Sampling time is something you decide and implement. Not sure what you mean by the above statement. Or do you want to make the sampling time "deterministic" - meaning it has the topmost priority and will never be missed ?

For your application, the Serial interrupt must have the topmost priority and as you have seen, when the ADC code is placed in Main, it did solve the problem.

In any case do not place the ADC code in the interrupt. But one good way is to ,

> Start an ADC conversion WHEN required from MAIN.
> Use the End of conversion signal from the ADC as an external interrupt to the MCU and maybe place the code to read the ADC inside THAT ISR. And keep that ISR as small as possible.

These are general tips - unless we know what sampling time, what ADC, what MCU and what clock - further advice is difficult.

Raghu