A reliable method of sample introduction is pre-sented for online gas chromatography with a special ap-plication to in situ field portable atmospheric sampling in-struments. A traditional multi-port valve is replaced with a valveless sample introduction interface that offers the advan-tage of long-term reliability and stable sample transfer effi-ciency. An engineering design model is presented and tested that allows customizing this pressure-switching-based device for other applications. Flow model accuracy is within mea-surement accuracy (1 %) when parameters are tuned for an ambient-pressure detector and 15 % accurate when applied to a vacuum-based detector. Laboratory comparisons made between the two methods of sample introduction using a ther-mal desorption aerosol gas chromatograph (TAG) show that the new interface has approximately 3 times greater repro-ducibility maintained over the equivalent of a week of contin-uous sampling. Field performance results for two versions of the valveless interface used in the in situ instrument demon-strate typically less than 2 % week −1 response trending and a zero failure rate during field deployments ranging up to 4 weeks of continuous sampling. Extension of the valveless in-terface to dual collection cells is presented with less than 3 % cell-to-cell carryover.