Total calcium and magnesium determined in serum with an automated stopped-flow analyzer

We describe the measurement of total calcium and magnesium in serum with an automated microcomputer-controlled stopped-flow analyzer. The calcium method is based on the cresolphthalein complexone procedure, with 2-amino-3-methyl-1-propanol as the alkalinizing agent. The assay, performed on 60-fold prediluted samples, requires 50 microL of serum. Absorbance is measured at 580 nm for 1 s, after a 5-s delay. Response is linearly related to concentration 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 continuous-flow Technicon SMA II method. A sample throughput of as many as 260 samples per hour is possible. The magnesium determination, a complexometric procedure, involves magnesium/calmagite complex in an alkaline reagent mixture and ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid to eliminate calcium interference. Prediluted serum samples are used (100 microL of serum diluted 25-fold), and absorbance at 520 nm is linear with concentration to 50 mg/L. Within-run CVs were 0.5 to 1.1%, and day-to-day 1.3 to 3.8%; analytical recovery was 99.3%. Results compared well with those by atomic absorption spectrometry (r = 0.994). A delay time of 10 and a measurement time of 2.5 s allows for a throughput of as many as 180 samples per hour.