The document discusses developing a technique for real-time calibration of portable trace-gas sensors using an in-line reference cell. It aims to differentiate between sample and reference signals based on physical parameters of the gas and wavelength modulation spectroscopy modeling. Key challenges addressed are reducing measurement drift over time. The proposed method inserts a permanent low-pressure reference cell to experience the same optical effects as the sample. Simulations and experiments show this approach can minimize cross-talk between sample and reference signals depending on parameters like pressure, modulation depth, and harmonic detection order.