Unless there is something wrong with the instrument you should be able to do this without changing anything. Isotopes differ in mass by 1 Dalton at least, they can be resolved by an ordinary low resolution ion-trap or quadrupole. The isotope pattern in a low res mass spectrum is a valuable aid to structure elucidation.

Hi Nisar. At first, what MS do you use? Does it have a MALDI or ESI ionization source? At second, in what m/z range you want to see isotope distribution? If you have a linear MALDI-MS, you could see it only if your analytes have not more than 5-6 kDa masses. If your instrument is a ESI-TOF, the upper mass limit can be slightly higher, about 7-10 kDa. Unfortunately, to resolve a higher masses compounds, you need to use a TOF with reflectron or Orbitrap or FT-ICR.

P.S.: if you work with low molecular substances in MALDI-MS, try to decrease laser energy or use a matrix with small crystals.

Hi Nisar, I suggest you give a few more information on your instrument and the way you produce the ions. Otherwise it will be difficult to help. With simple home-built linear ToF-MS you should easily get something like a resolution of 200-300 u. But there are many ways of screwing it up, like  non-homogeneous fields, wrong voltages, too slow ion detectors etc.

Thank you dear all for nice discussion.

We have locally designed dual stage linear time of flight mass spectrometer.

i have selected 100V/cm for extraction and 500V/cm for ionization. and flight distance is 100cm. my instrument  can separate the four isotopes of Pb. but i want to get very sharp peaks. there is Noise in the signal. i want to reduce noise and get sharp peaks