A Study Of Element Behavior In Organic Medium By Inductively Coupled Plasma Mass Spectroscopy

During the last decades inductively coupled plasma-mass spectrometry (ICP-MS) has been developed into an accurate and sensitive technique for multi-element determinations in a range of sample matrices. Compared with AFS, AAS and ICP-AES, ICP-MS offers better sensitivity and excellent accuracy with multi-element and isotope ratio measurement capabilities. Organic solvents are widely used for the purposes of preconcentration and separation of trace elements in analytical chemistry. In this study, the effects of EDTA, acetic acid, 2-propanol, methanol and methane on the behaviors of elements in ICP-MS were investigated. The effects of ICP-MS operation parameters, concentration of organic reagent on analyte signals were studied. The possible mechanism of the effect of organic medium on analyte signals were discussed in detail using carry-over effect caused by volatile organic solvent.The main results were summarized as follows:1. Experimental results show that the signal intensities of elements are enhanced in the EDTA medium. The enhancing factor decrease with increasing mass. The yields of oxides are reduced significantly in EDTA medium. Compared with 2% nitric acid solution, a lower nebulizer gas flow rate is needed for EDTA solutions to obtain the maximum signal. The easily ionizable element in EDTA matrix is also contributed to the decrease of analyte signal intensities.2. The analyte signal intensities were suppressed in a higher concentration of acetic acid. However, signal enhancements occur under certain conditions. The matrix induced-changes highly depend on the nebulizer gas flow rate and power. Higher concentration of acetic acid is thought to result in local cooling in the central channel of the plasma. As the concentration of acetic acid increases from 2% to 5%, analyte signal intensities decrease and oxide ratio increase, especially in the high nebulizer gas flow rate. Higher incident power and low nebulizer gas flow rate allow the plasma to accept a change in the concentration (2% to 5%) of acetic acid without significant variation of analyte signal. 3. Experimental results show that the most of analyte signals were enhanced when the concentration of 2-propanol medium was less than 100mmol/L, especially for elements with ionization energy greater than 9eV. However, the analyte signal intensities were suppressed when the concentration of 2-propanol medium was more than 100mmol/L. The behaviors of elements were identified with distinguish factor of matrix effect (DF) and these elements were assorted into three clusters. The mechanism of analyte suppression and enhancement effects in 2-propanol medium was discussed according. 4. Enhancement or suppression of analyte signals in the presence of methanol and acetone depends on volatility of the compound, its concentration, mass and ionization potential of analyte and operating conditions of ICP-MS. Presence of a low concentration of methanol or acetone enhances the intensities of elements in order of decreasing mass. This may be related to the spatial shift of the zone of maximum ion density in plasma, which in turn affects the extraction of ion fromthe plasma to the sampling cone. In contrast, the more volatile acetone more easily depresses signals of elements from low mass(7Li) to high mass(238U). A high concentration of methanol also depresses intensities of all elements due to its cooling effect on the central channel of the plasma. The enhancement effect of methanol and acetone appears to be more related to the amount of carbon present in the plasma than the difference in the functional group of the organic solvents. Carry-over experiments of methanol and acetone show signal enhancement of analyte is not due to the improvement in the nebulization-transport process of the sample. The oxide productivity decreases in the presence of methanol, the level of which depends on the nebulizer gas flow rate used. However the reduced oxide productivity is insufficient to account for the signal enhancement. The C+-analyte atom charge transfer...