Reduction of polyatomic ion interferences in inductively coupled plasma mass spectrometry with cryogenic desolvation
A new desolvation scheme for the introduction of aqueous and organic samples into an argon inductively coupled plasma (ICP) is described. The aerosol generated by the nebulizer is heated (+140°C) and cooled (-80°C) repeatedly. The dried aerosol is then injected into an inductively coupled plasma mass spectrometer (ICP-MS);The metal oxide count rate ratios (MO[superscript]+/M[superscript]+) were drastically reduced to ≤ 0.05% for all the elements tested with the exception of the UO[superscript]+/U[superscript]+ ratio which was reduced to 0.1%. However, the addition of ~2% of acetylene or H[subscript]2 to the aerosol gas flow with cryogenic desolvation reduced the UO[superscript]+/U[superscript]+ ratios down to 0.05%. The addition of acetylene to the aerosol gas flow caused only a slight increase in the analyte sensitivity (~20%) while the addition of H[subscript]2 increased the analyte sensitivity five fold. Moreover, the cryogenic loops also trapped Cl[superscript]- species as HCl, which made possible the determination of V and As in samples containing a concentrated salt matrix;This multiple cryogenic desolvation system was validated with the analysis of seawater and urine reference samples. No sample preparation was necessary, except for dilution with 1% HNO[subscript]3 to bring the total dissolved solids concentration below 0.1%. Accurate isotope ratios were also obtained for Fe, Ni, and Cd in the presence of Ca and Mo, which generally cause interferences;Samples dissolved in organic solvents are particularly troublesome for ICP-MS; the plasma becomes unstable and carbon deposits on the interface. Cryogenic desolvation was applied with methanol, ethanol, acetone, and acetonitrile. The plasma was stable with all the solvents at the same operating conditions as for aqueous samples. Some C[subscript]2 emission and carbon deposition were observed when acetone and ethanol were nebulized. No green C[subscript]2 band emission was observed in the plasma and only minor carbon deposition was observed on the sampling cone when methanol and acetonitrile were used. Analyte sensitivities from metal nitrate salts in methanol were similar to each other and to the sensitivities from aqueous metal solutions. Considerable memory effects were observed from several metal complexes. ftn*This work was performed at the Ames Laboratory under contract no. W-7405-ENG-82 with the U.S. Department of Energy. The United States government has assigned DOE report no. IS-T-1659 to this dissertation.