In a various fields such as electronics industry, it is required that trace amounts of impurity existing in a raw material can be determined. When an analytical method is not sensitive enough for trace amounts of impurity to be determined, the impurity is concentrated by various kinds of preconcentration techniques prior to the determination of the impurity amounts.
In case of an absorptiometry, one of conventional methods to improve a sensitivity is described on page 345 in a report "Talanta", Vol. 32, No. 5, 1985. In this report, granular solid such as ion-exchanger, which is weighed out prior to use, is added to a sample solution, whereby an impurity contained in the sample solution is adsorbed and concentrated on the granular solid. And then, the color of the impurity is developed in the solid.
After separated from the solution, the solid is charged into a filling portion of a measuring cell. Next, a light absorption of the solid phase is measured at definite wavelengths directly. And then, the amounts of impurity are determined by using a calibration graph.
Generally, the smaller amount of granular solid is employed in the analytical method, the higher concentration of a trace impurity is obtained in the solid.
The conventional method for trace analysis of impurity, however, has disadvantages, that is, it is difficult to aliquot a small amount of the solid with high reproducibility due to its hygroscopic property. In particular, an influence of a moisture absorption in aliquotting the solid is pronounced when the weight of the solid used in the analytical method is less than 0.1 g.
Further, when the filling portion capacity of the light absorption measuring cell is much larger than the volume of the solid used, a part of light beam passes through an unpacked area with the solid and directly reaches to a photo detector of a spectrophotometer. As a result, the absorbance obtained is unreliable.