We present an spectroscopic study of a sample of bright gamma-ray blazars. Our dataset includes spectra of a sample of about 10 targets, all of them Flat-Spectrum Radio Quasars (FSRQs). The spectra were obtained nearly periodically during the period from 2008 to 2018. Our aim is to search for variations in the luminosity of the emission lines, related to variations in the continuum flux. We have used a fitting code (PyQSOFit) specifically developed to model the optical spectra of quasars. It permits to decompose the emission in different components, such as power-law continuum, host galaxy, FeII emission and line emission. Error parameter estimation is obtained running Monte-Carlo iterations. We will present preliminary results of the line measurements and its temporal variability. We also present the correlation between line luminosity changes with the optical flux variability and also with the gamma-ray luminosity. Most variability studies disregard the possible connection between the relativistic jet flares and the changes in the emission from the Broad Line Region. However, evidences for such events have been found in few individual FSRQs, ours is an attempt to extend the search in a larger sample and covering a more expanded time period.