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In this paper we present new ALMA observations towards the proto-planethosting transitional disc of Herbig Ae/Be star HD 100546. This includesresolved 1.3 mm continuum, $^{13}$CO and the first detection of C$^{18}$O inthis disc, which displays azimuthal asymmetry in regions spatially coincidentwith structures previously identified in HST images related to spiral arms. Thelower limit on the mass of the dust disc is calculated to be9.6x10$^{-4}$M$_\odot$. A firm lower-limit on the total gas mass calculatedfrom optically thin, mid-plane tracing C$^{18}$O (2-1) emission is0.018M$_\odot$ assuming ISM abundances. These mass estimates provide anestimate of gas-to-dust ratio in the disc of 19, the ratio will increase ifC$^{18}$O is relatively under-abundant in the disc compared to CO and H2.Through deprojection and azimuthal averaging of the image plane we detect 1.3mm continuum emission out to 290+/-10 au,$^{13}$CO to 390+/-10 au and C$^{18}$Oto 300+/-10au. We measure a radially increasing millimetre spectral indexbetween wavelengths of 867$\mu$m and 1.3 mm, which shows that grain sizesincrease towards the star, with solid particles growing to cm scales in theinner disc.

Asymmetric mid-plane gas in ALMA images of HD~100546