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 pri sm to pre-select a spectral region equivalent to a single order for admissi on into an echelle grating monochromator. This design makes possible unambi guous wavelength selection without a restriction in entrance slit height. A ll wavelengths between 165 and 900 nm can be accessed by rotation of the di spersing elements within a range of +/- 2.4 degrees. A novel approach is ta ken to provide dynamic wavelength stabilization based on the simultaneous m easurement of a neon reference spectrum. A dual back illuminated CCD was cu stom 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 detec tor 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 suf ficient for ICP-OES applications. This temperature is maintained by means o f 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 viewin g 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 demonstrat ed to function effectively over a wide ambient temperature range. As a resu lt, measured spectral line intensity was stable to within +/- 2% over a per iod of several days during which ambient temperature was varied cyclically between 15 and 35 degrees C. The analytical performance in both axial and r adial 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 i n short integration times. With axial viewing of the plasma and 5 s integra tion time, 3 sigma detection limits measured for P (177.434 nm), Tl (190.80 1 nm) and Se (196.026 nm) using a cross-flow nebulizer were 3.0, 2.2 and 2. 7 mu g L-1, respectively.