Micellar improvement of the calmagite compleximetric measurement of magnesium in plasma.

Magnesium in plasma is determined by diluting a 50-microL sample with 5.0 mL of stable calmagite reagent containing the amphoteric detergent Empigen BB, and buffered with 2-amino-2-methyl-1-propanol at pH 11.5. The calcium response is masked with strontium-buffered [ethylene-bis(oxyethylenenitrilo)]tetraacetate (EGTA), while iron is masked with triethanolamine. The detergent causes an increased separation of the test and blank spectral absorbance bands. The test absorbance peaks, which may be measured immediately, are at 520 nm, while the blank peak shifts from 610 to 655 nm on addition of detergent. The blank absorbance at 520 nm is relatively decreased by the detergent. This allows the use of a more concentrated calmagite reagent, which in turn extends the linear relation of absorbance to magnesium concentration to 5 mmol/L in plasma. This method (y) has been compared with atomic absorption procedure (x) of McDonald and Watson (Clin, Chim. Acta 14: 233, 1966) and gives favorable regression statistics (y = 1.001X + 0.057 mmol/L).