The Fundamentals of AAS - MEP Instruments

2.2.4 The detector in HR-CS AASIn HR-CS AAS a linear CCD array with typically 512 pixels (picture elements) is used as thedetector, 200 of which are used for analytical purposes. Each individual pixel is evaluatedindependently, so that the equipment basically works with 200 independent detectors. All the 200pixels are illuminated simultaneously (for 1-10 ms) and read out simultaneously. The nextillumination is already being carried out during signal evaluation, making possible a very rapidmeasurement frequency. Fig. 2-20 is an example of typical detector readout, where the measuredvalue for each pixel in the environment of the analytical line for sodium at 330.237 nm is shown.The absorption line is essentially covered by three pixels, while the other pixels only exhibit thestatistical variation of the baseline, i.e. the noise.Fig. 2-20: Readout of the integrated absorbance measured by each individual pixel in the environment of the analytical line of Na 330.237 nmAs only three pixels are used in most cases to measure atomic absorption, the others may be usedfor correction purposes. As all pixels are illuminated and read out simultaneously, any variation ofthe intensity that is independent on wavelength, such as variations in lamp emission, can bedetected and eliminated through the use of correction pixels. This results in an extremely stablesystem with low noise level and significantly improved signal-to-noise ratios. The same correctionsystem also eliminates any continuous (over wavelength) background absorption, as will bediscussed in a later section.Another decisive advantage of this measurement principle with 200 detectors is that the entireenvironment of the analytical line becomes ‘visible’ at high resolution. This way it is possible forexample to recognize and avoid spectral interferences much more easily.Fundamentals , Instrumentation and Techniques of Atomic Absorption Spectrometry 22/ 65Analytik Jena AG | Konrad-Zuse-Straße 1 | 07745 Jena / Germany | info@analytik-jena.com | www.analytik-jena.com