Micro-method for the estimation of calcium by AutoAnalyser

The standard AutoAnalyser technique for the determination of calcium in plasma or other biological materials is based on the photometry of calcium murexide.Using this method we have been unable to obtain sufficient sensitivity to measure accurately the calcium content of plasma.Other disadvantages of this method include base-line drift due to instability of murexide, and consider- able contamination from sample to sample.Moreover, the estimation involves the diffusion of calcium across a semi-permeable membrane, and the diffusion rate of calcium from an aqueous standard differs from that of a protein-containing solution such as plasma.Wieme and van Raepenbusch (1962) described an AutoAnalyser method using plasmacorinth B instead of murexide, based on the manual method of Kingsley and Robnett (1957, 1958).We have increased the sensitivity of their method by eliminating the 37°C.water-bath, by using a more favourable wavelength, and by employing a 15 mm.flow cuvette.The plasmacorinth B concentration has been reduced to lower the blank value.These modi- fications permit the estimation of calcium concentration in 0-2 ml. of plasma.The method is also suitable for the estimation of calcium concentration in both urine and faeces.We have confirmed that interference by magnesium and phosphate is very small. REAGENTSAll the distilled water used was further purified by deionization.40% w/v SODIUM HYDROXIDE This solution contains 0-4 % w/v AP 14 surface-active agent (I.C.I.).PLASMACORINTH B STOCK SOLUTION (200 mg./100 ml.)Plasmacorinth B (G. Gurr), 500 mg., is dissolved in about 200 ml. of distilled water, containing 0-75 ml. of 0-1N hydrochloric acid, with vigorous stirring for about one hour, and then made up to 250 ml. with distilled water.Stored in a dark bottle at room temperature the solution is stable for at least three months.PLASMACORINTH B WORKING SOLUTION (40 mg./100 ml.) Plasmacorinth B stock solution, 20 ml., is diluted to 100 ml. with distilled water with the subsequent addition of 0 5 ml. of 0-1N hydrochloric acid.Stored in a dark bottle at room temperature this solution is stable for one week.This quantity is sufficient for 90 minutes running time.'AutoAnalyser is the trade name of Technicon Instruments Co.