Electron-probe microanalysis for the drug purity assessment with respect to heavy metals

Abstract

The content of heavy metals is among the criteria used to characterize the quality of drugs. The State Pharmacopeia of the Russian Federation (Ed. XI) recommends using sulfide method [ 1 ] in testing drugs for the "heavy metal content" criteflon. However, the procedure suggested in the State Pharmacopeia for this purpose is nonsix~ific and does not allow quantitative determination of the concentration of heavy metals. For this reason, other methods are sometimes offered for testing drugs with respect to the heavy metal content [ 1 -4]. The purpose of this work was to develop a procedure based on the electron-probe X-ray microanalysis (EPMA) for drug purity testing. This method is based on measuring the specman of characteristic X-ray emission from chemical elements, excited in the sample by a probing beam of high-energy electrons. The physical principles of EPMA are described in detail in [5 -7]. EXPERIMENTAL PART Sample PreparationThe samples for analysis were presented by the substances of levomycetin, nicodine, isoniazid, and ethacridine lactate used for drug production by the "Akrikhin" Pharmaceutical Chemistry Joint-Stock Company. The method of preparing powdered samples for EPMA was described elsewhere [8]. The powdered drugs were placed between two quartz plates and compacted by slight pressing. This microtablet (with a weight not exceeding 0. l mg) was transferred onto a sample holder made of a spectraIIy pure carbon. The holder with the sample was inserted into a chamber of a Balzers Micro BA3 vacuum stage (Liechtenstein). In order to render the sample conducting, the microtablet surface was coated with a 30-nm thick vacuum-deposited carbon layer. Then the carbon holder with the prepared drug sample was transferred into an electron microscope operated in the EPMA mode. 673 Preparation of Dextran StandardThe sensitivity was calibrated with the aid of dextran standards containing preset amounts of analyzed elements, modeling the physicochemical conditions accompanying the interaction of electron probe with organic drugs. The method of preparing standards and calculating sensitivities was described in detail elsewhere [9].In the first step, a solution of water-soluble salts was prepared using 5 ml of bidistilled water and a mixture of 15 mg CuSO4 9 5H20, 10 mg NiSO4 9 7H20, 14 mg CdSO4 9 8H20, 14 mg ZnSO4-7H20, and 12 mg CoCl 2. Then a 250 mg aliquote of dextran was dissolved in I ml of the salt solution. A droplet of this dextran standard was applied onto a perforated gold substrate, frozen in liquid nitrogen, and lyophilized by being held for 18 h in a vacuum of 5 x 10 -4 Pa at 200 K [10] (the principle and parameters of a low-temperature vacuum lyophilizer system were described in [11]). As a result, we obtained standard samples comprising powdered dextran with preset concentrations of Cu, Ni, Cd, Zn, and Co. The subsequent procedure of preparing the standard for EPMA measurements was the same as that described above for the drugs. Electron-Probe MicroanalyzerThe elemental analysis by EPMA is based on r...