There’s a good chance that you could use Raman Spectroscopy to do this analysis. Because Raman has very fine peaks and it is qualitative, it is a very good technique to determine an analyte within a mixture. You would need to obtain the Raman spectra of the active ingredient and the expedients in the mixture, and determine a suitable peak or peaks to measure. PLS is a good Chemometrics technique to use.

Colleagues of mine have done similar work – check their article on ResearchGate:

Noninvasive, Quantitative Analysis of Drug Mixtures in Containers Using Spatially Offset Raman Spectroscopy (SORS) and Multivariate Statistical Analysis

In general the minimum conditions you need to meet in order to detect a Raman signal include; (I) the materials you are interested to study have active Raman mode(s). This means the vibrational modes of the atoms and molecules should involve changes in the polarizability. Note that sometimes vibrational modes of materials may be associated with dipole radiation, but no polarizability changes, which means in such cases those modes are IR active and Raman forbidden modes, (II) The Raman active modes are strong enough to be observed even at the presence of noise or overlap with our peaks, (III) The excitation source used for the Raman scattering has a penetration depth that can reach the materials of interest for example when sample structure being studies is such that it has different layers that are made of various materials, and the material you may be interested is not the layer close to the sample surface.

Thank you Khushdeep Goyal