Development of an oxygen-gradient system to overcome plasma instabilities during HPLC/ICP-MS measurements using gradient elution

Abstract

The coupling of HPLC to ICP-MS detection is nowadays often applied in the field of speciation analysis and usually realized without major difficulties. However, when using organic solvent gradients in reversed phase liquid chromatography (RP-LC), plasma instabilities occur. Although different approaches have been described in the literature, the overcoming of plasma instabilities remains still challenging. One common procedure to damp the occurring plasma instabilities is the addition of a constant oxygen flow to the aerosol or plasma gases in order to prevent the reduction of organic constituents to elemental carbon in the ICP leading otherwise to cone clogging. The main disadvantage of this approach, however, is a large shift in the oxide ratios of the elements under investigation during the gradient, resulting in a loss of sensitivity during ICP-MS detection. To overcome this drawback, we investigated a new approach based on the addition of a matched oxygen gradient using a mass-flow controller, which is controlled by a newly developed software. The addition of an oxygen flow matching the organic solvent gradient leads to a more stable signal, lower (constant) oxide ratios (around 5%) and a higher S/N ratio. Based on empirical data, attained during several pretests, an oxygen gradient was matched with a solvent gradient. To show the applicability of the new approach the RP-LC/ICP-MS analysis of two platinum species was carried out using the matched oxygen gradient and contrasted to the application of a constant oxygen flow; with the new approach a 2–4 times higher S/N ratio and a more stable baseline within the chromatograms were achieved.