Developments with the oscillating capillary nebulizer—effects of spray chamber design, droplet size and turbulence on analytical signals and analyte transport efficiency of selected biochemically important organoselenium compounds

Abstract

Silica and polymer PEEK^® capillaries were used to modify an oscillating capillary nebulizer (OCN) in order to make it sturdier and more durable for routine analyses, and also to improve response sensitivity. The performance of four modified OCNs was compared to that of the original OCN design and also to a Meinhard High Efficient Nebulizer (HEN), using an identical single-pass cylindrical spray chamber for all the nebulizers. The effects of spray chamber design, droplet size and turbulence on analyte transport efficiency of low molecular weight organoselenium compounds were also investigated. Three organoselenium standards: (1) selenocystamine; (2) selenomethionine; and (3) trimethylselenonium iodide were used as test compounds with inductively coupled plasma-mass spectrometry (FI/ICP-MS). Droplet size distributions and Sauter mean diameters of aerosols were measured by laser Fraunhofer diffraction. An OCN fabricated with PEEK^® gas capillary, with either a thicker capillary wall or a larger inner diameter liquid capillary tube, performed better than the original OCN design. The optimal improvement of analytical signal of 3–4 times, corresponding to a 2.5–3.0 times improvement in analyte transport efficiency was found, compared to the original OCN design and also to a standard Meinhard HEN. Contrary to literature reports, we observed that moderate increases in mean droplet sizes resulted in improved analyte transport efficiencies and higher signal intensities for the volatile compounds tested. However, nebulizers producing aerosols with Sauter mean diameters, D_[3,2], greater than approximately 9.5 µm gave lower transport efficiencies and reduced signal intensities. Although greater turbulence in the gas flow inside a single-pass spray chamber predictably decreased net analyte transport efficiencies, the overall loss of aerosol was unexpectedly 3–5 times greater for small droplets (<3 µm) than for larger droplets (>3 µm, <9 µm).