RATIONALE: Many chemical processes operate at unsteady state and require a rapid mass analysis of transients with high resolution. An atmospheric pressure interface multi-reflection time-of-flight mass spectrometer (API-MRTOF-MS) has the potential to be a powerful process analytical tool. METHODS: The ion flight path of the API-MRTOF-MS was extended from meters to over one kilometer, and the mass resolution was increased to an ultra-high level. Furthermore, the mass analysis could be done at around ten milliseconds due to the rapidity of TOFMS. The mass analyzer reflects ions via electrostatic mirrors and directs them along a folded flight path, significantly increasing flight distances and theoretically with no upper limit. The powerful API, equipped with two quadrupoles, makes the API-MRTOF-MS suitable for complex mass spectrometry methods with alternative ion selection schemes. RESULTS: A mass resolution of 116,050 for Cs ⁺ ions is achieved within a total time-of-flight of only 18 ms. An ion transmission efficiency of over 50% was achieved after 600 cycles. CONCLUSIONS: The performance of the API-MRTOF-MS demonstrates that it is exceedingly suitable for high-resolution process analysis, particularly for transient process analysis, due to its fast analysis speed and high-selectivity.