Desorption electro-flow focusing ionization (DEFFI), a desorption-based ambient ion source, was developed 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 high velocity fluid jet. An electric field is applied across this jet formation region, producing high velocity charged droplets that impinge on an analyte laden surface. The sample desorption and ionization processes operate similar to desorption electrospray ionization (DESI). The DEFFI characterization presented here 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. The DEFFI mass spectrometric response identified regimes of maximum analyte signal intensity and optimal signal-to-noise ratio as a function of applied potential. Transmitted current measurements and a scaling analysis were developed to identify the regimes of operation observed by the MS response. This analysis identified the functional relationship between DEFFI parameters and the optimal applied potential. In addition, the DEFFI response was directly compared to DESI-MS under similar operating conditions. This comparison established a wider 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, clearly demonstrating DEFFIs field compatibility.
Pub Type: Journals
Flow focusing, Ion source, Mass spectrometry, Desorption ionization