P. Mader

The reactions of cystine at mercury electrodes

Abstract Cystine in aqueous solutions reacts chemically with mercury, forming the surface-bound cysteine mercuric thiolate. When the potential of mercury is made less positive this compound is transformed (at a partially free mercury surface in a currentless process and at an occupied electrode under the passage of current) into cysteine mercurous thiolate. The latter compound is also formed in a reversible interaction of cystine with mercury in the potential range where cystine is adsorbed. At the negative end of this range the electroreduction of cystine occurs by the reduction of cysteine mercurous thiolate. At negatively charged mercury, where cystine is not adsorbed, two electrons are transferred directly to the cystine SS bond in an overall irreversible process. On the positive side the dissolution of mercury into solutions containing cystine starts by anodic formation of slightly soluble mercuric cystinate near the electrode surface.

The anodic reactions at mercury electrodes due to cysteine

Abstract The detailed mechanism of anodic reactions of cysteine on mercury electrodes has been ascertained by means of electrocapillary measurements together with d.c. and a.c. polarography and voltammetry. The reactions consist of electro-oxidation of mercury in two separate steps to cystein mercurous and mercuric thiolates respectively which are both strongly adsorbed at the electrode. At low coverages of the electrode by the mercurous product the mercuric thiolate is formed in the potential region of the second step by a non-faradaic surface process of disproportionation of the adsorbed mercurous thiolate; in consequence, only one faradaic step is then observed. The replacement of mercurous by mercuric thiolate at the electrode surface is accompanied by a pronounced change of electrode capacity. The path of the electrode reaction is determined by the form of mercury thiolates in the adsorbed state which depends strongly on the pH of the solution. In the first step in acidic solutions a compact film is formed of perpendicularly oriented mercurous cysteine thiolate molecules held together by strong lateral interaction, in alkaline solutions cysteine forms chelates with S and N atoms binding monomeric monovalent mercury. At positive potentials preceding the mass dissolution of mercury a direct formation of cysteine mercuric thiolate occurs in the solution, followed by its adsorption at the electrode surface.

Czechoslovakian biological certified reference materials and their use in the analytical quality assurance system in a trace element laboratory

SummaryThe Trace Element Laboratory of the University of Agriculture in Prague (CSFR) participated successfully in interlaboratory experiments on the determination of the contents of trace elements in seven Czechoslovakian reference materials. Czechoslovakian certified reference material CRM 12-02-01 (Bovine liver) was used for the development of a new analytical method using the Dry Mineralizer Apion as well as in analytical quality assessment of data in the determination of Cd, Pb, and Hg contents in routinely analyzed animal tissues.

Catalytic hydrogen activity of the low-molecular weight thiols: kinetic control of the brdička catalytic current☆

Abstract At sufficient excess of cobalt, the Brdicka polarographic catalytic hydrogen currents of low molecular weight thiols frequently exhibit typical kinetic characteristics. A detailed investigation of these currents has revealed also a marked adsorption component. A mechanism which leads to the formation of the Brdicka currents was suggested on the basis of the changes of these currents with a number of factors. Under the speicified conditons the Brdicka catalytic wave may be distorted; the observed deformations are either due to the modification of the proper Brdicka current, or they are a consequence of the simultaneous appearance of another type of the catalytic hydrogen current, which can also exist in the transition metal/ligand systems.

Combination of classical dry ashing with stripping voltammetry in trace element analysis of biological materials: review of literature published after 1978

Critical statements have appeared recently in the literature concerning the need for classical dry ashing in trace element analysis of biological materials. In contrast, respected institutions (AOAC, Nordic Committee on Food Analysis, etc.) as well as numerous other laboratories have developed, verified, and/or successfully used classical dry ashing in practical analyses of a number of materials of biological origin. Hence, it is desirable to find out under which conditions the latter decomposition technique yields good and accurate results. Since electroanalytical techniques are among the most demanding with regard to the completeness of the biological matrix removal, we decided to critically review the literature published after 1978 in which classical dry ashing is combined with some version of electroanalytical measurement. It emerged from this review that in particular the charring step requires careful performing. When performed well, classical dry ashing leads to complete removal of the organic matrix and to accurate analytical results for a number of determined elements.

