The mass of gas in protoplanetary discs is a quantity of great interest for assessing their planet formation potential .
Disc gas masses are , however , traditionally inferred from measured dust masses by applying an assumed standard gas-to-dust ratio of g / d = 100 .
Furthermore , measuring gas masses based on CO observations has been hindered by the effects of CO freeze-out .
Here we present a novel approach to study the mid-plane gas by combining C ^ { 18 } O line modelling , CO snowline observations and the spectral energy distribution ( SED ) and selectively study the inner tens of au where freeze-out is not relevant .
We apply the modelling technique to the disc around the Herbig Ae star HD 163296 with particular focus on the regions within the CO snowline radius , measured to be at 90 au in this disc .
Our models yield the mass of C ^ { 18 } O in this inner disc region of M _ { \text { C } ^ { 18 } \text { O } } ( < 90 \text { au } ) \sim 2 \times 10 ^ { -8 } M _ { \odot } .
We find that most of our models yield a notably low g / d < 20 , especially in the disc mid-plane ( g / d < 1 ) .
Our only models with a more interstellar medium ( ISM ) -like g / d require C ^ { 18 } O to be underabundant with respect to the ISM abundances and a significant depletion of sub-micron grains , which is not supported by scattered light observations .
Our technique can be applied to a range of discs and opens up a possibility of measuring gas and dust masses in discs within the CO snowline location without making assumptions about the gas-to-dust ratio .