The Design, Fabrication and Performance Evaluation of a Scanning Monochromator for Rare Earth Elements Analysis

A direct reading spectrometer which can determine several elements in rare earth samples has been fabricated and tested. The spectrometer is based on Czerny-Turner type of configuration and consists of a plane grating of frequency 3600 grooves/mm as a dispersing element, concave spherical mirror of focal length 1.191 m as a collimator and a concave spherical mirror of focal length 0.973 m as a focusing element. The design of unequal focal lengths for collimating mirror and focusing mirror has been chosen to correct the comatic aberration at the wave length of 3500 Å. The reciprocal linear dispersion of the monochromator is 2.5 Å /mm and the resolution achieved is 0.23 Å.For setting various wavelengths at the exit slit, the grating is rotated by a stepper motor. A sine drive assembly is used for linearising the wavelength scale. The wavelength repeatability of the isntrument is found to be 0.2 Å and the maximum deviation from the wavelength linearity is 2.8 Å over the wavelength range of 2000 Å to 4500 Å. The Wavelength at the exit slit is detected by a photomultiplier tube. The data acquisition is done through PC/AT.The performance of the spectrometer has been evaluated with an ICP excitation source. Synthetic standards of Rare Earth Elements containing Ce,Sm,Dy,Er,Gd,Yb,Y,Mo have been analysed. The sample in the form of solution is nebulised into a plasma formed by the ICP excitation source. The plasma is generated in argon atmosphere by a 27.12 MHz RF generator at 1.0 KW forward power. The typical detection limits for elements like Mo,Mn and Ce are 5.6, 1.5 and 37.0 ng/ml respectively. These detection limits are found to be close to the values given in the literature.