エネルギー分散型蛍光Ｘ線法の石油への応用　ＩＩＩ　　ファンダメンタルパラメーター法による油中多元素同時定量

Reliable analysis of multi-elements in a variety of lubricating oils has been achieved with fundamental parameters method, and, using an energy dispersive X-ray fluorescence spectrometry, EDXRF (Fig. 2). A minimum number of standard (in order to meet increased accuracy, plural standards were used in this study, Table 3) for each element of interest is required for calibration purposes.<br>A theoretical formula (Eq. 1) was introduced by assuming that every lubricating oil consists of elements to be analyzed and hydrocarbons whose carbon to hydrogen weight ratio (C/H) is 6.5. Also, it was assumed that the observed spectral lines include primary fluorescent X-ray only. It is first necessary to determine the value of P_i in equation (1) for each element i of interest. This can be done by putting all concentrations C_j's of components in a standard, and, the spectral intensity I_i measured for the standard into the equation. For unknown samples, an iterative technique was employed. In the first iteration, the concentrations are approximated by the measured intensity ratios. The carbon and hydrogen concentrations can then be calculated with Eqs. (2) and (3), respectively. After a new set of concentrations is obtained, these are compared with the previous approximation using Eq. (4). If the resulting change in any C_i value is smaller than 0.001, the results are accepted as the final estimate of composition. If any C_i value changes by 0.001 or more, the C_i's are used in the next iteration.<br>A computer program has been written which collects X-ray data and converts the raw X-ray intensities to chemical composition. All constants such as X-ray line energies, effective energies and mass absorption coefficients were calculated internally. All data are corrected for dead time, background, peak overlap if necessary (equations 6, 7 and 8, and Table 4) and absorption. All analyses were performed in a helium atmosphere under the analytical conditions shown in bTable 2. The results of repeatability on a prepared sample are shown in Table 5. Coefficients of variation (CV%) of the concentrations of eight elements are in the range of 1.2 to 5.0%. The results (Fig. 3) obtained by the present method show fairly satisfactory agreement with the standard method cited in various literature.<br>The choice of a 6.5 for C/H value seems to be reasonable, because C/H values for most of the commercially available lubricating oils are distributed within a limited range (Fig. 1). In addition, changes of ±0.5 in the C/H value have little effect on both the mass absorption coefficients (Table 1) and the quantitative results (Fig. 3).