CHARACTERIZATION OF A COMPUTERIZED PHOTODIODE ARRAY SPECTROMETER FOR GAS-CHROMATOGRAPHY ATOMIC EMISSION-SPECTROMETRY

An atomic emission detector (AED) for gas chromatography offers improved sensitivity and selectivity over earlier AEDs for all major elements found in gas chromatographic effluents. It is based on a novel water-cooled, microwave-powered plasma light source, which is described in a companion paper. A photodiode array (PDA) improves selectivity by several orders of magnitude by means of real-time multipoint background correction. A computer-controlled spectrometer, based on a concave holographic grating, has a flat focal plane along which the PDA can be moved. The wavelength range Is from 160 to 800 nm. With the PDA, special algorithms provide automated wavelength calibration, autofocusing and measurements of intensity, wavelength, and line width. Wavelength measurements are precise to 0.004 nm, and wavelength changes to positions far from calibration wavelengths are accurate to 0.008 nm. The detection of an elemental emission line and Its spectral background makes use of many simultaneous signals from the PDA. These signals are combined Into a pair of digital filters, called a recipe. The recipe structure provides optimal signal detection and has null response to, or is orthogonal to, the background spectrum. The signal and background portions of a chromatogram are recorded separately, so that the selectivity can be improved after the run is completed. The minimum detectable level is 20 pg/s for chlorine and 13 pg/s for bromine. An advanced type of recipe has null response to two different Interfering spectra. This Improves selectivities under routine conditions by a factor between 3 and 10. For chlorine and bromine, molar selectivites with respect to carbon of 24 000 and 13 000, respectively, have been achieved. Spectra are acquired continuously during a chromatogram. They are used to confirm the elemental Identification of GC peaks and to make new recipes. An example is given where these spectra allow a subtle spectral change that degraded selectivity to be diagnosed and corrected. The effects of misalignment and of drift on performance are measured. Improvements in sensitivity and selectivity are reported for sulfur and for chlorine detection. Recipes are tested for sensitivity to wavelength errors and for sensitivity to shifts between multiple instruments. Over several days, responses are typically constant to 1 %. © 1990, American Chemical Society. All rights reserved.