The performance of Time-of-Flight Mass Spectrometer (TOF-MS) has been studied with laser produced photoions of various densities using a reflectron TOF-MS built in our laboratory. In this, the source of atoms is a resistively heated atomic beam source. As the atomic species reach the interaction region, the interaction with pulsed laser results in ions. Samarium was used as the source element to observe all its isotopes by resonant non selective excitation of one of the ground state transitions. However, as the temperature of the source is increased, the collective plasma effect of ions and electrons becomes important. As a result of this the Time-of-Flight signal (in linear mode) of Sm isotopes became poorly resolved from a well resolved condition. The number density of the ions produced in these conditions and hence the plasma parameters were calculated. The plasma parameters like Debye length (λ_D), the number density (N_D), and frequency (f_pi) confirms the collective effect. In order to avoid this undesired effect, the optimum operating condition with respect to oven temperature and laser intensity was estimated. .