Total Calcium and Magnesium Determined in Serum with an Automated Stopped-Flow Analyzer

We describe the measurement of total calcium and mag-nesium in serum with an automated microcomputer-con-trolled stopped-flow analyzer. The calcium method is based on the cresolphthalein complexone procedure, with 2-amino-2-methy-1-propanol as the alkalinizing agent. The assay, performed on 60-fold prediluted samples, requires 50 tL of serum. Absorbance is measured at 580 nm for 1 s, after a 5-s delay. Response is linearly related to con-centration up to 5 mmol/L; analytical recovery averaged 97.8%. Within-day CVs were 0.7 to 1.5%, day-to-day CVs 1.8 to 2.5 %. Results compared well with those by con-tinuous-flow Technicon SMA II method. A sample throughput of as many as 260 samples per hour is possi-ble. The magnesium determination, a complexometric procedure, involves magnesium/calmagite complex in a