Desorption electro-flow focusing ionization (DEFFI), a desorption-based ambient ion source, was developed, characterized, and evaluated as a possible source for field deployable ambient pressure mass spectrometry (APMS). DEFFI, based on an electro-flow focusing system, provides a unique configuration for the generation of highly charged energetic droplets for sample analysis and ionization. A concentrically flowing carrier gas focuses the liquid emanating from a capillary through a small orifice, generating a steady fluid jet. An electric field is applied across this jet formation region, producing high velocity charged droplets that impinge on an analyte laden surface. This configuration separates the jet charging region from the external environment, eliminating detrimental effects from droplet space charge or target surface charging. The sample desorption and ionization processes operate similar to desorption electrospray ionization (DESI). DEFFI demonstrated strong signal intensities and improved signal-to-noise ratios in both positive and negative mode mass spectrometry for narcotics, i.e., cocaine, and explosives, i.e., cyclotrimethylenetrinitramine (RDX), respectively. A characterization of DEFFI ionization mechanisms identified operation regimes of both electrospray and corona discharge based analyte ionization, as well as limitations in overall signal. In addition, the DEFFI response was directly compared to DESI-MS under similar operating conditions. This comparison established a wider and more stable optimal operating range, while requiring an order of magnitude lower applied gas pressure and applied potential for DEFFI than DESI. These reductions are due to the physical mode of jet formation and geometric configuration differences between DEFFI and DESI, pointing to a potential benefit of DEFFI-MS for field implementation.