Measurement of total calcium in serum by atomic absorption spectrophotometry, with use of a strontium internal reference.

A method for total calcium in serum was optimized, a dual double-beam atomic absorption spectrophotometer being used, with strontium as an internal reference. The effect of HCI and lanthanum concentrations on the signals of both calcium and strontium was determined. The final analysis was unaffected by known sources of interference in serum at a 50-fold dilution of serum in 50mmolar HCI containing 10mmol LaCI3and 0.12 mmol of SrCI2 per liter. Within-run precision for 30 replicate analyses performed during 30 mm gave a relative standard deviation (RSD) of 0.36% for the internal strontium reference mode vs. 0.72% RSD for the single-channel mode. Nine consecutive multiconcentration control pools were measured repeatedly for two years; the median RSD was 2.3%. With meticulous technique, long.term variability for total calcium in serum approached 1%. Accuracy is believed to be limited only by the precision, since systematic bias was not detected from recovery studies and comparisons with results of ashing procedures or oxalate precipitation techniques.