A microprocessor controlled scanning polarograph for solution labile compounds

Introduction The progressively greater number of analytical tests required on a sample and the larger number of samples required for significance of the data demand higher productivity to minimise increases in labour and equipment costs. At the same time, the precision and accuracy of the data must be maintained at the highest possible levels to assure validity of the results. Within the author’s laboratories, automation of equipment has been used to meet these objectives of increased efficiency and high quality [1, 2, 3]. Polarography, one of the few routine analytical techniques not previously automated in the author’s laboratory, is a labour and equipment intensive technique suitable for automation. This paper describes the development of an automated polarographic unit and compares its performance with nonautomated procedures. Two considerations are important in the polarographic assay procedures used. First, the cephalosporin antiobiotics, which comprise the largest quantity of polarographic samples tested [6], undergo decomposition in solution. Thus, an accurate determination of their purity is possible only when the sample is dissolved just prior to analysis [4, 5, 6]. Second, the solution concentration is directly related to the polarographic diffusion limited current which has been baseline corrected for residual current contributions. Two methods of corrections for residual currents are available: (1) extrapolation of the residual current baseline of the polarogram of the sample being analysed, and (2) measurement of the residual current on a blank solution. The former method corrects for residual currents due to impurities in the sample or other matrix effects but it is more complex than the latter technique where all currents are measured at a constant potential. Several papers have been published describing automated polarographic systems. The largest number of these systems utilise available Auto-Analyzer (Technicon Instruments Corp, Tarrytown, New York) components. A three-part paper by Lund and Opheim [7, 8, 9] is a good example of such flowthrough type systems. The dissolved sample is aspirated from the sample cup and pumped through the flow cell. A less complex system described by Flann [10] utilizes a modified Technicon Sampler II. A redesigned sampler arm places the deaeration probes and the electrodes into the proper cups. Both of these systems utilize previously dissolved samples (i.e. aged solutions) and measure currents at a constant potential. A commercial unit (Automatic Cell Sequencer, model 316, E.G. & G. Princeton Applied Research, Princeton, New Jersey) works on a principle similar to Flann’s system except that the entire polarogram is recorded. A system which automates the dissolution, deaeration and scanning of the sample has been reported by Cullen et al ]. The system has been used successfully to analyse pharmaceuticals in their finished form. The dissolution of the sample is accomplished with a Technicon Solidprep sampler. The Solidprep transfers the dry sample from the sample cup to the dissolution chamber. However, problems with this transfer operation have been documented [12, 13]. If the transfer of the sample is not complete, the accuracy and precision of the assay will be reduced. The problems associated with the transfer of the dry sample from the sample cup to the dissolution chamber have been overcome by the authors’ group by the development of a sampler unit which dissolves the samples in the removable glass weighing vessels. The design of the sampler also makes it possible to dissolve the sample just prior to analysis to minimise decomposition. The sampler unit has been interfaced with a 3-electrode polarograph (model 174A, E.G. & G. Princeton Applied Research, Princeton, New Jersey) for analysis of samples.