  We investigate whether the parsec-scale jets of quasars 3C 345 and 3C 279 are
dominated by a normal (proton-electron) plasma or a pair (electron-positron)
plasma. We first present a new method to compute the kinetic luminosity of a
conical jet by using the core size observed at a single very long baseline
interferometry frequency. The deduced kinetic luminosity gives electron
densities of individual radio-emitting components as a function of the
composition. We next constrain the electron density independently by using the
theory of synchrotron self-absorption. Comparing the two densities, we can
discriminate the composition. We then apply this procedure to the five
components in the 3C 345 jet and find that they are pair-plasma dominated at 14
epochs out of the total 19 epochs at which the turnover frequencies are
reported, provided that the bulk Lorentz factor is less than 15 throughout the
jet. We also investigate the composition of the 3C 279 jet and demonstrate that
its two components are likely pair-plasma dominated at all the four epochs,
provided that their Doppler factors are less than 10, which are consistent with
observations. The conclusions do not depend on the lower cutoff energy of
radiating particles.
