Research And Application Of Electron Probe Microanalytical Method For Minor And Trace Elements

Electron probe microanalyzer (EPMA) is a non-destructive, in situ and high spatial resolution analytical tool applied for determining the chemical compositions of samples. It is utilized in many fields. Especially, with the more and more applications of minor and trace elements in recent years, it is need to obtain more accurate and precious analytical results and decrease the limit of detection. Compared with major elements analysis, minor and trace elements analysis is more complicated. The intensity of X-ray peak on trace element is close to the background's when trace element analyzed. The analytical results will be affected obviously by the little interference or the positional errors on measurement. The measurement conditions and analytical method are different between the major elements and trace elements. Therefore, accurate measurement and control software was designed in this work. The key methods and technique which affect the anlaytical precision and accuracy of trace elements were studied deeply, such as background measurement and subtraction, interference correction and analytical conditions. Moreover, these methods were uitilized to analyze the chemical concentrations of U, Th and Pb on monazites in this work.The results indicated that it improved the accuracy of data and decreased the error of Chemical Th-U-Total Pb Isochron Method (CHIME). The following contents of research were involved in this work:1. Design of a measurement and control software on electron probe. In order to measure the characteristic X-ray intensity accurately and get more information, a measurement and control software was developed based on Windows2000 operating system by Visual C++ 6.0 language on EMX-SM7 electron probe. The software was developed with the open mechanism of serial port communication and the control mechanism in real-time database system. Not only the main functions in previous software but also the flexible functions of measurement and control were implemented. These functions included that analytical positions could be set at random and the different conditions of measurement could be selected for major and minor elements in a single measurement according to analysts' needs. Meanwhile, the acquiring data could be handled with various correct methods. The database was visited at any time, so as to modify the software and add functions without developing the software system once more.2. The research of measurement conditions about elements in electron probe microanalysis. The different measurement conditions of major and minor elements were proposed to improve analytical accuracy in a set of elements. In addition, the measurement time and current should be increased according to the detection limit. The X-ray spectrum intensity of trace elements should be accumulated by several times. The method improved the statistical precision and decreased the effect of strong electron beam for the analyticl compositions of the sample.3. Research of background analytical method about trace elements on electron probe. The imprecise background measurement can bring the error of quantitative analysis. It becomes more and more evidence with the lower compositions of elements, because of the interference and non-linear of background. The background positions were set on the both side of the selected peaks and the intensity of background were calculated by average in the traditional software system. The method isn't suit for non-linear background of trace elements. So the method of adding measurement points and least-squares fitting of background was adopted to reduce the interference of background measurement and improve the reasonable of background intensity.4. Research of interference correction method about trace elements. The traditional method of interference correction which is obtained by a single standard sample was instead of the method of "interference coefficient curve fitting" correction, which was measured by several interference standards. The accurate interference coefficient was obtained by this method and the accuracy of quantitative analysis was improved.5. The measurement method of area integral spectrum was studied for trace elements analysis. The method of area integral spectrum was proposed to analyze trace elements. The vary-step area integral spectrum was proposed to resolve the shift, the curve and the interference of the measurement spectrum. The method was proved that was fit for the samples which had a little interference and wasn't sensitive for strong electron beam.6. Research of CHIME on electron probe. EPMA dating of monazites is limited by the precision and accuracy of U, Th and Pb chemical compositions. The chemical compositions of U, Th and Pb are close to the detection limit of EPMA in monazite. Moreover, there are some rare earth elements in monazite. The interference of X-ray spectrum has become an unavoidable problem. So it is the key of CHIME how to reduce the interference of spectrum and how to lower the detection limit of trace elements in monazite.There are many factors which influence the accuracy of CHIME, such as, sample preparation and coating, the selection of standard samples, the measurement conditions of trace elements, the selection of diffraction crystal, etc. These factors were discussed here. The analytical control methods were proposed to improve the analytical accuracy and reduce the error of CHIME.In order to decrease the damage of strong electron beam for the sample surface and the influence of sample compositions in trace elements analysis, the method of accumulating X-ray intensities of trace elements several times and different measurement conditions of major and minor elements were proposed.The detailed spectrum interference of monazite analysis was listed. It is useful to select the positions of background and the measurement X-ray line of elements. The unavoidable interference of PbMa and UM|3 were corrected by the application of "interference coefficient fitting curve", which was measured by several standard samples of different content of Th. The more accurate interference correction coefficient was obtained.The accurate measurement which acquired the background intensities of different background positions and fitting curve of background correction were proposed to reduce the interference of spectrum and background non-linear for the measurement of UMP and PbMa. The method improved the rationality of intensity measurement on X-ray spectrum and the correctness of analytical results.The monazite samples which had been analyzed on Isotope mass spectrometer were analyzed on electron probe. Compared with previous methods, the methods in this work decreased the analytical errors about 5% to 10%, which included in the interference correction, the measurement and correction of background, the selection of measurement conditions, etc.