Stabilizing the Asymmetric Waveform: Firmware and Automated LC Tuning in the First Commercial c-FAIMS Device (2004–2005)

High-field asymmetric waveform ion mobility spectrometry (FAIMS) separates gas-phase ions by how their mobility changes in a bath gas under a strong, rapidly alternating electric field. With an asymmetric field waveform and a DC compensation voltage, this permits selection of ions by physical shape. Its resolution and reproducibility depend on keeping the high voltage asymmetric waveform exactly right. In the prototype cylindrical FAIMS (c-FAIMS) system at Ionalytics Corporation, the resonant LC circuit that generated this waveform drifted with temperature, load and component tolerance, and had to be continuously tuned by hand. This note describes, from the inventor's perspective, the firmware and automated tuning methods developed at Ionalytics Corporation in 2004–2005 for the Selectra c-FAIMS device. The core method is patented as US 7,058,528. These methods held the waveform stable without operator intervention, so FAIMS could be used as a routine add-on to mass spectrometers. The note sets out the engineering problem, the approach and its practical impact, as a citable record to accompany the patent.