In this paper we present new ALMA observations towards the proto-planet hosting transitional disc of Herbig Ae/Be star HD 100546 .
This includes resolved 1.3 mm continuum , ^ { 13 } CO and the first detection of C ^ { 18 } O in this disc , which displays azimuthal asymmetry in regions spatially coincident with structures previously identified in HST images related to spiral arms .
The lower limit on the mass of the dust disc is calculated to be 9.6 \times 10 ^ { -4 } ~ { } M _ { \odot } .
A firm lower-limit on the total gas mass calculated from optically thin , mid-plane tracing C ^ { 18 } O ( 2-1 ) emission is 0.018 M _ { \odot } assuming ISM abundances .
These mass estimates provide an estimate of gas-to-dust ratio in the disc of 19 , the ratio will increase if C ^ { 18 } O is relatively under-abundant in the disc compared to CO and H _ { 2 } .
Through deprojection and azimuthal averaging of the image plane we detect 1.3 mm continuum emission out to 290 \pm 10 au , ^ { 13 } CO to 390 \pm 10 au and C ^ { 18 } O to 300 \pm 10 au .
We measure a radially increasing millimetre spectral index between wavelengths of 867 \mu m and 1.3 mm , which shows that grain sizes increase towards the star , with solid particles growing to cm scales in the inner disc .