Qualification of a Voltammetry Measurement System

Doctoral thesis German OPEN
Scholz, Sunna (2014)
  • Publisher: Freie Universität Berlin Universitätsbibliothek, Garystr. 39, 14195 Berlin
  • Subject: qualification | ascorbic acid | voltammetry | DropSens | analytical instrument | screen printed electrodes | 543 Analytical chemistry | paracetamol | potentiostat | 543 Analytische Chemie
    • ddc: ddc:543

The voltammetry measurement system consisting of a potentiostat μStat 200, electrode strips and accessories has been scrutinized, following the four phases of instrument qualification. These are design qualification, installation qualification, operational qualification and performance qualification in succeeding order. As design qualification operational specifications of the voltammetry measurement system have been collected. These were the innovative technique of screen printing electrodes, disposable electrode strips, and that the electrode materials are nontoxic in contrast to mercury. Furthermore, the possibility was of interest, that measurements could be carry out outdoors, because of the small size and weight of the potentiostat. During the following installation qualification all delivered parts of the voltammetry measurement system have been checked and documented. The installation of software and hardware has been successful. The operational qualification consists of the testing of the parts of the voltammetry measurement system as well as the overall handling. The potentiostat has been tested and functions according to the specifications of the manufacturer. With regard to the electrodes only their surface properties could be studied. Because of the screen printing, they resemble a net like structure, which is not an ideal surface for an electrode. Moreover, the electrodes surfaces are hydrophobic, which leads to difficulties applying smaller volumes than 100 μL on an electrode strip. The reference electrode made out of silver is in direct contact with the sample solution and may be oxidized by it. Accessories like the measurement cell and the flow cell are functional, but due to their low weight they have to be secured in place. The requirement of the design qualification, to use the appliance outside the lab, is limited, since the small and light parts have to be set on a dry, stable and vibration-free surface. Testing of the software revealed numerous deficiencies. Divers errors occurred during execution of method parameters as well as in the course of processing and saving curves. As operational qualification various applications have been carried out on the voltammetry measurement system. The suitability of the system has been judged by the outcome. Since peak height and potential change with every measurement on an electrode strip DRP-110, the precision of CV measurements of one drop of Paracetamol solution has been tested. Measurements of 20 drops on one electrode strip showed rel. SD of 2,5% for the peak height and 13% for peak potential. The precision of those measurements can be increased by conditioning of the electrodes. The Pretreatment offered in the software does not decrease the SD. A pretreatment of the electrodes through CV scans of the buffer solution showed equally good results as the omitting the first five measurement with the electrodes for the calculations. Both approaches decreased rel. SD of the peak potentials by half and the rel. SD. of the peak height even to less than 1 %. The repeatability of 15 measurements following 5 conditioning runs on 20 electrode strips has been tested. The results were not significantly different and belonged to the same population with a probability of 95 %. The intermediate precision was checked against a different potentiostat (Trace Analyzer). The evaluation of the curves with the Trace Analyzer-software proved the series of measurements to be not significantly different from each other. A direct comparison of the potentiostats and control systems of the μStat and the Trace Analyzer by calibration curves of Paracetamol resulted in better performance of the Trace Analyzer. The changing of the electrode surface through oxidation and adsorption could be utilized to improve the sensitivity of the detection method of Nimodipin. With an adsorption time of 60 seconds and DPV measurement a LOD of 1,2 ng/L could be achieved, which had only been possible with adsorptive stripping methods at the hanging mercury drop electrode [18]. By means of the calibration curve LOD and LOQ of Paracetamol and 4-Aminophenol have been determined. The standard deviation for the procedure was 9,0 · 10−6 g/L for Paracetamol and 2,6 · 10−5 g/L for 4-Aminophenol. The LOD and LOQ were 1,0 · 10−5 g/L and 3,1 · 10−5 g/L for Paracetamol and 2,3 · 10−5 g/L and 7,0 · 10−5 g/L for 4-Aminophenol after evaluation of the curves with DropView. Additionally, the curves have been evaluated after convolution and derivation, which did not improve the limits. Testing for 4-Aminophenol as an impurity of Paracetamol resulted in a slightly higher LOD of 5,0 · 10−5 g/L, which corresponds to a yield of 52 ppm or respectively 75 ppm Paracetamol. On the basis of the oxidation of oxalic acid the properties of carbon nano tubes (DRP-110CNT) and Co-Phthalocyanine coated carbon electrodes (DRP-440) have been compared with the regular carbon electrodes (DRP-110). The tests showed, that an oxidation of oxalic acid occurs only on the Co-Phthalocyanine coated carbon electrodes. The carbon nano tubes electrodes do not exhibit any advantage over the regular carbon electrodes. The long term stability of the electrodes was tested via titration with the platinum working and counter electrodes (DRP-550). Monoamperometric as well as biamperomtric indication of the titration of ascorbic acid with iodine solution was used. The electrode strips were either connected to the μStat or to the titration system Titrando. Furthermore double platinum electrodes, silver ring electrodes and silver/silver chloride reference electrodes were used for titrations at the Titrando. The voltammetry measurement system is not applicable for the evaluation of titrations, because the yields are significantly different from the reference method. The yields of the mono- and biamperometic indications with the electrode strip were not significantly different from the reference methods. Due to the changing of the silver electrode only the biamperometric indication should be used. The electrodes are stable during the duration of the titration and react fast enough to the concentration change of the solution.
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