Photonic elastic-backscatter fingerprinting of exhaled-breath metabolites for non-invasive disease screening

Optical breath diagnostics normally require molecule-specific spectroscopy. We ask instead how much diagnostic structure survives in the least expensive optical readout available: a single fixed-wavelength diode laser and a dual-axis position-sensing photodiode, giving three channels — back-scattered intensity ( $$\:I$$ ) and two beam-position coordinates ( $$\:x,y$$ ). Seven calibration classes, comprising five disease-linked breath-metabolite mixtures (disease-linked surrogates) covering established gas concentration ranges (in ppb / ppm), a healthy-air surrogate and an ambient control, were injected at constant mass flow, into a sealed chamber and recorded at 25 kHz, later down sampled to 10 Hz for each run for easing the computational load, over nineteen runs. Detection of any injection is effectively certain within seconds: mean Mahalanobis distances from the ambient cluster are 176 to 447, against a within-cluster mean of 1.6. Concentration is recovered by cubic intensity–concentration calibrations across nine gas channels, with coefficients of determination from 0.966 to 0.997. All seven class centroids separate, and a trajectory-aware nearest-template rule assigns 73.7% of runs correctly on resubstitution. Under leave-one-run-out validation, however, accuracy falls to 52.6% against a chance level of 14.3%, and the beam-deflection channels are shown to be dominated by a common pressure-driven refractive-index gradient rather than by composition - an artefact of the design and architecture of the experiment which can be easily modified to improve accuracy. The three channel readout ( $$\:I,x,y$$ ), therefore establishes a validated lower bound for minimal optics, and quantifies precisely what separates that bound from classification-grade breath screening.