High performance concentric nebulizer for low-flow rate liquid sample introduction to ICP-MS

Abstract

A high performance concentric nebulizer (HPCN) was developed for sample introduction to inductively coupled plasma mass spectrometer (ICP-MS) at liquid flow rates of less than 10 µL min⁻¹. The HPCN has a triple tube concentric structure and consists of a concentric type nebulizer body and a fused silica glass capillary (i.d./o.d.: 50 µm/150 µm) mounted in the center of the nebulizer body as a sample introduction capillary. The nebulizing performance was greatly improved by tapering the liquid capillary (tip orifice i.d.: 20 µm, tip wall thickness: <8 µm) and by setting the liquid capillary tip at a proper position. When the set position was recessed by 25 µm to 50 µm with respect to the nebulizer nozzle tip, a liquid jet was produced inside the nozzle by a flow focusing effect. This phenomenon gives a fine aerosol generation. At a liquid flow rate of 5 µL min⁻¹ and a nebulizer gas flow rate of 1 L min⁻¹, the Sauter mean diameter D_3,2 of the primary drops generated by the HPCN (2.4 µm) was almost the same as that generated by the HEN-120-A.01 (2.6 µm) and was smaller than those generated by the HEN-90-A.01 (4.6 µm), CEI-100 (5.9 µm), and AriMist-HP (6.3 µm). The sensitivity in ICP-MS with the HPCN was over two-fold higher than those with the other microflow nebulizers, except for the HEN-120-A.01. The HEN-120-A.01 provides almost the same sensitivity as the HPCN, but it requires two-fold higher gas pressure than the HPCN. The HPCN can continuously and stably nebulize 1% NaCl solution for over 8 h without clogging. The HPCN combined with an on-axis cylinder chamber was applied to capillary liquid chromatography-ICP-MS, by which the speciation of eight arsenic compounds was demonstrated. We concluded the HPCN is a useful nebulizer for less than 10 µL min⁻¹ sample introduction to ICP-MS.