|The processing of emissions with a temporal variety in the optical emission spectrometry (OES) is not a widespread application nowadays. The acquisition of such a “transient” emission signal results from a large number of applications. Consequently, a chromatographic separation leads to an irregular entry /registration in the detector unit, for instance, into a spectrometric plasma excitation unit. As a customary example, the optical emission detector (AED) of Hewlett Packard can be named.|
Concerning a special application in the field of environment, a system was searched making the quantitative detection of transient halogen emissions possible . For this purpose, an acquisition software and an analysation software meeting the respective requirements has been developed for the PC 2000 on the basis of a LABVIEW-driver.
For the application 2 spectrometers (PC 2000 Master + Slave) have been used. The utilizable spectral range for the Master card was between 720 nm and 960 nm; for the Slave card 450 – 720. Both cards used gratings with 1200 lines /mm . By means of the 5 µm slit an optical resolution of less than 0,4 nm has been reached.
The measurement set-up with the microwave plasma chamber and the injection block is depicted in the following illustration:
Illustration 1: picture of the measurement set-up for the creation and acquisition of transient halogen signals
In the first step, the most suitable emission lines for fluorine (685,603 nm), chlorine (837,762 nm), brome (889,762 nm) and iodine (804,093 nm) and their separation from the total spectrum was investigated. These lines should have sufficient intensity to detect smaller concentrations but at the same time enable a satisfactory linear workspace when detection with a CCD-detector of the spectrometer takes place. Since the LABVIEW-driver allowed merely qualitative results, the development of a software which makes possible a transient acquisition as well as an analysis (including underground correction) of the signals. For all halogens, this should occur simultaneously.
To fulfil the above-mentioned conditions, the optimisation of the integration time of the CCD is vital. As one can note from illustration 2, intensive emissions lead to a Blooming (in this case from 765 nm up to 775 nm as well as from 785 nm to 792 nm) when integration time is too long.
Illustration 2: emission spectrum of a microwave-induced plasma of 720 nm – 850 nm
In case of considerably decreased integration times emissions of weak intensity are no longer acquired sufficiently. The optimisation of the detecting time has occurred for all halogens and allowed their linear acquisition without Blooming over a concentration range of 3 decades.
In the second step, the underground correction as well as the analysis of the emission with a temporal variety, leading to the graph illustrated below:
Illustration 3: graph of an analysed transient chlorine emission
In the third step, the measurement procedure has been handed to an integration routine solely developed for it, allowing the quantification of the measuring signal.
In the following development steps the expansion of the detection software with the Sentronic spectrometer PC 2000 of up to 8 variable emission lines which can be optimised for simultaneous and transient detection in the atomic spectrometry has succeeded. Thus, an efficient detector for multielement detection with a discontinuous sample presentation is available.
"Applied atomic spectroscopy" at the Institute for Chemo- and Bisosensor technology, Münster (ICB Münster) - Andre Klostermeier (developer of the above-mentioned software)