In Impactor API sources, a nebulised solvent stream impacts asymmetrically on a cylindrical target which is held in close proximity and at a high potential. These sources can give rise to enhanced sensitivity for polar and non-polar analytes at flow rates that are compatible with UPLC/MS (0.02 - 2.0mL/min). Here, we examine the unique aerodynamic features that result from the “cylinder in cross-flow” geometry and give a perspective on how these features may enhance API sensitivity. Particular emphasis will be given to the phenomena of high-We droplet impacts, the Coanda effect, turbulence and surface microvorticity.
Data will be presented that compare the Impactor API source performance with a conventional, nitrogen-assisted ESI source on a triple quadrupole MS instrument. Droplet velocity and size distribution data have been determined using phase Doppler anemometry (PDA) and laser diffraction particle sizing (LDPS) techniques. Data will also be presented from photographic and electron microscopy observations of both the spray dynamics and the impactor surface, respectively. These observations provide direct evidence of Coanda flow, turbulence and microvorticity at the curved surface of the target.
A direct comparison of Impactor API versus ESI at flow rates of 0.6mL/min reveal typical Impactor signal enhancements in the range 6.4 - 2.1 for the MRM analysis of verapamil as the mobile phase is varied from 100% aqueous to 100% organic, respectively. From droplet sizing data and PDA velocity measurements, a scheme will be presented that illustrates how the size distribution could be beneficially shifted to smaller diameters by the effect of high Weber number impacts on the hot target surface. Photographic evidence will be presented that shows Coanda streamlines related to the off-axis geometry and the onset of turbulence at the flow separation point. Furthermore, electron micrographs of a used target reveal evidence of counter-rotating microvortices in the region of the flow separation point with a disturbance wavelength of approximately 20µm. It is suggested that this observation shows some correlation with classical cross-flow theory where the wavelength is inversely proportional to the square root of the Reynolds number of the incident gas flow.
A study of aerodynamic effects which are unique to an Impactor API source.