measured shortly after derivatisation and other after a much longer time period.

Efficiency is more complicated: The only way I see is to switch to a method (e.g. HPLC-based) that can measure both, the derivatised and underivatised compunds, even this can be complicated as the charge and polarity disappears. And then do the same, compare different time points and check if the ratio changes. However efficiency is usually reported to be very high. If you are aiming in quantifying your compound you don'nt need to know if efficiancy is 98 or 99.5%. The only thing you have to be sure that efficiancy of derivatisation of your test-compound is the same as of the standard with known concentration. This can be seen as granted for added (internal) standards.

Thank you so much