Humidity-induced spectral shift in a cross-dispersion echelle spectrometer and its theoretical investigation

The relationship between ambient relative humidity H and the position shift of a spectral line was investigated both experimentally and theoretically. An echelle-based ICP emission spectrometer equipped with a CID detector was used for experimental verification of the derived model. The shift of a spectral line is quantitatively described by two defined spectral shift functions: delta lambda x(x, lamda, H) (in the x direction of the CID detector) and delta lambda y(y, lambda, H) (in the y direction of the CID detector). Experimental results indicate that delta lamda x(x, lambda, H) does not change with a variation in ambient relative humidity, but delta lamda y(y, lambda, H) does. A spectral shift equation, i.e., an empirical second-order polynomial equation, can be used to describe the relationship between delta lamda y(y, lambda, H) and H. Based on the classical dipole model, classical mechanics and electrodynamics the empirical spectral-shift equation involving delta lambda y(y, lambda, H) and H was theoretically deduced. The theoretical result is in good agreement with the experimental findings. The theoretical results indicate that the coefficients of the empirical spectral-shift equation are related to the basic physical parameters of materials and the geometric configuration of the echelle CID ICP-AES, and also provide physical meaning to the coefficients of the empirical shift equation obtained experimentally.