Development of a flat sheet membrane desolvator for plasma atomic emission and mass spectrometries.

摘要

Aqueous and organic solvent loading to plasmas used for atomic spectrometry have deleterious effects on the analytical performance. In particular, low power plasma are the most affected. These problems have limited the successes of direct solution nebulization and HPLC interfacing with plasmas for element selective detection. Also, inductively coupled plasma-mass spectrometry (ICP-MS) solvent loading causes polyatomic ion interference at masses of interest. This dissertation describes the details of the design, development, characterization and application of a flat sheet membrane desolvator (FSMD) for solvent removal with atomic emission and mass spectrometric techniques. Interfacing of a low power He-MIP with an HPLC for element selective detection is also discussed.;The FSMD was used to enhance the analytical performance of both low power (120 W helium microwave induced) and high power (1.3 kW, argon inductively coupled) plasmas. This improvement was more significant with the hard to excite nonmetals and metalloid (such as chlorine, phosphorus and boron) than with the metals (copper, zinc, lead and cadmium). Also, with the FSMD, the dependency of analytical characteristics (detection limits, sensitivity, and %RSD) on nature of solvents was significantly diminished.;The FSMD was utilized to interface a high performance liquid chromatograph with a low power, 120 W, helium microwave induced plasma for nonmetal element selective detection. Chlorine detection limits of 74 ng/s (as 2,6, dichlorobenzamide) and 143 ng/s (as 5,7-dichloro-8-hydroxyquinoline) were observed with complete resolution of the chromatographic peaks. This and other preliminary results confirms the feasibility of this hyphenation of techniques.;Finally, the FSMD was used for aqueous desolvation with direct solution nebulization into an ICP-MS. The effect of desolvation on the metal oxide ion formation and plasma cooling was investigated. Strong oxide formers (Ce and La) and a weak oxide former (Ba) were used as probes. Results showed that the removal of aqueous solvent leads to the reduction of hydrogen content of the plasma with two consequences: de-thermalization of the plasma and formation of oxides of other elements became more favorable. Other recent reports in the literature seem to support this observation too.;A flat sheet membrane desolvator was designed and characterized. Both vacuum pressure and countercurrent gas flows were investigated as a means of removing waste solvent out of the desolvator. 100% desolvation efficiency was observed for water in both cases. With the vacuum pressure driven FSMD, desolvation efficiencies of 92 and 74% were obtained for methanol and acetone, respectively. The highest preconcentration (analyte-to-solvent ratio) factors observed were 7 with the vacuum and 590 with the countercurrent gas flow driven FSMD. Results showed that the vacuum pump used with the FSMD contributed significantly to the plasma noise and also decreases the nebulizer gas flow rate. These and other aforementioned reasons makes the use of countercurrent gas flow the preferred method for solvent removal with the FSMD.;Overall, the flat sheet membrane desolvator was used successfully to remove both aqueous and organic solvents from aerosols thus enabling the introduction of analytes into the plasmas as dried particulates. The result is that the analytical performance of the plasma systems, particularly the low power plasma, was improved significantly.