transfer function of a sampler in the s domain
ADCs don't have a transfer function. An ideal ADC is an ideal sampler with a sampling period T.
Real ADCs are not ideal, they have some sampling delays, jitter, quantization noise, thermal noise, etc. Usually the sampling delays are really small compared to the dynamic of the systems, so we don't model them. Quantization noise and thermal noise should not be a factor unless the signal you measure is really small. And jitter is not an issue unless the signal frequency is really high.
So bottom line, if your ADC is really fast compared to the dynamics of your system and if the measured signal is inside the dynamic range of your ADC, you could pretend that the ADC is not there.