We test the usefulness of the intermediate ionisation lines AlIII 1860 and CIII] 1909 as reliable virial mass estimators for quasars. We identify a sample of 309 quasars from the SDSS DR16 in the redshift range 1.2<z<1.4 to have [OII] 3728 recorded on the same spectrum of AlIII 1860, SiIII 1890, and CIII] 1909. We set the systemic quasar redshift using careful measurements of [OII]. We then classified the sources as Population A, ex- treme Population A (xA) and Population B, and analysed the 1900{AA} blend using multi-component models to look for systematic line shifts of the AlIII and CIII] along the quasar main sequence. We do not find significant shifts of the AlIII line peak in Pop. B and the wide majority of Pop. A. For Pop. xA, a small median blueshift of -250km/s was observed, motiva- ting a decomposition of the AlIII line profile into a virialized component centred at rest frame and a blueshifted component for an outflow emission. For Pop. B objects, we proved the empirical necessity to fit a redshifted very broad component (VBC), clearly seen in CIII], and analysed the physical implications on a Pop. B composite spectrum using CLOUDY simulations. We find consistent black hole mass estimations using AlIII and CIII] as virial estimators for the bulk of Population A. AlIII (and even CIII]) is a reliable virial black hole mass estimator for Pop. A and B objects. xA sources deserve special attention due to the significant blueshifted excess observed in the line profile of AlIII, although not as large as those observed in CIV 1549.

Cone search capability for table J/A+A/669/A135/tableb1 (Measurements of the individual spectral fits)