Direct Mass Spectrometric Analysis of Zinc and Cadmium in Water by Microwave Plasma Torch Coupled with a Linear Ion Trap Mass Spectrometer

• MPT as a kind of new plasma ion source is directly applied in the analysis of metal ions in liquids.
• The mass spectrometric features of Zn and Cd were presented both in positive and negative modes.
• We showed that the negative mode is more suitable for the analysis of zinc and cadmium in water.

Microwave plasma torch (MPT) is a simple and low power-consumption ambient ion source. When MPT mass spectrometry is applied in the detection of some metal elements, the metallic ions exhibit some novel features which are different from traditional ICP mass spectrometry and may be helpful for the metal element analysis. Zinc and cadmium both are important transition metals with wide applications in modern industry. Here, we presented the research results of the MPT mass spectra of zinc and cadmium elements by a linear ion trap mass spectrometer (LTQ) both in positive and negative modes. Without any sample pretreatment, the aqueous samples were pneumatically nebulized and the aerosols were generated. The dried aerosols were introduced into the plasma through the central tube of the MPT to generate the complex metallic ions. For the positive composite ions of zinc and cadmium, the general forms are the hybrid of M(NO3)·nOH·mH2O, thus the MPT mass spectra are extremely complicated and unsuitable for the quantitative monitoring. However, the MPT negative ionic of zinc and cadmium is quite regular in the forms of M(NO3). We also showed that the limit of detection (LOD) for zinc and cadmium in negative mode can be achieved at the level of 10 μg/L (ppb). Therefore, the negative mode MPT mass spectrometry can be used as one of the alternative supplements of ICP-MS applied in on-line monitoring of zinc and cadmium metal ions in water at trace levels, which will be promising in the field analysis of metal elements and the rapid water quality inspection.