INFLUENCE OF THE PRACTICAL RESOLUTION ON LIMITS OF DETECTION USING HIGH LINE NUMBER GRATINGS IN INDUCTIVELY COUPLED PLASMA ATOMIC EMISSION-SPECTROMETRY

The signal to background ratio and the relative standard deviation (RSD) of the background were studied in order to optimize the limits of detection in inductively coupled plasma atomic emission spectrometry. Czerny-Turner type monochromators equipped with high line number gratings (2400 and 3600 line mm-1) working in the first and second orders were used. Because of the high incident and diffraction angles, leading to a magnification factor different from one, the influence of the slit width adjustment was studied. Symmetrical and asymmetrical slit widths were considered. It is shown that high signal to background ratios were obtained with slit widths of 10-mu-m. A degradation of the RSD of the background was observed for the narrow slit widths. This was not due to the dark current noise of the photomultiplier tube but rather to the limited number of photons. A longer integration time could alleviate this limitation. This was illustrated for the Cd I 228 nm line, which is one of the narrowest observed in this radiation source.