Application Of Organic Matrix Assisted SN/LA-ICP-MS In The Continental Crust Abundance Investingation

This work describes the application of organic matrix assisted SN/LA-ICP-MS in the investigation of continental crust abundance. The accurate analytical data are the basis of geochemical research, which have put forward new challege for the sample analytical method, especially in the investigation of crust abundance for the difficulty element. In this paper, we have entered into detailed discussion about the organic solvent induced signal enhancement effects in SN/LA-ICP-MS. Using organic matrix modifier, a new method was developed for the direct determination of difficulty element such as As and Te in geological samples by ICP-MS. The determined sample data was then used to evaluate the continental crust abundance. This work was carried out from the following five aspects.1. Organic matrix effect in ICP-MS. In this section, the effects of organic matrix in inductively coupled plasma mass spectrometry are reviewed. The sample preparation, sample introduction techniques, choice of instrument operation conditions, mechanisms of signal enhancement and typical applications of organic compounds in ICP-MS are introduced in detail.2. Volatile organic solvent induced signal enhancements in ICP-MS: A case study of methanol and acetone. Methanol and acetone were used to study effects of organic matrix on signal intensities of elements from ~7Li to ~238U and oxide yields in inductively coupled plasma mass spectrometry (ICP-MS). Presence of a low concentration of methanol or acetone enhances the intensities of elements in order of decreasing mass. This is attributed to the spatial shift of the zone of maximum ion density, which, in turn, affects the extraction of ions from the plasma to the sampling cone. The possible effect of liquid methanol or acetone on nebulization and/or transport efficiency was avoided by using carry-over effect experiment. The enhancement effect of methanol and acetone appears to be more related to the amount of carbon present in the plasma than the difference between the functional groups of organic solvents. The oxide yield decreases in thepresence of methanol, the magnitude of which depends on the nebulizer gas flow rate used. However, the reduced oxide yield is insufficient to account for the signal enhancement. Our results for 75As and 78Se agree with the C +-species~analyte atom charge transfer reaction hypothesis.3. Improved aerosol particle vaporization in LA-ICP-MS by the addition of methanol. The effects of adding methanol aerosol into the plasma central gas or auxiliary gas on analyte signals from 7Li to 238U were studied in LA-ICP-MS. Enhancement of analyte signals by the presence of methanol and water clearly depends on the first ionization potential of elements. There is a general trend of increasing signal enhancement factor with increasing ionization potential and decreasing mass number by the presence of methanol or water aerosol. Under the given instrument conditions, the signal intensities of elements were improved by a factor of 1.3 to 17 and 1.3 to 5 by the presence of methanol and water, respectively. There is a general trend of increasing peak widths of the carrier gas flow rate at half maximum with decreasing mass number in LA-produced dry aerosol. The significantly narrowed peak width of low mass element (from bigger than 1.00 1/min to 0.30 1/min) and shifted higher optimum carrier gas flow rate (from 1.20 l/min to 1.50-1.60 1/min) by the presence of 50% methanol aerosol may partially account for the trend of increasing signal enhancement factor with decreasing mass number. Our results demonstrate that a certain amount of methanol and water solvent is useful for analyte excitation in an Ar-ICP, since hydrogen released from the methanol and water enhances the energy transfer inside the plasma. The improved Th/U ratios and argon signals by the presence of 50% methanol and water aerosol in LA-ICP-MS support the better energy transfer reaction.4. Suppression of interferences for direct determination of As, Te, etc. in geological samples by ICP-MS. We developed a new method for direct determination of As and Te in geological samples using ICP-MS by reduction of interferences, without preconcentration, separation and use of hydride generation technique. Concentrations of HNO3 have a significant effect on the arsenic signal. This type of interferences cannot be corrected by internal standards (Rh and In) because the signal suppression due to HNO3 is apparently dependent on the first ionization potential of elements. Addition of 4% (v/v) ethanol to 1-10% (v/v) HNO3 was found to be an excellent method for reducing this typeof matrix effect from 30-40% to less than 5% for high first ionization potential elements 75As (9.81 eV), 82Se (9.75 eV), and I26Te (9.01 eV). Direct determination of arsenic and tellurium in geological samples by ICP-MS is often complicated by the interferences of Nd2+, Sm2+, Eu2+ and ArCl+ on As and Xe on Te, and the high first ionization potentials of As (9.81 eV) and Te (9.01 eV) also results in relatively low analytical sensitivity in ICP-MS. It is shown that both problems can be overcome by a combination of a 4% ethanol matrix modifier with nebulizer gas flow rate adjustment. The instrument background intensities were found to be significantly reduced by the addition of 4% (v/v) ethanol, and generally dropped with increasing nebulizer gas flow rate in both solutions. Figures of merit of the developed method were validated by analyzing a series of international rock standard reference materials. In this section, we have also evaluated the pressurized acid digestion-ICP-MS for the determination of Sb, In, Tl, Bi, Cd, W, Mo and Sn in 18 international geological reference materials.5. The investigation of continental crust abundance. The upper continental crust abundance of Li, Be, B, Ga, Ge, As, Rb, Nb, Mo, Cd, In, Sn, Sb, Te, Cs, Ta, W, Tl, Bi and U except As, Cd, Sb and Mo were obtained by using the relationships between the trace element composition of sendimentary rocks and upper continental crust. The middle and lower continental crust abundance of these elements were abtained through the study of amphibolite-facies gneiss samples and granulite samples, respectively. By correlation with geophysical data, we also provide these 20 elements abundance in the bulk continental crust.