  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, extreme 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 -250
km/s was observed, motivating 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.
