A scanning echelle monochromator for ICP-OES with dynamic wavelength stabilization and CCD detection

The design and evaluation of a scanning spectrometer with CCD detection are described. The spectrometer is a double monochromator employing a CaF2 prism to pre-select a spectral region equivalent to a single order for admission into an echelle grating monochromator. This design makes possible unambiguous wavelength selection without a restriction in entrance slit height. All wavelengths between 165 and 900 nm can be accessed by rotation of the dispersing elements within a range of +/- 2.4 degrees. A novel approach is taken to provide dynamic wavelength stabilization based on the simultaneous measurement of a neon reference spectrum. A dual back illuminated CCD was custom designed to match the characteristics of the spectrometer, provide for the simultaneous measurement of background in the vicinity of the analyte line and to facilitate the measurement of the reference spectrum. The detector exhibits a quantum efficiency greater than 50% throughout the UV range and a very low specific dark current so that cooling to -8 degrees C is sufficient for ICP-OES applications. This temperature is maintained by means of an integrated single stage Peltier cooling element. The spectrometer has a measured spectral bandpass of 0.007 nm at 200 nm. The dual monochromator design results in very low levels of diffuse stray light: with axial viewing in an ICP, a 10 000 mg L-1 Ca solution causes a shift in baseline at 193.696 nm equivalent to the peak height measured for a solution containing 75 mu g L-1 As. The dynamic approach to wavelength stabilization is demonstrated to function effectively over a wide ambient temperature range. As a result, measured spectral line intensity was stable to within +/- 2% over a period of several days during which ambient temperature was varied cyclically between 15 and 35 degrees C. The analytical performance in both axial and radial viewing modes is in keeping with the design and is limited by plasma background shot noise when simultaneous background correction is used. The high geometric radiation throughput provides for high measurement quality in short integration times. With axial viewing of the plasma and 5 s integration time, 3 sigma detection limits measured for P (177.434 nm), Tl (190.801 nm) and Se (196.026 nm) using a cross-flow nebulizer were 3.0, 2.2 and 2.7 mu g L-1, respectively.