In mass spectrometer, the resolution required was defined as
M/(delta M), So, it means for two larger M.W. molecules, the resolution required was larger than small M.W. molecules even the delta M was was the same.....

But why ??!!
why the instrument should have better resolution to distinguish different larger molecules ??!!

Thx...[?]

Don't know much about mass spectroscopy.

But if I had to guess...

For small molecules, the empirical formula can be calculated from the MW. There's only one formula possible for the MW 130.1418, for example. Even mass spectrometers with low resolution can figure it out for small molecules. WIth large, particularly biological molecules there are more combinations and permutations that can give very similar MW's. So having a high resolution probably helps here. But I'm not sure. I don't handle many compounds more then several hundred daltons.

However, I still don't understand in some sense.

For example, if 2 molecules, the first has atomic mass 22.01 and the second one has 22.00, the mass resolution required to distinguish THE TWO molecule was 22/0.01. Another 2 molecule have atomic mass 122.01 and 122.00, the resolution required for distinguish this TWO VALUE (PEAK) was 122/0.01.

As the delta M was 0.01, in common sense, what the instrument required was it can distinguish 0.01 mass difference. The mass of the peak were not related, if just consider two peak. In fact, mass resolution was defined as "the maximum resolution required to distinguish of two peak"

[?]

Remember, in real life, you have crud in your data. You have error, you have deviations. It will require a higher level of resolution to distinguish between two peaks as their standard deviations may increase.

Now you may feel pity for all those people doing mass spec of polymers and proteins.