Preparation of the bovine liver 12-02-01 reference material and the certification of element contents from an interlaboratory comparison

SummaryThe preparation of the bovine liver 12-02-01 reference material, its stabilization and homogeneity testing is presented. Evaluation of an interlaboratory comparison in which 31 laboratories took part using several analytical methods is described. From the results of the intercomparison, the contents of the elements Cd, Cu, Hg, Mn, Pb, Se, and Zn were certified, while for the elements Br, Co, Fe, K, Mo, Na, and Rb, information values were derived. Non-recommended values for the elements Ag, As, Ca, Cl, Cr, Cs, La, Mg, S, Sb, Sn, Sr, and V were also evaluated.

Effect of Surfactants on the polarographic Brdicka activity of cysteine

Abstract The catalytic brdicka activity of cysteine is greatly modified by a number of electrochemically inactive surfactants. Regardless of their chemical nature and electrical charge, all surfactants which are adsorbed at the electrode surface also at potentials of the brdicka catalytic hydrogen wave bring about a gradual increase in the height of the brdicka currents at low degree of coverage of the electrode surface by the adsorbed surfactant. When the degree of this coverage exceeds a certain value, the stimulating effect gradually diminishes with further increasing surfactant concentration. Finally, when the electrode becomes fully covered, the brdicka currents become suppressed compared to their magnitude in the complete absence of the surfactant. The possibility of the self—modifying effect of the proteinic catalysts on their brdicka catalytic activity is discussed.

Preparation of the Bovine Kidney and Bovine Muscle reference materials and the certification of element contents from interlaboratory comparisons

Results obtained in interlaboratory comparisons on candidate reference materials Bovine Kidney and Bovine Muscle, in which more than 32 laboratories participated employing atomic spectrometry, electrochemical, and nuclear analytical techniques, have been evaluated using combined statistical and analytical considerations. Certified and/or information values for the elements Br, Ca, Cd, Co, Cr, Cu, Fe, Hg, K, Mg, Mn, Mo, Na, P, Pb, Rb, Se, and Zn have been established. Information on contents of the elements Al, As, Au, Cs, Li, Ni, Sb, Sn, Sr, and V is also presented.

Cadmium, lead and mercury levels in feeding yeast produced in Czechoslovakia

Ninety-six samples of the feeding yeast known as VITEX were analyzed for Cd, Pb and Hg content during 1987-1989. Cadmium content ranged from 0.30 to 5.12 mg/kg(-1), lead content from 0.21 to 3.01 mg/kg(-1) and mercury content from 0.008 to 0.187 mg/kg(-1). Our findings meet the current government standards (max. allowed Pb = 5.00, Cd = 0.50 and Hg = 0.100 mg/kg(-1)) only for lead, and with five exceptions, for mercury. With two exceptions, all cadmium levels found in the samples exceeded the limit. One raw material - the wood chips - was shown to be the main source of cadmium in the technological process. Relatively high Hg contents were measured in the wood chips (up to 0.155 mg/kg(-1)); the highest Hg level (1.105 mg/kg(-1)) however was found in a sample of KOH.

International plant‐analytical exchange and quality assurance in a trace element laboratory

Abstract Our results of analyses of the plant samples within the International Plant‐analytical Exchange (IPE) program are discussed here. IPE is organized as proficiency test by the Wageningen Agricultural University (NL) and our laboratory has been participating in it since the middle of 1992. The agreement of our data with statistical estimates of true values derived from the compiled results supplied by all laboratories is satisfactory and confirms that procedures used in our laboratory and the whole analytical system lead to accurate results for plant materials. Moreover, remaining part of plant material No. 980 (Gerbera) is used with success as internal reference material in quality control (control charts) in our trace element laboratory.